Post by Mark Bishop.
Cone fields of the Tartarus Colles of Mars (~190° W, 26° N) comprise part of the volcanic province of the Elysium Rise. They lie amongst the mesas, ridges, small knobs and hills from which the region derives its name. Considerable interest exists in regard to cone location and origin, as their occurrence may be associated with recent volcanism, as well as the occurrence of near surface ground ice; the presence of which has consequences for past and present climate, astrobiological activity, and future exploration.
Image 1 from the narrow angle Mars Orbiter Camera (MOC) shows a suspected ‘meltwater’ pathway within the cone field [Area #1 inset (A)]. A broad (350-450 m wide) and shallow (10-20 m deep) channel of smooth terrain has partitioned the cone group. Intra-channel cones show a partial outline and erosion mostly of their northern flanks. Area #2 [inset (B)] shows the edge of a mesa and a surrounding ‘moat’ of smooth terrain between it and the cone field; a possible relic lobate debris apron (see for example, the PGWG June featured image; Hauber et al., 2008). Area #3 [inset (C)] identifies larger ‘outlier’ cones that are separated from the main cone group by smooth terrain. Smooth terrain surrounds each cone, although the annular form of this is best seen in Image 2.
Image 2 shows the prevalence of smooth terrain amongst this cone group, but a lesser association with meltwater-like channels compared with Area #1. Area #4 [inset (A)] shows cones that have annuli of smooth terrain and a relatively featureless zone within these perimeters. The crater floors also appear similarly flat, smooth and featureless at this higher spatial resolution; an attribute which may assist the eventual determination of cone origin. Area #5 [inset (B)] shows the conjoining, contiguous nature of the cones. This is a common feature across all fields, but is especially obvious here in the southern-most sectors of this image and for Area #1.
Collectively, these geomorphic features and the distribution of the so-called smooth terrain, attest to the interaction of periglacial-volcanic, or solely glacial-periglacial conditions within the Tartarus Colles, at a time of high axial obliquity, and maybe as recently as during the last 1 Ma (Mellon and Jakosky, 1995; Head et al., 2003). Such environments may have contributed to regolith instability from which episodic meltwater erosion and mass movements have resulted. These processes could originate from either the thawing of frozen ground by thermal activity related to the neighboring volcanic province of Elysium Mons, or in the absence of coincident volcanism and a frozen regolith, the recession of periglacial conditions owing to seasonal contrasts or a waning obliquity. In due course, the region has been blanketed by an aeolian mantle that has further masked and subdued or ‘softened’ the terrain. In both scenarios the thawing of frozen ground is a plausible mechanism from which meltwater pathways (the suspected broad, shallow channels and smooth terrain) have been generated by overland (sheet) flow, and that are now seen as ‘dust’ mantled swaths of featureless denuded terrain, devoid of cones. Although cone origin is ambiguous at this time, the best candidates for such an environmental setting are rootless cones (Greeley and Fagents, 2001; Lanagan, 2001) and/or pingos (Page and Murray, 2006; Page, 2007).
Bishop, M.A. 2008. Higher-order neighbor analysis of Tartarus Colles cone groups, Mars: The application of geographical indices to the understanding of cone pattern evolution. Icarus 197, 73-83. doi:10.1016/j.icarus.2008.04.003 [Abstract]
Greeley, R., Fagents, S.A., 2001. Icelandic pseudocraters as analogs to some volcanic cones on Mars. Journal of Geophysical Research 106, E9, 20527-20546 [Abstract]
Hauber, E., S. van Gasselt, M. Chapman, and G. Neukum, 2008. Geomorphic evidence for former lobate debris aprons at low latitudes on Mars: Indicators of the Martian paleoclimate, Journal of Geophysical Research 113, E02007, doi:10.1029/2007JE002897. [Abstract]
Head, J.W., Mustard, J.F., Kreslavsky, M.A., Milliken, R.E., Marchant, D.R., 2003. Recent ice ages on Mars. Nature 426, 797-802. [Abstract]
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Mellon, M.T., Jakosky, B.M., 1995. The distribution and behaviour of Martian ground ice during past and present epochs. Journal of Geophysical Research 100, E6, 11,781-11.799. [Abstract]
Page, D.P., 2007. Recent low-latitude freeze-thaw on Mars. Icarus 189, doi:10.1016/j.icarus.2007.01.005. [Abstract]
Page, D.P., Murray, J.B., 2006. Stratigraphical and morphological evidence for pingo genesis in the Cerberus plains. Icarus 183, 46-54. [Abstract]