Radiation: What is Happening in Japan?

As I watched the horrific events unfold in Japan, I noticed that the concept of radiation was not well understood among much of the media. Most of us are not experts in this field but with the safety of nuclear energy now being openly questioned in many countries, perhaps it is time for us to gain a basic understanding so we can make informed choices for ourselves.

PART 1: PHYSIOLOGICAL EFFECTS OF RADIATION, an overview

What is Radiation and What Does it Do To Our Cells?

Radiation consists of energetic particles. These particles range from photons of visible light to X-rays to very energetic electrons that can damage both our cells and the DNA inside our cells. The kind of radiation that people worry about most is often called ionizing radiation. These are very energetic particles that have enough energy to ionize an atom or molecule and this is what damages our cells. Ionization means that the energetic particle knocks out an electron. This actually happens in your cells at a low but chronic rate because of background radiation. Usually cells can detect and repair the damage or they can program themselves to die off and eliminate the potential genetic damage. However, sometimes cells can undergo a DNA mutation that is passed on and this can occasionally contribute to future cancer (if the cells are sperm or eggs, DNA damage can lead to birth defects). This is why up to 4000 people exposed to radiation during the Chernobyl nuclear accident later developed thyroid cancer and other genetic anomalies such as neural tube defects, Down syndrome and other chromosomal aberrations.

Radiation Poisoning

In addition to chronic exposure, people can also experience a single large dose of radiation, called acute radiation poisoning, for example, from nuclear explosions or handling a highly radioactive source in which a brief high exposure occurs. The amount of time between exposure and the development of initial symptoms of radiation poisoning often indicates how much radiation was absorbed. The first symptoms include nausea and vomiting and then diarrhea, usually within 24 to 48 hours. The cells of the gastrointestinal tract are rapidly dividing cells so they are destroyed first. These symptoms can occur when exposure is as low as 1 Sv (a Sievert is an SI unit of dose equivalent and is meant to quantify the biological effects of ionizing radiation). Blood cells, reproductive cells and hair cells also divide rapidly so they are also affected soon after exposure. Blood cell damage, if severe enough, can quickly lead to sepsis and death. Sepsis usually occurs when the bloodstream is overwhelmed by bacteria, but it can also occur when the bloodstream is full of dead and damaged cells. Either case stimulates a body-wide immune response so severe that organ systems can be damaged or shut down altogether. Rapid development of fever and vomiting within minutes of exposure is a sign of severe exposure. The good news is that even in severe cases, about half of all exposure victims survive (with severe sepsis being the leading cause of death).

From Ordinary to Dangerous Exposure Levels

For a comparison between exposure levels and physiological symptoms see this online chart. For a comparison between acute exposure levels and more ordinary radiation levels associated with X-rays, high altitude flights, etc., see this online chart.

PART 2: THE SCIENCE OF RADIATION

Radioactive Elements

Our universe is made up of 118 known elements, many of which are radioactive. Radioactive elements have unstable atomic nuclei. These atoms emit radiation as they spontaneously transform from a high-energy unstable state into a lower energy stable state, which represents either a new isotope of the same element or a new element altogether. Many of these reactions cascade, creating different intermediate, and often also radioactive, elements along the way, eventually stopping at atoms with stable nuclei. Many elements come in more than one isotope, each of which may vary in nuclear stability. This means that an element, carbon for example, can come in isotopes that, although they all have the same number of protons in their nuclei, differ in their number of neutrons. Carbon-12 is the most abundant carbon isotope, with 6 protons and 6 neutrons, creating the most stable nuclear arrangement. Carbon-13 is also stable but less abundant. It has 6 protons and 7 neutrons. Carbon-14, used in radiocarbon dating, has a slightly unwieldy nucleus of 6 protons and 8 neutrons, which is unstable. Each carbon-14 nucleus will eventually decay into a nitrogen nucleus; the carbon atom will become a nitrogen atom. 

Both atoms have the same mass number, the same total number of protons and neutrons, but carbon-14 has an unstable arrangement of 6 protons and 8 neutrons and nitrogen has a very stable arrangement of 7 each of protons and neutrons. Carbon-14 has a half-life of about 5700 years. This is the time it takes for half of any given sample of carbon-14 to decay into nitrogen-14. In this process, a neutron is converted into a proton. When it does so, a high-energy electron and a massless particle called a neutrino are emitted. The emission of high-energy electrons is called beta decay. Neutrino radiation passes through us everyday (it comes from the Sun). In fact, neutrinos pass right through the Earth as though it is invisible to them. These particles are harmless to us, but beta radiation is not. Luckily, carbon-14 only occurs in trace amounts and it has a very slow decay rate, so it contributes very little to the background radiation to which we are exposed. Still, our bodies at any given time contain a few atoms of this radioactive element. It's part of our proteins, carbohydrates and our fat and it's part of the carbon dioxide in our lungs and tissues. An occasional high-energy electron blasts a cell or two and the damage is quickly repaired.

There are three kinds of radioactive decay: beta, gamma and alpha, and they are all emitted as part of different decay processes. Beta radiation is the emission of an electron. Alpha radiation is the emission of a helium nucleus, also called an alpha particle. Gamma radiation is the emission of a very high-energy photon. A particular radioactive element can emit different kinds of radiation as it decays through different decay modes. Different kinds of radiation have different dangers associated with them.

Beta Radiation

In this kind of radiation, or decay as it is also called, either an electron (or a positron, the electron's antimatter counterpart) is emitted. These beta particles can have energies ranging from a few MeV (mega-electron volts) up to a hundred MeV and their speed can vary, in some cases approaching the speed of light. All beta radiation is ionizing radiation.*  That means that beta particles have enough energy to knock an electron out of an atom or molecule and, as a result, damage living tissue by either changing the structure of biological molecules and altering their functions or making them nonfunctional. Beta particles can also strike molecules of DNA and create spontaneous mutations which can lead to cancer, or birth defects if the DNA is in an ovum or sperm cell. Although the energy of beta particles varies, most beta particles can be blocked by an aluminum sheet. Beta radiation can penetrate living tissue but not very far. However, this kind of radiation can be especially dangerous because, as it is made up of charged particles, electrons, it is strongly ionizing.

