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Home , Endurance (crater), Late heavy bombardment, Terrain softening

Early Impact Bombardment (2008) 3030.pdf

1 2 THE LATE HEAVY BOMBARDMENT: POSSIBLE INFLUENCE ON . D. M. Burt , L. P. Knauth , and K. H. Wohletz3 1School of and Space Exploration, Arizona State University, Box 871404, Tempe, AZ 85287-1404, [email protected], 2same, [email protected], 3Los Alamos National Laboratory, Los Alamos, NM 87545, [email protected].

Introduction: From orbit, Mars appears to be as (acid fog model) owing to atmospheric enrichment in heavily cratered as Earth’s . The cratering record volcanogenic sulfur dioxide (SO2) to provide the is best exposed on the ancient Southern Highlands; in greenhouse warming that carbon dioxide (CO2) the Northern Plains it is largely covered by a thin ve- couldn’t. neer of younger sediment and lavas. The cra- Inasmuch as these “warm, wet, acid” geological tering is widely assumed to date from the same episode features coincide in time and space with the craters of of bombardment that cratered the Moon (the so-called the LHB, a simpler hypothesis might be that they are Lunar Cataclysm). On the basis of directly related. That is, the LHB itself can probably of returned lunar impact melts, this episode has tenta- account for most of them, and especially their transient tively been assigned to the interval 4.0-3.8 Ga. The , although local volcanism, especially in the Late Heavy Bombardment (LHB) on Mars and other region, was occurring at the same time. terrestrial planets, if it occurred, probably spanned the Geological Changes. Before the onset of the LHB same geologically short time interval. (i.e., prior to 4.0 Ga), Mars probably had more of an Somewhat surprisingly, given the wide discussion atmosphere and hydrosphere than at present; it also of the LHB for the Moon and , current and appears to have had a magnetic field (but none since). past geological literature on Mars tends to ignore this Given its distance from the Sun, and apparent inability apparent spike in cratering, and rather implicitly as- to retain a thick atmosphere, most surface water was sumes that the bombardment of Mars was continuous probably present as ice, nevertheless. Freezing of sur- from its formation at about 4.5 Ga until about 3.8 Ga, face water would concentrate soluble salts in dense an interval called the . In light of the LHB, brines beneath the ice; salts would crystallize if tem- the Noachian interval of Mars may actually have been peratures were cold enough or enough ice sublimed. In rather short, with the record of the first half-billion other words, brine freezing (probably with ice subli- having been largely destroyed or buried (as on mation) provides a possible alternative to direct brine Earth). evaporation for crystallizing salts [1]. The excellent preservation of the martian cratering Following each major impact, or at the height of record from 3.8 Ga on probably implies that Mars has the LHB (when many smaller impacts followed in been dry and cold since then. Other than continued close succession), enough steam should have been cratering at a much reduced rate, the major geological generated to create a temporary greenhouse, and con- process appears to have been extremely slow erosion densation of and alteration by this steam could explain and deposition by the wind. Important local contribu- contemporaneous water-related features - clays, drain- tions were made by basaltic volcanism, landslides and age networks, and lakes. If the impact target was rich debris flows, ground ice (leading to ), in iron sulfides or various sulfate salts, the steam con- glaciers (including rock glaciers), catastrophic flood- densate could have been acid (i.e., acid rain). How- ing in , and extremely minor chemical ever, such acidity would have been ephemeral, given and physical weathering. Still, the ancient craters are that Mars consists of basic silicates (silica or SiO2 all there. The major reason for their preservation is combined with MgO, FeO, CaO, and some Al2O3). probably the LHB itself. That is, the liquid acid would have reacted with (neu- Compared to Earth, Mars is small and much farther tralized itself against) basaltic rock, unless flash freez- from the Sun. Whatever its nature beforehand, the ing or evaporation preserved it in the form of crystal- catastrophic cratering of the LHB, in addition to com- line ferric acid sulfates, such as those found by the two pletely resurfacing the planet, should have resulted in rovers. Another way to create this mine dump mineral- catastrophic loss of hydrosphere and atmosphere to ogy [2] would be for the impact to scatter shattered space. How then to explain the widespread evidence of iron sulfides, which could later oxidize during damp ancient drainage networks, crater lakes, buried clay diagenesis (the Roger Burns method). Neutral salts horizons, and surface sulfates (including acid sul- could similarly result from impact scattering of salty fates)? These features are widely cited as evidence that target materials or flash evaporation of brines. The Noachian Mars was warm as well as wet, and further- important point is that acid surface waters are not re- more was literally bathed in sulfuric acid, supposedly quired to make sulfate salts (“evaporites”), because Early Solar System Impact Bombardment (2008) 3030.pdf

