FIGURE 2.1.1 Major events in the geological histories of Earth and Mars over the last 4.5 billion years. The timing of the boundaries between Mars's three major named geological eras is highly uncertain because of the absence of an absolute calibration of the ages of martian surface features. Moreover, the geological record of the earliest events in martian history, those of the so-called pre-Noachian era, has been largely erased by subsequent events, including the heavy bombardment that took place during the Noachian era. Diagram courtesy of Michael H. Carr, U.S. Geological Survey.

Elysium region where several large fluvial channels emerge from graben radial to the volcanos there.32 One possibility is that release of groundwater was caused by propagation of dikes radial to the volcanos through the local cryosphere and hydrosphere. Large bodies of water must have been left behind after the floods.33 A thick cryosphere was probably present when the floods formed. Any terminal lakes or seas would thus have frozen, and ultimately the ice would have sublimated away or been buried. Observational evidence for any such oceans is sparse, although there is good evidence in the northern plains for burial of pre-flood craters and ridges by sediments,34,35 and several features suggest the former presence of ice in the low areas at the ends of the channels (Figure 2.6).36 It has been suggested that formation of a large flood would have temporarily changed global climates by injecting large amounts of H2O and CO2 into the atmosphere, but failure to detect carbonates is troubling. However, carbonate could be distributed uniformly throughout a crust, emplaced by circulation of water, and could hold a several-bar CO2 atmosphere without being detectable spectroscopically. Also, thermal-emission spectrometer results have

The oldest geological features recognizable on Mars belong to the Noachian era, named after Noachis Terra in the southern highlands. In fact, most of Mars's rugged southern highlands are of Noachian age. This era was characterized by very high cratering rates and the formation of the major impact basins (e.g., Hellas and Argyre); major episodes of volcanic activity; and the formation of the oldest valley networks. Lying immediately atop Noachian geological units are features of the Hesperian era. Much of the northern lowlands and, in particular, the ridged plains are of Hesperian age. The youngest major surface features on Mars belong to the Amazonian era, so named because they are typified by the plains and volcanic materials of Amazonis Planitia. The surfaces of the prominent volcanos of Elysium and Tharsis Montes are of Amazonian age.

The absolute ages of martian features and thus the time history of the planet's evolution are currently uncertain. Converting the relative chronology implied by stratigraphy relationships requires that the absolute ages of key surface features be determined, and this will almost certainly require a sample-return mission. In the meantime, observations of the density of craters in a region can be used as a means of estimating that region's absolute age.2 In other words, if a surface has a greater density of craters than an adjacent region and the rate at which impacts have occurred over time is known, then the ages of the surfaces can be estimated. Unfortunately, this technique is dependent on imperfect models of the cratering rate at Mars through time, which are, in turn, extrapolated from the known absolute chronology of the Moon.

Despite the very great uncertainties, particularly in the dating of the boundary between the Hesperian and the Amazonian, researchers estimate that the age limits of the major geological eras on Mars are as follows:

• Noachian era, ~4.1 billion to ~3.7 billion years ago;

• Hesperian era, ~3.7 billion to ~3 billion years ago; and

• Amazonian era, ~3 billion years ago to the present day.

Features older than the Noachian are usually referred to by the informal designation of pre-Noachian. Virtually nothing is known about this important period of martian history because the geological record of this time has been erased by later events. Unfortunately, this lost era includes the time in martian history when conditions might have most closely resembled those on Earth. By the end of the Noachian era, Mars was firmly established on a global evolutionary track that was significantly different from that followed by Earth over the subsequent 3.7 billion years.

1 K.L. Tanaka, "The Stratigraphy of Mars," Journal of Geological Research 91:E139-E158, 1986.

2W.K. Hartmann and G. Neukum, "Cratering Chronology and the Evolution of Mars," pp. 165-194 in Chronology and Evolution of Mars, R. Kallenbach, J. Geiss, and W.K. Hartmann (eds.), Kluwer, Dordrecht, the Netherlands, 2001.

detected carbonates in the dust, and the martian meteorite ALH 84001 has several percent carbonates by weight, indicating that carbonates are present although the total quantity is unknown.

