When people talk about the search for aliens, many think of relatively intelligent creatures capable of creating civilisations and sending signals through space. That search involves listening for communications coming from the stars. If intelligent aliens were closer to home and within our own solar system we surely would have noticed them by now since we’ve sent out probes to most of the planets and moons. We seem to be alone.
Astrobiologists aren’t only concerned with intelligent life. Here on Earth, intelligent life is brand new. In fact, even multicellular life is fairly new as single-celled organisms were all that existed for billions of years. We know there aren’t giant civilisations building cities elsewhere in the solar system, but what about microscopic life?
Life elsewhere in the universe doesn’t have to be similar to our own. We don’t know how they will function and we might not be able to comprehend them. But since we only have one data point, we tend to think about life that’s at least slightly similar to our own in that it’s carbon-based, needs stable environmental conditions, and thrives in a water world. Naturally, the search for life here in our solar system focuses extraterrestrial oceans. Here are the 4 places astrobiologists think are most likely to harbour alien life in the solar system.
It’s hard to imagine the dry, dusty landscape of Mars being suitable for life but in the past there was a huge, deep ocean covering most of the northern hemisphere. The rovers driving around Mars today haven’t found life yet but they have found abundant evidence that Mars was a warmer, wetter world for billions of years. Orbital missions are also finding that Mars isn’t as arid as you might think.
See the dark lines that appear in the above GIF? They’re visible when it’s warmer and vanish when it’s cooler. This is just one of many lines of evidence for water flowing on Mars today. It’s not clear where the water is coming from but periodic changes at the surface show that not all of the grooves and channels are ancient. The Curiosity rover has analysed the surface soil and found that it’s 2% water by weight.
Life as we know it needs water so there’s always hope on Mars. The current rovers have looked for microscopic life and haven’t found anything but they aren’t optimally designed for the task. NASA’s next Mars rover launches in 2020 and will use better equipment to search for microscopic organisms. Even if no life exists on Mars today, the rovers might be able to find evidence of life existing in the past. It’s likely we’ll never be able to say with confidence until we get humans there to dig up and do some real astropaleontology.
Earth is a water world, yeah? More ocean than land? Europa, one of Jupiter’s moons, seems to have a liquid ocean bigger than all the oceans on Earth put together. Scientists are in agreement that there’s an ocean under there but disagree on how thick the crust is. Regardless of how thick the crust is, the ocean will be very dark so something analogous to photosynthesis wouldn’t evolve.
The Hubble Space Telescope detected plumes of water shooting out from the surface of the moon but the first evidence of Europa being a secret water world came from the relatively young and smooth surface. It seems hard to believe that a huge ocean could remain as liquid that far from the sun and under a thick ice crust but it’s likely that interactions with Europa’s neighbours explains the source of heat keeping the moon active. The tidal flexing caused by the gravity of Jupiter and other Jovian moons seems to be generating enough heat for the ocean and also plate tectonics, which have only been found there and on Earth.
Unlike Mars, we’ve never landed anything on Europa but ambitious missions have been proposed. The difficulty is getting through the ice crust and down to the ocean. Some people have suggested using nuclear power to burn through and then release a submarine. NASA is funding the research for a future project called Icy-Moon Cryovolcano Explorer (ICE) that hopes to land on Europa, walk a rover down into an ice volcano, and then release a submarine that can explore the depths. We’ll have to be careful not to contaminate the extraterrestrial ocean with our own microbes.
Jupiter isn’t the only planet with moons that are targets in the search for life. Saturn has some fascinating candidates too including Enceladus, which could have a subsurface ocean like Europa. Nobody expected it to until giant plumes were seen erupting from the moon when Cassini flew by in 2005. “Make a list of the requirements for terrestrial-type life — liquid water, organics, minerals, energy and chemical gradients and Cassini has found evidence for all of them in the plume,” said Jonathan Lunine, director of the Center for Radiophysics and Space Research at Cornell University.
Cassini has flown through the plumes many times and collected so much data that we can be pretty sure that Enceladus is similar to Europa with water deposits below the surface. Cassini has spotted over 100 geysers so far. If these plumes of water and ice come from an ocean below, more modern equipment could be used to fly through them and check for signs of life itself.
When exploring Saturn, Cassini also visited Titan and captured these stunning images of sunlight reflecting off a liquid surface. Cassini dropped a lander, Huygens, down onto the moon where it found shorelines, drainage channels, and other features that sound very Earth-like. Images from orbit reveal the only known liquid lakes found off Earth, some bigger than any of our own.
Lakes? Constantly topped up by rain? Surely this is the jackpot! Sadly there’s a catch: the hydrocarbon lakes appear to consist of liquid methane and ethane rather than water. It’s not what we have here but doesn’t rule out the possibility of life. For starters, life might be able to evolve to use liquids other than water. Secondly, there could be water in there too. On Earth scientists have found microbes surviving in the biggest asphalt lake, Pitch Lake. The organisms live inside tiny water bubbles within the oil, showing once again that we can find life in the most unlikely places on Earth.
Although the most striking feature of Titan is the presence of hydrocarbon lakes, there’s also evidence of yet another subsurface ocean. As with the other watery moons, Titan’s gravity gives tell-tale signs that there’s an extremely salty ocean below the surface. Jupiter’s Europa is thought to have more water than Earth but Titan is thought to have more than both combined. Perhaps someday there will be astronauts sailing the methane lakes as scientists explore the depths of the hidden water ocean.
Some people might see the search as a pipe dream but it makes sense to look. We’ll learn so much even if we don’t find life. Studying these lifeless oceans can help us learn more about early conditions in Earth’s oceans. We also see a lot of sights that make no sense on neighbouring planets and moons. We’re very biased by our own world and we’re likely to find geological processes elsewhere in the solar system that are alien to us. Searching for life will not be a waste of time or resources.
However, what if we did find life of some sort? Finding the independent existence of life on another world would change our general outlook on life in the universe; if biogenesis can occur twice just in our solar system, the universe is probably teeming with life.
These efforts will change how we view exoplanets. If life never existed anywhere else in the solar system, then watery exoplanets might not necessarily be great places to find life. But if we were to find life on one of Saturn’s moons, we’ll see the countless Earth-like words in a totally different way. We’re already beginning to plan for interstellar missions, so finding life here would dictate the first worlds we should send interstellar probes to.
Main image © NASA Astrobiology Institute