Iodine-131 and The Need for Iodine Pills

The beta radiation from iodine-131, a common and hazardous product of nuclear fission, can only penetrate between 0.6 to 2 mm of tissue. This isn't very far. The hazard with I-131 is not its penetration but how ionizing its beta particles are. 90% of tissue damage from this isotope comes from this ionization. Only 10% tissue damage comes from the far more penetrating gamma radiation that is also emitted as it decays (this damage occurs further away from contact). Iodine-131 becomes dangerous when it becomes airborne. This can happen when a nuclear plant or bomb explodes and sends radioactive dust into the atmosphere. It can be breathed in and it can land on water, plants and animals, which are ingested. This radioactive dust then accumulates in the thyroid where the iodine we consume is concentrated and used for thyroid functioning. As radioactive iodine decays inside the thyroid it damages it and this damage can later on lead to thyroid cancer. The risk of developing thyroid cancer is highest among the youngest, so fetuses and children are at high risk. Fortunately, people at risk of I-131 exposure can take iodine pills. This reduces the thyroid's retention of the radioactive iodine. It's then flushed out of the body in the urine before it can do much damage. Thousands of cases of thyroid cancer could have been prevented if these simple inexpensive pills had been available to the people living near the Chernobyl nuclear plant just before or immediately after the explosion there. The half-life of iodine-131 is 8 days so only a short pill regimen is needed. Within weeks, food and water exposed to iodine-131 are no longer hazardous.

*Non-ionizing radiation, in contrast, does not carry enough energy per particle or photon to ionize atoms or cells but it can nonetheless be dangerous. UV radiation from the Sun is an example of nonionizing radiation that can damage skin cells enough to cause cancer.

Gamma Radiation

Gamma radiation is different from beta radiation because it is made up of photons of electromagnetic radiation rather than particles with mass such as electrons. But, like beta radiation, gamma ray photons are emitted during some kinds of radioactive decay and they are a form of ionizing (tissue-damaging) radiation. Gamma rays have a very short wavelength and very high energy.

Health Risks of Gamma Radiation

To measure the biological effect of gamma rays, the SI unit of equivalent dose, the sievert, can be used, much like beta radiation. You might also see it described in units of absorbed dose, in grays, and for gamma rays this measurement is numerically equivalent to the sievert.

The highly energized electrons of beta radiation are rapidly slowed to non-dangerous levels in the body because the charged electrons interact electromagnetically with the atoms in the body's tissues. Gamma rays, being photons, are not appreciably slowed down as they pass through materials. However, they can be effectively blocked by materials with a high atomic number or by materials of high density. Passing through such materials, some photons are bounced off the relatively large nuclei of these barrier atoms, scattering and losing energy. Lead is very dense and it is the most effective gamma ray barrier but aluminum, concrete and soil can also be effective. Like beta radiation protection, the thickness of the barrier required depends on the energy of the radiation, in this case the energy of the particular gamma photons.

Beta particles can only penetrate a millimeter or so of the body, so they tend to cause radiation burns on the skin if someone is in direct contact with a radioactive source such as iodine-131, gamma rays easily penetrate the whole body, causing diffuse damage and, as a result, radiation sickness and increased cancer risk, rather than burns. However, please don't confuse external exposure with internal exposure. For example, iodine-131-laced dust, if inhaled or taken internally through contaminated food or water, can cause much of the same kinds of diffuse damage and radiation sickness as gamma radiation exposure even though the penetrating ability of the beta particles is very minimal.

Gamma rays are not particles so they cannot by themselves contaminate anything. This is why irradiated food poses no radiation health risk and medical equipment sterilized by gamma rays pose no threat. The radiation in these cases kills all bacteria. As well, very high-energy gamma rays do not tend to be as damaging to the body as those that are lower energy because at very high energy they simply pass right through the body as though it was invisible to them, without interacting with any of the body's atoms and molecules.

Where Gamma Rays Come From

Gamma rays tend to be emitted along with other forms of radiation from a radioactive source immediately following other kinds of decay reactions. The reason for this is that, when an atomic nucleus emits an alpha (you will learn about these particles shortly) or beta particle, the daughter nucleus is often left in an excited state. It will spontaneously move to a lower energy state by emitting a gamma photon. This is the same kind of process that happens in a light bulb. When electricity passes through the tungsten filament in an old-fashioned light bulb, the tungsten atoms emit photons in the visible spectrum (light) as they are excited and move to a lower energy state (the atoms also emit infrared photons - longer wavelength photons - and that's why the bulb gets hot and it's not very efficient).

Alpha Radiation

Alpha radioactive decay happens when an atomic nucleus emits an alpha particle. An alpha particle is the same thing as a free helium nucleus; it has 2 protons and 2 neutrons bound tightly together. For example, uranium-235 decays into thorium-231, emitting an alpha particle in the process. Alpha particles are always emitted at a relatively low velocity. They have a narrow range of possible kinetic energies. Because they are highly charged, having a charge of +2, and because they have a relatively high mass, they can't travel far from their source, only about 2 centimeters through air. Even when in direct contact with skin, alpha particles cause little damage because they can't penetrate tough skin cells. However, if atoms of an alpha-emitting radioisotope are ingested, they can cause extreme cellular damage and possibly cancer because they are extremely ionizing. Radon, for example, is a naturally occurring radioactive gas found in some soils and rocks. If it is inhaled it can damage lung tissue and lead to lung cancer. You might have heard of the famous 2006 radiation poisoning case of Russian dissident Alexander Litvinenko. He died of radiation poisoning caused by the ingestion of polonium-210, an important ingredient in nuclear weapon triggers (these will be discussed below). One theory is that a small sample of polonium-210 was smuggled into Great Britain by an unknown Russian official and placed in his tea. The radiation in his body was difficult to detect because hospitals are equipped only with gamma ray detectors and polonium-210 emits only alpha particles.

PART 3: NUCLEAR BOMBS, NUCLEAR REACTORS AND URANIUM-235

The Uranium-235 Chain Reaction

An example of a gamma ray source is uranium-235, a common radioactive isotope used in nuclear reactor fuel rods. It's not a scary green glowing liquid like you see in the movies or on The Simpson's. You can actually hold it in your hands with minimal protection because it has a very long half-life of 700 million years. So how is it used as fuel and bombs?

Uranium-235 is fissile, which means it can undergo a fission chain reaction - the kind of reaction that is sustained inside nuclear reactors.  This is what its fission chain reaction looks like. The fission chain reaction of uranium-235 is often called the actinium series, and if you take a look at the diagram here, you will see that many elements, which are themselves radioactive, are created and decay along with the uranium isotope. As the process unfolds, alpha, beta and gamma radiation are all emitted from various decay reactions, along with a sustained creation of free neutrons. For example, lead-211, an alpha decay product of polonium-215, decays through beta^- decay, emitting a positron, into bismuth-211. As I mentioned, Uranium-235 has a very long half-life. This refers to its spontaneous fission rate. This decay happens all by itself, given enough time. Uranium-235 is special in that it is one of few materials that can also undergo induced fission. This means that it can be induced to decay very rapidly. This decay reaction is governed not by the uranium's very stable decay constant but by bombardment mechanics. What it needs is to be struck by a free neutron and a nuclear fission reaction is started. The uranium-235 atom readily absorbs the extra neutron and immediately becomes so unstable it splits, creating products, which are themselves unstable, and they rapidly split too, until stable nuclei are eventually formed. The mixture of daughter elements formed can take fractions of a second to months to days up to many thousands of years to decay and that is why spent nuclear fuel is so hazardous. Each daughter atom, when it has just been created, is in an excited state so a gamma photon is emitted. That is why nuclear power plants must be shielded by water and then by thick concrete. A great deal of gamma radiation is continuously emitted in every direction, just as light is emitted from a light bulb. Plant workers must also be protected from the alpha and beta radiation that is also emitted. Water is very effective in stopping both of these particles.

