Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | The Cryosphere Discuss., 8, 2957–2994, 2014 www.the-cryosphere-discuss.net/8/2957/2014/ doi:10.5194/tcd-8-2957-2014 TCD © Author(s) 2014. CC Attribution 3.0 License. 8, 2957–2994, 2014 This discussion paper is/has been under review for the journal The Cryosphere (TC). Glacier-like forms on Please refer to the corresponding final paper in TC if available. Mars Glacier-like forms on Mars B. Hubbard et al. B. Hubbard, C. Souness, and S. Brough Title Page Department of Geography and Earth Sciences, Aberystwyth University, Aberystwyth, UK Abstract Introduction Received: 7 May 2014 – Accepted: 16 May 2014 – Published: 5 June 2014 Conclusions References Correspondence to: B. Hubbard ([email protected]) Tables Figures Published by Copernicus Publications on behalf of the European Geosciences Union. J I J I Back Close Full Screen / Esc Printer-friendly Version Interactive Discussion 2957 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Abstract TCD Over 1300 glacier-like forms (GLFs) are located in Mars’ mid-latitudes. These GLFs are visually similar to terrestrial valley glaciers, being predominantly composed of ice-dust 8, 2957–2994, 2014 mixtures and showing signs of downhill viscous deformation and of an expanded former 5 extent. However, several fundamental aspects of their behaviour are virtually unknown, Glacier-like forms on including temporal and spatial variations in mass balance, ice motion, landscape ero- Mars sion and deposition, and hydrology. Here, we investigate the physical glaciology of martian GLFs. We use satellite-based images of specific examples and case studies B. Hubbard et al. to build on existing knowledge relating to: (i) GLF current and former extent, exempli- 10 fied via a GLF located in Phlegra Montes; (ii) indicators of GLF motion, focusing on the presence of surface crevasses on several GLFs; (iii) processes of GLF debris transfer, Title Page focusing on mapping and interpreting boulder trains on one GLF located in Protonilus Abstract Introduction Mensae, the analysis of which suggests a minimum GLF flow speed of 7.5 mm a−1, and (iv) GLF hydrology, focusing on possible supraglacial gulley networks on several Conclusions References 15 GLFs. On the basis of this information we summarise the current state of knowledge of Tables Figures the glaciology of martian GLFs and identify future research avenues. J I 1 Introduction J I Numerous studies have remarked upon and investigated the similarities between ice- Back Close rich landforms on Mars and Earth (e.g., Colaprete and Jakosky, 1998; Marchant and Full Screen / Esc 20 Head, 2003; Forget et al., 2006). Glacier-like forms (GLFs), which comprise one partic- ular sub-group of these features, are strikingly similar in planform appearance to terres- Printer-friendly Version trial valley glaciers (Fig. 1). However, despite this similarity, the fundamental glaciology of martian GLFs remains largely unknown. Improving this knowledge would enhance Interactive Discussion our understanding both of the specific landforms concerned and of broader planetary 25 issues such as (i) how Mars’ present-day landscape was formed, (ii) the presence and phase state of H2O on Mars’ surface, and (iii) how Mars’ climate has changed in geo- 2958 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | logically recent times. The aim of this paper is to summarize and develop our under- standing of the fundamental physical glaciology of Mars’ GLFs. As well as summarising TCD existing knowledge, we provide new observations and interpretations of glacial land- 8, 2957–2994, 2014 forms on Mars and outline potential avenues for future research. 5 1.1 Background Glacier-like forms on Mars 1.1.1 GLF classification, location and form B. Hubbard et al. Mars’ mid-latitude regions, between ∼ 20◦ and ∼ 60◦ N and S, host numerous land- forms and surface deposits that bear a striking resemblance to small-scale terrestrial ice masses (e.g., Souness et al., 2012). These landforms, being composed predom- Title Page 10 inantly of H2O ice (Holt et al., 2008; Plaut et al., 2009) and exhibiting surface mor- phologies consistent with viscous flow (Marchant and Head, 2003; Head et al., 2010), Abstract Introduction have come to be known collectively as viscous flow features or VFFs (Milliken et al., Conclusions References 2003; Souness and Hubbard, 2012). Glacier-like forms, or GLFs, are a distinctive sub- Tables Figures type of VFF that are elongate and similar in appearance and overall morphology to 15 terrestrial valley glaciers. GLFs thereby generally form in small cirque-like alcoves or valleys, appear to flow downslope between bounding sidewalls, and terminate in a dis- J I tinctive tongue which may or may not feed into a higher order ice-rich terrain type. GLFs J I thereby represent the lowest-order component of what Head et al. (2010) referred to