Impact-induced overland fluid flow and channelized erosion at Lyot Crater, Mars

Authors


Abstract

[1] Lyot Crater is one of the youngest impact basins > 200 km in diameter on Mars. Although published hydrological models suggest that impact-related groundwater release might have occurred at Lyot, no geomorphic evidence for such activity has been previously identified. Here, we use images acquired predominantly by the Mars Reconnaissance Orbiter (MRO) Context Camera (CTX) and Mars Odyssey Thermal Emission Imaging Spectrometer (THEMIS) visible and infrared subsystems to document an extensive channeled scabland extending to the north, west, and east of Lyot, covering an area of ∼300,000 km2. The configuration and morphology suggests that the channel-forming fluid was water derived from the target substrate and/or the surrounding terrain. Possible formation mechanisms are groundwater mobilization by seismic energy from the impact event and dewatering of the ejecta blanket.

1. Setting and Previous Research

[2] Lyot Crater, a 215 km-diameter impact basin located at 50.7°N, 330.8°W (Figure 1a), is considered to be one of the youngest large impact basins on Mars based upon (1) the presence of hummocky floor deposits, continuous lobate ejecta deposits, and a well-preserved secondary crater field [Hawke and Mouginis-Mark, 1980; Mouginis-Mark et al., 1981], (2) its superposition of terrain considered to be Early Amazonian in age [Tanaka, 1986; Greeley and Guest, 1987; Tanaka et al., 2005], and (3) crater counts of the basin and its ejecta (absolute age estimates of ∼1.6 Ga, ∼3.3 Ga, [Dickson et al., 2009] and ∼3.4 Ga [Werner, 2008] in the Hartmann [2005], Neukum [Ivanov, 2001], and Hartmann and Neukum [2001] isochron systems, respectively).

Figure 1.

(a) Lyot Crater and surrounding terrain (MOLA gridded topography (128 px/deg) over MOLA shaded relief map). The Lyot primary ejecta blanket is outlined in white. Thick black lines denote channels and areas of channel scour, while dotted lines delineate candidate areas of channel scour (mapped using MRO CTX and THEMIS images). Yellow boxes mark the locations of Figures 1b–1d and 2a2b. Contour lines denote 250 m intervals. Projection is transverse mercator with center longitude of 330°W. (b) Braided channels north/northwest of Lyot. Mosaic of CTX images B19_016959_2370_XN_57N333W, B20_017315_2369_XI_56N334W, B22_018383_2364_XI_56N334W, and G01_018449_2364_XI_56N333W (centered near 57.48°N, 334.15°W). (c) Detail of channels indicated by the white box in Figure 1b. Subframe of CTX image G01_018449_2364_XI_56N333W. (d) CTX image B20_017565_2362_XI_56N319W showing an area of channel scour with superposed pedestalized craters northeast of Lyot (centered near 58.35°N, 319.90°W).

[3] Owing to its relatively young age and low elevation (−7.03 km below mean areoid [Smith et al., 2001]), Russell and Head [2002] considered Lyot a key place to look for evidence of impact-induced groundwater release. They examined images acquired by the Viking orbiters and the Mars Global Surveyor (MGS) Mars Orbiter Camera (MOC) in conjunction with MGS Mars Orbiter Laser Altimeter (MOLA) data, focusing upon the basin interior and the ejecta blanket immediately outside the crater rim. No evidence for groundwater release within or overtopping the rim of Lyot was found.

2. Observations

[4] CTX image B18_016748_2352_XN_55N332W, acquired on 25 February 2010, shows a large braided channel near the edge of the Lyot ejecta blanket. This observation prompted an examination of all images acquired by the Viking orbiters, MOC, THEMIS, Mars Express High Resolution Stereo Camera (HRSC), and CTX, as well as MOLA topographic data covering the region from 40–70°N and 295–10°W in search of similar features. Unfortunately, much of these existing data were unusable due to annual storm activity in the northern mid- to high-latitudes that persists throughout much of the martian year. Therefore, during northern spring/summer of 2010, we began a systematic CTX mapping campaign for the region surrounding Lyot in an attempt to cover as much terrain as possible during the short clear-atmosphere window between Ls 85–145°.

[5] Analysis of CTX and THEMIS VIS and IR images shows an extensive channeled scabland (i.e., morphologies similar to the Channeled Scabland of the Columbia River Plateau, Washington [Bretz et al., 1956]) on the plains surrounding Lyot (Figure 1a). The region consists of channels displaying braided reaches (Figures 1b1d) and large areas of scour without well-defined channels. Individual channel widths range from ∼0.1–1 km, while some collections of channels and areas of scour span > 30 km. Channel gradients are relatively shallow (∼0.07°) and depths are typically on the order of meters based on MOLA gridded topographic data. In some locations, channels cut across craters and their ejecta, while elsewhere they divert around pre-existing topography (Figures 1b1d). No distinct distal depositional facies are observed. Small segments of three of these channels were mapped (using primarily the gridded 128 pixels/degree MOLA digital elevation model (DEM)) but not discussed by Tanaka et al. [2005].

