Friday, December 14, 2012

Curiosity On Mars

The Curiosity Rover on Mars, shown below, made headlines in the last few days.

The image above is a mosaic self-portrait taken by the rover's imager at Rocknest, an area in Gale Crater on Mars, taken in October 2012.

What we know about Earth, Mars, other planets in our solar system and even other planets in our universe is increasing every day but deep mysteries remain. Mars offers a tremendously valuable natural laboratory in which scientists can explore fundamental questions about how life on Earth came to exist.

The image above compares the size of Earth and Mars in true colour.

Understanding Mars (and Earth) will help researchers figure out what to look for as they look for signs of life in the universe. I doubt there is a human alive that hasn't wondered, are we alone? We are asking not only if there is life elsewhere but also how life got its start here on Earth.

Humans have been listening to the cosmos for decades and we haven't heard a peep from anyone or anything, leaving the question of whether we're alone or not unanswered. Meanwhile, there are many questions about the nature of life that might have answers here on our own planet or in our own solar system. What defines life? What does life need to exist in terms of environment, nutrients, energy, and basic bodily building blocks or molecules? Is liquid water or oxygen necessary for life? What about radiation? What chemical reactions are absolutely required for life? Must it be based on DNA, protein, or even carbon, like life on Earth? These questions help astronomers look for possible signs of life in the universe. And they help scientists know what questions to ask and what to look for on Mars. Mars is a rocky planet similar to Earth, and it seems very likely to have had liquid water during some period in its past. The image below is an artist's impression of what a wetter Mars might have looked like billions of years ago, based on geological data.


Mars should have had many of the same raw materials that Earth once did when the first simple unicellular organisms began to evolve here. Figuring out what the raw ingredients for life were on Earth and then looking for signs they once existed, or still exist, on Mars is a logical step in answering the question, are we alone, and it is exactly what Curiosity is designed to do.

What Curiosity Should Look For On Mars

Scientists are using what we know about life on Earth as the starting point, so they are looking for signs of organic (carbon-based) life. That search began with looking for obvious signs of life on Mars such as fossilized organisms like bacteria, as well as atmospheric and geological signatures of living organisms. For example, oxygen in our atmosphere is a signature of abundant photosynthetic plant life on Earth. Molecular oxygen (O2) gas is highly reactive. It quickly disappears from the atmosphere by reacting with other gas molecules and oxidizing rock. If all photosynthesis suddenly stopped, almost all the free atmospheric oxygen on Earth would gradually be depleted. Although oxygen is necessary for multicellular life on Earth, Mars could have harboured life even though it has an oxygen-deficient atmosphere. The first simple organisms on Earth, called anaerobes, evolved when Earth had almost no oxygen in its atmosphere. In fact, a gradual oxygen build-up in the atmosphere by cyanobacteria colonizing at the time was toxic to them, causing Earth's first major extinction event.

No obvious signs of life on Mars have been found but that doesn't mean that some pre-life organic molecules didn't form. Biochemists have made progress toward understanding how very simple life can evolve from the right mix of complex organic molecules in a favourable environment, but no one has yet been able to replicate the millions of years of natural and spontaneous organic chemistry that led to the first life on Earth.

Any sign of organic molecules that could be building blocks of life on Mars might give us a clue that life can and will form given the right ingredients, environment and time. Current geological evidence suggests that billions of years ago, Earth was a tumultuous place where volcanoes raged, filling the sky with lightning. It was almost completely covered by a shallow warm sea, rich in dissolved carbon dioxide, methane, ammonia, hydrogen sulphide and hydrogen cyanide. Most researchers believe the environment began to stabilize quickly and nucleotides, which are basic components of RNA (shown below right), formed and replicated spontaneously followed by ribosomes and proteins. These complex organic structures were eventually enclosed in a primitive membrane and developed a way to reproduce themselves, forming the first simple unicellular organisms.

LIke DNA, RNA is a remarkable biological molecule that codes and decodes information and regulates the expression of genes. The molecule itself is an enzyme and a chemical catalyst, which means it can self-duplicate and it can  enhance the creation of other molecules such as helping to build peptides from amino acids, both important properties of life. Unlike DNA, RNA is a single strand, like the hairpin strand shown right. It's used as a messenger molecule in our bodies, helping to turn the genetic information in our DNA into proteins. Because RNA is simpler than DNA it may have predated it, being the first genetic material to be function inside simple virus-like and bacteria-like organisms.

If scientists could find signs that a similar process at least began on Mars, we could begin to answer a fundamental question about the universe - is life inevitable? This very human question makes the current Curiosity Mission so compelling.

Past Rover Missons To Mars Found Evidence Of Liquid Water

The Curiosity rover is a car-size robotic rover that was launched in November, 2011 It landed in Gale Crater on Mars in August this year (2012). It was originally on a two-year mission but that mission was extended indefinitely just a few days ago. Curiosity (the one to the right) is the latest of three generations of Mars rovers, shown below.

The examples above are test rovers, all from NASA's Jet Propulsion Laboratory. The smallest rover, front center, is the flight spare of the Sojourner, which landed on Mars in 1997. To the left is the test rover for Spirit and Opportunity, which both landed on Mars in 2004 for a planned 90-(Martian) day mission. Spirit became stuck in 2009 and ceased communications in 2010 but Opportunity is still active on Mars, moving, gathering information and reporting back to Earth. Whereas Opportunity is a solar-powered rover, Curiosity is fueled by a radioisotope thermoelectric generator. It converts the heat generated by radioactive plutonium into electricity. The plutonium decays slowly (half-life of 87.7 years) so the electricity available 14 years from now will be only slightly reduced from 125 watts of power to 100 watts, its minimum expected lifetime.

All the rovers were designed to test the rocks, soil (also called sand but properly called regolith) and atmosphere of Mars in a hands-on way. After seeing what looked like dry riverbeds and other large water-related structures on the surface of Mars (shown below), scientists wanted to look for signs of past water activity such as precipitation, evaporation and sedimentation and to look for minerals that are known to be created only in the presence of water.

The image left shows streamline islands in Maja Vallis on Mars, taken by Viking. The image bottom left shows intricately branched channels, also taken by Viking. For more Viking images and information, try the online publication by the Viking Orbiter Imaging Team.

The identical Viking 1 and Viking 2 landers carried out the first experiments to look for signs of life on Mars in the late 1970's. They carried out gas chromatography, a gas exchange experiment, a labelled release experiment and a pyrolytic release experiment (all described in the preceding link). Organic compounds are common on asteroids, meteorites and comets so researchers expected to find them on the surface of Mars too, but they didn't find anything organic except chloromethane and dichloromethane. One theoretical explanation is that the surface of Mars, exposed to strong ultraviolet (UV) radiation, has built up a strongly oxidizing layer of regolith. One of these oxidants is perchlorate which breaks organic molecules apart, leaving chloromethane and dichloromethane as products. Perchlorate was later discovered on Mars in 2008 by the Wet Chemistry Lab onboard the Phoenix Mars Lander. Most researchers found the Viking organic molecule findings inconclusive, spurring the beginning of Mars rover exploration, where samples can be taken from a variety of geological sites.