Back Close as Mars’ integrated glacial landsystem. According to this model, GLFs flow and may 20 merge downslope to form broad, rampart-like lobate debris aprons (LDAs) (Squyres, Full Screen / Esc 1978, 1979). LDAs may, in turn, coalesce, typically from opposing valley walls, to form lineated valley fills (LVFs), which take the form of complex and contorted surfaces that Printer-friendly Version often exhibit no obvious flow direction. Interactive Discussion In their inventory of Mars’ GLFs, Souness et al. (2012) inspected > 8000 images of 25 the martian surface and identified 1309 individual forms, reporting the location (Fig. 2) and basic morphometry of each. Hereafter, we refer to specific GLFs through their clas- sification number in this inventory, available as a supplement accompanying Souness 2959 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | et al. (2012). Of the total population, 727 GLFs (56 %) were found in the Northern Hemisphere and 582 (44 %) in the Southern Hemisphere, with GLFs showing a pref- TCD erence for the mid-latitudes (centred on a mean latitude of 39.3◦ in the north and 8, 2957–2994, 2014 −40.7◦ in the south). The spatial distribution of GLFs showed high clustering in sev- 5 eral locations of both the northern and Southern Hemisphere, for example along the so-called “fretted terrains” (Sharp, 1973) of Deuteronilus Mensae, Protonilus Mensae Glacier-like forms on and Nili Fossae in the north and around the Hellas Planitia impact crater in the south Mars (Fig. 2). GLF morphometry was found to be remarkably similar between the two hemi- spheres, with a mean GLF length of 4.91 km in the north and 4.35 km in the south, B. Hubbard et al. 10 and a mean GLF width of 1.26 km in the north and 1.34 km in the south. Similar to on Earth, a pronounced preference for a poleward orientation was also found, with GLFs Title Page having a mean bearing of 26.6◦ (NNE) in the Northern Hemisphere and 173.1◦ (SSE) in the Southern Hemisphere – indicating a strong sensitivity to insolation. These inter- Abstract Introduction hemispheric similarities in distribution and morphometry indicate that all martian GLFs Conclusions References 15 share a high degree of commonality in terms of composition and formation. These are considered below. Tables Figures 1.1.2 GLF composition J I The precise composition of GLFs is still unknown due to the fact that they are almost J I ubiquitously covered in a layer of dust-rich regolith. Hubbard et al. (2011) noted that Back Close 20 boulder incisions into the unconsolidated surface of GLF #948 located in the north wall of Crater Greg, Eastern Hellas, were some decimetres deep, representing a minimum Full Screen / Esc surface dust thickness at this location. There have been very few direct observations of the interior of GLFs, but Dundas and Byrne (2010) reported the fortuitous capture Printer-friendly Version of very recent meteorite strikes that indicated the presence of relatively clean (i.e., Interactive Discussion 25 debris poor) massive ice at a depth of some centimetres to metres below the surface. Furthermore, data from the shallow subsurface radar (SHARAD) sensor, mounted on the Mars Reconnaissance Orbiter (MRO), suggest that many VFFs (including GLFs) may well be composed of massive H2O ice with minimal lithic content (Holt et al., 2960 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | 2008; Plaut et al., 2009). These findings led to the widespread acceptance that H2O ice accounts for the dominant portion of GLF mass. However, the presence of a lithic TCD component has been demonstrated by ice fade through sublimation following recent 8, 2957–2994, 2014 impact exposures (Dundas and Byrne, 2010), and the precise proportions of ice-rock 5 mixture, particularly at depth, are still unknown. Glacier-like forms on 1.1.3 GLF formation Mars A continuing point of discussion relates to precisely how and when GLFs formed. It B. Hubbard et al. is generally agreed that GLFs are now largely relict forms dating to a past, but rela- tively recent, martian ice age (see Kargel, 2004). This late-Amazonian ice age, likely 10 extending from ∼ 2 Ma BP to younger than 0.5 Ma BP (Head et al., 2003), is thought to Title Page have been initiated by increased planetary obliquity which led to the melting of Mars’ Abstract Introduction polar caps, the release of moisture into the atmosphere and its precipitation as snow or condensation above or within the ground at lower latitudes (e.g., Forget et al., 2006; Conclusions References Hudson et al., 2009; Schon et al., 2009). This ice deposition extends well into Mars’ Tables Figures 15 mid-latitudes, where it appears to have survived, preserved beneath surface regolith, until the present day. Still, the mechanisms by which GLFs first accumulated sufficient J I ice-rich mass to flow downslope and acquire their distinctive surface morphologies re- main uncertain.