JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 117, E04003, doi:10.1029/2011JE004005, 2012 A chronology of early Mars climatic evolution from impact crater degradation N. Mangold,1 S. Adeli,1,2 S. Conway,1 V. Ansan,1 and B. Langlais1 Received 13 October 2011; revised 9 January 2012; accepted 18 February 2012; published 12 April 2012. [1] The degradation of impact craters provides a powerful tool to analyze surface processes in the Martian past. Previous studies concluded that large impact craters (20–200 km in diameter) were strongly degraded by fluvial erosion during early Martian history. Our goal is to study the progression of crater degradation through time with a particular emphasis on the craters with alluvial fans and on the relative chronology of these craters. The geometric properties of 283 craters of >20 km in diameter were analyzed in two highlands of Mars, north of Hellas Planitia, and south of Margaritifer Terra, both known to contain craters with alluvial fans. Three classes were defined from morphology: strongly degraded craters with fluvial landforms and without ejecta (type I), gently degraded craters with fluvial landforms and preserved ejecta (type II), and fresh craters with ejecta and no fluvial landforms (type III). Our main result is that the type II craters that present alluvial fans have characteristics closer to fresh craters (type III) than degraded craters (type I). The distinctive degradation characteristics of these classes allowed us to determine a temporal distribution: Type I craters were formed and degraded between 4 Gyr and 3.7 Gyr and type II craters with alluvial fans were formed between Early Hesperian and Early Amazonian (3.7 to 3.3 Gyr). This chronology is corroborated by crosscutting relationships of individual type II craters, which postdate Late Noachian valley networks. The sharp transition at 3.7 Gyr suggests a quick change in climatic conditions that could correspond to the cessation of the dynamo. Citation: Mangold, N., S. Adeli, S. Conway, V. Ansan, and B. Langlais (2012), A chronology of early Mars climatic evolution from impact crater degradation, J. Geophys. Res., 117, E04003, doi:10.1029/2011JE004005. 1. Introduction consistent with the observed topography. Degraded craters are therefore one of the main lines of evidence for a warmer [2] Impact crater degradation provides a powerful tool to climate on early Mars. Other lines of evidence include, the analyze past Martian climate. Previous studies concluded extensive identification of phyllosilicates in the Noachian that large impact craters were strongly degraded during early crust [e.g., Poulet et al., 2005; Bibring et al., 2006; Loizeau Martian history (<3 Gyr), whereas younger craters are only et al., 2007; Mangold et al., 2007; Mustard et al., 2009; weakly degraded [Craddock and Maxwell, 1990; Craddock Dehouck et al., 2010] and fluvial valley networks [e.g., et al., 1997]. Based on Viking data, this ancient degra- Carr, 1996; Craddock and Howard,2002;Howard et al., dation was attributed to fluvial erosion, because no other 2005; Irwin et al., 2005; Ansan and Mangold, 2006; Fassett process (i.e., eolian or glacial activity, volcanism) could and Head, 2008, 2011]. adequately reproduce the topographic profiles of degraded [3] Initial studies of crater degradation were conducted craters [Craddock et al., 1997]. Specifically, craters with no using Viking images morphologic interpretation and topog- or low rims must have been modified by erosion, and not by raphy from photoclinometric profiles, i.e., using radiometric an aggradational process such as volcanic or aeolian filling variation in Viking images [Craddock et al., 1997]. Global [Craddock and Maxwell, 1990; Craddock and Maxwell, altimetry and recent high-resolution imagery enable us to 1993]. Forsberg-Taylor et al. [2004] performed numerical revisit this work with much better data sets. These data simulations of crater degradation by fluvial and eolian pro- allowed us to pick out fine details in the morphology of cess and they also concluded that fluvial erosion was most degraded craters. In particular it allowed us to identify pre- served impact ejecta, which is strong evidence for limited degradation, and fluvial landforms on rims. These details 1Laboratoire Planétologie et Géodynamique de Nantes, LPGN/CNRS were particularly pertinent in the case of the craters with UMR6112 and Université de Nantes, Nantes, France. 