All life as we know it requires liquid water, at least at some stage. If direct evidence of past liquid water was found, researchers could focus on determining if there was ever a life-conducive environment on Mars. Meanwhile, the Sojourner rover along with the Pathfinder lander, shown below, was launched in 1996 to test the idea of sending a robotic rover and to explore the Martian atmosphere and surface. It had cameras, a meteorological station to investigate the Martian atmosphere and an X-ray spectrometer to analyze soils and rocks.

Workers at the Jet Propulsion Laboratory are shown above closing up the metal petals of the Pathfinder Lander, enclosing the Sojourner rover inside, visible on the nearest "petal" before its launch.

The Sojourner data suggested that Mars did in fact have a warmer wetter past with a thicker atmosphere and liquid water. The Spirit and Opportunity rovers were then designed to expand on the successful Sojourner rover prototype and test the hypothesis of a once wet and warm Mars further. Below, one of the rovers is shown inside its lander's petals.

These rovers have a robotic arm, panoramic cameras, three different spectrometers to test rock and soil, a rock abrasion tool to remove dust and examine fresh material underneath, and a microscopic imager. The rovers were designed to travel widely and test many different geological sites. They found that most of Mars' rock appeared to be volcanic in origin and its soil, or regolith, comes from the weathering of these rocks. They found significant nickel in some soils, suggesting that the regolith came from meteoric impacts as well. Chemical analysis showed that the volcanic rocks have been slightly altered by tiny amounts of water and that coatings and cracks in the rocks contain water-deposited minerals. They also found that dust on the planet, which covers all surfaces, is magnetic because it was shown to contain the mineral magnetite. Some component in the dust, possibly sulphate minerals, also contains chemically bound water. They found additional chemical confirmation of water in water-specific minerals such as goethite and carbonates. In a region called the Columbia Hills, they found clear evidence of weathering caused by liquid water.

Knowing that liquid water was once present on Mars, the Curiosity mission is designed to go one step further by looking specifically for evidence of chemistry linked to, or a possible precursor of, organic life on Mars.

How To Get A Big Robotic Science Lab To Mars

This rover is big (the size of a small car and weighing about 900 kg), so it could not be placed in a lander. Instead, it had to be put directly inside its aeroshell (the protective container on the spaceship that protects the unit from space). It also couldn't be slowed down and cushioned upon landing on Mars using airbags, like the Pathfinder mission and the Mars Exploration Rover mission used. This landing had to be precisely guided and soft, and it had to be pre-programmed in advance because of the time delay between Earth and Mars.

First, the aeroshell containing the rover separated from the cruise stage of the rocket, which provided power, communications and propulsion during the long flight to Mars. Thrusters on the aeroshell then fired to place it within a 20 by 7 km landing ellipse and align its heat shield. A supersonic parachute deployed when the aeroshell slowed down enough by friction with the Martian atmosphere. All the previous rover landings used similar parachutes but the Martian atmosphere is so thin these parachutes are not enough to slow the aeroshell sufficiently. The returning command module used in the Apollo missions could rely on atmospheric braking and parachutes alone because Earth's atmosphere is much thicker. The aeroshell's heat shield separated and fell away and the rover (attached to a descent stage platform) dropped out of the aeroshell. Variable thrust rockets on the descent stage platform above the rover further slowed the descent, using a radar altimeter feeding data to the rover's flight computer as it navigated itself down. Soon afterward, a sky crane lowered Curiosity under the descent stage while Curiosity transformed from its stowed configuration to its landing configuration, locking its wheels in place. The sky crane slowed to a complete stop as Curiosity touched down and soon after freed itself and flew away to crash land elsewhere. This new procedure, shown below, is an important advance in lander technology, allowing more delicate testing lab equipment to get to far-off planets and study them.
Curiosity's landing, coined "the seven minutes of terror," is discussed by NASA engineers this five-minute video:

Curiosity Is A Complete Geological Testing Lab

Curiosity needs to be able to obtain various kinds of surface and internal rock and soil samples from various regions and test them in various ways. What really sets Curiosity apart from its predecessors is its instruments. It is essentially a mobile geologic testing lab. First it uses high-resolution cameras to look for geological features of interest. Gale Crater is a good place to look. Curiosity's landing site is marked by the tiny green dot in the image (created by combining data from three Mars orbiters) below.

In the crater's center is a huge mountain informally called Mount Sharp and from it large outflow channels, possibly carved by once-flowing water, extend into the plains below. This is where researchers hope to find signs of organic material and possibly even signs of extinct primitive life, which might have had a chance to evolve in this once-wet environment. When Curiosity finds a feature of interest here, it vaporizes a tiny part of it's surface with a laser and examines the emission spectrum from it, telling it what elements are present. If the composition is interesting, the rover can swing over a microscope and an X-ray spectrometer to examine it more closely. An X-ray spectrometer uses an X-ray to excite the electrons in the atoms of the material. The spectrum given off by the excited electrons is specific to each element present and provides a confirmation and better detail of the elemental makeup of the material. Finally, Curiosity can drill into the material and place a powdered sample into one of two mobile labs, the SAM or the CheMin.

SAM (Sample Analysis at Mars), shown below, is a suite of instruments - a mass spectrometer, a gas chromatograph and a tuneable laser spectrometer.

These instruments can identify gases present in the rock as well as any organic material present. The tuneable laser spectrometer can precisely measure isotope ratios in carbon and oxygen in any carbon dioxide or methane present in the sample. This can tell the researchers if the organic material has a biological or geochemical origin. Carbon has two stable isotopes, carbon-12 and carbon-13. Carbon-12 makes up 99% of all the carbon on Earth but it is even more concentrated in biological material because biochemical reactions favour carbon-12 over carbon-13, so an overabundance of carbon-12 in a material suggests it has a biological origin, at least a biology familiar to us on Earth.

CheMin (Chemistry and Minerology Instrument) consists of an X-ray powder diffraction instrument and an X-ray fluorescence instrument. It is shown below being installed into Curiosity. The inlet funnel for samples is sticking out at the bottom.

These instruments can identify and quantify minerals present in rock samples and by doing so they can assess whether water was involved in their formation or deposition or whether water has altered the rock at any point in its history. CheMin is especially focused on looking for any possible bio-signature minerals in rock samples. These include minerals that are created only through biological processes. Examples here on Earth are coal, oil, chalk, limestone, pearls and amber. While scientists don't expect to find these materials, they can look at the abundance and isotopic composition of various metals involved in redox reactions common in biology such as iron, chromium, and some rare earth elements as well as sulphur and oxygen isotope ratios in minerals that suggest biological activity. Of course researchers are also on the lookout for any microfossils such as tiny microscopic objects that resemble spores or bacteria. Such a find could be a definitive sign of past life on Mars.

Using these instruments, Curiosity is focused on looking for additional geological signs of past or present liquid water on or near Mars' surface as well as geological signs of past or present life in an area of Mars (Gale Crater) that is most likely to have harboured it. Right off the mark, Curiosity has kept researchers busy, sending back evidence of an ancient riverbed in Gale Crater, monitoring dust storms, measuring radiation levels and, most importantly, analyzing its first sample of Martian soil. Below is an image of the results of the first analysis of soil from the CheMin's X-ray powder diffraction instrument.