[6] The channels cover an area of ∼300,000 km2 and extend > 300 km beyond the ejecta margins into the surrounding plains to the north, west, and east of Lyot, following the local topography and regional slope; similar features are not observed to the south among the Deuteronilus Mensae (Figure 1a). This is consistent with the observed distribution of ejecta and secondaries, which extend predominantly to the north. No obvious secondary craters are observed to the south and southwest of Lyot beyond the immediate vicinity of the crater. Southeast of the basin, we have identified secondary craters between the mesas of Deuteronilus Mensae and upon the cratered highlands beyond the mesas; no secondary craters occur atop the aprons associated with the mesas or the lineated valley fill material. Lyot ejecta to the southeast appears to be partially buried by the lineated valley fill material (also noted by Russell and Head [2002]). These relationships imply that the Lyot impact event post-dates the formation of the fretted terrain but pre-dates the emplacement of the aprons and valley fill material (interpreted to be ice-rich by Plaut et al. [2009, 2010]).

3. Interpretations and Discussion

[7] The channels associated with Lyot appear to be the result of fluvial erosion, most likely by liquid water (which includes the possibility of brines, acids, etc.). The assumption that the liquid was water, as opposed to some other fluid, is based on the preponderance of evidence that various forms of H2O are present on Mars today in its atmosphere [Spinrad et al., 1963; Barker et al., 1970; Farmer et al., 1977; Jakosky and Farmer, 1982], seasonal frost [Pimentel et al., 1974], polar caps [Kieffer et al., 1976; Titus et al., 2003], middle and high latitude regolith [Boynton et al., 2002; Smith et al., 2009; Holt et al., 2008; Plaut et al., 2009; Byrne et al., 2009], and minerals [Gendrin et al., 2005; Mustard et al., 2008]. The release of liquid water during impact events from either groundwater or melting of ground ice has been invoked to explain observed crater morphologies on Mars [e.g., Head and Roth, 1976; Mouginis-Mark, 1979]. Impact modeling efforts also predict the release of water for impacts on Mars [e.g., Segura et al., 2002; Stewart and Ahrens, 2003; Stewart et al., 2004; Wang et al., 2005].

[8] Based on the geographic location of the channels relative to Lyot and their braided morphology, we propose that the observed channels were carved by sediment-laden water derived from the target substrate and/or the surrounding terrain via either mobilization of shallow groundwater by seismic energy from the impact event or dewatering of the ejecta blanket. It is unlikely that the channels pre-date the Lyot impact, with their source buried beneath the ejecta blanket, based on the close geographic relationship to Lyot and the absence of morphologically similar channels in the northern plains beyond the immediate vicinity of Lyot. The channels are also unlikely to be related to the valleys and fretted terrain to the south of Lyot as CTX has covered > 90% of the fretted terrain of Mars and we have not observed channels such as those at Lyot in conjunction with fretted terrain elsewhere.

[9] Earthquakes can seismically trigger the flow of shallow groundwater in two main ways: ejection of confined groundwater and/or upwelling of an unconfined shallow water table. In the former case, the water is confined by an impermeable layer (such as permafrost or settling and compaction caused by the quake) and is typically released by jetting or spouting, resulting in fissures and/or mounds referred to as sand or mud volcanoes [e.g., Waller, 1968]. The latter case produces widespread non-eruptive water-sediment flows due to a rising water table and soil liquefaction [e.g., Waller, 1968]. Soil liquefaction during seismic events results from the compaction of saturated soil by cycles of shearing [e.g., Terzaghi et al., 1996], causing an increase in pore pressure, and in turn a reduction in shear strength of the soil. Evidence for liquefaction associated with terrestrial impact events has been documented [Underwood, 1976; Warme and Kuehner, 1998; Terry et al., 2001]. Seismically-triggered groundwater flow can occur over great distances; the 1964 Great Alaskan Earthquake triggered the ejection of water-saturated sediments >400 km from the epicenter [Waller, 1968]. The lack of fissures and cones in the vicinity of Lyot suggests that water was released non-explosively. Therefore, if seismic triggering was responsible, the groundwater involved in carving the channels was likely very shallow in the subsurface and was unconfined. This is contrary to the model proposed by Clifford [1993], in which martian groundwater is confined by kilometers-thick permafrost. Calculations by Wang et al. [2005] demonstrated that impacts into saturated soil on Mars would be capable of producing pore pressures in excess of lithostatic, allowing for flooding without the need for a confining cryosphere to pressurize groundwater. The lack of evidence for water ponding within Lyot [Russell and Head, 2002] may also point to an unconfined water source, as it suggests that there was no significant artesian release of groundwater after the impact event. However, evidence of ponding may be obscured by the pervasive mantling material within the crater.

