Geology of the terrestrial planets with implications to astrobiology and mission design

Although Mercury, Venus, Earth/Moon, and Mars originated from similar material accreted in close regions of the planetary nebula, the worlds that developed after 4.6 billion years of Solar System evolution are not much alike. Mercury and the Moon, for example, are without a protecting atmosphere and were not exposed to liquid water on their surface for at least 4 billion years. The surfaces of Venus and Mars are desiccated and are presently not a suitable habitat for life, but reservoirs of liquid water remain in the atmosphere of Venus and the subsurface of Mars, and microbial organisms may have adapted to survive in these ecological niches. The search for water and life on any of the terrestrial planets is intrinsically connected to their geological history. Missions should be designed to explore any potential past and present habitats. In addition, ices on Mercury and the Moon should be explored for remnant biogenic material from the early evolution of life on Earth (and elsewhere). Especially desirable are sample return missions and missions that include hierarchical architectures with scalable degrees of mission operation autonomy, which will allow optimal r econnaissance of planetary environments, including surface and subsurface environments.