This X-ray diffraction image reveals the presence of crystalline minerals such as feldspar, pyroxenes and olivine mixed with amorphous material likely to be volcanic glass. X-rays beamed at the sample are scattered by the atoms in it. Each mineral shows up as a unique scattering pattern, a set of rings. The colours, right, represent the intensity of the X-ray beam, red being most intense in the center. X-ray diffraction gives scientists not only the chemical composition of rocks and sand but it also reads the mineral's internal structure, how its crystals are arranged. It tells them much more about what's present. For example, the presence of carbon could mean diamond or graphite and they have very different structures (and properties), which this X-ray diffraction instrument can distinguish. Knowing what minerals are present can reveal much about the geological evolution of the rocks and sand, and scientists suspect they will be able to piece together a collection of younger and older mineral samples, which could show a gradual transition from a wet environment to a very dry one. Olivine is especially interesting because, in the presence of water, it weathers into a material called iddingsite, which is a combination of clay minerals, iron oxides and ferrihydrites. Comparing the presence of iddingsite with olivine could tell researchers how much water was once present and the rate at which it disappeared.

In this five-minute video, Curiosity chief scientist and geologist, John Grotzinger, describes how Curiosity took and analyzed its first sample:

The first mineral sample taken by Curiosity is similar to volcanic (basalt) soils in Hawaii. Feldspar is a very common mineral in Earth's crust (60% of it) that crystallizes from magma. Pyroxenes and olivine are also very common. Together they make up most of Earth's upper mantle (where olivine is protected from weathering by water). Seeing them on the plains of Gale Crater on Mars was not unexpected.

Life as we know it is built from carbon-based compounds called organic compounds. They make up proteins, carbohydrates and DNA, for example. Below are some very simple building blocks of these compounds. Below left is glucose, a simple sugar and part of many complex carbohydrates. Below right is an amino acid, part of a protein polymer. 20 different amino acids make up proteins in living organisms. "R" stands for a functional group that makes each amino acid unique. Far below left is adenosine, one of four nucleotides of DNA.

These kinds of molecules are especially important to scientists looking for signs of life on Mars. They all contain carbon backbones, as well as oxygen and hydrogen. These elements are present in most planetary atmospheres as well as in sand and rock. It is their distinct chemical arrangements that set them apart as organic compounds, something a good X-ray diffraction device can discern. Neither these molecules nor any organic molecules that are involved in their formation have been definitely discovered yet on Mars, a surprise to researchers. But that doesn't mean there aren't any on Mars.

While the chemical bonds in complex organic molecules, especially DNA, can be broken by ultraviolet (UV) radiation, UV radiation as well as the energy from lightning, likely to have been plentiful thanks to significant early volcanic activity on both planets, might have provided the energy needed to create the first simple amino acids and sugars.

Meteorites - A Bonus Sample Set From Mars

Scientists have a few tantalizing clues about what kind of environment ancient Mars might have been. Little bits of the planet have been raining down on Earth for millions of years, originating from ancient Martian impacts. Most known Martian meteorites have been radiometrically aged to be between 0.5 million and 1.5 billion years old. This is the time they were dislodged from the planet and shot into space. The rock inside them, however, can be much older, on the scale of billions of years old. One meteorite in particular caused a big stir in the scientific community when it was discovered in 1985. It was found in Alan Hills in Antarctica and it is called ALH 84001. This meteorite appears to have been ejected from Mars about 16 million years ago and arrived on Earth 13,000 years ago. Cracks in it are filled with carbonate materials that imply the presence of liquid water. These minerals have been aged to between 4 and 3.6 billion years old. They also found evidence for polycyclic aromatic hydrocarbons (PAHs) and tiny tubular and ovoid structures that some but not all researchers think could be microfossils of something called nanobacteria. Unfortunately this meteorite hasn't proved that ancient life existed on Mars. PAHs are atmospheric pollutants on Earth. Although the concentration of PAHs in the meteorite seems to be higher away from the surface that was exposed to air, it still could be of Earth origin. If the PAH's are of Mars origin, they might hold more promise. These complex organic compounds, detected in interstellar space, consist of multiple aromatic carbon rings that might be created in carbon/hydrogen rich cores of nebulae. They may be hydrogenated, oxygenated and hydroxylated into even more complex compounds such as amino acids and nucleotides when they are exposed to the conditions of interstellar space. Some researchers thinks fullerenes of these molecules created in nebulae might have seeded Earth with the raw materials of life. The idea that the tiny ovoid structures in the Martian meteorite might by nanobacteria is controversial. The existence of nanobacteria on Earth is widely disputed and many researchers think they may be too small to house RNA, DNA's smaller simpler cousin.

While the argument for ancient Martian life is ongoing, the argument for a once-wet Mars is very strong based on a variety of data that includes this meteorite. In 2011, researchers completed isotopic analysis on the rock that indicates its carbonates precipitated at a temperature of 18°C from water that contained dissolved carbon dioxide from the atmosphere. The isotopic ratios suggest a sequential deposition of carbonate from a gradually evaporating body of shallow water.

Other Martian meteorites also have what appear to some researchers to be microscopic fossilized life forms and most tested meteorites contain organic molecules. The latest testing comes from Andrew Steele et al., of the Carnegie Institution for Science. They showed that complex organic molecules containing reduced carbon - carbon bonded to hydrogen or itself - are present inside and throughout the meteorites they've tested and they are of Martian origin rather than from contamination in our biosphere. However, the formation of these particular organic molecules was most likely part of a volcanic process that traps carbon in crystals of cooling magma, a non-biological origin. Complex organic molecules are precursors to life on Earth and researchers have shown that Mars not only had warm shallow water at some point in the past but it was also doing some organic chemistry on its own, creating complex organic molecules, at least while water was present. Using this Martian meteor data, Curiosity will focus on finding and studying pools of reduced organic carbon on Mars to learn more about how it is created and how to distinguish it from organic molecules of biological origin.

What Curiosity Found So Far

The hunt for signs of life on Mars is not as easy as it might seem at first glance. Without obvious fossils or other evidence of life on Mars, researchers have to rely on the biochemistry and geology of all potential biomarkers on Mars, which makes up a complex list of what to look for. Added to that is uncertainty about how to extrapolate from Earth's almost-lost evidence of pre-life chemistry (and steps from non-life to life that still aren't understood) to what might be almost lost on Mars billions of years after the fact. What Curiosity found when it scooped up its first sample of Martian sand and tested it clearly excited the scientific community, but it left some of the rest of us feeling underwhelmed, partly because this sample is just a small piece of a huge and complex puzzle.

So far Curiosity's onboard lab is working very well and it has identified a complex chemistry in the first windblown sand samples it's tested, located in an area called Rocknest, a fairly flat part of Gale Crater. It found perchlorate, Ca(ClO4)2, an oxidizing molecule I mentioned earlier which the Phoenix Mars Lander identified. It is also an energy-rich molecule, a component of rocket fuel, and in theory it could be used as an energy source for microbes if they exist or existed. Curiosity has also found chlorinated methane compounds, which are organic molecules of non-biological origin. It identified four gases that were released when it heated its first Rocknest sample: water vapour, oxygen gas, sulfur dioxide gas and carbon dioxide gas. The oxygen gas could be from the breakdown of perchlorate in the sample. The water it detected does not mean the sample was damp at all. It is water molecules chemically bound to dust and sand grains. The levels of water, however, were higher than the researchers expected. So far, its analysis of Rocknest sand reveals several kinds of chlorinated methane compounds such as CH3CL, CH2CL2 and CHCl3, as well as sulfur compounds such as hydrogen sulfide. The carbon-containing compounds are technically organic molecules but they are not biological organic compounds in and of themselves. The presence of CH2Cl2 however is potentially interesting because it is an intermediate step involved in organic chain propagation, shown below, where carbon backbone chains can be created in the presence of UV radiation.