Free neutrons are not found under ordinary circumstances. They are all tightly bound up inside atoms, squeezed together by the strong force (even the strong force is not enough to hold the nuclei of highly unstable elements together and that is why they decay). A slow neutron bombards a uranium-235 nucleus and a self-sustaining nuclear fission reaction ensues.

Free Neutrons - A Fourth Kind of Radiation

Concentrated beams of free neutrons are used in many physics experiments and are sometimes used in medicine to treat cancer. Neutron radiation is indirectly ionizing radiation. Neutrons themselves aren't electrically charged, but when they are absorbed by atoms in the body, gamma photons are emitted and these high-energy photons in turn ionize nearby atoms, stripping an electron off each one and that is when biological molecules, of which atoms are part, can be damaged. Neutron radiation is extremely dangerous. Free neutrons have very high, but variable, energy, so they easily pass through most materials but they interact enough with most atoms in cells to cause damage. In reactors, neutron radiation contributes to the overall gamma radiation that is emitted. Fortunately, water and concrete are effective barriers. Because water molecules are small, they are particularly effective in scattering the neutrons and slowing them down, their energy being absorbed by the water as it is ionized. This special interaction with water, however, makes neutron radiation particularly effective in causing cancer or death compared to an equivalent exposure to gamma ray or beta radiation, because our bodies are composed mostly of water. Free neutrons are also dangerous because they can induce radioactivity in other atoms. When free neutrons are captured by other atomic nuclei, unstable isotopes are created which then become radiation hazards, themselves. This public safety article lists some of the most common isotopes created by this process, called neutron activation. Neutron activation is the principle behind the neutron bomb. This nuclear weapon, if detonated, leaves buildings and infrastructure intact while killing people and animals.

How An Atomic Bomb Works

The atomic bomb that was dropped on Hiroshima in 1945 was made of highly enriched uranium-235. It consisted of two subcritical masses of uraunium-235. To detonate the bomb, one of the pieces was fired at the other down a gun barrel inside the bomb casing, to create a critical mass. The critical mass of a fissionable material depends on many factors including density, shape, enrichment, purity and temperature. Once a piece of material reaches critical mass it can sustain a nuclear fission reaction by itself.

The bullet also struck a small polonium/beryllium generator. This is what creates the free neutron to kick-start the fission explosion.  The generator is kept separate and enclosed by a layer of foil. When the foil is broken by the bullet, the polonium-210, which spontaneously emits alpha particles, is able to provide free alpha particles to collide with beryllium-9 to produce beryllium-8 and free neutrons. The uranium-235 now had enough critical mass to sustain a reaction long enough to discharge explosive energy and it had the necessary free neutron trigger to set the explosion in motion.

How A Nuclear Reactor Works

Nuclear reactors aren't all that different from coal power plants. They both heat water into pressurized steam and the steam drives a turbine generator to make electricity. The difference is in how the water is heated.

First of all, you might be wondering where uranium-235 comes from. It is the most common fissionable material used in both nuclear plants and nuclear bombs. It is a common element and has been part of Earth since it was born. Its atoms were created in supernovae and eventually became part of the Solar system. Because uranium is unstable, it has been decaying, very slowly, ever since its stellar birth. The Earth had much more uranium-235 when it first formed billions of years ago.

Uranium ore only needs to be enriched by about 2-3% before it can be used as fuel, compared to weapons-grade uranium, which needs to be at least 90% pure Uranium-235. This is because a bomb is small and it needs to be very pure to achieve critical mass.

The enriched uranium is formed into pellets that in turn are formed into rods and assembled into bundles. The bundles are submerged in water. The water acts as a coolant (as well as a radiation shield). Left to its own devices, the uranium, once it has begun fission, would heat up and eventually lead to a meltdown. In addition to water, the rods are kept from overheating by control rods made of neutron-absorbing material. These can be inserted into the bundles to a varying degree to control the rate of reaction. The absorption of free neutrons slows the reaction. The control rods can be completely lowered into the bundles. When they are completely lowered they can't completely stop the reaction but they can slow it down to about 6%. The water, while acting as a coolant and radiation shield, also transfers the heat energy in the form of pressurized steam to drive the turbine and spin the generator.

What's Happening in Japan?

For a continuously updated reactor-by-reactor status  report on the 6 Fukushima reactors refer to this link. Three of these reactors are at  INES level 5 (out of a possible 7 with Chernobyl being a 7) as of March 21, 2011. Some nuclear authorities have labeled them level 6. Click on the INES link to find out what this means in terms of current threat level.

Before I discuss what is known so far about the Fukushima nuclear accident, I recommend that you take a look at what happened at Three Mile Island. You will find some of what happened there eerily similar to what is happening now. In the Japanese nuclear reactors at the Fukushima nuclear plant, the control rods automatically lowered into the fuel bundles to stop the reaction soon after the earthquake struck, a process called an emergency shutdown. The problem is that the rods were extremely hot when the reaction was stopped and the residual 6% reaction rate continued to heat the water. The fuel rods are housed in zirconium. This metal is used because it doesn't normally react with water and it doesn't absorb neutrons so it doesn't impede the reactions. The plant lost electrical power when the tsunami swamped it several minutes later. And, somehow, backup power was also lost. Without power, the plant couldn't continue to automatically keep the water tanks in the core filled. In the core, water is continuously lost to steam, from the heat given off by the decay reactions. As the core heated up to about 2200 F, the zirconium housing began to chemically react with the water that was left to produce zirconium oxide and hydrogen gas. This reaction is exothermic so it contributed even more heat to the core. The pressure of hydrogen gas increased until it explosively blew the reactor container apart, further destabilizing the reactor and making it even more difficult to keep the fuel rods under water and preventing them from melting. This CNN video explains that three separate safety systems unexpectedly failed at these reactors which resulted in catastrophe.

