1MS3-1-1 THE GEOLOGIC HISTORY OF MARS James W. Head1, Michael H. Carr2, 1Department of Geological Sciences, Brown University, Providence, R.I. 02912 USA; 2U. S. Geological Survey, Menlo Park, CA 94025 USA. Contact: [email protected]. Introduction: Our current understanding of the geologic history of Mars (Fig. 1) pro- vides insight into a number of outstanding questions that need to be addressed in future Mars exploration missions. Here we review the general themes in our current understanding of the geologic history of Mars [1] and identify a list of key questions that will help to guide future exploration planning. Current Understanding of the Geologic His tory of Mars: Mars accumulated and differentiated into crust, mantle and core within a few tens of mil lions of years of Solar System formation. Formation of the Hellas basin, which has been adopted as the base of the Noachian period, is estimated to have occurred around 4.1 to 3.8 Gyr ago, de- pending on whether or not the planet experienced a late cata clysm. Little is known of the pre-Noachian period except that it was 1) characterized by a magnetic field, and 2) subject to numerous large basin-forming impacts, probably including one that formed the global dichotomy. The Noachian period, which ended around 3.7 Gyr ago, was characterized by high rates of cratering, erosion, and valley network forma tion. During the Noachain, most of the Tharsis rise (volumetrically) formed and surface conditions were at least episodically such as to cause widespread production of hydrous weath- ering products such as phyllosilicates. The precise environment of origin of the phyl- losilicates (surface alteration by water, deep groundwater circulation, impact-energy related, all of the above?) is not well understood, however. Ex tensive sulfate deposits accumulated late in the era. Average erosion rates in the Noachian, though high com- pared with later epochs, fell short of the lowest average terrestrial rates. The record suggests that the warmer, wetter conditions necessary for fluvial ac tivity were met only occasionally, such as might occur if caused by large impacts or volcanic erup tions. At the end of the Noachian, rates of impact, valley formation, weathering, and erosion all dropped precipitously but volcanism continued at a relatively high average rate throughout the Hesperian, resulting in the resurfacing of at least 30% of the planet. Large water floods (the outflow channels) formed episodically, possibly leaving behind large bodies of water, perhaps even forming a sea or ocean in the northern lowlands lasting for an unknown duration. The major canyon system formed. The observations suggest that the change at the end of the Noachian suppressed most aqueous activ- ity at the surface other than large floods, and resulted in growth of a thick cryosphere. However, the presence of discrete sul fate rich deposits, sulfate concentrations in soils, and the occasional presence of Hesperian valley net works indicate that water activity did not decline to zero. After the end of the Hesperian around 3 Gyr ago the pace of geologic activity slowed further. The average rate of volcanism during the Amazonian was approximately a fac- tor of ten lower than in the Hes perian and activity was confined largely to Tharsis and Elysium. The main era of water flooding was over, although small floods (such as at Cerberus Fos sae) occurred episodically until geologically recent times. Canyon devel- opment was largely restricted to formation of large landslides. Erosion and weather ing rates remained extremely low. The most distinc tive characteristic of the Amazonian is formation of features that have been attributed to the presence, accumulation, and movement of ice. Included are the polar layered deposits, glacial deposits on vol- canoes, ice-rich veneers at high latitudes, and a variety of landforms in the 30-550 latitude belts, including lo-bate debris aprons, lineated valley fill and concentric crater fill. Pedestal craters record the presence of thick mantles of mid-to high-latitude ice deposits at many times throughout the Amazonian. Most of the gullies on steep slopes also formed late in this era. The rate of formation of the ice-related features and the gullies probably varied as changes in obliquity affected ice stability relations. Collapse of the CO2 atmosphere may have occurred from time to time in the Amazonian. The current configuration of spin axis/orbital parameters (and thus climate-related deposits) is almost certainly not typical of the Ama zonian as a whole. Lessons for Future Exploration: The advent of the international Mars exploration pro- gram during the last two decades has brought untold new knowl edge of, and perspec- tives on, the geologic history of Mars. It has also raised important new questions [2], which form the basis of future research and mission planning. Among