UV radiation (which bombards the surface of Mars and once bombarded Earth's surface before the ozone layer formed) cleaves chlorine gas into two free radicals. These radicals react with methane creating a longer carbon chain, shown right. This process can continue, creating long and complex carbon chains

It is possible, however, that the carbon detected may be from Earth and carried by Curiosity to Mars. The SAM detector is extremely sensitive. The chlorine, however, is almost certainly Martian. NASA now claims it has had no definitive detection of methane on Mars (yet). Methane (CH4) itself is an interesting molecule connected to life. 90% of the methane on Earth comes from the biological processes of life. It is not a stable compound, reacting quickly with other gases and breaking down in the presence of UV radiation. Methane is not expected to be present in any quantity on Mars if life does not exist there. Very trace amounts could come from comet impacts or chemical reactions underground between rocks and hot water. Volcanoes can pump out significant amounts of methane but none have been active on Mars for billions of years. Some experiments on Mars have detected higher than expected, but transient, methane levels in Mars' atmosphere, making researchers curious. Could there by a microbial source of methane, perhaps underground? On Earth, biologically produced methane tends to come with ethane while non-biological volcanic methane usually comes with sulfur dioxide. This might help provide some clues while exploring Mars's ongoing methane mystery.

Meanwhile we can be justified feeling a little let down that there are no definitive signs of any life or pre-life chemistry on Mars (yet). Scientific exploration can sometimes advance a rate much slower than we'd like it to. NASA and various other research teams around the world are successfully building a foundation for the future of Mars exploration that could get much more interesting when they eventually send humans to the planet. Also, Curiosity is just a few months into its two-year mission and it has not reached its main destination yet. Curiosity's main mission is to go to Mount Sharp, the huge mountain in the center of Gale crater. Mount Sharp, rising 5.5 km from the crater floor, appears to be an enormous mound of eroded sedimentary layered rock. These layers are especially interesting, considering that, being composed of sedimentary layers, they must come from a wet environment where they were sequentially deposited over a long period of time, about two billion years. This suggests that the crater may have once been filled with water, forming a large lake in the distant past. Sand and rock studies here will hopefully tell researchers whether it was once a lake or not.

Currently two rovers are operating on Mars - Curiosity and Opportunity - and three orbiters are surveying the planet - Mars Odyssey, Mars Express, and the Mars Reconnaissance Orbiter. NASA plans to send a new orbiter called MAVEN next year to analyze the atmosphere in greater detail, hoping to understand better Mars' dramatic climate change and its loss of water and most of its atmosphere. The European Space Agency (ESA) plans to send a Phoenix-like lander, called the ExoMars rover, to Mars in 2018. It will be equipped with drilling equipment that could drill about two metres deep into Martian rock for samples, looking for signs of bioorganic molecules using, among other instruments, an organic molecule analyzer. This instrument will have two operating modes - laser desorption mass spectrometry and gas chromatography mass spectrometry. Gas chromatography will identify all the volatile gases that are released as a sample is heated to 900°C. The gases will then be analyzed further in a mass spectrometer. In the other  mode, a special laser will ionize part of the sample surface and a mass spectrometer will analyze those ions. Researchers hope to determine the isotopic composition and the chirality of any organic molecules they identify, which will help determine if they come from a living or nonliving source. I mentioned the isotope connection to life earlier. Chirality is another interesting quality of all building blocks of living organisms. These molecules all have the same handedness. Amino acids are left-handed and the sugars in nucleotides are right-handed.  For example, below are two forms of alanine, an amino acid.

These two forms, called optical isomers, are mirror images of each other. Only one isomer is found in almost all living organisms - the L-isomer (some bacteria are a rare exception, having the mirror-image D-alanine in their cell walls). L-alanine is the one on the left, above. Why they are this way remains a mystery. Although Mars's surface is probably far too hostile for any life (too cold and dry with intense ultraviolet radiation), microbes might conceivably survive underground in protected rock crevices, for example, and this rover will be designed to find them if they are, or were, there.

NASA just announced it plans to send a new robotic science rover to Mars in 2020. It will utilize much of the successful technology developed for Curiosity, keeping costs and risks down. There will be an open competition for the payload and instruments on the new rover and a team will be set up to outline the new mission's scientific objectives.

The Curiosity mission was expensive. It cost NASA about 2.5 billion dollars. NASA, the ESA and other space agencies around the world are facing the pressure of tightening budgets and financially uncertain futures. Though NASA has plans to send a new rover, mentioned above, several future NASA Mars exploration mission plans have been cancelled or postponed. However, there still seems to be a healthy desire to explore Mars, to understand its evolution as a planet and to look for signs of life there. On the bright side, tight budgets and challenges often force ingenuity on researchers. The question, like a bright juicy carrot, remains - are we alone?

Friday, November 30, 2012

Atoms Part 1: How Atoms Are Made

The very first atom, a simple union of a proton with an electron, appeared when the universe was just about 380,000 years old. It was hydrogen.

Big Bang Synthesis

Just after the Big Bang, our universe started out as a tiny seething environment filled with unimaginable energy. In just a millisecond, tiny particles called quarks formed. They come in two stable kinds - called up and down. The temperature was several trillion degrees Celsius. These quarks, attracted to each other by a fundamental force in the universe, called the strong force, quickly formed little groups of three quarks each, called neutrons and protons. About seven times more protons were created than neutrons. Neutrons contain two down quarks and one up quark. Protons contain one down quark and two up quarks.

Free protons are very stable. Each one, made right after the Big Bang, will outlast our universe. But free neutrons are unstable. They have a half-life of just over 10 minutes. This means that of all free neutrons today, only half will remain 10 minutes from now. What happens to them? They decay into stable protons. A down quark decays into an up quark.

Few free neutrons had a chance to decay, however. A few seconds after the Big Bang, almost every free neutron was paired up with another neutron and two protons to make a helium-4 nucleus. Helium-4 nuclei are very strongly bound. The strong force again attracted them to each other just as it attracted quarks together. By the time the universe was about three minutes old, the process of making atomic nuclei had stopped altogether, and once bound inside nuclei, neutrons became stable.

The universe had enough time to make a handful of simple atomic nuclei: a lot of helium nuclei, along with trace amounts of deuterium and lithium nuclei. The majority of protons didn't bind into nuclei at all. They remained free instead. Around these protons and nuclei, electrons and photons zipped around in all directions. A photon is a particle of light and radiation. The universe had to cool down much more before electrons could settle into orbits around nuclei to create atoms. It took about 380,000 years. When the electrons settled into atoms, photons were freed up to fly away in all directions. This is where something you may have heard of, the cosmic background radiation, came from. These photons started off as highly energetic radiation called gamma rays. While they traveled, space itself expanded as the universe expanded. This expansion stretched their wavelengths longer. These photon travelers from that distant time reach us today as faint (long wavelength) microwave background noise. You can see it as snow on an old fashioned TV when it isn't tuned to any station.

