As I’ve mentioned previously, my career is based around looking for alien life in the universe. Naturally, this brings up the very pertinent question of “Where exactly does one look for aliens?”
The answer, surprisingly, is “pretty much all over the place.” And with good reason – here on Earth, living organisms have been found in some of the most seemingly inhospitable places, which suggests that life is, above all, tenacious in the extreme.
Where to begin, then? Why not in our own backyard? As it turns out, there are more than a few places in our own Solar System that might harbor life. So, without further adieu, let’s take a guided tour of the Solar System’s hottest real estate, moving from the inner planets outwards.
Venus
Venus may seem like a surprising candidate – the surface is hot enough to melt lead, the atmospheric pressure is crushing, and it rains sulfuric acid. But Venus was not always so grim. It is thought that it may have had oceans for the first two billion years of its history, before the growing intensity of the young sun triggered a runaway greenhouse effect that boiled them off. Life may have been able to get a toehold in these early seas, as it did on Earth.
But where could such life have fled to under the onslaught of rising temperatures? Curiously, it turns out that while the surface may be utterly inhabitable, at ~50km above the ground, the atmosphere of Venus is remarkably Earth-like in temperature and pressure. It’s still fairly acidic, there’s no oxygen, and it’s still on the warm side, but there are organisms on Earth that will quite happily live in similar conditions. UV radiation would be a problem – however, interestingly enough, cylcooctasulfate – a sulfur compound that absorbs UV rays and re-emits them as visible light, and that’s used by terrestrial microbes as “sun screen” – is found in the Venusian atmosphere at an altitude of 50km.
Earth
No, I’m not suggesting Earth’s been invaded – I’m instead referring to the idea of the shadow biosphere. The basic premise of the shadow biosphere is that we assume that all life on Earth is biochemically similar to us (e.g., it uses the same types of proteins and DNA, same chemical reactions, and so forth), and therefore we would fail to detect microbes that used radically different biochemistry. The microbes wouldn’t be “aliens”, per se, as it’s assumed that they would’ve evolved here on Earth – but such a finding would still be incredibly significant, as it would suggest that life may developed independently on Earth, multiple times.
Supporters of the shadow biosphere hypothesis point to the fact that the vast majority of microbes can’t actually be cultured in a laboratory, and as a result, we know very little about them. There have been searches for “weird life”, including, most notoriously, GFAJ-1. GFAJ-1 was initially reported to use arsenic in the construction of its DNA (as opposed to phosphorus, which is what all known life uses instead). However, after its discovery was announced, further experimentation couldn’t detect the presence of arsenic in its DNA, and biochemical modeling suggested that DNA using arsenic wouldn’t actually be chemically stable. The search goes on.
Mars
This list obviously wouldn’t be complete without everyone’s favorite red planet, Mars. Mars has long held a fascination, in part due to early observations of channels or canals on the surface (these were later revealed to be the result of an optical illusion). As it turns out, such a reputation might be warranted – Mars is the most Earth-like planet in our Solar System, and shows evidence of being a much warmer, wetter planet in its past (most notably, the presence of dry river networks and lake beds). In the present day, there also appears to be seasonal flows of liquid brine or extremely salty water, most likely the result of salts absorbing water vapor from the atmosphere.
As I mentioned in my previous essay, methane has also been detected in the Martian atmosphere. Since methane isn’t chemically stable under Martian conditions, something must actively producing it. Stranger yet, the production appears to be sporadic, suggesting that this is the result of an active process. While there are purely geological processes that can produce methane, here on Earth, the vast majority of methane is produced by microbes, which obviously raises suspicions.
It’s unlikely the Martian microbes – if they exist – are living on the surface, due to the high flux of radiation. Instead, they’ll most likely be found in deep subsurface habitats or aquifers, or potentially underneath the polar ice caps. Future missions to Mars (notably the ESA’s ExoMars and NASA’s Mars 2020 Rover) will hopefully give us better answers to the age old question of life on Mars.
Europa
Moving into the outer Solar System, Europa is one of the four major moons of Jupiter, and is covered entirely by a thick layer of ice. It’s been a target of great interest to astrobiologists since the data from the Voyager missions suggested the presence of a vast ocean underneath the ice layer. The thickness of the ice shell and the depth of the ocean is subject to debate, but it’s thought that it could be as much as 100 miles deep, and encompass a volume of water twice the size of all of Earth’s oceans. Given the importance of liquid water to life as we know it, this obviously makes it a potential candidate for habitability.
Due to the complete absence of sunlight underneath the ice shell, if there’s life on Europa, it’s probably clustered around hydrothermal vents, much like the vent ecosystems seen on ocean floors here on Earth. These vents are driven by volcanic heating driven by the intense tidal forces of Jupiter, which also keeps the ocean from freezing, and is also most likely responsible for the alleged plumes of water erupting from the surface.
Several missions are planned to study Europa – ESA’s Jupiter Icy Moons Explorer and NASA’s Europa Multi-Flyby Mission, which will hopefully be able to measure the thickness of the ice shell, gather more data on the chemical composition of the surface, and sample the surface plumes (if they exist). Proposals have been circulating to actually drill down and explore the ocean, but such a mission is a while off.
Enceladus
Similar to Europa, Enceladus is an ice covered moon orbiting Saturn. It features extensive plumes of water erupting from its southern hemisphere, thought to originate in a subsurface ocean. The exact mechanisms driving the plumes hasn’t been determined, but there’s likely hydrothermal activity in play. Since the plumes are so extensive, the Cassini mission in orbit around Saturn has been able to conveniently sample some of the erupted material, and discovered that it has a high salt content (suggesting hydrothermal activity) and traces of simple organic compounds. Given the presence of organics, liquid water, and a likely energy source, Enceladus has become a hot topic amongst astrobiologists, and will hopefully be the target of future exploration
Titan
Another moon of Saturn, Titan is the second largest moon in the Solar System, and the only one with a dense atmosphere. The atmosphere is made up of a mixture of nitrogen, methane, and a mixture of organic compounds. Titan is a chilly -355 degrees Fahrenheit, so cold that methane is liquid at the surface. In fact, the most interesting thing about Titan is that liquid methane takes the place of water – there are rivers and lakes of the stuff.
Consequently, unlike the other worlds we’ve looked at, if there’s life on Titan, it’s very different from the water-based life we’re familiar with. Potential biochemical pathways have been identified for the Titanian atmosphere, and, interestingly enough, some of the features in Titan’s atmospheric composition would be consistent with presence of metabolizing organisms. Nonetheless, life on Titan remains a much more speculative topic, and will require further exploration of this mysterious, haze shrouded moon.
Conclusion
While Earth may be the most habitable world in our Solar System, it isn’t the only place life might have evolved. No alien life has been conclusively detected, but the hunt is on. The most exciting aspect of this search is that if life evolved independently, multiple times within the same solar system, it suggests that the emergence of life is a common event.
In other words, if we discover that our Solar System is teeming with life, it’s likely that so is the rest of the galaxy.
Tessa is a 28 year old PhD student, and perhaps the world’s only queer trans astrobiologist. A nerd going way back, her interests include science fiction, space exploration, sustainability, science communication, and feminism and gender. Her hobbies also include horseback riding, playing the flute, social dancing, knitting, and occasional attempts at writing fiction. She currently resides in Tempe, AZ with her even nerdier fiancee and a mastiff mix who thinks he’s a lapdog. She tweets occasionally @spacermase.
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