[10] Another hypothesis for the formation of the channels associated with Lyot is that they were carved by dewatering of the ejecta, similar to the dewatering of large volcanic landslides on Earth [e.g., Janda et al., 1981; Tilling et al., 1990]. When the main moving mass of a saturated landslide stops, residual water within the body of the flow moves through interstices and along the interface between the landslide and the underlying surface. Pore pressure and compaction often force water upwards to create surface ponds that can coalesce to form surficial streams that incise channels. Water flowing across the surface of a landslide can eventually reach the margins and flow out over the toe. Segregated water within the landslide can also travel along the basal interface and emerge at the landslide margins as a hyperconcentrated flow [e.g., Lawson, 1982; Pierson and Scott, 1985]. These dewatering streams are load-rich, discharge-poor systems, which typically leads to braiding, as seen in the Lyot channels (Figures 1b1d). The best terrestrial examples of these processes are lahars discharging from volcanic landslides; the North Fork Toutle lahar from the 18 May 1980 Mount St. Helens eruption was fed by a dewatering debris avalanche [Janda et al., 1981; Pierson and Scott, 1985; Tilling et al., 1990].

[11] The general lack of large channels incising the Lyot ejecta blanket, with the exception of a few places near the margins (Figure 2), suggests that if ejecta dewatering was responsible for the formation of the channels, the water was predominantly transported along the basal ejecta-plains interface. Dewatering of impact melt sheets has been suggested to form fluidized features (e.g. mudflows, alluvial fans, channelized features) associated with other young impacts on Mars, including Zunil, Tooting, and Zumba, but these craters are significantly smaller (<30 km diameter) and the features are all located within the craters and/or their ejecta blankets near the crater rim [Tornabene et al., 2007]. However, the well-preserved edges of the continuous ejecta blanket (e.g. no evidence of undermining and collapse), the fully-formed nature of the channels near the ejecta blanket margins, and the areal extent of channels and scour may argue against a dewatering origin. Therefore, we prefer the interpretation that seismic energy from the impact event was the primary formation mechanism of the channels.

Figure 2.

Example of channels cutting the Lyot ejecta blanket near 55.97°N, 333.28°W. (a) Mosaic of CTX images G01_018594_2364_XI_56N333W and G01_018739_2370_XI_57N333W. (b) MOLA topography overlaid atop the CTX mosaic from Figure 2a. White arrows denote the portion of the Lyot ejecta blanket incised by channels (marked with black arrows). Projection is transverse mercator with a center longitude of 333°.

[12] Channels carved into portions of the continuous ejecta deposit of the 60 km-diameter Sinton Crater were proposed by Morgan and Head [2009] to have formed from the melting of surface ice by blanketing of hot ejecta and late-stage impact-related hydrothermal springs. However, these channels occur only atop of and cut into the ejecta blanket and originate near the rim of Sinton as opposed to beyond the margins of the ejecta blanket, as is the case with Lyot.

[13] Impact-induced rain is expected for impactors >2 km under current martian conditions if water is liberated from the target material [Stewart et al., 2004]. This process has been proposed to explain fluvial features in Mojave Crater [Williams and Malin, 2008], as well as ancient valley networks [Toon et al., 2010]. While the Lyot impact likely led to precipitation of H2O, the abrupt channel heads and lack of tributaries suggests that atmospheric precipitation was not the primary formation mechanism for the observed channels.

[14] Channels have also been reported in the vicinity of the youthful (Late Hesperian/Early Amazonian [Cabrol et al., 2001]) 140 km-diameter Hale Crater [Tornabene et al., 2008; Edgett et al., 2008, Philippoff et al., 2009]. Some channels cut the Hale ejecta blanket, while others appear to originate at or beyond the ejecta margins. These channels have previously been attributed to melting of ice in the target material by superheated impact melt [Tornabene et al., 2008] or remobilization of saturated ejecta [Philippoff et al., 2009]. However, the presence of channels in the vicinity of Hale that do not originate on or at the margins of the ejecta blanket but are similar in morphology to those that do may suggest that channel formation at Hale was triggered by seismic energy from the impact acting upon shallow, subsurface groundwater. The presence of groundwater in the area at various times in martian history is evidenced by the presence of a number of gullies (of the type described by Malin and Edgett [2000]) within Hale and in surrounding craters and nearby Nirgal Vallis, which appears to have formed due to groundwater sapping based on its morphology [e.g., Sharp and Malin, 1975].

4. Implications

[15] The features documented here constitute some of the best candidates for impact-induced overland fluid flow observed on Mars to date. This is significant as Lyot is likely the youngest large impact basin on Mars [Werner, 2008]. The presence of these channels supports the models that predict the release of liquid water for impacts of this magnitude on Mars [e.g., Head and Roth, 1976; Mouginis-Mark, 1979; Stewart and Ahrens, 2003; Stewart et al., 2004] and may suggest that unconfined groundwater was present in the shallow subsurface at the time of the Lyot impact.

Acknowledgments

[16] We thank the MRO CTX and Mars Odyssey THEMIS and operations teams for acquiring the images that were central to this study. This work was funded by the NASA Mars Reconnaissance Orbiter project (contract 1275776).

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