Hydrogen is the lightest and simplest elemental atom in the universe. It is assigned the atomic number 1 because it has just one proton in its nucleus. Every single kind of atom, or element, has its own atomic number. This hints at something very important about atoms. The number of protons in an atom's nucleus determines what kind of atom it is - it is what makes iron, with 26 protons, different from copper, with 29 protons, for example:
Of all the mass of all the atoms in the universe, 75% comes from hydrogen. Before stars formed, the only atoms in the universe were hydrogen, helium and lithium:
The early atom picture is just a bit more complex than this, however. Each of these three kinds of atom comes in different types, called isotopes. To understand what an isotope is, let's take hydrogen as an example. The simplest hydrogen atom contains a nucleus made of just one proton. However, all atomic nuclei larger than this hydrogen atom also contain neutrons. The number of neutrons in an atomic nucleus determines which isotope it is.

The simplest isotope of hydrogen is hydrogen-1 or 1H. Other isotopes of hydrogen were also made when hydrogen-1 was made: hydrogen-2, called deuterium, and hydrogen-3, called tritium, for example:
Protons and neutrons (shown in orange and purple, respectively, in this diagrams) in a nucleus attract each other through a glue-like force called the nuclear strong force, but protons naturally repel each other because they are of the same positive charge. These two forces compete with each other. Neutrons tend to stabilize a nucleus because they attract each other and protons equally. Too many neutrons, however, can destabilize the arrangement. When an atomic nucleus takes on more and more neutrons, it tends to get more and more unstable. The nucleus becomes unwieldy. Deuterium, like hydrogen-1, is stable. These atoms, once they were formed billions of years ago never decay. Tritium, on the other hand, isn't stable. It is radioactive, and it has a half-life of about 12 years. This means that in 12 years, half of all tritium atoms today will have decayed into another, stable, atom called helium-3. An unstable tritium nucleus, with one proton and two neutrons, decays into a stable helium-3 nucleus, with one neutron and two protons. A neutron decays into a proton, releasing an electron in the process.

All the tritium in the universe would have disappeared billions of years ago if it were not made naturally when cosmic rays interact with gases. Neutron-heavy hydrogen-4, 5, 6 and even hydrogen-7 have been made in the lab, but they are very unstable isotopes and they last just tiny fractions of a second before decaying into more stable atoms.

Besides stable hydrogen-1 and deuterium, helium-3, helium-4 and lithium-7 atoms, all of which are also stable, were made when atoms began to form in the universe. Almost a quarter of the atomic mass of the universe is made up of helium-4, with trace amounts of deuterium, helium-3 and lithium-7. No other elemental atoms were made at this time. They didn't exist in the universe until the first stars formed about 200 million years later, and began to fuse lighter elements into heavier ones.

Star Core Synthesis

During most of a star's life, hydrogen fuses into helium under the enormous pressure and heat deep inside its core. Small stars, like our Sun, will eventually run out of hydrogen and nuclear fusion will stop. Only in very massive stars, much larger than our Sun, do carbon, oxygen and even iron atoms form in significant quantity, all from successive fusion reactions.

The production of carbon atoms requires even more energy than the fusion of hydrogen into helium. When a large star begins to run out of hydrogen, its core begins to collapse. This pressure heats it up even further. Helium nuclei begin to fuse fast enough to compete with the decay of their unstable product, beryllium-8. If they don't fuse fast enough, beryllium-8 simply decays back into two helium nuclei before any new fusion reaction can happen. Now, however, some beryllium-8 sticks around long enough to fuse with other helium nuclei. When they do, stable carbon-12 nuclei are created. Three helium nuclei fuse into a carbon nucleus. The two fusion reactions involved in making carbon occur almost simultaneously:

Carbon nuclei can fuse with other helium nuclei to create oxygen nuclei, and so on. Carbon atoms didn't form right after the Big Bang because the universe cooled down before carbon nuclei could form.

When we think about atomic nuclei fusing with each other to make bigger nuclei, we are describing a special high-energy environment inside a star. Here, the pressure and temperature are so high that atoms don't exist as atoms, with electrons orbiting nuclei. Here, the electrons are just too excited. They fly around freely. When this happens you get a special state of matter called plasma. Free nuclei float around in a "soup" of highly energized electrons flying around in all directions. This plasma within a star is similar to the plasma that filled the universe just before atoms formed.

In the plasma inside very large stars, all the available carbon nuclei eventually fuse into even larger nuclei such as oxygen, sodium and magnesium. Once the carbon is gone, the star contracts once again and gets even hotter. Oxygen fuses into silicon and sulfur nuclei as well as other elements. Eventually the core of the star contains nothing left but sulfur and silicon. It contracts further and it gets even hotter. It soon has enough energy to make even heavier elements. Each new nucleus is made by fusing a helium nucleus to an existing nucleus, in a step-by-step fashion. For example, a silicon nucleus fuses with a helium nucleus to make sulfur. The sulfur nucleus fuses with another helium nucleus to make argon. The argon nucleus fuses with another helium nucleus to make calcium, and so on. This process continues along to create larger and larger nuclei. It all happens very fast. The entire step-by-step process lasts about five days in total, until a nickel-56 nucleus is eventually made. Then it suddenly stops. This is when things get very interesting for the large star.

Stars burn brightly for many millions of years because massive amounts of energy are released every time light elements such as hydrogen fuse into helium. This energy is called nuclear binding energy. It's the energy needed to remove a proton or a neutron from a nucleus and it's the same energy that's released when a proton or a neutron is added to a nucleus.

As atomic nuclei get larger and larger, the increase in binding energy as protons and neutrons are added to the nucleus begins to wane, and eventually it doesn't increase at all. It becomes negative. The graph below charts binding energy as a function of nucleus size. Initially, as protons and neutrons are added to the nucleus, the binding energy of the nucleus increases dramatically. See the sharp upward incline to the left. You can see the increase start to wane at around carbon and oxygen nucleus size. (Notice the little jagged up-tick in energy at helium - this shows how much binding energy this particular nuclear arrangement has. This nucleus is especially tightly bonded together.) Now look at iron-56. This nucleus has the highest binding energy in this graph:

Actually, nuclei with 58 and 62 nucleons have the highest possible binding energy of all nuclei. We don't get them here in this massive star, because each new element must be a multiple of 4 (a helium nucleus has 2 protons and 2 neutrons). We get nickel-56 instead (that's 14 helium nuclei that have bound up together). As nickel-56 tries to fuse with one more helium nucleus to make the next largest element, zinc-60, the process actually requires energy instead of producing it. Rather than releasing energy when neutrons and protons are added, heavier elements release energy when protons and neutrons are removed ? fission rather than fusion is favoured. Nickel-56, therefore, is the last element this star or any star can make in its core.