As the zirconium housing deteriorates, the fuel rods can leak. The pellets in these rods are not metal, they are a compressed powder of uranium oxide, so the rods per say don't melt. This is not good news though. Some of the gas products of the fission reaction are radioactive xenon and radioactive krypton. These gases normally accumulate inside the rods, but once the rods are compromised, they can escape into the reactor building. They are heavier than air so they will sink to the bottom of the building and if it's airtight they won't get out. Some uranium compounds will also escape but these are also very heavy and they will not disperse much unless there is an explosion or fire. The most dangerous by-products are the radioactive isotopes iodine-131, cesium-137 and strontium-90. They are lighter and can disperse into the atmosphere, with strontium-90 being slightly heavier and less dispersible than the other two. As mentioned, in the beta radiation section of this article, iodine-131 can be taken up and concentrated in the thyroid gland where it can damage cells and possibly lead to thyroid cancer. Cesium-137, with a half-life of around 30 years, can be mistaken for potassium inside living organisms and passed up the food chain and concentrated, all the while emitting ionizing radiation. Cesium detectors are usually used to assess how much radiation is leaking from a reactor. Strontium-90, also with a half-life of about 30 years, is less volatile than the other two isotopes but it is especially dangerous because it mimics calcium in people and animals, so it is taken up by and deposited in bones, destroying rapidly dividing bone marrow cells and potentially causing cancer. The hydrogen explosions serve to disperse the radioactive materials into the air. Technicians have attempted to add boron to the coolant water because it absorbs free neutrons and slows the reaction rate. If the core can't be cooled down, eventually, the powdery fuel and its reaction products, along with what is left of the zirconium housing, will melt into a lava-like soup called corium. Very thick steel on the reactor bottom will dissipate much of the heat from the mixture, but if nothing is done to cool the mixture, the steel will melt and it will all come into contact with the thick concrete housing below. The concrete might hold. If it doesn't the mass will eventually disperse in the ground below, the heat will slowly dissipate, and the nuclear reaction will slow to a stop.

No one can predict how much radioactive material will be dispersed from the Japanese reactors. It's also very difficult to predict exactly what materials will be dispersed because there are so many possible chemical reaction products and fission by-products, each of the latter possibly emitting beta, gamma and/or alpha radiation. Neutron radiation is another significant hazard from a reactor that is no longer properly shielded, along with the various radioactive isotopes this radiation can induce.

Health risks are also very difficult to assess in the population nearby in Fukushima prefecture because air-born radiation will travel along prevailing winds with radioactive particles deposited via raindrops, snow or dust along the way onto water reservoirs, crops, animals and people. The Japanese earthquake and tsunami disaster is already documented and being constantly updated here at Wikipedia.

I am horrified, as is everyone, by the unfolding disaster in Japan. I believe we are deeply indebted to the people there who are, unfortunately, teaching us how to endure and how to prevent future disasters, perhaps through better education, more active public involvement and better nuclear plant design.

Finally, I have two videos so share with you that are frightening to watch but valuable as an informative guide to the dangers of nuclear energy that we should all be aware of.

The first one is 6 minutes long, from RT News Station and it aired on March 17, 2011. It's an assessment of the current state of the 6 nuclear reactors in the Fukushima power plant and what might be happening right now:

This next video is called Inside Chernobyl's Sarcophagus. It's a followup on the 1991 NOVA documentary called Suicide Mission To Chernobyl and it's 46 minutes long. Well worth a viewing to compare what is going on how in Japan with what is considered the worst nuclear accident in history.

Our Solar System Part 1: Earth

An Exploration of How Earth Was Formed and How Life First Took Hold

A Violent Beginning

Looking at this photo of Earth taken during the Apollo 17 lunar mission, you might get the impression that Earth has always been this beautiful blue life-filled planet.

Far from it. Our planet was born of fire and violence, and life, once it took hold, hung on tenaciously in one form or another over several mass extinctions. Many events came together in just the right order to allow us to exist here today.

4.6 billion years ago, Earth and the entire solar system itself, was a cloud of dust and gas composed of hydrogen and helium as well as heavier elements spewed away from supernovas, near the edge of a galaxy called the Milky Way. A nearby supernova may have recently exploded, creating a shock wave that created localized pockets of denser matter, dense enough to trigger this cloud to collapse. It began to rotate, thanks to the angular momentum of its particle atoms and gravity, and inertia slowly flattened it into a disk, called a protoplanetary disk. It might have looked something like this artist's conception.

Material began to concentrate in the center and heat up. Its rotational speed increased much like how a skater rotates faster as she draws her arms in. The force of gravity and angular momentum conspired to increase the overall energy of this central mass and, with no way for the energy to escape into the vacuum of space surrounding it, eventually its temperature and pressure became so great that hydrogen atoms began to fuse, igniting a nuclear fusion reaction. At this point the central mass called a protostar, ignited into a star, our Sun. Internal energy, directed outward, soon balanced the inward force of gravity to create a stable state called hydrostatic equilibrium. Radiation from the blast blew much of the dust and gas outward but not all. Bits of small dust-like grains composed of silicon, magnesium, aluminum and oxygen as well as other trace elements collided with each other and accreted. These chunks are preserved in chondrite meteorites. These bits formed larger and larger chunks of matter as they collided with and captured each other within their orbital neighbourhoods, eventually forming planets, moons and asteroids. The thick dust around the newly formed Sun would have made it appear red and opaque as chunks of planetesimals were colliding with each other to form Earth. The accretion process is illustrated here in this artist's rendering of Earth forming from a disk of accreting chunks of rock.

When Earth was forming from an accreting mass of orbiting matter, it regularly melted from the energetic impacts and this process caused the matter inside Earth to differentiate according to mass. 

A core evolved from the densest elements, mostly iron (89%) but other heavy elements as well, including radioactive elements. A mantle composed of a mixture of oxygen (45%), silicon (22%) and magnesium (23%) as well as many other elements, formed around the core. Finally, a crust composed of lighter elements and compounds, most of which is silicon dioxide (61%) and aluminum oxide (16%), would eventually form as the planet cooled.  This process is called planetary differentiation.

The Moon is Born From Catastrophe

Earth had just formed. It was about 100 million years old and it had just differentiated into core and mantle but no crust existed yet because it was still completely molten, when it collided with a Mars-size planet called Theia, shown here in this artist's depiction. Theia is the name of the mythical Greek titan who gave birth to the moon goddess, Selene.

Computer simulations suggest that Theia's iron core sank into Earth's core on impact while Theia's mantle and a great deal of Earth's mantle were ejected into orbit around Earth. It probably took less than a thousand years for this material to coalesce into the Moon. Earth gained significant mass and angular momentum from this collision. After impact it was rotating so fast a day was only 5 hours long. And the new Moon loomed very large on the horizon. It was about ten times closer, about 30,000 km away. This is what Earth might have looked like then.

