“The Universe is not only queerer than we suppose, but queerer than we can suppose.” So said J.B.S. Haldane in Possible Worlds and Other Papers in 1927. At the beginning of the twentieth century, the astronomer Percival Lowell mesmerized the world with his vision of a Mars crisscrossed with canals, the home of an ancient, advanced race trying to survive on a dying planet. Then, in 1965, the first space probe to send back photographs from Mars, the Mariner 4, showed, not canals, but a world that looked much like the moon, pockmarked by craters, cold, with a thin wisp of an atmosphere. It seemed to demonstrate beyond all doubt that nothing could be living there. Around the same time, the space probes that visitede Venus revealed a world bathed in sulfuric acid clouds with surface temperatures surpassing the boiling point of lead, at over 800 degrees Fahrenheit. The solar system beyond earth seemed clearly dead and lifeless. Only our home world appeared able to support life.
But in the last two decades, the outlook for life beyond Earth in our solar system has undergone a radical transformation. Where before, it seemed impossible that life could survive out there, scientists are now beginning to speculate that life—and not just life as we know it, but life as we don’t know it—may be abundant in our solar system.
Mars, rather than being a dead, lifeless analogue of the moon, turned out to be a place that at some time in its distant past had abundant flowing water that formed rivers and streams, lakes and seas. In fact, about one third of its surface was covered with water. Even now, there are vast ice fields at its poles. Scientists wonder if perhaps there might be life clinging to underground pockets of liquid water. On top of that, there were anomalous readings from an experiment on the Viking 1 lander in 1976. Even now, there are burps of methane coming from the planet that either indicate there is continuing volcanic activity—or that something alive is producing the gas, since methane on Earth comes from only those two sources.
Venus seems utterly lifeless on the surface, based on the high surface temperatures and poisonous composition of its atmosphere. But readings of its upper atmosphere show conditions that are not quite so harsh. And oddly enough, the atmosphere contains particles that seems to have the characteristics of living cells. But much more study is needed to be sure.
Europa, one of the largest moons of Jupiter that was originally discovered by Galileo back in 1610, has turned out to be covered with ice. Based on the cracked surface and readings from the Galileo space probe that circled Jupiter from 1995 to 2003, most scientists believe that there is a subsurface ocean of liquid water beneath the ice perhaps as deep as 60 miles. If this is true, then Europa has more liquid water than the Earth does. And where there is liquid water and energy, the chances that life as we know it could exist are much better. Recent measurements indicate that not only could microbial life survive, but the amount of oxygen and energy in the system would allow fish-sized organisms to prosper. Meanwhile, another moon of Jupiter, Ganymede, like Europa, also has a subsurface ocean. Thus, there are prospects of life there as well.
Further out in the Solar system, Encyladis, a large moon of Saturn, has geysers of water shooting from the surface and spraying out into space. Once again, liquid water under its icy surface indicates a chance that life could survive.
Weirdest of all is Titan, the largest moon around Saturn. It has a thick atmosphere. Temperatures are always 250 degrees below zero Fahrenheit. Water is solid and rock-like. But methane is a liquid and Titan has an abundance of that liquid. It flows down rivers and forms lakes and seas. The methane lake Kraken, near the north pole, is larger than the Caspian Sea. The methane on Titan cycles around the planet just the way water cycles on Earth. It evaporates from the surface, condenses into clouds and then rains back down onto the land. The surface of Titan is covered with organic compounds. The photographs of the surface, sent back by the probe Huygens as it descended on parachute, reveal a world that looks more like Earth than any other, with its rolling hills, and jagged coastlines.
Recently, scientists have speculated on how life could exist on Titan. Such life would depend upon liquid methane and ethane like we depend upon water. It would breathe in hydrogen, breathe out methane, and eat acetylene. If such life existed on Titan, then the scientists predicted that Titan’s atmosphere would be depleted of hydrogen near the surface. The acetylene that the atmosphere produced should also decrease near the surface. Subsequently, recent measurements from the Cassini space probe in orbit around Saturn have indicated that exactly those things are happening there. Does that mean Titan has life, but not life as we know it here on Earth?
At this point, all we have is speculation based on some rather unusual bits of data for the worlds beyond Earth in our solar system. Alternative explanations dependent upon non-biological processes could easily explain all the data. But then again, maybe not. Occam’s razor offers no help yet in discovering what the truth is. We simply don’t have enough information yet to make the judgment. Future space probes will ultimately answer the question of the existence of life elsewhere in our solar system, one way or another.