the outstand- ing questions are: What is the history of the magnetic field? What is the history of water on Mars, its total abundance, and its acquisition (accretion, degassing) and loss with time? What was the nature of the Noachian climate? Was it warm and wet, cold *) All the abstracts are the original authors’ versions abst 1 1MS3-1-1 *and wet, or some combi nation that changed with time? Was Noachian val ley net- work formation episodic or continuous? Were there oceans in the northern lowlands in the Noa chian? What factors caused changes in the Noachian climate? Was there a late heavy bombardment and, if so, what were its consequences? How did the seem- ingly pervasive aqueous formation of Noachian crustal rocks occur and over what time scales? What caused the change in surface chemistry and water flux at the end of the Noachian? When did the major canyons of Valles Marineris form and how, and at what rate did they evolve? Are the sediments on the floor of the canyons Noachian, or later, in age, and how did they originate? Were there extensive lakes in the canyons and if so, how were they related to outflow channel formation? Were outflow channels catastrophic or episodic, and how much water and sediment were emplaced in the northern lowlands (ponds, lakes or seas)? What was the fate of the out flow channel effluent? What caused the change from phyllosilicate to sulfate deposits around the Noachian-Hesperian boundary? When did Mars become characterized by a global cryosphere and how much groundwater remains today in the subsur face? How has the thickness and latitudinal distribu tion of the cryosphere changed with time? What was the flux of volcanism during the Hesperian; was it a steady decline from the Noachian, or a peak related to the emplacement of Hesperian ridged plains? Why was volcanism centralized to the Tharsis and Elysium regions? What is the spin-axis/orbital pa rameter history of Mars and how has this controlled the distribution of volatiles in the surface and subsur face? Under what atmospheric and astronomical parameter conditions do tropical mountain glaciers, mid-latitude glaciers, and large circumpolar ice caps form and what do these deposits tell us about the climate history of Mars? What it the age of the cur rent polar ice deposits and why do they apparently differ? What is the origin of the north polar basal unit? How long does it take to form the individual polar layers and what processes of deposition and sublimation are involved? Answering these funda- mental questions requires the sophisticated, multi discipline approach that is necessary for developing an in-depth understanding of the geologic history of Mars. Reaching this new understanding requires a robust future international Mars exploration pro gram. Fig. 1. Geologic history of Mars. References: [1] Carr, M. H. and Head, J. W., 2010. Geologic history of Mars, Earth and Planetary Science Letters 294,185–203. [2] Head, J.W., 2007. The geology of Mars: New insights and outstanding questions, In: Chapman, M., (Ed), The Geology of Mars: Evidence from Earth-Based Analogs, Cam bridge University Press, 2007, pp. 1-46. abst 2 1MS3-1-2 EPISODICITY OF VOLCANIC AND FLUVIAL PROCESSES ON MARS G. Neukum1, A.T. Basilevsky1,2, and the HRSC Team, 1Freie Universitaet Berlin, Malteserstr., 74-100, Haus D, 12249 Berlin Germany; 2Vernadsky Institute, Kosygin Str., 19, 119991 Moscow, Russia. Contact: [email protected] We have investigated 12 typical regions on Mars (Figure 1) in detail with respect to their geologic evolution through time on the basis of detailed geologic mapping and determining age relationships through crater counting techniques using imagery. New data in combination with previously obtained data have been analyzed by way of a refined method of crater- ing age extraction that also gives fine details of periods of resurfacing (Figure 2). We have found that there has been volcanic and fluvial/gla- cial geologic activity on the Martian surface at all times from >4 Ga ago until present. Fig. 1. Regions of this study. This activity shows episodic pulses in intensity of both volcanic and fluvial/glacial processes at ~3.8–3.3 Ga, 2.0–1.8 Ga, 1.6 to 1.2 Ga, ~800 to 300 m.y., ~200 m.y., and ~100 m.y., and a pos- sible weaker phase around ~2.5–2.2 Ga ago (Figure 3). In between these episodes, there was relative quies- Fig. 2. Crater counting methodology cence of volcanic and/or flu- vial/glacial activity. The epi- sodes we find on the Martian surface in the crater frequency analyses of HRSC, MOC and THEMIS data coincide with some age groups of the Martian meteorites (~1.3 Ga, ~600–300 m.y., ~170 m.y.). It appears that the surface activity expressions and their episodicity are related to the interior evo- lution of the planet when convection in the asymptotic stationary state changes from the so-called stagnant-lid regime to an episodic behavior.