The Moon's orbit is expanding at a rate of 3.8 centimetres per year and as it does so, Earth's rate of rotation slows, gaining 2 milliseconds every one hundred years. The reason for this is that the Moon's gravitational attraction literally squishes Earth into an egg shape (actually the distortion is only on the scale of a few meters) and there is a lag time for Earth to reform a sphere shape so the Earth's and Moon's bumps don't perfectly face each other. This creates a torque on the spin of the Earth and a similar back reaction on the Moon, which slows down Earth's rotation and injects energy into the Moon's orbit, pushing it away. All the heat lost from the rhythmic deformation of Earth also translates into lost rotational energy. The moon used to rotate faster but tidal forces slowed its rotation until it became tidally locked with Earth, happening just 50 million years after its formation. That is why we see only one face of it. The moon will continue to move away from the Earth until it is about 1.6 times the distance it is now but its rate of orbit expansion has slowed so much it will take 15 billion years to do so. By then, the Earth and the moon will have long been engulfed by the Sun when it expands into a red giant and the Sun itself will be nothing but a brown dwarf dying star remnant. 

Add Ingredient For Life #1: A Magnetosphere

A part of the differentiation process, which was well under way when Theia collided with Earth, is what is called the iron catastrophe, in which the spinning molten metal outer core created Earth's magnetosphere, shown here.

Earth's magnetosphere formed when the Earth was about 500 million years old, not long after the Theia impact. Contrary to the name "catastrophe," the creation of the magnetosphere was essential for life to take hold on Earth. Without it, solar radiation would have stripped away the solar radiation-deflecting atmosphere and any surface water would have dispersed into space before life could evolve.  

Add Ingredient for Life #2: An Atmosphere

Planets as large as Earth had enough gravitational pull to hang onto gases that came from outgassing within the mantle and from comet impacts. These captured gases created Earth's first atmosphere, which replaced a thin haze of hydrogen and helium, leftover from Earth's formation, with carbon dioxide, nitrogen and water vapour. Water vapour increased in abundance with each comet impact (and there is new evidence that asteroids may have contributed much of Earth's water in addition to the contributions from icy comets. Earth was heavily bombarded with comets and asteroids during this violent and chaotic phase of the solar system's evolution.

Hydrogen and helium gases, being very light, dissipated into space as they formed and were replaced over time with the heavier gases from mantle outgassing. As the mantle began to settle, volcanic outgassing contributed to the atmosphere tremendous amounts of carbon dioxide (CO₂), as well as hydrogen sulfide, sulfur dioxide, methane and some ammonia, and additional water vapour released from mineral hydrates within the mantle.

As the Earth began to cool, the surface solidified into a crust and vast amounts of accumulated water vapour in the primordial atmosphere condensed and rained down to form the first ocean. Surface temperatures then were about 230°C. Liquid water existed only because the CO₂-heavy atmosphere was extremely dense, creating enough pressure to prevent water from vapourizing.  Frequent super-cyclonic storms raged across the surface, fueled by the Earth's rapid rotation and energetic atmosphere. The Moon, much closer then, may have created tides of superheated water up to a thousand times higher than those today, perhaps 100 km high, crashing over land every few hours! Frequent cosmic collisions continued up to about 3.8 billion years ago. Their impacts regularly re-vapourized part or even the entire ocean, creating high altitude clouds that completely enveloped the planet. As the bombardment slowed, clouds dissipated as water vapour rained out of the atmosphere into the ocean.

Earth As a Giant Organic Chemistry Lab

Volcanic gases such as carbon dioxide, sulfur dioxide and hydrogen chloride readily dissolved in the ocean into acids that would have been neutralized by various minerals. Eventually the ocean became a reducing soup of organic compounds in an environment energized by intense UV radiation. Even though the young Sun was fainter then, most UV radiation reached the surface because the ozone layer had not built up yet. Volcanic activity and intense radioactivity within the young core also contributed energy to the environment. Intense lightning from volcanic eruptions may have played an important part in producing HCN (hydrogen cyanide) from CH₃ (methyl groups) and NH₃ (ammonia) or N₂ (nitrogen gas). HCN is essential for the synthesis of amino acids and nucleobases, parts of the building blocks of proteins and DNA, respectively. Small chemically reactive intermediate molecules such as formaldehydes, ethylene, cyanoacetate and acetylene, which can recombine into more complex intermediates that can in turn form stable biochemicals, would have formed under these conditions, but we don't know how concentrated they might have been in the primordial ocean because it is very difficult to pinpoint what the temperature and pH of that environment was. However, we do know that amino acids did in fact form because life made its first remarkable appearance about 3.8 billion years ago. Experiments such as the Miller-Urey experiment, which involve simulating the conditions of this primordial environment, have successfully created over 20 different amino acids.

Oxygen and Nitrogen

The earliest atmosphere contained very little oxygen gas. What little oxygen was present came from the dissociation of water vapour in the upper atmosphere by ultraviolet radiation. Much of this oxygen would have been photochemically converted into what was then a very thin ozone layer, further reducing the abundance of it in the atmosphere.

Researchers have puzzled over the abundance of nitrogen in our atmosphere. It was a predominant material during Earth's formation, probably in solid form, much of which may have been released as gas from the mantle through volcanic eruptions. Nitrogen is an inert gas that is quite stable under solar radiation so it may have built up slowly and remained in the atmosphere so that now it comprises about 78% of the atmospheric content today.

So far I have described the Hadean (a name aptly derived from Hades or Hell) era Earth, a period lasting from Earth's formation to about 3.8 billion years ago when life was about to appear for the first time.

A few rocks surviving from this very early period have been discovered in Greenland, Canada and Australia. The oldest dated rocks, zircons, have been dated at 4.4 billion years old. Life evolved when there was no oxygen and no protection from UV radiation, when volcanoes ravaged the world and the ocean was a roiling soup. The sky was an ominous reddish colour and the ocean was dead grey. It is possible that life arose more than once under these conditions as the primordial ocean repeatedly vapourized and condensed during repeated asteroid impacts. Life may have evolved in shallow clay-rich waters and/or near hydrothermal vents that spew out concentrated ammonia and methane, two molecules that could serve as building blocks for more complex organic compounds. Some theorists also propose that life may have gotten its start when primitive RNA-like sugar-rich molecules hitched rides on the backs of meteorites. It is difficult to model how these sugar molecules, which form the backbone of RNA and are called ribose, could have been created from Earth's primordial ingredients.

A Molecule That Can Copy Itself

In the chaotic and energetic environment, a molecule somehow gained the ability to replicate itself. An early form of abiotic (meaning nonliving or pre-life) evolution has been postulated in which different potential methods of replication were attempted and either improved upon or eliminated based on the natural tendency of any system to move toward the lowest possible energy state. It is possible that within the ocean's organic soup, the energy from lightning and UV (ultraviolet) radiation drove reactions creating more and more complex molecules from simple compounds such as methane and ammonia. Among these molecules were amino acids and nucleobases, the building blocks of life. Reactions occurred randomly, and by chance a replicator molecule was formed, perhaps something much simpler than but superseded by DNA, perhaps with RNA as an intermediate. DNA is now life's universal replicator molecule, except for some viruses and prions.

