The formation and evolution of youthful gullies on Mars: Gullies as the late-stage phase of Mars’ most recent ice ageIntroductionGlobal distribution of gulliesLatitudeElevationOrientationSlopeLocal slopeGlobal roughnessThe geology of gullies and their environmentLocal geologic contextEpisodic formation of gulliesAssociation between gullies and bedrock layeringRelationship to other young features on MarsAge of gulliesTerrestrial analogsGullies in terrestrial polar environmentsApplications to MarsSynthesisObservations that must be addressed by gully formation/evolution modelsGlobal three-dimensional distributionDetailed geology of gullies and their environmentAge and evolution of gully systemsAn assessment of theories for formation and evolution of gullies on MarsGroundwater expulsion from a confined aquiferAccumulation and melting of surface ice and the regeneration of gulliesAcknowledgmentsReferencesThe formation and evolution of youthful gullies on Mars: Gulliesas the late-stage phase of Mars’ most recent ice ageJames L. Dickson*, James W. HeadDepartment of Geological Sciences, Brown University, Providence, RI 02912, USAarticle infoArticle history:Received 22 January 2009Revised 29 April 2009Accepted 24 June 2009Available online 27 June 2009Keywords:MarsMars, climateMars, surfaceGeological processesabstractGullies are extremely young erosional/depositional systems on Mars that have been carved by an agentthat was likely to have been comprised in part by liquid water [Malin, M.C., Edgett, K.S., 2000. Evidencefor recent groundwater seepage and surface runoff on Mars. Science 288, 2330–2335; McEwen, A.S. et al.,2007. A closer look at water-related geologic activity on Mars. Science 317, 1706–1709]. The strong lat-itude and orientation dependencies that have been documented for gullies require (1) a volatile near thesurface, and (2) that insolation is an important factor for forming gullies. These constraints have led totwo categories of interpretations for the source of the volatiles: (1) liquid water at depth beneath themelting isotherm that erupts suddenly (‘‘groundwater”), and (2) ice at the surface or within the upper-most layer of soil that melts during optimal insolation conditions (‘‘surface/near-surface melting”). In thiscontribution we synthesize global, hemispheric, regional and local studies of gullies across Mars and out-line the criteria that must be met by any successful explanation for the formation of gullies. We furtherdocument trends in both hemispheres that emphasize the importance of top-down melting of recent ice-rich deposits and the cold-trapping of atmospherically-derived H2O frost/snow as important componentsin the formation of gullies. This provides context for the incorporation of high-resolution multi-spectraland hyper-spectral data from the Mars Reconnaissance Orbiter that show that (1) cold-trapping of sea-sonal H2O frost occurs at the alcove/channel-level on contemporary Mars; (2) gullies are episodicallyactive systems; (3) gullies preferentially form in the presence of deposits plausibly interpreted as rem-nants of the Late Amazonian emplacement of ice-rich material; and (4) gully channels frequently ema-nate from the crest of alcoves instead of the base, showing that alcove generation is not necessarily aproduct of undermining and collapse at these locations, a prediction of the groundwater model. We inter-pret these various lines of evidence to mean that the majority of gullies on Mars are explained by the epi-sodic melting of atmospherically emplaced snow/ice under spin-axis/orbital conditions characteristic ofthe last several Myr.Published by Elsevier Inc.1. IntroductionThe Amazonian period of Mars has generally been characterizedas having climatic conditions similar to those observed today:Mars has been a low-temperature, low-pressure, hyperarid envi-ronment dominated by eolian activity since the Hesperian, over 3Gyr ago (Carr, 1996; Golombek et al., 2006; Marchant and Head,2007). While this impression is likely accurate for globally aver-aged conditions over time, the discovery of extremely young gul-lies in middle/high-latitudes provides compelling evidence thatconditions were adequate at certain locations at certain times forshort-duration surficial flow of liquid water within the last severalmillion years (Malin and Edgett, 2000). While other channel-carv-ing agents have been proposed (Musselwhite et al., 2001; Treiman,2003; Shinbrot et al., 2004), liquid water is most consistent withthe resultant morphology of gully landforms (Malin and Edgett,2000; McEwen et al., 2007)(Fig. 1). Dry flows on steep slopes onMars have been observed for decades at all latitudes, but lati-tude-dependent gullies bear little resemblance to these features(Fig. 2; equator-facing slope in Fig. 1a) (Malin and Edgett, 2000).While dry granular flows are capable of producing channels on ex-tremely steep slopes (Shinbrot et al., 2004), the fine-scale braidedchannels, streamlined islands on channel floors, meanders, and ter-races documented by HiRISE data provide morphologies that onEarth are only observed in the presence of liquid water (McEwenet al., 2007; Schon and Head, 2009). The occurrence of gullies onlyat mid- and high-latitudes (Malin and Edgett, 2000; Milliken et al.,2003) and on slopes below the angle of repose (Heldmann andMellon, 2004; Dickson et al., 2007a; Heldmann et al., 2007) furtherimplicate liquid water as a channel-carving agent.Malin and Edgett (2000) noted and documented that (1) aver-age conditions are below the triple-point where most gullies occur,(2) gullies are found mostly on cold, poleward-facing slopes, (3)0019-1035/$ - see front matter Published by Elsevier Inc.doi:10.1016/j.icarus.2009.06.018* Corresponding author.E-mail address: [email protected] (J.L. Dickson).Icarus 204 (2009) 63–86Contents lists available at ScienceDirectIcarusjournal homepage: www.elsevier.com/locate/icarusgullies are broadly clustered in certain regions (Newton crater, DaoVallis, etc.), and (4) many gullies appear to emerge from distinctlayers of bedrock. These properties were used to argue that gulliesmost likely formed from the sudden outburst of water from a shal-low aquifer a few hundred meters beneath the surface, eruptingexplosively along crater/valley walls, entraining debris while carv-ing sinuous channels and producing depositional aprons, and ceas-ing when water within the system is exhausted. The polewardorientation of most