We hear a lot about the prospect of microorganisms existing on Mars and probes of increasing complexity that will be going there in the years to come, but the life search will also examine the outer Solar System. Having an environment with liquid water makes the possibility of life forms more plausible. For this reason, astrobiologists consider the space extending from the orbit of Venus to the orbit of Mars around the Sun as a “Goldilocks zone”. Venus is super-hot at the surface because of an extreme greenhouse effect from high levels of carbon dioxide in its atmosphere, but it’s actually far enough from the Sun that it would be cool enough for liquid water if its atmosphere were different. Mars is cold, but warm enough for liquid water; in fact, scientists are almost certain that water flows on the Martian surface from time to time.
Moving out to the orbits of Jupiter and Saturn, the outer Solar System, sunlight is too weak for liquid water to flow on the surface of a planet or moon, so it’s beyond the traditional limit of a Goldilocks zone, but liquid water can exist below the surface when there is a source of heat other than the Sun. Over the last 20 years, planetary scientists have accumulated evidence of large amounts of liquid water existing under the icy surfaces of worlds in the outer Solar System. The two that have received the most attention are Jupiter’s moon Europa and Saturn’s moon Enceladus, because measurements from space probes show that each has an entire ocean. The ocean is covered with ice, which keeps the liquid water beneath from evaporating into space, but while flying by Enceladus, NASA’s Cassini probe has photographed plumes of water gushing from geysers in the Enceladus ice. This means that ocean of Enceladus may be accessible to a probe that could be sent in the near future. As opposed to being blocked by the ice the covers it, the ocean may be reached through a geyser, and the fact that there are geysers in the first place suggests that the portions of the ice layer might not be so thick that a robot from Earth couldn’t drill through it and reach the ocean.
What keeps these moons warm enough to maintain global oceans of water below their freezing surfaces? The answer is called tidal heating. Europa travels around Jupiter in an elongated orbit, causing Jupiter’s immense gravity to tug on the moon with varying amounts of strength as Europa moves. Thus, Europa is constantly squeezed and stretched and this generates heat deep inside the moon. The same thing happens to Enceladus as it orbits around Saturn.
As for missions to these moons, NASA is currently preparing a Europa mission to launch in the early 2020s. Instruments are being selected to study Europa from orbit to acquire details about its environment relevant to the search for life forms, such as whether the subsurface ocean releases plumes of water through the ice as Enceladus does. The Europa mission may include a small landing craft to conduct experiments on the ice surface, and possibly on protruding liquid water if any plumes are located. It’s not clear yet what type of experiments would go on such a lander for the Europa mission, but NASA has begun flirting with an idea of mission to Enceladus specifically to seek life forms. It would be called the Enceladus Life Finder (ELF) and it would be a followup to the Cassini probe orbiting Saturn now that will continue to perform Enceladus flybys. Launch of the ELF would probably take place well into the 2020s, or beyond, but the experiments to search for life could be fairly sophisticated. Based on recent presentations at the Lunar and Planetary Science Conference, held each year in Houston, ELF tests could be looking for evidence that water samples on Enceladus act preferentially on certain molecules versus others, for instance on fatty acids with on odd number of carbon atom versus those with an even number of carbon atoms. Experiments looking for differences in chemical interactions between water samples and molecules that are chemically “left handed” verses those that are chemically “right handed” also could be in the works. This level of sophistication would be important to avoid scenarios similar to what happened in NASA’s Project Viking when one test seemed to have detected life, but other tests seemed to detect chemical activity in Martian dust that was not the result of life.
Written by David Warmflash
David is an astrobiologist and science writer. He received his M.D. from Tel Aviv University Sackler School of Medicine, and has done post doctoral work at Brandeis University, the University of Pennsylvania, and the Johnson Space Center, where he was part of the NASA's first cohort of astrobiology training fellows. He has been involved in science outreach for more than a decade and since 2002 has collaborated with The Planetary Society on studying the effects of the space environment on small organisms.