A Cell Membrane

Some kind of protective envelope to house the replicator molecule would have been needed and this would likely have come from a primitive phospholipid bilayer sphere, which can form spontaneously when phospholipid molecules are placed in water. Eventually a self-contained organism, a single cell prokaryote, evolved, which used DNA as its genetic code, RNA for information transfer and protein synthesis and enzymes to catalyze reactions. There may have been many kinds of protocells and this one line out-survived the others and evolved. Many of these terms may be new to you. Exploringorigins.org provides an excellent and easy to understand tutorial on this entire process, explaining the importance of RNA, DNA and proteins and how they may have come together to create the first living cell under the extreme conditions of early Earth. It comes with many short animations to illustrate the processes involved.

An Energy Source For Life

The first cells to evolve likely used surrounding organic molecules for energy, much like many extremophiles do today. An example of an extremphile, are primitive bacteria-like unicellular organisms called endoliths that survive by feeding on traces of iron, potassium or sulphur. They can survive long ice ages by simply slowing down their cellular processes.

A Much Better Energy Source For Life: Sunlight

At some point, about 3 billion years ago, cells evolved a new strategy for capturing and using the energy in sunlight, and this evolution drastically changed the atmosphere and climate of Earth. Highly successful cyanobacteria (bacteria that obtain their energy from sunlight) utilized a new cellular process called photosynthesis, using CO₂ and water as raw materials to create energy-rich sugars, with oxygen as a byproduct. These bacteria evolved in shallow water, trapping sedimentary grains in their bacterial biofilms to create large structures called stromatolites. Stromatolites were the first life to colonize Earth. A few very rare colonies still exist in hypersaline lakes where animals can't graze on them. The oxygen gas released from enormous colonies of stromatolites was captured by organic matter and by dissolved iron in the ocean, but as these minerals became saturated with oxygen, it began to accumulate in the atmosphere. This process is sometimes called the Oxygen Catastrophe because it probably caused the greatest extinction event of all time. Oxygen would have been toxic to most other kinds of bacteria because it destroys organic compounds. As free oxygen combined with atmospheric methane, a very potent greenhouse gas, and as atmospheric carbon dioxide diminished through photosynthesis, eventually a glaciation event was triggered that was so severe that Earth was entirely enveloped in ice, the first of several "Snowball Earth" episodes to come. This ice age, called the Huronian glaciation, began about 2.3 billion years ago and lasted between 300 and 400 million years. The Oxygen Catastrophe also marked the beginning to a new opportunity for life to evolve because until now, life was energetically limited to fermentation reactions. Now cells could take advantage of the much more effective metabolic process of respiration. The geology of Earth changed dramatically as minerals became oxidized. Mitochondria, organelles acting like little cellular power plants, evolved soon afterward, which could turn sunlight into highly concentrated energy storage molecules called ATP. A new kind of cell called a eukaryote, housing all of these enhancements, made its first appearance about 2 billion years ago. Because it had more energy at its disposal, it could grow larger and even more complex. Meanwhile, some of the oxygen was converted into ozone through ultraviolet radiation, creating a UV-protective layer in the upper atmosphere which allowed cells to colonize the surface of the ocean, and later, on land as well. Before this, the DNA in cells would have been vulnerable to high rates of lethal mutations caused by the radiation. At this time, volcanic islands began to coalesce into one large supercontinent called Nuna or Columbia, which existed around 2 billion years ago and began to fragment 1.6 billion years ago, starting a pattern of repeated supercontinent assembly and fragmentation driven by plate tectonics that continues to occur today, ending with the latest supercontinent, Pangaea. This brief but interesting National Geographic video  illustrates Earth's early formation.

Life Hangs In There Through Extremes

The three types of unicellular organisms, archaea (a group that contains, for example, extremophiles and methanogens which live in our gut), bacteria, and eukaryotes (at that time confined to living in the water) continued to diversify, possibly during the extreme cold of the ice age as well as after conditions warmed. By about 1 billion years ago, plant, animal and fungi lines had all split and multicellular life was beginning to evolve as cells began to accumulate in colonies and a division of labour began to take place. The first simple multicellular organisms such as green algae and sponges began to evolve.

Multicellular Life Evolves Twice!

Another supercontinent called Rodinia formed about 1.1 billion years ago, and like Nuna, it was entirely barren because no terrestrial life yet existed. This supercontinent was centered on the equator and some theorists believe that because of its location, the rate of chemical weathering of its rock increased, sequestering CO₂ and removing its greenhouse gas function from the atmosphere. Two more Snowball Earth episodes followed, around 710 and 640 million years ago. Permafrost decreased chemical weathering and as atmospheric CO₂ levels gradually built back up through volcanic activity, each ice age ended. This ushered in a period of intense evolution of multicellular life forms called Ediacara biota. Plants and animals with tissues performing specific functions evolved. Fossil evidence suggests that these early organisms were completely replaced by those of the later Cambrian explosion, because most current body plans of animals appear only in the Cambrian fossil record and not in the older Ediacaran period. You might be wondering how the fossils of these soft-bodied animals could exist today at all. Life on Earth is made up of organic compounds. These molecules are based on carbon to which atoms of oxygen, hydrogen, nitrogen and other elements are attached. When some of these animals died, their soft bodies were quickly buried in sand, preserving them very well. Hydrogen, nitrogen and oxygen are volatile elements and they were driven off of the fossils over time. What remained is carbon and so the overall shape of each organism was preserved in ancient rocks as a carbon layer, an imprint. Canada has an excellent collection of these fossils. I urge you to click on this link and browse the online exhibit.  This website gives you an idea of how these animals might have lived and what they might have looked like. At around 540 million years ago these once quite plentiful older fossils, representing impossible to classify Ediacaran organisms with discs, tubes, mud-filled bags and quilted mattress-like bodies, vanished. No one is sure why but perhaps these organisms represent a failed first attempt at multicellularity. By about 510 million years ago, the Cambrian explosion was underway. Most of the major phyla of organisms made their appearances as the rate of evolution accelerated and the diversity of life exploded and organisms eventually exploited every available ecological niche. This 32-minute online lecture explores Cambrian life in detail. It is still unclear what triggered the Ediacaran die-out and the Cambrian explosion, but perhaps fluctuating atmospheric oxygen levels contributed to them.

Land Plants Appear

About 510 million years ago, at the start of Cambrian explosion, land plants made their first appearance, evolving from branched filamentous algae that lived in shallow waters. This 5-minute video

gives you an idea of just how essential plants are to all life on Earth. There is some evidence that simple photosynthetic algae lived in fresh water depressions and lakes perhaps as early as 1 billion years ago but they did not colonize in high enough numbers to impact atmospheric gases. Several more mass extinctions, triggered by climate changes or catastrophic events or a combination of these, would threaten life but not obliterate it in the eons to come.

