Icarus 228 (2014) 96–120 Contents lists available at ScienceDirect Icarus journal homepage: www.elsevier.com/locate/icarus Comprehensive analysis of glaciated martian crater Greg ⇑ William K. Hartmann a, , Veronique Ansan b, Daniel C. Berman a, Nicolas Mangold b, François Forget c a Planetary Science Institute, 1700 East Fort Lowell Road, Suite 106, Tucson, AZ 85719-2395, USA b Université de Nantes, 1, quai de Tourville, BP 13522, 44035 Nantes cedex 1, France c Laboratoire de Météorologie Dynamique, CNRS/UPMC/IPSL, 4 place Jussieu, BP99, 75252 Paris Cedex 05, France article info abstract Article history: The 66-km diameter martian crater, Greg, east of Hellas, hosts various distinctive features, including den- Received 13 August 2012 dritic valleys filled with chevron-textured masses (south wall), and lobate tongues a few kilometers long Revised 20 September 2013 (north wall). We analyze these features by various quantitative techniques to illuminate martian geologic Accepted 21 September 2013 and climatic history. Crater retention model ages indicate that Greg is at least 1–3 Gy old, but surface lay- Available online 2 October 2013 ers of mantles and glacial features are orders of magnitude younger. Properties of the dendritic valleys, combined with climate models, suggest that fluvial activity began under a thicker, warmer atmosphere, Keywords: soon after the crater’s formation. The oldest exposed fluvial systems have surface crater retention ages of Mars a few hundred My, indicating runoff in recent geologic time. Much of Greg is covered by ice-rich mantle Mars, climate Ices deposits, for which we infer gradual accumulation and depths of order 30–85 m; they mask pre-existing Cratering landforms. The lobate tongues are interpreted as glaciers with mean slope of 10.2 ± 2.3° and average thickness of 33 ± 19 m. Our calculations and data suggest that these glaciers were originally ice-rich and that their surface layers have been depleted by volatile loss. The glaciers probably formed when ice-rich mantle deposits reached critical thickness and flowed downhill. The top 5–10 m of the mantle and glaciers show crater survival times of order a few My to 15 My, which, remarkably, is the time since the last 1–4 episodes of obliquity >45°. Global climate models affirm that Greg lies in one of two non- polar areas with extremes of ice deposition during high-obliquity epochs. This match with observations supports the use of such models in studies of planetary climate change. Ó 2013 Elsevier Inc. All rights reserved. 1. Introduction evolve over much longer periods? How thick is the mantling in this area? Was liquid water involved during the glacial evolution? This paper focuses on a 66-km-diameter crater in the region Here, we analyze the evolution of Greg crater’s ice-related and li- east of Hellas basin. Located at 38.5°S and 113.1°E (246.9°W), it quid water-related landforms. We see this study as more than an contains an unusual concentration of features that we interpret investigation of one martian crater, however. Rather, we attempt as glacial landforms (see Section 2). In 2010, the crater was named to demonstrate how a number of different types of quantitative Greg. It lies in a region several hundred km wide, notable for debris measurement, from crater counts and topographic measurements aprons, which have been long considered suggestive of ice flow to climate models, can be combined in a carefully chosen diagnos- phenomena (Squyres, 1978; Lucchitta, 1981; Crown et al., 1992). tic landscape to unravel larger issues of martian geologic/climatic Independent of geomorphology, analysis of obliquity change evolution. Section 2 summarizes the overall context. Section 3 pre- indicated that insolation and climatic conditions vary with time sents our methodology. Section 4 presents geomorphologic obser- (Laskar et al., 2002, 2004; Touma and Wisdom, 1993). Forget vations and interpretation of crater counts on four main terrain et al. (2006) ran global climate models and found that Greg lies types inside Greg crater. Section 5 summarizes the chronology in one of two martian non-polar locales of highest ice deposition and evolution, and Section 6 discusses the results with regard to rate during periods of high obliquity. Fassett et al. (2010), in study- the climatic evolution of Mars. ing various features of the region, aptly called the area an ‘‘epicen- ter of ice accumulation.’’ Forget et al. (2006) noted that the putative glacial features ap- 2. Background and context pear geologically young, but had no absolute age estimates. Are they formed as recently as the last obliquity cycles, or did they In 2001 or 2002, the then-unnamed crater, Greg (Figs. 1 and 2), was noted independently by two of us (WKH and DCB), using Mars ⇑ Corresponding author. Fax: +1 520 622 8060. Global Surveyor’s Mars Observer Camera images, as hosting an E-mail address: [email protected] (W.K. Hartmann). unusual concentration of lobate tongue-shaped features (Fig. 3) 0019-1035/$ - see front matter Ó 2013 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.icarus.2013.09.016 W.K. Hartmann et al. / Icarus 228 (2014) 96–120 97 Fig. 1. Crater Greg (38.5°S, 113°E) located on the eastern part of Hellas basin. (a) Location on global topographic MOLA map. (b) THEMIS IR day image mosaic of Greg and Reull Vallis region at resolution of 100 m/pixel. (c) THEMIS IR night image mosaic. on its north inner (pole-facing) wall (Hartmann et al., 2002). Later, terrestrial glaciers, and described the tongues as possible glaciers visible-light THEMIS images revealed additional, adjacent lobate or rock glaciers. They emphasized arcuate ridges, such as shown tongues on that same wall. The features, typically 1 Â 4 km, give in Fig. 4a. This is not to say that all features discussed as ‘‘gullies’’ a strong visual impression of downhill flow. Such features are not are associated with glaciations (Dundas et al., 2012), but rather unique, but these are the best-developed and most clustered that the type of glacial formations we see on Greg’s north wall examples. Similar but less well developed features were discussed likely are associated with the type of crater-wall gullies defined by Milliken et al. (2003) and Berman et al. (2009). by Malin and Edgett (2000). Hartmann et al. (2002, 2003) suggested that lobate tongues are Head et al. (2005) documented evidence of glaciation at low lat- a residual, short-lived glacial form in which some of the original ice itudes where glaciation had been considered unlikely under pres- has sublimed, and that km-scale arcuate ridges at the tongue’s base ent conditions. Berman et al. (2005, 2009) surveyed mid-latitude strongly resemble arcuate ridges at the base of martian hillside craters and found strong latitude dependence among the kinds of gullies, suggesting a relationship between lobate tongues and hill- features found in Greg. In their study area east of Hellas, they found side gullies. These ideas were developed further by Hartmann et al. 24 craters that had lobate tongues (23% of those surveyed); Fig. 4a (2003) and Hartmann (2003a,b), which referred to a lobate tongue shows an example. Equatorward of 45°, 100% of the tongues faced as a possible rock glacier. Hartmann et al. (2003) suggested that as the pole (as in Greg, at 38°). Recent Malin–Edgett crater-wall gul- flow proceeds, ‘‘ice is continually brought within a few meters of lies followed a similar pattern (Balme et al., 2006; Berman et al., the porous layers of surface debris, so that the ice sublimes and 2009), supporting the possible connection of glaciers, the mantling the ice content of the formation decreases, with the ice-rich central process, and gullies. These gullies may indeed be features created part of the flow collapsing due to sublimation loss of the ice.’’ That by basal melting beneath the ice-dominated lobate tongues. Ber- paper indicated a young age for glaciers, citing calculations by Tur- man et al. (2005) showed that when arcuate ridges occur at the tle et al. (2001) that such masses of ice or ice/soil might flow on bases of gullied walls, the crater floors tend to slope downhill away timescales as short as 10,000 years. Milliken et al. (2003) also cal- from the gullied wall, indicating substantial mass wasting from culated short timescales for glacial flow on Mars, not more than walls onto the floor (Fig. 4a). Berman et al. (2009) showed that 10 My. The short calculated flow times, however, do not prove that Greg’s floor indeed slopes downhill away from the north wall, as the present glacial bulk interior is very young, as it may result from seen in Fig. 2e. many cycles of advance, retreat, and recharge, reaching some near- A strong morphologic relationship of lobate tongues to ice-rich equilibrium as its initial ice was depleted by sublimation, but re- debris aprons is also now clear. For example, Fig. 4c and d shows a charged by mantling. lobate tongue and remnant arcuate ridges on the pole-facing Hartmann (2005, Fig. 14) gave the first crater counts on the lo- slopes of a mountain surrounded by a prominent debris apron, bate tongues and found young crater retention model ages (sur- roughly 500 km north of Greg. As with Greg, the lobate tongue vival times) of order 10–100 My for the larger craters (D 31– and arcuate ridges are only on the pole-facing slope. Radar data 63 m), but even younger survival times of 3–10 My for smaller cra- showed that debris aprons in this area contain relatively pure ters (D = 11–31 m).