LATE WATER CYCLES on EARLY MARS and THEIR POSSIBLE CAUSES. R. P. Irwin III1 and Y. Matsubara1, 1Center for Earth and Planetary S

Eighth International Conference on Mars (2014) 1446.pdf

LATE WATER CYCLES ON EARLY MARS AND THEIR POSSIBLE CAUSES. R. P. Irwin III1 and Y. Matsubara1, 1Center for Earth and Planetary Studies, National Air and Space Museum, Smithsonian Institution, MRC 315, 6th St. at Independence Ave. SW, Washington DC 20013, [email protected], [email protected].

Introduction: Studies of Martian degraded impact craters and valley networks based on Mariner and Vi- king data found a concurrent decline in crater degradation and fluvial erosion around the Noachian to Hespe- rian boundary [1,2]. These findings suggested a major climate change around that time from a warmer, wetter early Mars to the cold, dry conditions seen today. Martian impact craters have a range of degradation states, and stratigraphically younger craters are gener- ally less degraded than older ones of the same size, indicating that crater degradation was progressive with time and not the result of a single erosional spike [3]. In this context, one would expect to see some late No- achian or early Hesperian craters with an overall fresh morphology (i.e., little interior fill, a raised rim, visible ejecta, and possibly a secondary crater field) but with some fluvial dissection. A number of impact craters with these characteristics have been noted [4,5]. Some of the larger ones have locally deep dissection of their rims and large alluvial fans on their floors [4]. New observations based on Mars Reconnaissance Fig. 1. Buta crater (11 km in diameter) and an un- Orbiter imagery show that fluvial erosion of fresh im- named 6 km primary crater that superimpose Holden pact craters is more widespread than was previously crater secondaries but have dissected ejecta. recognized and not confined to local areas. Here we provide examples in four regions around Mars, note complexities in the local stratigraphy, and make some preliminary interpretations regarding an intermittent water cycle on early Mars. Observations: The cratered highlands between about 15–30° S contain some of the most obvious fresh craters with fluvial dissection, but such erosion is found on Hesperian surfaces elsewhere as well. Margaritifer and Noachis Terrae. Holden and Bakhuysen are two examples of large craters with both fluvial erosion and preserved secondary fields. These characteristics and their superimposed crater counts suggest a Hesperian age [6–8]. Holden crater’s (26.0º S, 34.0º W, 153 km diameter) western rim has deeply dissected alcoves that sourced a large alluvial bajada [4]. Inside the crater, younger alluvial fan deposits postdate flood deposits from Uzboi Vallis, which overlie light-toned stratigraphy. The latter may be contemporary with older fan development [7]. We have identified multiple primary craters that post-date Holden crater or its ejecta but have either fluvial dissection or infilling by light-toned stratigraphy (Fig. 1) [9,10]. These craters formed after Fig. 2. A fluvially breached and partly infilled 2 km the Holden impact but before the end of fluvial erosion crater on the eastern rim of Bakhuysen crater. in the area. Eighth International Conference on Mars (2014) 1446.pdf

