The 8th International Conference on Mars was, as head convener David Beaty put it, "the largest gathering of Martians in the 4.5 billion year history of the Earth." However, when the largest gathering of Martians in the history of Mars happened -- if ever -- remains uncertain. In many ways, our space program has been driven by the search for extraterrestrial life, and Mars has been a particularly popular hunting ground. The conference provided an opportunity for the community to reflect on what we've learned so far.
While conditions on the red planet's surface are certainly inhospitable today, Martian geology hints at a more promising past. The evidence for flowing water on our neighboring planet is plentiful, meaning that surface temperatures and pressures would have been higher and early Mars may have more closely resembled Earth. And since life has an obvious stronghold here, Mars has a good case for being a likely place to discover how life might have come about early in our solar system's history.
Finding life elsewhere in our solar system requires hard thought about just what exactly we are looking for. David Des Marais, of NASA Ames Research Center, started off the July 16 session about rover-scale geology (PDF) by discussing life in the context of Mars. Before starting to define or look for habitable environments, tasks both current and future missions are charged with, we need a working definition for life and an appreciation for how life on another planet might be similar or different from on Earth.
The requirements for life might be boiled down to four main elements: the raw materials required in chemical reactions, a solvent to promote said reactions, an active metabolism to maintain its disequilibrium state, and the capability for adaptive response to changes in its environment and promote its own survival. Each of these criteria might manifest somewhat differently from on Earth, but we have to start somewhere. Firstly, what is life made of? The most common elements found in the solar system—carbon, hydrogen, oxygen, nitrogen, potassium and silicon—are likely candidates, due to a combination of their abundance and how they bond with other atoms. Carbon, in particular, with its versatile bonding capabilities, seems a logical building block for life beyond Earth, as carbon can form a vast number of complex molecules. This structural versatility means that life would have a wide variety of forms at its disposal for building cellular structures. Silicon is another commonly imagined fundamental block, but the rigidity of its bonds and its preference for forming tetrahedrally coordinated structures limit its versatility, leaving carbon at the forefront of likely foundational elements. Assuming carbon-based life forms on Mars, and elsewhere in our solar system, is likely a safe bet.
We also assume that water will be a necessary component for life on Mars and other Earth-like planets, again based on water's role in Earth's organic material and water's chemical properties. Water is another versatile molecule that can promote complex chemical structures and reactions. Without some sort of solvent, the ongoing chemical reactions necessary to create and repair biological molecules are limited. Water is also a relatively common material, and is liquid at the usual temperature range assumed to be advantageous for life. There are other potential liquid solvents—methane on Titan, for example—but they are typically only liquids at much colder temperatures, perhaps limiting life as we know it. In the context of life on Mars, the evidence for plentiful liquid water early in the planet’s history is therefore encouraging.
Of course, life is more than just the presence of complex molecules. In order to be alive, the organism has to be able to control its self-repair and create copies of itself to pass on its aliveness. On earth we have DNA and a complex chemistry designed to copy the encoded "how-to" manuals, and something similar would be expected on Mars. Further, this copying method should probably include some means of improvement, perhaps via Darwinian evolution, because if an organism's environment changes but the organism doesn't, it's likely that the organism will lose its "alive" status.
These last two criteria may be the most difficult aspect of characterizing past life on another planet. We may be able to find organic molecules that may indicate the presence of life, but concluding that they unambiguously mean that there was life may still be a stretch. Given that there are components of the geologic record on Earth whose biological implications are still contested, it might be presumptuous to assume that organic molecules equal life. Finding fossils of macroscopic organisms would of course be a whole different ball game, but no one is really holding their breath. We'll likely have to make do with the organic chemistry for now.
The last element to consider with regards to finding life on Mars is the preservation potential of the habitat and the biological signatures for which we search. It is generally agreed that if Mars was ever inhabited, it would have been early in the planet’s history when conditions were more amenable, meaning that any evidence for life must have survived billions of years. The greatest threat to biological markers on Mars is likely the radiation environment at the surface. High-energy rays—in the form of cosmic, gamma and UV radiation—can rapidly break down complex organic molecules and erase unambiguous clues about life on the red planet. Therefore, we need to look where it is both old and new; that is, where the rock deposits were formed in the early, wet periods of Mars' history, but where these rocks have been buried for most of their history and are only recently (in geologic terms) exposed at the surface by erosion, such that they have been protected from surface radiation and weathering.
Both rovers currently driving around on Mars, Opportunity and Curiosity, have reported finding potentially habitable environments, the clues contained in the ancient rock record. The final session of the conference (PDF) focused on exciting science to come, including updates on the next generation of rovers, which intend to take the next steps towards detecting biological material and identifying life.
In 2018, the European Space Agency, along with collaboration with Russia and NASA, intend to send a rover capable of drilling deep into the Martian surface (where deep is up to 2 meters) and exploring the subsurface. A deep drill might penetrate below the radiation-cauterized horizon to where organic traces have been protected and preserved, therefore skirting the issue of preservation in landing site selection. This mission, called ExoMars, is a two-stage endeavor, with an orbiter to study trace gases in the atmosphere, along with the descent probe intended as a technology demonstration to prove that landing on the surface is possible, in 2016, and the rover/landing platform following two years later. The rover will be about the size of Curiosity and carry, in addition to its drilling platform, a substantial science payload to examine surface rocks, probe the subsurface remotely, and analyze samples, providing a well-rounded laboratory designed to detect and characterize signs of life. Despite the rover's subsurface capabilities, choosing the landing site is still a complicated task. To make the most of its time on the surface, the rover is not planned to stray more than a kilometer or so from the landing location, and will instead very thoroughly characterize the nearby landscape. Operations on the surface of Mars take a long time; if a part malfunctions, there is no hardware store to run to, so every movement of the robotic arm, every drive, and certainly every drill will need to be thoroughly vetted for risks. Given the time, effort and expense required for every operation, having considerable geologic variation with the rover’s range is highly desirable, and selecting the landing site will be an exciting prospect in the next few years. [Note: It now appears ExoMars may be delayed to 2020. --Ed.]
NASA will also have to think about the destination for its next robotic exploration on Mars. This mission, planned for 2020 (and creatively called Mars 2020) will expand on the current directive to find and characterize habitable environments, and is planned to actually collect and store promising samples for return to Earth sometime in the future. Laboratories here on Earth can perform investigations on Martian samples unrivaled by anything we can send to Mars, and returning pristine samples to Earth would advance our understanding of our neighbor planet by leaps perhaps comparable to Neil Armstrong's step on the moon. At the very least it would be a spectacular return on investment. Again, the choice of a landing site, still in the early stages, will require balancing geologic and safety considerations to pick the juiciest location. The instrument payload will be announced this month. [In fact, it's expected to be announced tomorrow. --Ed.]
The planetary science community is going to have to rally in order to make the sample return mission a reality, but in the mean time, mission designers are gearing up to collect and study the best samples possible. The 8th Mars conference was a valuable opportunity for the Martian community to synthesize the big advancements in the past 7 years and develop overall goals and broad themes that will guide the next few years of Mars planetary science. Understanding habitable environments and the potential for life on the red planet is certainly one of the big ones.
Vago J. L., Rodionov D. S., Witasse O., Kminek G., Lorenzoni L., the Landing Site Selection Working Group and the ExoMars Team. (2014) ExoMars Status and Landing Site Selection. Eighth International Conference on Mars, abstract #1105.
Farley K., and Schulte M. Update on the Mars 2020 Mission. Eighth International Conference on Mars, invited talk.