Our Changing View of Mars
Mars formed in the solar nebula and differentiated into a crust, mantle, and core. During the subsequent Noachian period, which began about 4 billion years ago, impact cratering, volcanism, and erosion were extensive. River valleys cut into surfaces already eroded and altered by water, some pervasively. The Hesperian period, starting about 3.7 billion years ago, saw continued extensive volcanism. Erosion and the formation of river valleys slowed dramatically, but catastrophic floods occurred; some of those floods emanated from subsurface reservoirs and produced temporary lakes and seas. Tectonic activity opened large canyons. Aqueous alteration of surface materials was more localized and reflected a growing scarcity of water.
The Amazonian period, starting about 3 billion years ago and continuing today, is icy and dusty. Water remains in the climate system, but predominantly as ice that is redistributed across Mars’s surface in climate cycles induced by the planet’s orbit and orientation to the Sun. Erosion rates are extremely slow, with wind in the thin atmosphere being the dominant agent. The decreasing abundance and stability of liquid water over time are tied to the gradual loss to space of much of the early Noachian atmosphere and the planet’s water inventory via thermal escape and ionization processes. The ionization processes were enhanced as the planet’s interior cooled and the magnetic field that deflects charged particles from the solar wind diminished. Declining global temperatures and the thinning atmosphere makes any liquid water on the planet’s surface vulnerable to freezing or evaporation.
The image above shows an ancient delta within the 45-m-diameter Jezero Crater, as photographed by the Mars Reconnaissance Orbiter in January 2007. The outside rim of the crater appears white. Lays of sedimentary rock on the crater floor contain spectral evidence of hydrous clay minerals and carbonates (green) among igneous minerals (yellow and blue). The sediments are thought to have come from nearby highlands, transported by water billions of years ago when the crater was a lake basin. The presence of clay minerals in such a deltaic setting is favorable for the accumulation preservation of organic material.
If life ever took hold on Mars, what environmental niches did it occupy? And what limited it? Did it ever become pervasive enough to modify its environment on the scale of Earth’s oxygenation, for example? Was it resilient to the loss of Mars’s atmosphere and hydrosphere? If life never took hold, what is that telling us, given Mars’s likely similarity to early Earth and the availability of habitable environments? The poet T. S. Eliot tells us that the end of exploration is to arrive where we started and to know the place for the first time. The end of Mars exploration is not in sight, but it is safe to say that we are learning more each day about where we started.
 See Ashwin Vasavada, “Our Changing View of Mars,” Physics Today (70, 3, pp. 34-41).