Hello? Earth Calling . . . PART 2

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Are Aliens Calling Us?

As our detailed exoplanet catalogue builds up, there is always the possibility that we could pick up an alien signal. A detectable electromagnetic (EM) signal would have to come from nearby because the signal (with no loss due to reflection or diffraction by any close by particles or obstacles) rapidly loses strength according to the inverse square law. Of the estimated 1400 stars within 50 light years of Earth, 40 have been confirmed to have planet systems. How many of these might support intelligent life? How many of those might be using EM radiation to send a signal? Would it be powerful enough to detect? 

Earth is a miniscule target from even 50 light-years away. A signal beam from this distance would have to be aimed right at Earth in order to be detected. Electromagnetic waves might not be the only way to signal. Other potential methods that an alien civilization could use to signal or communicate are discussed on this Wikipedia entry. Radio waves seem likely to be used because they are long enough to pass through gas clouds and atmospheres without being reflected or diffracted. They can travel unmolested a long way through interstellar space. Optical lasers have also been proposed. Perhaps a signal could utilize a series of laser bursts, but at least the ones we can make would be too weak to detect over a nearby star's luminescence. If the aliens can manipulate immense energies, they could use gamma ray bursts or even gravitational waves. Alien civilizations could also inscribe a message in matter and send it over long distances. NASA used this general approach (the Golden Record) on the Voyager spacecraft. An alien technology might be able to inscribe microprobes and program them to travel to every habitable star system within a certain radius. Intelligent aliens could also unintendedly leave technosignatures of their existence. For example, light reflected from large space mirrors or Dyson spheres or the presence of excess chemicals or radiation in one location could offer clues to their presence. The technical difficulties of sending out a strong signature over thousands of light-years doesn't rule out the possibility that millions of intelligent alien civilizations are signaling within our galaxy but, depending on how far away they are, it could take thousands to millions of years to receive a signal (if it is powerful enough). By then, the signal would tell us only that intelligent life existed long ago far away. Still, that discovery alone would change how we view ourselves in this universe. We are making the assumption here that intelligent aliens would want to develop signaling technology and use it. We have only our own species as a motivation reference.

Are We Calling Aliens?

How would an alien species know we are here? Earth being monitored by intelligent technically superior aliens has been a long running theme in science fiction. An unfriendly advanced alien species could plunder Earth War-Of-The-Worlds-style for resources such as water, killing us in the process. Or, aliens could be friendly. In the movie, Contact, aliens bounced our first TV signals back to us as a "Hey, we hear you Earth!" and, encoded inside them, they gifted us with wormhole spacecraft technology. Signals from our radios* and TV's have been spreading out into space around us for several decades, which could potentially alert aliens within 60 or so light-years to our presence. Some wonder, as in Contact, whether we should be advertising our existence to potential enemies. There are several reasons why we probably have nothing to worry about. First, our sphere of influence is tiny. As mentioned, broadcast signals grow increasingly weak as they travel from their source. Second, while TV and radio signals were once broadcast from huge ground stations at thousands of watts, almost all are now transmitted via satellites at around 75 watts, and they are directed from aerials aimed down at Earth rather than being shot out in all directions including space. There may have been a window of a few decades when perhaps someone listening from as far as the Gliese star system might have heard our broadcasts but they would have needed sensitive technology to receive it because it would have been literally billions of times weaker then what receivers on Earth picked up. More likely, Earth would be discovered through indirect observation similar to what we are using to detect exoplanets now, or through more advanced direct imaging technologies. Here too the odds are not great because Earth is a very small planet and would be difficult to detect from tens to hundreds of light-years away. Still, aliens with biochemistries like ours might be excited to discover us. Earth's liquid water and ice could be detected by their reflectivity and the presence of our atmospheric (biologically produced) oxygen gas could be detected by studying the absorption spectrum of sunlight reflected from Earth.

*Just a brief refresher on radio waves and sound travel: The opener sequence of the Contact movie was misleading. Sound does not travel through space because it is a mechanical wave that requires a medium (such as air or water or even solid materials) to propagate. Radio waves travel through space as electromagnetic (EM) radiation. Shortwave radio and some AM radio frequencies are too short to pass through Earth's ionosphere. Instead they bounce off the ionosphere as if it were a solid barrier and right back to Earth. Longer wavelength FM radio and TV frequencies travel right through the ionosphere into space.

Our magnetosphere might be the first Earth signal detected by correctly situated aliens.

This is an artist's not-to-scale rendition of Earth's magnetosphere (with the purple outline) created for NASA.

Recent data from the European Space Agency's Cluster mission suggests that if aliens listened to Earth, they might hear a loud and obnoxious series of chirps and whistles if they were in the right location in space. Intense radiation, called auroral kilometric radiation emanates from our planet in a narrow beam of extremely intense (1 – 10 million watt) radio waves. The radiation comes from solar wind particles striking Earth's magnetic field and accelerating along it (called cyclotron radiation). Luckily for us our ionosphere blocks it. Otherwise the noise would easily drown out all of our broadcasts. Only satellites in high orbits above the ionosphere, such as the FAST satellite, can detect them. On the flip side, similar magnetosphere radiation could be used to detect exoplanets, at least those that are correctly oriented to Earth. It is not a biosignature (proof of life), however, but magnetospheres are protective bubbles that prevent planetary atmospheres from gradually blowing away in their star's stellar wind. The presence of one around a planet is good sign, but not proof, that life could exist on it.

Picking The Right Planet: What Would Alien Life Look Like?