Earth Teaches Us How To Look for Life On Other Planets

The atmospheric effects of life on Earth have helped astronomers to define their search for possible markers of life on extrasolar planets. The presence of liquid water and gases such as methane, carbon dioxide and especially oxygen mark the possibility that life which uses biochemical pathways similar to those used on Earth could be supported. An increasing understanding of how life evolved on Earth may help astronomers refine their search toward eventually finding even more specific biomarkers of life.

Next up: Part 2: Mars

Our Solar System Part 2: Mars

Mars, the fourth planet from the Sun, is named after the Roman god of war and agricultural guardian. This is a mosaic image of the planet taken by Viking 1 in 1980. It was first seen close up by Mariner 4 as it flew by the planet in 1965. Until then, many researchers believed the light and dark patches and long channels visible in this image were signs of liquid water in the form of seas and rivers, possibly even irrigation channels, suggesting that intelligent life lives there. We now know that Mars is far less hospitable than we once thought and yet, after decades of intensive study, Mars still intrigues us with its many mysteries. 

Water

Water does, in fact, exist on Mars. The lines and depressions you see in the image above are optical illusions. But, there is a large quantity of water ice at the poles, as revealed by radar data from two current missions, Mars Express and Mars Reconnaissance Orbiter. In fact, there is so much water ice in the southern polar cap that, if melted, it would cover the whole planet in water 11 meters deep. However, liquid water can not exist for long on the surface of Mars because any liquid water would quickly freeze and sublimate into the atmosphere. As well, the surface temperature is, on average, too cold for water in its liquid state and the atmospheric pressure is far too low (it's about equal to the air pressure on Earth at 35 km above sea level, that’s about 3 times higher up than a typical jet's cruise altitude) to support liquid water. At this extremely low pressure it just sublimates and disperses into space, although a very tiny amount can and does exist as water vapour in Mars' thin atmosphere.

These are two colour images taken by NASA's Phoenix Mars Lander in 2008. The white patches reveal water ice just beneath the planet’s surface regolith.

The right image was taken 4 days after the left one. If you look closely you can see that some ice has sublimated away during this time.

In the following NASA image you can see the northern polar cap of Mars very clearly.

This ice is not to be confused with the water ice I just mentioned. This is dry ice, or frozen carbon dioxide, CO₂. 

On Mars, during a pole's winter, the pole exists in continuous darkness. The surface becomes very cold, averaging around -87°C (compare this to the absolute coldest Earth temperature on record at -89°C) at Volstok Station in Antarctica in 1983). At this temperature, about a third of Mars' atmosphere condenses out (freezes) into solid dry ice. When Martian spring arrives, the dry ice will sublimate back into the atmosphere as it is exposed to sunlight and warms up. This seasonal cycle drives powerful winds, as fast as 400 km/h, sweeping off the poles and gives rise to high cirrus clouds, shown below as bluish wisps in this beautiful photograph of the Martian sky just before sunrise, taken by the Imager for Mars Pathfinder in 1996.

The thin atmosphere on Mars consists of almost all carbon dioxide, 95%, with 3% nitrogen and traces of argon, oxygen, water vapour and other gases. The atmosphere extends almost twice as high into the Martian sky as it does on Earth. This is because Mars has much lower gravity to keep the atmosphere contained, about 38% that of Earth.

Methane Mystery

Interestingly, Mars also has a tiny bit of methane in its atmosphere. Although methane is a bit chemically unstable, each methane molecule on Mars should last for hundreds of years, maintaining a fairly stable atmospheric level. However, observations show that its levels fluctuate yearly, and they coincide with the seasonally changing presence of atmospheric water vapour. It is estimated that Mars produces about 270 tons of methane per year and there is currently some heated speculation about where it is coming from. Asteroid impacts should contribute less than 0.8% of this amount per year. There is also the nagging question of what is destroying the methane because it also seems to have a strangely high turnover rate in the atmosphere. Methane can come from volcanic activity, comet impacts, microbial activity or mineral processes such as serpentinization. In this case, a mineral called olivine, which is abundant on Mars, reacts with water and carbon dioxide to create serpentine, magnetite (these are two minerals) and methane gas. There are other possible geological sources of methane on Mars as well.

None of these possibilities has yet been ruled out. Until recently, it was widely believed that Mars has not been volcanically active for billions of years. Yet new atmospheric data suggests that Mars might have experienced at least one volcanic eruption, with resultant water flowing on its surface, within the last 100 million years. For some researchers, in particular the European Space Agency, the coincidence of methane levels and water vapour hints at a biological methane source, especially since these two gases seem to be concentrated in three equatorial regions. The significance of this will be explained in a moment. But first things first: how can life on Mars exist? The surface of Mars is very inhospitable to life. It is constantly blasted with deadly UV radiation, micrometeorites and solar radiation because Mars has no protective magnetosphere to shield it (this is also why Mars' atmosphere is so thin; we will get into this in more detail in a moment). However, recent images from the imaging system onboard NASA's Mars Odyssey orbiter reveal seven possible cave entrances on the flanks of the Arsia Mons volcano. These caves, known as the seven sisters, might provide a haven for possible methanogenic microbes. Researchers at the European Space Agency believe that perhaps these caves are deep enough and warm enough to sustain some liquid water, and being caves they could shield microbes from radiation and micrometeors. To test this hypothesis, NASA will launch the Mars Science Laboratory in 2011 (landing on Mars about 9 months later). It will measure the isotopic proportions of carbon-12 and carbon-14 in Martian methane. The idea behind this measurement is that living cells absorb carbon-14 at the same rate as they absorb carbon-12 into their tissues. Their living cell ratio of carbon-12 to carbon-14 is the same as that of their surroundings. However, when the cells die, the ratio of carbon-14 to carbon-12 decreases as the unstable carbon-14 decays. Meanwhile the ratio in nonliving things, such as the atmosphere and rocks, stays the same. The two ratios can be compared and the once-living fossil can be dated. This dating method, though, is not perfect and it has upper and lower reliable time limits. As well, the atmosphere on Mars is not equivalent to Earth’s atmosphere, making this kind of test potentially unreliable. So, the ratio of ethane to methane will also be tested. A ratio less than 0.001 suggests a biological methane source, whereas nonbiological chemical reactions produce nearly equivalent amounts of ethane and methane. Perhaps when these results come back, we can put to rest one way or another the question of life on Mars. Personally, I hope there is life, even microbial life. Such a discovery would suggest that life on other planets in our and other solar systems might be more likely than we ever imagined.

