Habitability Models for Astrobiology.

Habitability has been generally defined as the capability of an environment to support life. Ecologists have been using Habitat Suitability Models (HSMs) for more than four decades to study the habitability of Earth from local to global scales. Astrobiologists have been proposing different habitability models for some time, with little integration and consistency among them, being different in function to those used by ecologists. Habitability models are not only used to determine whether environments are habitable, but they also are used to characterize what key factors are responsible for the gradual transition from low to high habitability states. Here we review and compare some of the different models used by ecologists and astrobiologists and suggest how they could be integrated into new habitability standards. Such standards will help improve the comparison and characterization of potentially habitable environments, prioritize target selections, and study correlations between habitability and biosignatures. Habitability models are the foundation of planetary habitability science, and the synergy between ecologists and astrobiologists is necessary to expand our understanding of the habitability of Earth, the Solar System, and extrasolar planets.

A globally fragmented and mobile lithosphere on Venus.

Venus has been thought to possess a globally continuous lithosphere, in contrast to the mosaic of mobile tectonic plates that characterizes Earth. However, the Venus surface has been extensively deformed, and convection of the underlying mantle, possibly acting in concert with a low-strength lower crust, has been suggested as a source of some surface horizontal strains. The extent of surface mobility on Venus driven by mantle convection, however, and the style and scale of its tectonic expression have been unclear. We report a globally distributed set of crustal blocks in the Venus lowlands that show evidence for having rotated and/or moved laterally relative to one another, akin to jostling pack ice. At least some of this deformation on Venus postdates the emplacement of the locally youngest plains materials. Lithospheric stresses calculated from interior viscous flow models consistent with long-wavelength gravity and topography are sufficient to drive brittle failure in the upper Venus crust in all areas where these blocks are present, confirming that interior convective motion can provide a mechanism for driving deformation at the surface. The limited but widespread lithospheric mobility of Venus, in marked contrast to the tectonic styles indicative of a static lithosphere on Mercury, the Moon, and Mars, may offer parallels to interior-surface coupling on the early Earth, when global heat flux was substantially higher, and the lithosphere generally thinner, than today.

Topographic Expressions of Large Thrust Faults on Mars.

On planets with little erosion, thrust faults produce broad, asymmetric, positive-relief, linear to arcuate ridges -often referred to as lobate scarps- that remain largely unaltered, such that their topographic expressions are a measure of the structural uplift caused by the displacement and associated country-rock deformation of the faults. Here we map and systematically assess the structural relief of 24 thrust faults across Mars to infer their growth behavior. Our mapping indicates that the majority of individual thrust faults have simple, linear map traces with lengths of up to ~450 km, but that some thrust faults form systems of up to 1400 km in length. For the most topographically pronounced landforms, the structural relief developed above the fault is as great as ~3400 m. We then relate topographic measurements to the displacement on the underlying fault planes to study the displacement variations along the fault length. We find a variety of displacement distribution shapes of the fault systems, which we attribute to differences in fault growth that include unrestricted and restricted growth, linkage, and/or fault interaction. Finally, we relate the maximum displacements (Dmax ) determined for each of the faults to their respective fault length (L) to establish a maximum displacement-to-length relationship. The observed scaling characteristics and order-of-magnitude scatter of our Dmax/L data are not uncommon for fault populations on Earth and tie in well with the map patterns, tectonic geomorphology, and systematic along-strike displacement distributions to have grown in a basement-block faulting style found in intra-plate tectonic settings on Earth.

Planetary science. Low-altitude magnetic field measurements by MESSENGER reveal Mercury's ancient crustal field.

Magnetized rocks can record the history of the magnetic field of a planet, a key constraint for understanding its evolution. From orbital vector magnetic field measurements of Mercury taken by the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft at altitudes below 150 kilometers, we have detected remanent magnetization in Mercury's crust. We infer a lower bound on the average age of magnetization of 3.7 to 3.9 billion years. Our findings indicate that a global magnetic field driven by dynamo processes in the fluid outer core operated early in Mercury's history. Ancient field strengths that range from those similar to Mercury's present dipole field to Earth-like values are consistent with the magnetic field observations and with the low iron content of Mercury's crust inferred from MESSENGER elemental composition data.

Flood volcanism in the northern high latitudes of Mercury revealed by MESSENGER.

MESSENGER observations from Mercury orbit reveal that a large contiguous expanse of smooth plains covers much of Mercury's high northern latitudes and occupies more than 6% of the planet's surface area. These plains are smooth, embay other landforms, are distinct in color, show several flow features, and partially or completely bury impact craters, the sizes of which indicate plains thicknesses of more than 1 kilometer and multiple phases of emplacement. These characteristics, as well as associated features, interpreted to have formed by thermal erosion, indicate emplacement in a flood-basalt style, consistent with x-ray spectrometric data indicating surface compositions intermediate between those of basalts and komatiites. The plains formed after the Caloris impact basin, confirming that volcanism was a globally extensive process in Mercury's post-heavy bombardment era.