With all this nickel being made, you may wonder why the cores of rocky planets and meteorites contain lots of iron-56 in them, not nickel. That's because nickel-56 is unstable. It has a half-life of about 6 days as it decays into cobalt-56, which is also unstable with a half-life of about 77 days. Cobalt-56 decays into iron-56, and that is a stable nucleus. It is the heaviest stable element made inside a star.

So what happens next when nickel-56 is made inside the star? And where do even larger atoms come from?

Supernova Synthesis

When stars are busy fusing nuclei, they are in a state of equilibrium. The outward force of thermonuclear fusion balances the inward force of gravity. When fusion reaches nickel-56 size, it stops altogether. The core begins to collapse once again but this time there's no new kind of fusion to ignite and restart the process. No more outward force is created to balance the crushing inward force of gravity. The star completely collapses in on itself. This collapse is catastrophic - it can reach a velocity as high as 70,000 km/second! The temperature and density of the core skyrocket. The inner core of the star eventually reaches a density comparable to that of an atomic nucleus. Consider that atoms are 99.9% empty space! Here, all that empty space has been squeezed out and atomic nuclei and electrons are packed in on each other. They are so tightly squeezed that protons begin to capture electrons and turn into neutrons. This process creates a dense hot neutron soup that can't decay any further. The temperature is about 6000 times higher than it was before the collapse. It is an environment where large and highly unstable neutron-rich nuclei form through a process called neutron capture, rather than through nuclear fusion. Free neutrons are basically squeezed into large nuclei so fast the nuclei don't have a chance to decay. There is an upper limit to nuclei formed this way. When the number of nucleons approaches 270 (that's larger than any even unstable atom scientists have created), the nucleus rapidly and spontaneously decays through fission, releasing a lot of energy, until it reaches a stable isotope.

The core collapses, creating a slew of new heavy unstable atoms all within a few seconds. The rapid collapse of the large star releases a staggering amount of gravitational potential energy. It is this energy that drives a supernova explosion. Heavy nuclei blow far and wide into surrounding space.

As the energy of explosion begins to subside, smaller unstable (radioactive) nuclei formed in the super-heated core undergo a series of nuclear decays (rather than fission) until they reach stable nuclei. Lead-204, with 82 protons and 122 neutrons, is the heaviest stable atomic nucleus in the universe. All elements in the periodic table with atomic numbers higher than 82 are unstable, although some of these have very long half-lives. Bismuth-209 for example, with 83 protons, has a half-life longer than the age of the universe, and therefore can be thought of for all intents and purposes as stable, and non-radioactive.

Every subsequent supernova seeds the universe with more and more large atomic nuclei. While the number of very small atoms remains about the same today as when they were created shortly after the Big Bang, the number of larger atoms continues to increase as they are created in the cores of large stars and during supernovae. These three kinds of atom creationBig Bang synthesis, star core synthesis and supernova synthesis - explain why there is so much hydrogen and helium in the universe, why there are significant amounts of oxygen, carbon and silicon - major components of rocky planets like Earth, and why heavy metals such as gold-79 (Au) and platinum-78 (Pt) are precious because they are present in the universe only in tiny amounts.

Next, take a look at Atoms Part 2, which explore how atoms emit light.

Thursday, November 29, 2012

Atoms Part 2: Atoms and Light

TV sets, the Northern Lights, fireflies and fireworks all have one thing in common: they emit light. The light comes from atoms. An atom can emit bright distinct colours.  How does an atom pull off this visually stunning trick?

Atomic Structure

Bright colourful displays of light owe themselves to the electrons in atoms - how they are arranged and how they move inside the atom.

Every atom comes in the same basic formula: protons, neutrons (except for hydrogen-1 which doesn't have one) and electrons. Below is a simple diagram of an oxygen-16 atom. Eight electrons surround a nucleus made up of eight protons and eight neutrons. The nucleus is drawn as a simple pink circle.

The atom is the basic building block of all matter in the universe, from gases, to liquids to solids. To form a nucleus, tiny fundamental particles called quarks bind together in groups of three to make protons and neutrons. Quarks come in six types but only two types - up and down quarks - are stable and make up atomic nuclei. Two up quarks and a down quark bind with each other through a fundamental glue-like force called the strong force to make a proton. Two down quarks and an up quark bind the same way to make a neutron. Protons and neutrons bind to each other inside a nucleus through the same strong force. As implied by the name, this force is strong. It won't let neutrons and protons budge unless enormous energy is applied to the atom.

The electrons in atoms are a different story. Electrons are attracted to the nucleus through a different fundamental force, called the electromagnetic force. This force is less intense. The electrons have a little wiggle room. An electron can absorb energy, either from colliding with another atom or by absorbing a tiny packet of light called a photon. An electron can move away from or closer to the nucleus depending on its energy. Negatively charged electrons are attracted to positively charged protons in the nucleus, but an electron will never fall or spiral down into the nucleus because it must maintain a specific minimum amount of energy. This minimum energy is based on the rules of quantum mechanics. In fact, the rules go even further: the electron not only has to maintain a certain minimum energy, preventing it from spiraling into the nucleus, it is also limited to specific values of higher energies too. Electron energy is quantized, which means it comes only in tiny specific amounts. It's like ordering coffee to go from a coffee shop - you can get a small, medium or large but not a med/large. That in-between price is not entered in the cash register.

An electron becomes excited when it absorbs energy.  When it is excited, it may move away from the nucleus a tiny bit. The distance it can move outward is, again, quantized into tiny packets, or specific distances. An electron might absorb a tiny amount of energy and still not become excited. It needs to absorb a specific amount of energy before it can move up into a higher energy state. It's like climbing the rungs of a ladder. Moving your foot up halfway between rungs won't get your body any higher up. To climb, you can move up only by the height of each rung, one rung at a time. I've redrawn the oxygen atom (right), this time adding two more rings in grey. These grey rings are two possible excited electron states. They are further away from the nucleus because they represent higher energy. Electrons can move into specific rings (distances) but not in between them.

Electrons Can Be Described As Wave Functions

The quantum mechanical arrangement of electrons in an atom is described by a complex mathematical formula called a wave function. What we need to know about the wave function is that, in a real atom, the exact locations of electrons can't be found; they can only be localized into clouds where they are likely to be found. These clouds are also called orbitals and they can take on some fairly complex shapes. It also means that you can measure the speed of an electron as it orbits in the atom, or you can measure its location, but you can't know both at the same time. Despite this uncertainty, you can describe the average distance of an electron from its nucleus. This is called an electron shell. It isn't a realistic description of how the electron moves around the nucleus, but it does give you an idea of how much average energy an electron possesses. Shells are represented by the rings around the nucleus like the ones we drew for the oxygen atom. Electron energy increases as you go outward in the rings. As a general rule, the innermost shell is filled with electrons first, before electrons occupy outer shells. This arrangement represents the lowest possible energy state, or ground state, of an atom. In the oxygen atom, the two black rings are ground state shells. Atoms, like all systems, tend to want to be in the lowest energy state possible.

Now I'll take a hydrogen atom, the simplest of all atoms, and use it to show you how it can emit light. Light emission from all other atoms works the same basic way. Here is what a simple electron shell diagram of a hydrogen atom looks like, right:

The electron in this atom is in its ground state; it occupies the closest possible electron shell. Although it looks like a simple ring here, this electron orbiting the proton nucleus forms a kind of three-dimensional standing wave. It's almost impossible to visualize, so we'll use a two-dimensional wave model to explore it.

