Valuing life detection missions

Recent discoveries imply that Early Mars was habitable for life-as-we-know-it; that Enceladus might be habitable; and that many stars have Earth-sized exoplanets whose insolation favors surface liquid water. These exciting discoveries make it more likely that spacecraft now under construction - Mars 2020, ExoMars rover, JWST, Europa Clipper - will find habitable, or formerly habitable, environments. Did these environments see life? Given finite resources (\$10bn/decade for the US ), how could we best test the hypothesis of a second origin of life? Here, we first state the case for and against flying life detection missions soon. Next, we assume that life detection missions will happen soon, and propose a framework for comparing the value of different life detection missions: Scientific value = (Reach x grasp x certainty x payoff) / \$ After discussing each term in this framework, we conclude that scientific value is maximized if life detection missions are flown as hypothesis tests. With hypothesis testing, even a nondetection is scientifically valuable.Comment: Accepted by "Astrobiology.

High and dry: billion-year trends in the aridity of river-forming climates on Mars

Mars' wet-to-dry transition is a major environmental catastrophe, yet the spatial pattern, tempo, and cause of drying are poorly constrained. We built a globally-distributed database of constraints on Mars late-stage paleolake size relative to catchment area (aridity index), and found evidence for climate zonation as Mars was drying out. Aridity increased over time in southern midlatitude highlands, where lakes became proportionally as small as in modern Nevada. Meanwhile, intermittently wetter climates persisted in equatorial and northern-midlatitude lowlands. This is consistent with a change in Mars' greenhouse effect that left highlands too cold for liquid water except during a brief melt season, or alternatively with a fall in Mars' groundwater table. The data are consistent with a switch of unknown cause in the dependence of aridity index on elevation, from high-and-wet early on, to high-and-dry later. These results sharpen our view of Mars' climate as surface conditions became increasingly stressing for life.Comment: Accepted by Geophysical Research Letter

Exoplanet secondary atmosphere loss and revival

The next step on the path toward another Earth is to find atmospheres similar to those of Earth and Venus - high-molecular-weight (secondary) atmospheres - on rocky exoplanets. Many rocky exoplanets are born with thick (> 10 kbar) H$_2$-dominated atmospheres but subsequently lose their H$_2$; this process has no known Solar System analog. We study the consequences of early loss of a thick H$_2$ atmosphere for subsequent occurrence of a high-molecular-weight atmosphere using a simple model of atmosphere evolution (including atmosphere loss to space, magma ocean crystallization, and volcanic outgassing). We also calculate atmosphere survival for rocky worlds that start with no H$_2$. Our results imply that most rocky exoplanets orbiting closer to their star than the Habitable Zone that were formed with thick H$_2$-dominated atmospheres lack high-molecular-weight atmospheres today. During early magma ocean crystallization, high-molecular-weight species usually do not form long-lived high-molecular-weight atmospheres; instead they are lost to space alongside H$_2$. This early volatile depletion also makes it more difficult for later volcanic outgassing to revive the atmosphere. However, atmospheres should persist on worlds that start with abundant volatiles (for example, waterworlds). Our results imply that in order to find high-molecular-weight atmospheres on warm exoplanets orbiting M-stars, we should target worlds that formed H$_2$-poor, that have anomalously large radii, or which orbit less active stars