2Now at DLR Institute of Planetary Research, Berlin, Germany. alluvial fans [Moore and Howard, 2005]. Indeed, post- Viking imagery showed the presence of alluvial fans in Copyright 2012 by the American Geophysical Union. some of these ancient craters, which are clear signatures of 0148-0227/12/2011JE004005 enhanced fluvial erosion and deposition in craters without E04003 1of22 E04003 MANGOLD ET AL.: MARS IMPACT CRATER DEGRADATION E04003 postdate the Hellas basin, because they are all superposed on its northern rim. [7] The SMT study site is composed of Noachian high- lands with local intercrater plains such as in Ladon basin. This region is less homogeneous than NHP in geological context, with tectonic features related to the SE margin of Tharsis, chaotic terrains and outflow channels. Well- developed valley networks are present mainly to the east of the area, e.g., Loire Vallis and Samara Vallis, but valleys also exist in the western area, sometimes with depositional fans, for example, in and around Holden and Eberswalde craters [Grant et al., 2008; Malin and Edgett, 2003; Pondrelli Figure 1. MOLA topography superimposed on MOLA hill et al., 2008]. The SMT region has a few craters which contain shade with our study regions marked: box A is southern large polygonal blocks, resembling chaos terrain, which Margaritifer Terra (SMT) and box B is north Hellas Planitia probably correspond to degradation related to subsurface pro- (NHP), and dotted gray boxes are those areas studied by cesses rather than climatic ones [e.g., Chapman and Tanaka, Moore and Howard [2005]. 2002, Rodriguez et al., 2005, Meresse et al., 2008]. Never- theless, ancient terrains predominate the area, which offers a good statistical set of >20 km diameter craters. standing bodies of water [Moore and Howard, 2005, Kraal 2.2. Data Sets and Methods et al., 2008]. Fans in these craters were interpreted as [8] The High Resolution Stereo Camera (HRSC) instru- being due to a climatic optimum, at the Noachian-Hesperian ment acquires images in five panchromatic channels under transition. As a consequence, we chose to focus our study on different observation angles, as well as four color channels at the craters with alluvial fans; their degradation stage, the a relatively high spatial resolution [Neukum and Jaumann, type of fluvial erosion, and on their chronology relative to 2004]. In this work we used only panchromatic nadir ima- other climatic markers such as the global valley networks. ges, with a maximum spatial scale from 10 to 40 m/pixel. [4] After having first presented the data sets, methods and CTX images (Context Camera [Malin et al., 2007]) were regions studied, we present a qualitative classification of the used to determine the detailed morphology of features within morphology of the impact craters studied using simple the two study areas, especially identification of ejecta and parameters such as the presence of fluvial erosion and of fluvial erosion. A few High Resolution Imaging System impact ejecta. Then, a quantitative analysis of these crater (HiRISE) images at 25 cm/pixel were used to focus on the classes is presented to link the morphometric degradation fine detail of some impact craters [McEwen et al., 2007]. with the geometric modification and tie it in to the chro- Thermal Emission Imaging System (THEMIS) images nology of Mars. Last, the results are discussed in broader ’ [Christensen et al., 2003] with a resolution of 100 m/pixel context with particular focus on Mars climatic evolution. were used to complement visible imagery, especially when the thermal properties display variations consistent with 2. Approach and Methods distinct geomorphic landforms, such as ejecta blankets. 2.1. Study Regions [9] Mars Orbiter Laser Altimeter (MOLA) gridded data at 463 m/pixel [Smith et al., 1999] were used to extract [5] Our study focused on two large areas of the Noachian topographic information for the surveyed craters. While this highlands where alluvial fans in craters were found by data set contains artifacts at full resolution, the use of craters Moore and Howard [2005]: Northern Hellas Planitia (NHP) >20 km limits these effects. To avoid errors caused by the at À17StoÀ30S latitude and 51Eto85E longitude distortion of a regional map projection, each crater was and Southern Margaritifer Terra (SMT) at 13Sto28S projected into a local sinusoidal projection, with the central latitude and 320W to 350W longitude (Figure 1). These meridian of the projection being the same as the longitude of two regions are slightly larger than those initially studied by the crater’s center. The rim of each crater was digitized as a Moore and Howard [2005], because we found more craters circle using the HRSC and CTX images and a slope map with alluvial fans in these surrounding areas, some of them derived from MOLA data. The crater’s center point and having been noted by Kraal et al. [2008] as well. The crater radius were estimated from these circles. The distance and properties were extracted for a total of 283 impact craters of angular displacement from the center point was calculated >20 km in diameter in the two studied areas, a number for every pixel up to 1.5 crater radii from the center point. providing better confidence in the statistics. Each crater was divided into eight 10 wide segments, facing [6] The NHP study site consists almost exclusively of the cardinal directions, north, NE, east, etc. (Figure 2). The Noachian highlands terrain including extensive valley net- width of the segments, was therefore always >3.4 km at the works, both SE of Huygens crater [Ansan et al., 2008] and in rim, thus includes both interpolated and noninterpolated the southern Tyrrhena region [Mest et al., 2010].