A standing wave is also called a stationary wave. Its ends are fixed in place. To get an idea of what one looks like, take a look at the collection of waves below left:

Every wave shown is a standing wave. The simplest, or fundamental, wave is shown top left. The wave function of an electron is like a standing wave with its two ends fixed together, and the point where they connect is called a node. It's the one place on the wave model where the electron's energy can be fixed in place, or localized.  The red dots in the animation below are nodes in a standing wave. The top wave (left) is followed by six waves that are harmonic overtones of the fundamental wave. It takes energy to increase the number of oscillations in a wave.  If you taped one end of a string to a smooth floor and snapped the other end back and forth horizontally in your hand, close to the floor, you would notice you have to snap harder to add more oscillations in the string.

Higher harmonic frequencies represent higher average electron energy. Each node, where the wave is stationary, is where the electron's energy can be localized, or pinned down to an exact value. Between nodes the electron's energy is diffuse, somewhere in the electron cloud.

From Wave Functions to Orbitals

A real hydrogen atom is not nearly as simple as my simple electron shell diagram suggests. Its single electron has a multitude of three-dimensional orbital shapes available to it. The catch is that these orbitals are all energy-dependent. When the hydrogen atom is in its (lowest energy) ground state, its single electron will always take the closest possible energy orbital. This orbital is analogous to the simplest fundamental standing wave, with one node (at its ends), shown at the top, above left.

If you do some fancy math by plotting the square of all the local amplitudes this electron can take along the simplest fundamental standing wave, you get an electron cloud density distribution that looks like the red/white mist over a black background at the top left in the diagram below. Again, this is a simpler two-dimensional image of what is really a three-dimensional shape. The brighter the area, the more likely you are to find the electron. The hydrogen atom's lowest energy state orbital is spherically symmetrical with one node. It is called 1s, shown at the top left in the diagram below:

(en:User:FlorianMarquardt; Wikipedia)

The electron in this state is very close to the nucleus. This image corresponds to my simple diagram of a single energy shell, shell #1, or n=1 as it is usually called. The simple energy shell atomic model, drawn with rings showing available electron energy levels, is called a Bohr model, after Niels Bohr.

An electron orbiting a nucleus can absorb energy, and if the electron absorbs enough energy (a minimum "packet" amount), it will move up into a higher energy orbital. I've shown this earlier for an oxygen atom, but I could show it for hydrogen too, by drawing an additional outer electron shell, shell n=2, with the electron now occupying that outer shell. I could also show the hydrogen atom more realistically using the orbital imagery above. This kind of imagery is based on the quantum mechanical model of the atom. In the early 1900's, Erwin Schrodinger combined the equations for the wave behaviour of the electron with Louis de Broglie's equation for wave-particle duality to come up with a mathematical model for the distribution of electrons in an atom, the "fancy math" I mentioned. The electron cloud orbital images above are the result of those calculations.

This new model for the atom no longer tells us where the electron is but where it might be. Our energized electron may now take one of two possible new orbitals, 2s or 2p. They are analogous to a standing wave with two nodes, one in the middle and the other one where the ends connect. We can add even more energy to our single electron, enough to allow it to move up into an even higher energy orbital. Now it has a choice of three possible orbitals: 3s, 3p or 3d. These orbitals correspond to a standing wave with three nodes. I could draw this simply by adding a third outermost circle to my Bohr atom drawing, representing an electron shell even further away from the nucleus. The nodes in the standing waves are represented as circles in the Bohr model. Each one represents a specific allowed energy of an electron. However, as we move up in energy states, this simple ring-type picture becomes less and less accurate. The electron can now take complex routes as it moves about the nucleus. Sometimes it is furthest away from the nucleus in its new higher energy orbital but other times it may not be.

Electrons, because they are standing wave functions, must have energies that correspond to the nodes in a standing wave. They aren't allowed to take on energies between these nodes (the "no's in my earlier Bohr excited oxygen diagram).

Each atom has a particular set of wave functions. Larger atoms usually have slightly closer 1s orbital electrons than a hydrogen atom does. The closest electron is more strongly attracted to a more strongly positive nuclear charge (more protons in it), and that means it has just a bit less potential energy. The standing wave function of a 1s orbital in a larger atom will be just a bit longer (a slightly longer wavelength has less energy) than the one for hydrogen. All of the larger atom's other electrons will therefore have slightly different wave functions too. The nodes will be shifted just a bit farther apart. Additional electrons also complicate orbital distances because the electrons interact with each other, sometimes in complex ways. Atoms all have the same basic arrangement (and shapes) of energy orbitals- 1s, 2s, 2p and so on, but each orbital will have its own specific energy unique to each atom. The energy "rungs" are special to each atom.

If we go back to the excited hydrogen atom, even higher energy orbitals are possible. In fact, if enough energy is supplied to the electron in the hydrogen atom,  it will move up each successive energy orbital and eventually leave the highest possible energy orbital altogether, leaving the proton nucleus by itself. The atom in this state, where one or more electrons have left it, is called ionized. The hydrogen ion has a charge of +1, thanks to the lone proton that's left. It is completely ionized because it lost all of its electrons. An oxygen atom would have to lose all eight of its electrons to be completely ionized.

The Basic Idea Behind an Emission Spectrum

An atom in which one or more electrons have moved up into higher state orbitals is called an excited atom. In time, the atom will eventually return to its ground or lowest energy state as its electrons return to their lowest possible energy orbitals. Sometimes this takes a tiny fraction of a second. In other cases it can take minutes, depending on the particular atom and the orbital involved. In order to return to ground state, the electron has to shed its excess energy somehow. It does this by emitting a tiny packet of light. That packet, called a photon, carries a specific amount of energy off with it. It is exactly the same amount of energy as the difference in energy shells. When an excited electron in the oxygen atom returns to normal, for example, it takes three quarters of a second to emit a green photon, if it is excited enough. Then it emits a red photon, taking a whole two minutes to do so, below left:

These photon energies match the energies of the two excited shells involved. If we could look at a spectrum of this light, we would see two bright bands of colour - one red and one green.

Emission Spectrum of Hydrogen

As we saw earlier, there are many energy shells available to the electron in a hydrogen atom. Even this simple atom, as a whole, doesn't emit just one colour of light, but it doesn't emit a continuous spectrum of colours like a rainbow either. It emits a sequence of bright spectral lines, each one unique to a particular drop in electron orbital energy in hydrogen atoms. Each elemental atom emits its own line spectrum, called an emission spectrum. An excited electron can drop down one, two, three or more energy orbitals in one go, like a person sliding down a ladder, one, two or three rungs at a time.

Excited atoms don't just emit light in the visible range either. Our hydrogen atom emits photons in both the ultraviolet (UV) and infrared range as well as various visible colours. The entire electromagnetic (EM) spectrum is shown below right.

A photon is a quantum packet of electromagnetic radiation. It can range in energy from a gamma photon to a radio wave photon. The wavelengths of gamma photons are so short (a few trillionths of a metre) that most people call them rays instead. Likewise, radio photon wavelengths are so long (generally up to 10 metres) that we call them waves instead. As the photon's energy decreases, its wavelength increases and its frequency decreases.