salty impact deposits could have derived their salts via in cross-beds in various near-surface horizons along impact reworking of salts of various origins from vari- the Rover traverse in . ous target areas [1]. The acid sulfates could be direct The most common (at least 50%) phase in these lapilli impact condensates or sulfide oxidation products. is the crystalline, specular, high temperature form of By the tail end of the LHB (that is, by the time of hematite (so-called gray hematite, with detected en- the near-surface geological interval investigated by the richment in Ni); their blue-gray color led to the spher- two rovers in Crater, and Opportunity at ules initially being called “blueberries”. Other than Meridiani Planum), most of the martian hydrosphere some doublets and a linear triplet, the spherules tend to and atmosphere had presumably already been lost to be unclumped and uniform in size (within a given ho- space (via impact erosion), and Mars would have been rizon); they show no evidence of concentration by cold and dry. In part, impacts rework older impact flowing or mixing groundwaters. Wind erosion has left deposits. Impact-generated steam would probably con- them exposed as a lag deposit uniformly exposed over dense as snow or ice, at least far from the impact site. an area hundreds of km across. Lack of exposure to liquid water presumably accounts Steam Alteration. A common phase in basaltic for the excellent preservation of features (including surge deposits (phreatomagmatic types, wherein steam metastable acid sulfates), lack of salt recrystallization, explosions result from explosive mixing of magma and and minimal erosion at these two surface sites. water) is yellow-orange palagonite, or hydrated and Impact Surge Deposits and Spherules. Although oxidized volcanic glass. Palagonite is believed to be it is smaller and colder and its surface is far older, extremely common all across Mars, and may partly be Mars does have two important features in common responsible for its distinctive color. Rather than form- with Earth - the presence of an atmosphere and of ing by volcanism, palagonite could have originated by abundant subsurface volatiles (mainly water on Earth, hydration and oxidation of basaltic impact melts in mainly ice on Mars). These features mean that the steamy impact surge clouds. LHB on Mars should have been distinct from the LHB Terrestrial impact cratering, in the presence of wa- on dry, atmosphereless bodies such as the Moon and ter or ice, commonly results in silica alteration and Mercury. The young martian rampart craters, believed deposition by hot springs. Given the low atmospheric to form via impacts into an icy substrate, reflect this pressure on Mars, acid fumarolic or steam alteration distinctness. On Earth, cross-bedded fine-grained sedi- should be more common than hot springs. Such altera- ments, locally containing various types of small spher- tion, followed by impact scattering, could account for ules (glassy condensates and accretionary lapilli), are the silica-rich fragmental horizon recently identified known to be deposited via explosions that vary from beneath , Gusev Crater. This horizon oc- nuclear to volcanic to impact-derived. These explo- curs above the one containing the spherules. sion-deposited sediments (so-called surge or base Conclusion: Impact surges seem to require either a surge deposits) can greatly resemble sediments depos- volatile-rich target or an existing atmosphere or both, ited by flowing water or wind, a fact that has led to as on Mars. By the tail end of the LHB, when the im- multiple misattributions [3]. In places, small radial pact surge deposits (our interpretation [4]) and spher- scours caused by vortices, or bomb sags caused by the ules at Meridiani Planum and Home Plate (Gusev Cra- landing of ballistic ejecta, can help identify such sedi- ter) formed, Mars was already dry and cold. Surface ments. In this regard, a bomb sag has tentatively been waters are not indicated at that time (i.e., by available identified in the cross-bedded surge beds at Home evidence either at Meridiani or Gusev), although they Plate, Gusev Crater and a deep scour is present at the probably were ephemerally present earlier in martian top of a large cross bed in the Burns Cliff exposure, history (especially during the most intense period of Crater, Meridiani Planum. bombardment). Mars is indeed an impact-dominated Surge deposits can vary from wet to dry, depend- planet, and many of its most interesting features appar- ing on the initial steam content. Spherical accretionary ently date from and probably were caused by the LHB. lapilli typically form in rather wet deposits, via con- References: [1] Knauth, L.P. and Burt, D.M. (2002) Icarus densation of sticky steam on particles in a turbulent, 158, 267. [2] Burt D.M et al. (2006) Eos, 87, 549. [3] Burt D.M. et dilute density cloud. Accretionary lapilli, unlike sedi- al. (2008) JVGR (in press). [4] Knauth L.P. et al. (2005) Nature, 438, mentary concretions, tend to be strictly size and shape 1123. limited and unclumped; they also can contain high temperature minerals. Millimetric spherules of un- specified composition occur in a distinctive horizon beneath Home Plate in Gusev Crater; somewhat larger (up to about 5mm) and more abundant spherules occur