Edinburgh Research Explorer Rootless cones on Mars indicating the presence of shallow equatorial ground ice in recent times Citation for published version: Lanagan, PD, McEwen, AS, Keszthelyi, LP & Thordarson, T 2001, 'Rootless cones on Mars indicating the presence of shallow equatorial ground ice in recent times', Geophysical Research Letters, vol. 28, no. 12, pp. 2365-2367. https://doi.org/10.1029/2001GL012932 Digital Object Identifier (DOI): 10.1029/2001GL012932 Link: Link to publication record in Edinburgh Research Explorer Document Version: Publisher's PDF, also known as Version of record Published In: Geophysical Research Letters Publisher Rights Statement: Published in Geophysical Research Letters by the American Geophysical Union (2001) General rights Copyright for the publications made accessible via the Edinburgh Research Explorer is retained by the author(s) and / or other copyright owners and it is a condition of accessing these publications that users recognise and abide by the legal requirements associated with these rights. Take down policy The University of Edinburgh has made every reasonable effort to ensure that Edinburgh Research Explorer content complies with UK legislation. If you believe that the public display of this file breaches copyright please contact [email protected] providing details, and we will remove access to the work immediately and investigate your claim. Download date: 01. Oct. 2021 GEOPHYSICAL RESEARCHLETTERS, VOL. 28, NO. 12, PAGES2365-2367, JUNE 15,2001 Rootless cones on Mars indicating the presence of shallow equatorial ground ice in recent times Peter D. Lanagan, Alfred S. McEwen, Laszlo P. Kesztheiyi Lunar and Planetary Laboratory, University of Arizona, Tucson, Arizona Thorvaldur Thordarson Department of Geology and Geophysics, University of Hawaii at Manoa, Honolulu, Hawaii Abstract. High resolutionMars Orbiter Camera (MOC) Observations of Rootless Cones on Mars images have revealed the existence of clusters of small cones and Earth in the Cerberus plains, Matte Valles, and Amazonis Plani- tia, Mars. These conesare similar in both morphology and The conesseen in MOC images range in size from 20-m to planar dimensions to the larger of Icelandic rootless cones, 300-m in basal diameter and have large summit craters with which form due to explosive interactions between surficial diametersabout half as wide as the bases(Fig. 2a). These lavas and near-surface groundwater. Impact crater size- dimensions are consistent with the more explosive of the frequency relationships indicate that surfaces upon which Icelandic rootless cones as less explosive cones have smaller the cones sit are no older than 10 Ma. If martian cones form and narrower summit craters. The martian cones are found in the same manner as terrestrial rootless cones, then equa- in clusters ranging from a few to many hundreds of cones torial ground ice or ground water must have been present (Fig. 2a). Suchclustering is typical of terrestrial examples near the surface in geologically recent times. (Fig. 2b). There are no strong alignmentsof conesthat would suggest a fissure vent beneath the lavas within these Introduction cone fields. Also, lavas do not issue from these cones, while lavas often issue from cinder cones. Like terrestrial rootless High resolutionMars Orbiter Camera (MOC) images cones, many of the martian cones contain nested craters. In have revealed clusters of small cones in the Cerberus plains, many cases,these cones sit on distinctive platy-ridged lavas Matte Valles, and AmazonisPlanitia (Fig. 1). The mor- similarto someflows in Iceland [Keszthelyiet al., 2000]. phologies and geologic settings of the martian cones are Regionally, clusters of cones are noted near the mouth consistent with those of rootless cones, constructional fea- of Matte Valles, in southern Amazonis Planitia, and in the tures resulting from a seriesof phreatomagmatic explosions southwestern parts of the Cerberus plains. The tops of ap- following the emplacement of a lava flow over a water-rich parent cones, which are embayed by lava, are seen in addi- surface[Thorarinsson, 1953]. Their formationinvolves me- tional locations. In many cases,cones are proximal to local chanical mixtures of lava with water- or ice-laden substrate fluvial features such as erosionalbanks, longitudinal grooves, in inflated lava flow fields [Thordarson,2000]. They are and scoured surfaces. Many hundreds of cones are near or called root!esscones, or pseudocraters, becausethey do not eroding out from beneath the Medusae Fossae Formation overlie a volcanic vent. (MFF). Although the origin of the MFF is under debate, Other structures interpreted to be rootless cones have these conessuggest it may have formed from tephra created been identifiedin Viking imageryof ChrysePlanitia [Gree- by interaction between lavas and surface volatiles. ley and Theilig,1978], Deuteronilus Mensae [Lucchita, 1978], Icelandic rootless cone fields form in regions where in- Acidalia Planitia [Allen, 1980; Frey et al., 1979] and Isidis flated lava flowsare emplacedover marshyterrains [Thor- Plani•ia [Frey and Jarosewich,1982]. Unlike the conesdis- darsonet al., 1992; Thordarson,2000]. The Icelandicroot- cussed in this paper, most of these cones measure 600-m less conestypically exhibit well-bedded basal unit of glassy across the base, or double the size of the largest terres- (quenched)ash to lapilli scoriawhich gradesupward into a trial rootlesscones [Chapman et al., 2000], and it is un- crudely-bedded sequenceof spatter-rich agglutinates, indi- clear if the Acidalia and Isidis cones rest on volcanic sur- cating that they are formed by sustained eruptions involving faces. Structures interpreted to be cones similar in size to disintegration of molten lava. Furthermore, Icelandic root- terrestrial rootless cones are observed in Viking images of lesscones have not undergone any post-formational shearing Elysium Planitia [Mouginis-Mark,1985]. However, those which demonstrates that they were deposited onto a station- smaller cones were observed at the limit of Viking image ary upper crust of the lava. These observations strongly sug- resolution, so their true characteristics are uncertain. The gest that the Icelandic cone groups are formed by rootless structures observed by MOC are the first clearly identified eruptions where the lava is fed through internal pathways martian cones having dimensions, morphologies, and geo- or lava tubes to the site of explosive interaction. Thordar- logic settings similar to terrestrial rootless cones. son [2000] also noted that rootlesscone groupshave only been found in association with inflated lava flows where lava tubes act as lateral feeders to the explosion site, and are Copyright2001 by the AmericanGeophysical Union. constrained to marshy regions where water-saturated sedi- Papernumber 2001GL012932. ments and lava may intimately mix. By analogy, rootless 0094-8276/01/2001GL012932505.00 cones on Mars may require sufhcient shallow ground ice to 2365 2366 LANAGAN ET AL.: MARTIAN ROOTLESS CONES 30 ............................. rains. 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