When it comes to whether or not life exists in Earth’s metaphorical backyard, Saturn’s largest moon Titan has often sparked the curiosity of researchers. But a new study out of London, Ont.’s Western University is shedding light on why when it comes to life elsewhere in our Solar System, we may have to keep looking.
Spearheaded by Western astrobiologist and member of Western’s Institute for Earth and Space Exploration Catherine Neish, a recent study shows that the subsurface of Titan — Saturn’s largest moon — is most likely a non-habitable environment incapable of supporting life.
A dip in the ocean
When it comes to the study of life outside of Earth, the icy moons of giant planets are of particular interest due to the belief that the moons have large subsurface oceans of liquid water.
On Titan for example, it’s believed that beneath its icy crust is an ocean more than 12 times the volume of the oceans on Earth.
“Life as we know it here on Earth needs water as a solvent, so planets and moons with lots of water are of interest when looking for extraterrestrial life,” said Neish.
Is anyone out there?
Published in the journal Astrobiology, Neish and her collaborators used data gathered from impact cratering to determine the amount of organic molecules that could be transferred from Titan’s organic-rich surface to its subsurface ocean.
Throughout Titan’s history, comets impacting the icy moon have melted its surface, creating pools of liquid water that go on to mix with surface organics. As a result, the melt is denser than its icy crust, so the heavier water sinks through the ice, and possibly all the way to Titan’s subsurface ocean.
According to the study, Neish and her collaborators determined how many comets of different sizes would strike Titan each year over its history by using the assumed rates of impacts on Titan’s surface. This allowed the researchers to predict the flow rate of water carrying organics that travel from Titan’s surface to its interior.
Neish and the team found the weight of organics transferred in this way is no more than 7,500 kg per year of glycine, the simplest of amino acids, and which makes up proteins in life. This is approximately the same mass as a male African elephant.
This is important, the study reads, due to the fact that all biomolecules, including glycine, use carbon as the backbone of their molecular structure.
“One elephant per year of glycine into an ocean 12 times the volume of Earth’s oceans is not sufficient to sustain life,” said Neish, who’s also a professor of Earth sciences. “In the past, people often assumed that water equals life, but they neglected the fact that life needs other elements, in particular carbon.”
Bad news for E.T.
Other icy worlds, such as Jupiter’s moons Europa and Ganymede and Saturn’s moon Enceladus, have very little carbon on their surfaces, and it remains unclear how much could be sourced from their interiors. Because Titan is the most organic-rich icy moon in the Solar System, if its subsurface ocean is not habitable, it therefore does not bode well for the habitability of other known icy worlds, according to the study.
“This work shows that it is very hard to transfer the carbon on Titan’s surface to its subsurface ocean – basically, it’s hard to have both the water and carbon needed for life in the same place,” said Neish.
The recent study however doesn’t just shed light on Titan’s probable lack of habitability, but also that for the Solar System’s four outer gas giants, Jupiter, Saturn, Uranus and Neptune.
“Unfortunately, we will now need to be a little less optimistic when searching for extraterrestrial lifeforms within our own Solar System,” said Neish. “The scientific community has been very excited about finding life in the icy worlds of the outer solar system, and this finding suggests that it may be less likely than we previously assumed.”