Martian Meteorites

Meanwhile, NASA has amassed a catalogue of 34 Martian meteorites. So far, these are the only physical samples of Mars we have because no probes sent to Mars have yet included return missions. That makes these meteorites extremely valuable to researchers. Scientists are fairly certain these meteorites are of Martian origin because they have the same elemental and isotopic compositions as rocks and gases as those analyzed on Mars. Most of these meteorites are quite young and this, along with the new atmospheric data mentioned above, suggests that Mars might have been volcanically active more recently than once believed. Rocks could have been spewed from volcanoes and launched into space, released from Mars' low gravity. For example, 7 nakhlite meteorites have been found so far. These unassuming-looking meteorites are named after El-Nakhla in Egypt where a large (10kg) meteorite was discovered in 1911. About a dozen of them fell as a meteor shower in the area. One is shown here.

These are all igneous rocks formed from basaltic magma about 1.3 billion years ago and ejected from Mars by an asteroid impact about 11 million years ago. The interesting thing about these meteorites is that they are all about the same age in terms of their formation age (1.3 billion years old) and their cosmic ray exposure age (11 million years). This points to a single origin of these rocks: a single location on Mars and a single impact. Perhaps most significantly for us is that the rocks formed 1.3 billion years ago, meaning that a volcano must have existed as recently as then. More recent data from the European Space Agency’s Mars Express orbiter suggests that some lava flows on Mars are as recent as 2 million years. All of this means that while Mars dos not appear to have ever had plate tectonic activity, it has, up to quite recently, been geologically active.

The most intriguing, and controversial, Martian meteorites contain what appear to be fossilized Martian microbes. The most convincing meteorite fell about 13,000 years ago in Antarctica. This is what it looks like.

It was discovered in 1984 and it made quite an international stir in 1996 when scientists announced it might contain fossilized Martian microbes. The rocks itself is very old; it is thought to have formed from molten rock about 4 billion years ago and later blasted off the Martian surface about 15 million years ago in an asteroid impact. It then floated in space until it landed in Antarctica. A rock this old may have come from a young wet Mars. The structures are very small, 20-100 nanometres (nm) in diameter, smaller than any known Earth bacteria, which are on average about a micrometer (1000 nm) in diameter. However, some very recently discovered round bacteria are as small as 400 nm. This bacteria, called Nanoarchaeum equitans, lives in boiling hot hydrothermal vents and has a simple archaic-appearing genome never seen before. Here is what the Martian candidates look like under a scanning electron microscope:

If they are tiny microbes, they are the first concrete evidence of extraterrestrial life. Researchers have done many tests on the rock to look for organic compounds, which would point to the presence of life processes. They have found some possible organic signatures of life such as amino acids and polycyclic aromatic hydrocarbons, but these compounds could be nonbiological in origin, or they could be the result of contamination by organic compounds within the Antarctic ice. As well, researchers have come up with both biological and nonbiological mechanisms by which these bacteria-like shapes could be formed. I will continue this debate in the following section.

Mars in The Beginning

Let’s now focus on early Mars – how was it different from today and could early Mars have supported possible life?

So far, I have hinted at an early Mars with liquid water (and an implied significant atmosphere) and active volcanic activity. Mars Express orbiter and Mars Reconnaissance orbiter have both found clay minerals that are signatures of a wet environment on Mars, at least in its southern highlands, where surface rocks are about 4 billion years old. Ancient dried up valley networks, and chaotic flood plains are also evident on Mars’ surface. This evidence of water along with the possible microbial fossils of a similar age in the meteorite mentioned above suggest that simple microbial life may have gotten a very early foothold on the young planet, possibly much earlier than life on Earth, which is thought to have begun about 3.5 billion years ago . However, we must keep in mind that a nonbiological origin of the “fossils” can also be argued. And we must add to this extensive evidence on the Martian surface of a meteorite bombardment about 3.9 billion years ago, at the time that our young Moon was bombarded by impacts. This makes a case for early tenuous Martian life to have been obliterated during this period of intense meteorite bombardment before it had any chance to evolve. Keeping in mind these scientific pros and cons, a large question remains: How did Mars ever hold onto liquid water (and an atmosphere) in the first place? What conditions existed then and not now?

 If you have read some of the other planetary articles in this series you may by thinking in the direction of magnetosphere. Mars doesn’t have one, and without a magnetosphere, any atmosphere formed on Mars would be rapidly ionized by solar UV radiation and then picked up and swept away by magnetized solar wind (solar wind is magnetized by the Sun’s very powerful magnetic field). In 1989, the Soviet Phobos probe directly measured Mars’ atmospheric erosion. When the data is extrapolated backwards 4 billion years (and changes in solar wind taken into consideration) it fully accounts for the planet’s lost atmosphere. You might argue that, for liquid water to exist on Mars, it must have also once had an atmosphere and it, therefore, must have once had some kind of magnetosphere to protect that atmosphere from solar erosion. You would be right: In 1998, magnetometers on NASA’s Mars Global Surveyor discovered the remnants of one. A series of magnetic loops are arrayed across the southern hemisphere. In these areas the surface magnetic field is about as strong as that on Earth. However, elsewhere the magnetic field is 100 to 1000 times weaker. The southern magnetic fields harbour localized pockets of gases ionized by solar UV radiation. Earths’ magnetosphere is created by an active dynamo, the result of electrical currents circulating in its liquid metal core. A similar dynamo once churned inside Mars, and we now even have evidence for when it stopped. A giant impact basin, about 4 billion years old, is demagnetized. This means that the crust that reformed after the impact was not under the influence of any magnetic field. This, in turn, means that the dynamo must have stopped before then. The question we are left with is why? Mars’ crust points to a possible answer that explains both the formation and the demagnetization of the impact basin. Its northern hemispheric crust is much thinner and lower in elevation than the crust in the southern hemisphere. This could be the site of what would be largest impact crater in the solar system, roughly the area of Europe, Asia and Australia combined, on a planet about one tenth the mass of Earth. Mars could have been struck by a meteor one tenth to two thirds the size of the Moon, depending on the velocity of the impact. The impact would have to have been violent enough to blow off a significant amount of crust off the northern hemisphere and not enough to melt the whole planet, as was the case with early Earth in which a planetoid is believed to have impacted early Earth and formed the Moon. It would also have to have been violent enough to disrupt Mars’ core dynamo. This is currently a hypothesis based on computer modeling, and as promising as it is, the scientists themselves admit that it needs more verification. Scientists are also currently looking at other possible mechanisms by which Mars lost its early magnetosphere.

We are left with a somewhat haunting image of a young newly formed planet, possibly endowed with all the ingredients necessary for life to take hold and evolve, violently jarred from its future path onto a different path that leads to dry dead planet, whose past is now a puzzle for us, the inhabitants, the evolutionary products of a much luckier planet, to solve. I leave you with this Cosmic Journeys 25 minute video called "Mars World That Never Was." It offers great imagery and a good recap of all that was discussed here.



Next up: Part 3: Venus