Life on Titan?
We are especially interested in Mars because it is a planet not that different from Earth. If there is, or has been life anything like ours anywhere in the Solar System, Mars is the most likely place.
On the other hand, how much like ours does life have to be?
Basically, any stable environment where there are chemicals that can be used to make bodies, and energy to drive them, living creatures of some sort are a possibility. Then of course, all this is about chemical based life, as ours is. There are probably many other options for living creatures, but it is easier to look for life we could actually recognize.
One possible site for this is Jupiter’s moon, Europa, which has a deep ocean hidden under its icy surface. Thanks to tidal heating by Jupiter, we believe there are probably geothermal vents at the bottom.
On Earth, such vents in the deep ocean support huge populations of exotic creatures. Once again though we are looking at life similar to what we find here. An even more intriguing world is Titan, Saturn’s largest moon.
Titan has a diameter of 5,149 km, which is quite a lot larger than our moon, which is 3475 km across. However, what makes Titan unique in the Solar System is that it has a thick atmosphere. Other moons, like ours, have atmospheres so thin they are almost a vacuum. At the surface, Titan’s atmospheric pressure is almost 1.5 times the pressure at the Earth’s surface.
It is made up mostly of nitrogen, about 97%. The other 3% is made up of methane, hydrogen and other gases. Astronomers knew about Titan’s atmosphere fairly early on. Even small telescopes showed it to have a unique colour, a sort of pinkish-brown. This is interesting because none of the atmospheric gases listed above have that colour.
However, when spacecraft had a close look, and the Huygens lander, named after Titan’s discoverer, Christiaan Huygens, settled onto its surface, trace amounts of ethane, diacetylene, methylacetylene, acetylene, propane, cyanoacetylene, hydrogen cyanide, carbon dioxide, carbon monoxide, cyanogen, argon and helium were detected. Reactions between these chemicals make the atmosphere a thick, brown organic fog.
The presence of complex organic molecules raises the possibilities of life. Experiments with gaseous mixtures like these show that ultra-violet light or electrical discharges can produce aminoacids, the building blocks of proteins.
In addition the processes of chemical-based life on Earth require a liquid in which all the chemical reactions can happen. For us that liquid is water.
However, Titan lies about 9.6 times further from the Sun than does the Earth. It receives roughly 1% of the solar heat and light that we do on Earth. This means Titan is cold, -179.5 C. At these temperatures water is a permanently frozen rock mineral.
It was therefore surprising when Huygens sent back pictures showing it had landed in a floodplain, which happened to be dry at the time but was clearly covered by occasional deep, fast flows of liquid. It sat on sandy ground surrounded by liquid-worn rocks and pebbles.
Both the pebbles and the “sand” were probably mostly ice. On Titan it snows or rains methane and other hydrocarbons, which are gases on Earth but snow or rain on Titan. This makes Titan and Earth the only known bodies in the Solar System that have something that occurs there as liquid, vapour and as a solid.
On Titan, hydrocarbons evaporate from lakes, fall as rain on the land, and then carve stream and river channels on their way back to the lakes, like the water cycle on Earth.
Could Titan offer an environment suitable for life?
In a way, finding Titanians could be more exciting than finding Martians, if that were at all possible.
The NASA Jet Propulsion Laboratory has produced a video of the Huygens probe landing: youtube.com/watch?v=msiLWxDayuA
—Mars is high in the southwest after dark. The Moon will be New on the 13th.
Ken Tapping is an astronomer with the National Research Council’s Dominion Radio
Astrophysical Observatory, in Penticton.