After the momentous discovery of hydrothermal vents in the ocean of Enceladus, the Saturnine moon has become the prime candidate to host life beyond Earth. The chemical blocks are all there and the conditions seem to be just right for simple organisms to form.
“Confirmation that the chemical energy for life exists within the ocean of a small moon of Saturn is an important milestone in our search for habitable worlds beyond Earth,” Linda Spilker, Cassini project scientist at NASA’s Jet Propulsion Laboratory in Pasadena, California, said in a statement.
If you were a betting person, your money should be on Enceladus. But there’s no planned mission to go back and confirm if there’s life or not on Enceladus, so before that’s tested, life could be spotted elsewhere.
Strong on the Enceladus discovery, Europa seems to be the second most likely place to find life in the Solar System. Europa is an icy moon of Jupiter known for its stripy, cracked surface. Researchers are confident Europa has plumes, and it's even possible its ocean is just as rich in heat and chemicals as Enceladus'.
NASA is planning an important mission, called Europa Clipper, that will do a more detailed analysis of the plumes than Cassini (which was designed and launched in the 1990s) could do on Enceladus.
“If there are plumes on Europa, as we now strongly suspect, with the Europa Clipper we will be ready for them,” added Jim Green, director of Planetary Science at NASA Headquarters.
Europa in all its glory. NASA/JPL-Caltech/SETI Institute
The ocean worlds are clearly intriguing. Life, in our current understanding, can’t thrive without water. But maybe life can survive in incredibly salty water under the soil of a frigid, deserted world – Mars.
Mars used to look a lot different than it does today. At one time, it was a water-rich planet with volcanos and an atmosphere just as dense as our own. Sure it was almost exclusively carbon dioxide, but that might not have mattered to microbial life. They had heat, they had the building blocks of life, and they had water.
They could have had it all, but they didn’t have a magnetic field. Over billions of years, the solar wind has blown Mars' atmosphere into space, turning a wet and warm world into a dry, cold wasteland. Could life have evolved during those epochs? Quite likely, yes. There’s evidence of localized surface water up to 650 million years ago. Plenty of time for life to form. Could life have evolved rapidly enough to survive the more extreme conditions of the modern Mars? We don’t know.
We have been sending probes to look for life since the 1970s, but we are still wondering if there’s life on Mars. Some believe that we have already found the evidence, but the smoking gun is still buried on the Red Planet. That’s for future missions to uncover.
Enceladus, Europa, and Mars are clearly the contenders for life beyond Earth. But if money wasn’t a problem, where else should we go looking for life?
Top of the list of the weirdest places is Pluto. The dwarf planet is the furthest world we have studied and has a treasure trove of unique and unusual features. There are some suggestions of an under-ice ocean on Pluto as well, but while Europa and Enceladus are heated by the gravitational dance between the gas giants and their Moon, poor Pluto is stuck with a syrupy ammonia ocean.
“Life can tolerate a lot of stuff: It can tolerate a lot of salt, extreme cold, extreme heat, etc. But I don’t think it can tolerate the amount of ammonia Pluto needs to prevent its ocean from freezing – ammonia is a superb antifreeze,” Professor William McKinnon said when discussing the possibility of life on Pluto.
“It’s no place for germs, much less fish or squid, or any life as we know it. But as with the methane seas on Titan – Saturn’s main moon – raises the question of whether some truly novel life forms could exist in these exotic, cold liquids.”
It’s not surprising that McKinnon mentioned Titan. It has rivers, lakes, clouds, winds, and rain. Yes, it has an average surface temperature of -179°C (-290°F) and all hydrological phenomena are made of liquid methane, not water. Still, some weird lifeforms might have found an ideal cozy niche on Titan.
Sunlight glints off of Titan's northern seas in this near-infrared, color mosaic from Cassini.NASA/JPL/Univ. Arizona/Univ. Idaho
If Titan is a bit too chilled, then life might prefer a warmer place such as Io, the volcanic moon of Jupiter. Life on the surface of Io is unlikely. Its surface temperature goes from thousands of degrees above zero to hundreds below. You can find lakes of molten rock and snowfields of frozen sulfur. Also, Jupiter blasts so much radiation at it that life would have a hard time surviving.
But underneath its yellowish exterior, things might be different. We know life can survive underground on Earth. Life on Io might have evolved underground, not thinking twice about its precarious position.
For the true extremophiles, there’s Venus. Once upon a time, it was a temperate planet, but now it's a hellish world where you will be crushed, burned, and melted. Literally. The pressure on the surface is the equivalent to being 1 kilometer (0.6 miles) underwater, the temperature is high enough to melt lead, and its atmosphere is rich in sulfuric acid.
Researchers think that Venus was once suitable for life and may even have had a shallow ocean. It is in the habitable zone, which provides the planet with the right amount of energy to kickstart life. But while Venus might have been habitable, its changes have been so dramatic that anything that might have been there is likely long gone. Unless the standard course of life is finding refuge underground.
All life on Earth, from the most resilient to the most delicate, is believed to have originated from a single ancestor in a warm water environment or some similar conditions. This might be the only way for life to form, or it might be one of many ways. We can only find out by actually going out there and looking for it.
Until then, we can speculate and imagine, balancing skepticism and excitement about what’s out there. After all, life, uh, finds a way.
Digital terrain model of the observation of brine water flowing on Mars. NASA/JPL/University of Arizona