The emission spectrum of the hydrogen atom (shown below) is a small sampler of the entire EM spectrum. This emission spectrum serves as a unique fingerprint for hydrogen atoms. An excited oxygen atom will have a different distinct emission spectrum. Don't worry about all the symbols at the top yet, but notice how small the visible range is compared to the whole spectrum (ignore the colours - they don't correspond to the visible spectrum). The hydrogen spectrum we can see is just a small fraction of all the possible emission photons from hydrogen atoms.

Emission Spectrum of a Hydrogen Atom

(OrangeDog; Wikipedia)

Each of the spectral lines you see above corresponds to one excited electron jump, returning down one or more energy levels. There are a lot of energy levels available to it, but there are forbidden areas or gaps where the electron cannot jump. These are the white spaces in between the lines (the spaces in between the standing wave "rungs") in the emission spectrum. The energy of electrons (and photons and other fundamental particles too) is quantized. It comes in discrete packets. Each jump corresponds to a photon of a particular wavelength (measured in nanometers, nm, billionths of a metre) being emitted. The wavelength can be calculated using a formula called the Rydberg formula. The quantized electron energy levels of the hydrogen atom give you a series of discrete spectral lines, rather than a whole rainbow of colours, when excited hydrogen returns to its ground state.

Now take a closer look at the top symbols. Looking from the top left, you can see Ly-alpha, Ba-alpha and Pa-alpha, and so on. These are series, or groups, of spectral lines: Lyman series, Balmer series and Paschen series. They are grouped according to the energy shells (average orbital energies) of the Bohr model:


This looks complicated but it isn't really. You'll notice lots of interesting energy trends in this diagram if you play with it a bit. For example, look at an electron jump from n = 6 to n = 1. This jump releases a 94 nm wavelength photon of energy. That's the shortest wavelength photon in the diagram. That means it's the highest energy photon, well into the UV range. There's a big difference in energy between an n = 6 orbital and an n = 1 orbital. If we tip things on their head, an electron would have to absorb the same energy as a 94 nm photon in order to jump up from n = 1 to n = 6. The atom would have to be struck by a fast moving particle or bombarded with extreme UV radiation. Now look at an electron jump from n = 2 up to n = 6. Much less energy is required here, equivalent to a 410 nm (visible violet) photon. The n = 2 electron is further from the nucleus. It's less attracted to it, which means it has more potential energy than an n = 1 electron does, so it's easier to push further up to n = 6 energy.

Most of these spectral line emissions are very faint and they can only be seen in the lab. There, a sample of pure hydrogen atoms is used, and it will contain atoms with electrons in various different excited states. A single electron in a single atom doesn't emit all the photons of all the series. A single electron will make just one or a few jumps to reach its ground state, emitting one to a few photons of EM radiation in the process, not all at once but one at a time.

Looking at the various wavelengths for electron single-shell jumps, there is also a trend: The electron needs much more energy to jump one shell up from the lowest shell closest to the nucleus (n = 1 to n = 2; 122 nm) than it does to jump from n=3 to n=4 (1875 nm), for example. It takes much more energy to pull the electron away when it's close to the nucleus, the attractive positive charge, than when it's farther away it. You are seeing Coulomb's inverse square law of charge interaction at work.

The Rydberg calculations are complex and only the relatively simple single electron emission of hydrogen atoms has been fully worked out. Spectra of atoms with just a few electrons have been worked out in some detail but it is much more difficult to do because electrons in each atom interact with each other, and that complicates the calculations.

You don't need to do any calculations to obtain a visible emission spectrum of any atom. You just need a pure source of the atoms, a way to excite them, and a prism called a spectroscope, to separate out the emitted photon wavelengths.

The Lyman series lines are all electron jumps down to the lowest energy shell. See how they all go down to n = 1? These emissions are all in the ultraviolet band of the electromagnetic spectrum, including a line at 122 nm wavelength, 103 nm wavelength and so on.

The Paschen series of hydrogen emission lines corresponds to electron jumps back down to the third energy shell. These lines are in the infrared range, from 820 nm to 1870 nm, so we can't see them either with the naked eye.

The Balmer series of spectral lines are jumps down to the second energy shell (the hydrogen is still in an excited state and the electron will eventually jump down to the n=1 lowest energy shell, emitting an invisible 122 nm UV photon in the process).

The Balmer emissions are mostly in the visible part of the spectrum, which means we can see most of them. The Balmer emission spectrum looks like this:

Emission Spectra Are Atom Footprints

There are actually six lines in the Balmer spectrum, above,  but you can only see four of them. The two invisible lines would be to the far left. They are just within the UV range, under 400 nm, so we can't see them. They represent electron energy shells n = 7 and n = 8. These two shells are farther out from the furthest shell (n = 6) shown in the emission series diagram earlier, and they represent two even higher energy states possible for the electron.

410 nm, 434 nm and 486 nm lines all fall into the visible violet/blue/green range and a far right line at 656 nm shows up as red to our eyes. In emission spectra, some lines are brighter than others. Red emission is especially strong from hydrogen atoms. It's a common electron jump. Click on this link to find out why. This colour alone, in fact, is used as a signature of hydrogen, the most common atom in the universe. It is what makes much of the Orion nebula, for example, appear reddish purple to us:

Hydrogen plasma glows the same reddish purple because its visible emission is in the Balmer series, mixing intense red with three fainter bluish tones, shown in the centre of the plasma tube below. Hydrogen atoms in the plasma state form a partially ionized gas containing hydrogen ions, electrons and neutral excited atoms. If you pass this light through a prism you will get the Balmer emission spectrum, shown above.

(Alchemist-hp (talk) (; Wikipedia)

Teachers can have fun with other atoms by performing a fairly simple flame test in the lab. When enough energy is applied to atoms (heat from a Bunsen burner for example), they glow with specific colours. Electrons are absorbing and releasing packets of energy according to their specific electron orbital energies. As they release energy, they release photons - they glow. Like hydrogen, each atom's colour is the combination of its visible emission spectrum lines (the wavelength energies of its electron jumps):

Emission Spectra Versus Absorption Spectra

A hydrogen atom must absorb the exact energy of a particular energy shell. This means it must absorb the energy equivalent to a particular wavelength photon in order to later emit it. Again this is because of the quantized packet nature of electrons (and photons). This means that hydrogen and other atoms give us absorption spectra that are the exact opposite of their emission spectra. The black lines in the absorption spectrum match the coloured lines in the emission spectrum below:

Emission Spectrum Of Oxygen

An atom with more electrons, such as oxygen, with eight electrons, emits a much more complex visible emission spectrum:

Notice an especially bright red line and green line in the spectrum above. They are common electron orbital shifts in this atom. The green colour is the colour of most Northern Lights displays. Less common red Northern Lights glow higher up in the atmosphere:

(this photo was taken from the International Space Station)

Up there, oxygen atoms are far enough apart that they have enough time to emit red photons before another atom strikes them and absorbs that energy. An oxygen atom's complete EM emission spectrum would contain hundreds of lines.

In Atoms Part 3, we'll continue to explore atoms and light, especially hydrogen. We focus next on the Sun, a massive ball of almost all hydrogen.