If we want to undertake the long-term mission of sending a probe to an exoplanet, the first step is to choose a planet that has promising evidence of life. What are the signals of life on a planet? To answer that question we must first figure out what life is. That answer is nowhere near as easy as one might guess, but this is also where things get very interesting. Astrobiology is a rapidly expanding interdisciplinary field that studies the origin and evolution of life on Earth as well as the possibility of life on other worlds. This field attempts to set the parameters of what life is and what it requires. NASA offers an online astrobiology magazine that features many great articles worth a read.

Naturally, the science starts right here at home on Earth, where life is abundant and comes in a stunning variety of forms. Not long after NASA was established in 1958, the agency began to look for the presence of life beyond Earth. It was a significant component of the Viking missions to Mars in the 1970's. At the same time, biologists were beginning to discover fascinating examples of microbial life on Earth that live without sunlight, in extreme cold or heat, under intense pressure, in extremely acidic or salty environments and even in highly radioactive environments. These microbes are now called extremophiles. These discoveries are rapidly expanding the list of potential environments where life could arise. The Curiosity rover has been looking for environmental conditions favourable for microbial life since it landed on Mars in 2012 as part of NASA's Mars Science Laboratory mission.

NASA's Curiosity Rover took this selfie on Aeolis Mons, Mars, in January 2016

Chemical analysis of Titan's atmosphere and surface by the Cassini-Huygens mission to Titan, Saturn's largest moon, reveals that even though Titan is far too cold to have surface liquid water, its atmosphere is thick, chemically active, and filled with a diverse assortment of organic compounds. This list of compounds includes complex chemicals such as polycyclic aromatic hydrocarbons (PAH's), which are composed of two or more carbon-based benzene rings. Other research suggests that PAH's could be possible starting materials for the non-biological synthesis of compounds such as amino acids and nucleotides. These are the raw materials for proteins and DNA (deoxyribonucleic acid), two biomolecules that are essential even to the simplest forms of life on Earth.

Shown left is a true-colour image of Titan's hazy brown hydrocarbon-rich atmosphere taken by the Cassini orbiting space probe. Arriving at Saturn in 2004, the Cassini orbiter mission is still active.

Liquid surface water has long been thought to be essential for life on a planet. It is life's solvent on Earth, the medium essential for all known biochemical reactions. All of the reactions that take place in our bodies, for example, occur in an aqueous (water) solution within cells and between cells. Astrobiologists are now exploring the possibility that on other worlds other molecules could take on a life solvent role. On Titan, hydrocarbons such as methane or ethane could act as a biological solvent. Unlike water, methane is nonpolar, and it is not as strong a solvent. This gives it advantages and disadvantages. It would not transport substances through a cell wall as easily as water does - disadvantage. It is less chemically reactive than water – advantage (and perhaps a surprise to some when they consider its flammability). Methane and ethane do not tend to break down large organic molecules as quickly as water does (through hydrolysis). This means that complex biomolecules would be more stable in such an environment, and its biochemistry could take better advantage of hydrogen bonding, a chemical reaction that is essential to building proteins, DNA and cellulose in plants. In fact, in 1981 Isaac Asimov, a biochemist as well as famous science fiction author, went further by suggesting that methane biochemistry might do away with proteins altogether. Poly-lipids could instead fulfill protein's role in a nonpolar solvent such as methane. A hypothetical cell membrane that could function in liquid methane was computer-modeled in 2015. Instead of phospholipids (compounds of phosphorus, carbon, hydrogen and oxygen), which build cell membranes on Earth, this membrane would be composed of carbon, nitrogen and hydrogen. It would have the same stability and flexibility as the phospholipid cell membrane used by life on Earth.

False-colour radar mosaic of Titan's north pole region, showing dark blue methane/ethane/dissolved nitrogen lakes, oceans and rivers taken by the Cassini orbiter.

Analogous to Earth's water cycle, Titan has a methane cycle, which transports energy as it cycles between an atmospheric gas and a liquid in its oceans, lakes and rivers. Imagine, under the gloomy thick atmosphere of Titan, a -180°C hydrocarbon river is filled with living organisms. Their biochemistry would be nothing like Earth life. Perhaps a cautious note to offer here is that because Titan is so cold far less free energy is available for biochemical reactions (life processes). Earth, in contrast, was downright hot when life first evolved here, as we will see later on. Still, as far as we know, life cannot be ruled out. The relatively low-cost Mare Explorer could be the first mission to directly explore Titan's hydrocarbon seas.

Right is an artists' concept of the Titan Mare Explorer probe, provided by NASA.

A lander would splash down on one of the planet's largest lakes, Ligeia Mare, and explore its chemistry and depth, as well as the surface atmosphere and local weather, while taking photos. It was set to launch in 2016 but it remains at the conceptual stage. Another proposed NASA mission could follow up on the Mare Explorer mission by exploring its largest hydrocarbon ocean, called Kraken Mare. While the Mare explorer would use a stationary floating probe, this Titan Submarine mission, which could begin as soon as 2038, would be mobile.

This cute 2015 video below, showcasing the Titan Submarine, was created by the NASA Glenn Research Centre:

Still in its conceptual stage as well, this unmanned submarine would investigate the ocean using a variety of probes. Let's hope that something gets underway soon so we can begin to answer our questions about life on this moon. Imagine the excitement if evidence for life with a biochemistry different from Earth were discovered. It would completely change how astobiologists approach the question of extraterrestrial life and it would greatly broaden our hope that we are not alone in the universe. Finding no signs of life there, though disappointing, would also be very useful data that coud be used to refine future missions to other worlds.

Next we will delve deeper into what life is, its chemistry and where it could evolve.