The Cassini Mission and Astrobiology

For the past year, the SETI Institute has been one of the lead teams in NASA's Astrobiology Institute (NAI), and this week many members of the SETI Institute have been in Boulder, CO for the biennial meeting of the NAI. The SETI Institute's team pulls together a dozen of our scientists and educators in Life in the Universe research, SETI research, and Education and Public Outreach to address some of the most important questions in astrobiology. Chief among these is to understand how the origin and evolution of life depends upon particular planetary environments, and how, in turn, planetary environments may themselves be shaped by biology. We are pursuing these questions by investigating a number of worlds in our own solar system, and by asking similar questions about worlds orbiting other stars.

Within our solar system, our work starts with the early Earth. As long as Earth remains the only world on which life is known to exist, astrobiology has little choice but to build on our knowledge of terrestrial biology. We must always be aware that life in other places may have found other ways to make its biology work, but exploring terrestrial life nevertheless gives us the starting point for many of the questions we ask.

Besides Earth, we are paying a great deal of attention to three other worlds in the solar system. (1) Mars, which once clearly had liquid water on its surface, and may still have for at least brief periods. The Red Planet may or may not harbor a subsurface biosphere. (2) Jupiter's moon Europa, which almost certainly is host to the solar system's second ocean, with the possibility that that ocean hosts at least microbial life, and (3) Saturn's moon Titan, the most mysterious of all, and a body that has recently been explored in detail by the Cassini spacecraft and the Huygens probe.

All these investigations will impact our SETI searches, because they will help us understand the kinds of planets and moons--and therefore the kinds of solar systems--that might harbor life. In particular, the Allen Telescope Array (ATA) will soon face the question of whether it should look for artificial signals from worlds orbiting red dwarfs - so-called M stars. Planets orbiting these stars face particular challenges to the possibility of liquid water oceans at their surfaces, and whether or not these worlds or their moons provide good venues for life remains controversial. The SETI Institute's NAI team pulls together a team of biologists, climate modelers, planetary scientists, astronomers and SETI researchers to examine these issues in detail over a series of workshops. The outcome will determine whether or not the ATA devotes resources to listening for signals from M stars.

The decision is a major one, as M stars comprise some 75% of the stars within the thousand or so light-years to be probed by the ATA. In effect, questions about SETI searches have driven our team to ask questions about planetary habitability. As is so often the case, SETI and other forms of astrobiology reinforce each other in important ways.

Much of our thinking about Mars, Europa and even M-star planets builds on an Earth-based paradigm for life - in particular, on a requirement for liquid water. Titan is the one world we are investigating in detail that in most respects falls outside this usual model. At around -180 C, the surface of Titan is too cold for anything other than occasional transient liquid water environments. But it is of great astrobiological interest nonetheless, for at least two reasons. The first is that the chemistry of its current atmosphere in some ways resembles that thought to have existed in the atmosphere of early Earth. Not only should understanding this chemistry help address important questions related to origins-of-life chemistry on early Earth, but it might even help us understand the reasons why early Earth's atmosphere slowly became more oxygen-rich over time. Understanding these processes is one of the objectives of the work of Emma Bakes, a Co-Investigator in the SETI team's NAI project.

The second reason that Titan is of great astrobiological interest is that it appears (we'll likely know for sure soon) to present us with an example of a world where there is a meteorological cycle based on liquid methane, rather than liquid water. Scientists have long speculated about whether life could be based on some liquid other than water. Life based on liquid hydrocarbons like methane or ethane would have to be extraordinarily different than life based on water; the solution chemistry would be entirely dissimilar to what we know. Such speculations are so difficult to assess that they would never have been sufficient to justify the Titan Huygens probe. But even though we are going to Saturn and Titan for less speculative reasons, the chemistry revealed by the Huygens probe should nevertheless help provide insight into the possibility - or the unlikelihood - of such truly different forms of life.