Bakhuysen crater (23.0º S, 15.8º W, 153 km diame- lands are elevated and multibasinal, and the only sur- ter) has significant rim dissection and large alluvial face flow path from the South Pole to equatorial lati- deposits [4]. At least two primary impact craters ap- tudes would require filling Argyre with water [20], so pear to postdate Bakhuysen but predate the end of ero- the southern highlands are also a trap [21]. The Hellas sion in the area, as shown by their dissection (Fig. 2). and Argyre basins both occupy mid-latitude positions Terrae Cimmeria and Sirenum. The region around that would have experienced less evaporation than if Gale crater has a compelling record of fluvial dissec- they had been at the equator. For these reasons, Mars tion both before and after the crustal dichotomy would have had little water in its tropical areas and boundary scarp formed. These indicators include val- should have been more arid than Earth regardless of its leys that were left hanging by the scarp formation but atmospheric mass. experienced later knickpoint retreat, and fluvial erosion One possible explanation for a variable Martian after the Aeolis Mons deposit formed within Gale water cycle has independent empirical and theoretical crater, which crosscuts the boundary scarp [11]. support. The axial obliquity of Mars is believed to In Terra Sirenum, Hesperian airfall mantling (the have varied over time, alternately moving water to the “Electris deposit”) postdated the major fluvial and la- poles during low-obliquity periods and to the mid- custrine activity in the region, but some dissection of latitudes when obliquity is high [22,23]. This mecha- that deposit took place later [12,13]. nism could have intermittently overcome the polar cold Discussion: The limited dissection of some impact traps and facilitated discrete periods of fluvial erosion. craters is not surprising, because large craters were still Ultimately a thinning atmosphere would have elimi- forming (at a declining rate) as the Martian climate nated the liquid phase from this cycle. changed during the Hesperian Period [14]. The more References: [1] Malin M. C. (1976) JGR, 81, significant findings are locations where fluvial erosion 4825–4845. [2] Craddock R. A. and Maxwell T. A. occurred both before and after two separate events that (1990) JGR, 95, 14265–14278. [3] Craddock R. A. and are not likely to have been closely spaced in time, and Maxwell T. A. (1993) JGR, 98, 3453–3468. [4] Moore which show little evidence for fluvial dissection in J. M. and Howard A. D. (2005) JGR, 110, doi: between the two. The occurrence of rare or complex 10.1029/2004JE002352. [5] Mangold N. et al. (2012) geologic events between larger impacts and the last JGR, 117, doi:10.1029/2011JE004005. [6] Mangold N. fluvial dissection suggests that the water cycle was et al. (2012) Icarus, 220, 530–551. [7] Irwin R. P. III active long after the impacts and was not their direct and Grant J. A. (2013) USGS Map I-3209. [8] Irwin R. consequence [8]. These sites suggest that Martian cli- P. III (2013) LPS 44, Abstract #2958. [9] Grant J. A. mate change involved multiple epochs of activity ra- and Wilson S. A. (2012) PSS, 72, 44–52. [10] Irwin R. ther than one monotonic decline. P. III et al., submitted, Geomorphology. [11] Irwin R. Published explanations for a variable hydrologic P. III et al. (2005) JGR, 110, doi:10.1029/ cycle have been varied and (to a degree) speculative. 2005JE002460. [12] Grant J. A. and Schultz P. H. These scenarios include an ambient hyperarid climate (1990) Icarus, 84, 166–195. [13] Irwin R. P. III et al. that was punctuated by relatively brief precipitation (2004) JGR, 109, doi:10.1029/2004JE002287. [14] following large impacts or volcanism [15,16]. The Hartmann W. K. and Neukum G. (2001) Space Sci. former explanation is inconsistent with our finding that Rev., 96, 165–194. [15] Baker V. R. et al. (1991) Na- fluvial erosion occurred long after three of the six larg- ture, 352, 589–594. [16] Segura T. L. et al. (2002) Sci- est post-Noachian impact craters and that the other ence, 298, 1977–1980. [17] Wordsworth R. et al. three craters did not experience substantial erosion at (2013) Icarus, 222, 1–19. [18] Andrews-Hanna J. C. all [8]. The volcanic explanation is consistent with the (2012) JGR, 117, doi:10.1029/2011JE003954. [19] very large amount of Hesperian volcanic resurfacing Clifford S. M. and Parker T. J. (2001) Icarus, 154, 40– on Mars, but most volcanic surfaces similarly have 79. [20] Parker T. J. (1985) M. S. Thesis, Calif. State little dissection. Univ. [21] Irwin R. P. III et al. (2011) JGR, 116, Much attention has focused on the early surface air doi:10.1029/2010JE003620. [22] Touma J. and Wis- pressure (a proxy for the atmospheric mass per unit dom J. (1993) Science, 259, 1294–1297. [23] Forget F. area) that would have enhanced greenhouse warming et al. (2006) Science, 311, 368–371. [17]. However, a more massive atmosphere does not imply an active water cycle by itself. Tharsis fixes the equator along the Martian crustal dichotomy boundary, which it superimposes [18], so the crustal dichotomy is a pole-to-pole slope that would have moved water to a cold trap at the North Pole [19]. The southern high-