Called “Habitable Worlds”, the meeting was sponsored by the Bioastronomy Commission of the International Astronomical Union, with organizers Alan Boss (Carnegie Institution), Karen Meech (U. Hawaii), and Thorsteinn Thorsteinsson (Iceland Hydrological Service). Following are a few highlights selected from among more than 200 papers presented.
Planet Formation. Traditionally, binary or multiple stars have been thought unsuitable for the formation of planetary systems. This is because the protoplanetary discs in binary systems are disturbed by the gravitational interactions between the stars. Alan Boss (Carnegie Institution) points out that new models for planet formation do not suffer from this problem, as they predict that disks in binary systems will cool more rapidly and become stable against such gravitational perturbations. Thus Boss suggests that binary stars (which constitute about half the stars seen in the night sky) should not be ruled out in searches for exoplanets.
Oxygen on Earth. The rise of atmospheric oxygen on Earth about 2.3 billion years ago is perhaps the most significant milestone in the biogeochemical evolution of Earth, as noted by Mark Claire and David Catling (U. Washington) and Kevin Zahnle (NASA Ames). They have modeled the sources and sinks for oxygen over the history of the Earth to help understand the timing of this key event. Since life existed on Earth long before the oxygenation of the atmosphere, it is not simply the presence of photosynthesis that leads to an oxygen-rich atmosphere. They conclude that a chemically reduced atmosphere with a large methane greenhouse could persist long after the evolution of oxygenic photosynthesis. They find that the oxic transition occurs when the amount of reduced gas generated by volcanism drops below the flux of oxygen from photosynthesis coupled with carbon burial. The timing of this crossover is set in their models primarily by levels of iron in the continental crust.
Fossil Microbes. One of the recent (and continuing) controversies in astrobiology concerns the identification of the earliest life on Earth. (Similar questions will arise, of course, in efforts to identify microbial fossils in returned martian samples). Bill Schopf (UCLA), who is a major player in these discussions, summarized recent advances in laboratory raman imagery that can help these identifications. He stressed that the presence of kerogens – organic materials that can sometimes survive for billions of years – provide a unique biological signature. Two-dimensional raman analysis can be used on a micrometer scale to identify kerogens in ancient rocks. Now such techniques have been extended to three dimensions to reveal possible cell walls. Schopf suggests that these lab tools will provide much insight into the biogenicity of putative ancient fossil microbes.
Microbe Shapes. Janet Siefert (Rice U.) explained how the development of cell shapes can be used to trace the evolution of microbial life. Recent work has shown that the bacterial domain relies on a cytoskeleton, which in turn is related to fundamental properties of the proteins that make up cell walls. This is a powerful technique because cell shape and size are among the easiest properties to measure, even for fossil microbes.
Life on Ice. Several authors compared the geology and biota of Iceland with that of Mars. Eric Gaidos (U. Hawaii) and his colleagues described a microbial community within the Grimsvotn subglacial lake of Iceland. Volcanic heat keeps this lake liquid and may produce hydrothermal systems that could support chemotrophic microbes (like those at deep sea vents). While they found life in the lake, including many cold-water microbes, these seem to be similar to other microbes in cold climates and do not offer evidence (yet) of such chemotrophic ecosystems.
Planetary Diversity. David Stevenson (Caltech) provided a theoretical overview of possible planets, over a range of mass from that of the Moon to ten times that of Jupiter, examining many possible locations for biology. He noted that it is not clear that an Earth-mass planet at the proper distance from its star will necessarily be habitable on long time scales; biological suitability depends critically on size and availability of water as well as location within a “habitable zone.” Water lubricates the crust and makes plate tectonics possible, which in turn provides geochemical cycling and probably is necessary for a magnetic field. True habitability implies liquid water and overall stability coupled with sustained thermodynamic disequilibrium. Since most liquid water will occur below the surface (for example, within the Galilean satellites of Jupiter), we should consider that environment as well as the surface when discussing habitability.
Mars Water. Given the timing of this meeting, a great deal of attention was devoted to the current exploration of Mars. Much of this discussion focused on the historical role of water on Mars. Jim Head (Brown U.) set the stage by describing the major transition that took place on Mars approximately 3.5 billion years ago. Before that time, the atmosphere, surface, and subsurface hydrological cycles were interconnected. Water levels rose and fell, and for some periods there was probably extensive water at the surface (perhaps with a crust of ice). Subsequently, and extending to the present, the development of a martian cryosphere (permafrost) has separated the interior water from the atmosphere and polar caps. Life may still exist in the deep subsurface, but it is not likely to communicate directly with the accessible surface of the planet. Head concluded that Mars does not have a global geochemical cycle today, but other speakers were not so sure.
Mars Gullies. Phil Christensen (ASU) and Jim Garvin (NASA HQ), among others, discussed the interpretation of the recent gullies that are seen from Mars orbit. These gullies are considered the most compelling evidence for liquid water at the surface in recent times. Approximately 20,000 gullies have been identified, so they are a ubiquitous feature and may have multiple origins. Many gullies, however, are associated with “pasted-on” deposits of dirty ice or snow. These deposits of frozen water probably form during periods when the Mars rotation axis is highly tilted, as happens at intervals of tens or hundreds of thousand of years. Under those conditions ice may migrate from the polar caps to pole-facing slopes at intermediate latitudes. This explanation would make the gullies part of a hydrological cycle involving the atmosphere and surface ice deposits, rather than indicating a connection with an interior reservoir of liquid water.
Mars Methane. One of the biggest questions in Mars studies today concerns reports, from three scientific teams, of detection of methane gas in the atmosphere. Methane is not in equilibrium with the highly oxidizing conditions on Mars. Jonathan Lunine (U. Arizona) noted that its lifetime in the atmosphere of Mars is only about 300 years. This is a problem, for either a localized volcanic source or a biological one. If true, the discovery of methane is one of the most important recent results of Mars studies. But all three detections were marginal, and they have not yet been published in refereed journals, so we must wait in anticipation for confirmation.
SETI. Finally, there were many papers at this meeting related to intelligent life. Lori Marino (Emory U.) discussed work in progress on the evolution of intelligence on Earth, addressing the question whether the emergence of intelligence is favored, based on mammal brain size over the past few tens of millions of years. Several SETI programs were presented, both the traditional microwave searches and more recent optical searches, which can be carried out with relatively small telescopes. Dan Werthimer (UC Berkeley) described the “SETI at Home” program, which involves 5 million public participants who analyze data on their home computers – a tremendous outreach success as well as a way to extend the SETI search. Frank Drake (SETI Institute) summarized 101 documented SETI programs carried out to date. Capabilities continue to grow very rapidly, but with no success so far in detecting an extraterrestrial civilization.