The martian canyons (e.g., Valles Marineris) are among the least understood features of the planet. Their relevance to liquid water and biology is that they may have once contained large lakes that ultimately drained catastrophically to the east to form outflow channels that connect to the canyons.37 They also provide access via spectroscopy to the deep subsurface, where liquid water might have been present. Several large channels also start in box canyons to the north of the main canyons, indicating that liquid water was present locally at elevations well above the floor of the main canyons. The central and eastern sections of the canyons contain thick stacks of layered sediments, rich in sulfates, which could have been deposited subaqueously. However, even if the canyons did once contain lakes, as appears likely, the source of the sediments, their mode of deposition, and the lifetime of the lakes all remain undetermined.

FIGURE 2.3 Warrego Vallis at 42°S, 267°E. The drainage density of this Noachian terrain is comparable to terrestrial values and implies precipitation and surface runoff. Image from the Thermal Emission Imaging System on the Mars Odyssey spacecraft courtesy of NASA/JPL/Arizona State University.

Although the rate of valley formation tailed off at the end of the Noachian, valleys continued to form at a low rate.38 Some of the most prominent valleys such as Nirgal Vallis and Nanedi Vallis are Hesperian in age, but both these valleys have characteristics that suggest that they formed mainly by groundwater sapping rather than by surface runoff as is the case for most of the Noachian valleys (Figure 2.7).39 Nevertheless, post-Noachian valley networks with runoff characteristics are found, as adjacent to Echus Chasma. In addition, several post-Noachian volcanos have surfaces that are highly dissected. In fact some of the most dissected surfaces anywhere are on volcanos. Several suggestions have been made to explain the young valleys on volcanos: that they formed by nuées ardentes or lava, that they formed during temporary warm periods caused by floods or large impacts, that they formed as a result of local conditions associated with volcanic eruptions, or that they resulted from the melting of ice deposited on the volcanos during periods of high obliquity, or after large floods. Whatever the cause, the presence of the valleys strongly supports the occasional temporary availability of liquid water on the volcano surfaces (Figure 2.8).

In summary, the present-day surface of Mars, with its cold temperatures, high ultraviolet flux, oxidizing conditions, and scarcity of liquid water and organics, is inhospitable to life as we know it. If life is present today, it likely is below the surface, protected from the harsh surface environment, or perhaps in exceedingly rare, localized environments driven by recent volcanic activity. The surface has experienced more benign conditions in the past, particularly the distant past. During the Noachian period, which ended around 3.7 billion years ago, liquid water was abundant at the surface, lakes were common, oceans may have been present, and the planet, like Earth, experienced high weathering rates with the production of clay minerals, and high rates of erosion and deposition, all consistent with warm, wet, habitable conditions.

At the end of the Noachian, conditions changed. Weathering and erosion rates declined rapidly to very low rates, which resulted in dominantly cold surface conditions and development of a thick cryosphere. Large floods episodically flowed across the surface leaving behind temporary lakes or seas, which could have proved temporary refuges. In addition, the planet intermittently experienced high obliquities that may have allowed accumulation of ice and snow at low latitudes, which on melting by sunlight or volcanic heat may have provided moist conditions in local areas. In all epochs, the combination of volcanism and water-rich conditions must have inevitably led to hydrothermal systems in which life could have thrived. Finally, accompanying these changes in physical processes were chemical changes.40 Weathering in the Noachian (>3.7 billion years ago) produced clay minerals, which have not been detected in the younger (3.0 billion to 3.7 billion years old) Hesperian rocks. Instead, many Hesperian deposits are rich in sulfates, many of which may have formed in highly acidic waters. Alteration of the younger (<3.0 billion years ago) Amazonian rocks is mainly by oxidation.

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