Geomorphological map of the lower NW flank of the Hecates Tholus volcano, Mars (scale 1:100,000)

Abstract

Hecates Tholus (centered at 32.1°N 150.2°E), is a shield volcano in the Elysium volcanic province, located in the Martian lowlands in the northern hemisphere. Images of this volcano acquired by the Context Camera (CTX) instrument on board NASA's Mars Reconnaissance Orbiter spacecraft show many glacial landforms that have gone unnoticed by previous authors. We present a geomorphological map of the lower northwestern flank of the Hecates Tholus volcano, at 1:100,000 scale, based on the use and analysis of CTX images with a resolution of 6 meters/pixel. The map is organized into a series of geomorphological units (surface cover) and elements (located at a point, along a line or distributed over an area) in order to provide a clearer understanding of the extent of glacial processes and the main dynamic element of this Martian glacial complex.

Keywords:

• Mars
• geomorphology
• glaciers
• remote sensing
• Hecates Tholus volcano

1. Introduction

Hecates Tholus is the only large volcano outside the Tharsis region for which various authors have found evidence of glacial activity – past or present. In addition, Hecates Tholus has received less attention from the glacio-geomorphological perspective than the other large volcanoes on Mars.

Most of the main volcanic edifices on Mars show several features that have been described as being produced by glacial processes (e.g., Head & Marchant, 2003; Kadish, Head, Parsons, & Marchant, 2008; Lucchita, 1981; Milkovich, Head, & Marchant, 2006; Shean, Head, & Marchant, 2005; Shean, Head, Fastook, & Marchant, 2007; Williams, 1978; Zimbelman & Edgett,1992). Olympus Mons, Arsia Mons, Ascraeus Mons or Pavonis Mons are some examples of Martian volcanoes showing putative glacial features on their flanks or in the nearby surrounding plains. However, all of these are located in the Tharsis volcanic region. Some of the other large volcanoes on Mars, Elysium Mons, Hecates Tholus and Albor Tholus, are located in the Elysium volcanic province (Figure 1), in the Martian lowlands in the northern hemisphere. Of these three volcanoes, glaciers have been described only on Hecates Tholus (e.g., Fassett & Head, 2006, 2007; Hauber et al., 2005; Helbert, Reiss, Hauber, & Benkhoff, 2005; Neukum et al., 2004), while relief possibly related to the interaction of magma, water and ice (such as sub-glacial volcanoes, mobergs, or chaotic terrains, among others) have been widely described in the Elysium volcanic province and the surrounding area (e.g., Carr, 2006; Chapman, 2003; Head, Marchant, Dickson, Kress, & Baker, 2010; Jaumman & Head, 2003; Levy, Head, & Marchant, 2009; Nussbaumer, Hauber, & Jaumann, 2004; Soare, Osinski, & Thomson, 2009; among others), marking the important role of water (probably also of ice) in this region of Mars.

Figure 1. (A) MOLA-derived topographic map of Mars showing the location of the Elysium volcanic province (EVP); (B) MOLA-derived shaded relief map of EVP and the location of Hecates Tholus shield volcano; and (C) Mosaic of THEMIS-IR daytime images of Hecates Tholus volcano and the location of the mapped area (shown in Figure 2).

Identification and mapping of glaciers on Mars is more difficult (than on Earth) because in all cases they are covered with glacial debris (dirty-ice) and the research is always dependent on the observation of images. This led to a decades-long discussion about the existence of glaciers on Mars (e.g., Carr, 2006; Shean et al., 2005; Zimbelman & Edgett, 1992). However, this discussion has been settled and glaciers have been identified. Head et al. (2010) published a geomorphological guide to identifying landforms on Mars in high-resolution images and many authors now accept the existence of glaciers throughout the history of the planet, including in times as recent as 2.1–0.4 Myr during the Late Amazonian (e.g., Carr & Head, 2010; Fassett, Dickson, Head, Levy, & Marchant, 2010; Head, Mustard, Kreslavsky, Milliken, & Marchant, 2003).

Figure 2. Composition of CTX images of the study area used here to develop the geomorphological cartography (images source: CTX/MRO/MSSS/NASA).

On the other hand, models of the evolution of glaciers (and especially changes in ice thickness) have been built on the basis of geomorphological observations of the tropical volcanoes on Mars, as well as on observations of ice deposits at different latitudes (e.g., Head, Marchant, Agnew, Fassett, & Kreslavsky, 2006a; Head, Nahm, Marchant, & Neukum, 2006b; Head et al., 2010). A wide variety of causes have been considered to explain the location, spatial distribution and ice thickness of the geomorphological features on the flanks and bases of the Martian volcanoes, including orbital, climatic and volcanic processes (e.g., Fastook, Head, Marchant, & Forget, 2008; Laskar, Levrard, & Mustard, 2002; Laskar et al., 2004; Wilson & Head, 2008).

Hecates Tholus (Figure 1) is a shield volcano located at 32.1°N 150.2°E (MC-7 quadrangle), in the Elysium volcanic province of Mars, in the Martian lowlands in the northern hemisphere. The edifice is about 8000 meters high above the surrounding plains (summit located at about 4600 meters above the Martian Datum). It is slightly elliptic, being 170 × 190 kilometers in diameter at the base, although the lower southern flanks appear to be covered by lava flows from the Elysium Mons volcano (e.g., de Pablo, 2009). Its most recent volcanic episode seems to have occurred between 1000 and 100 Myr (Neukum et al., 2004; Hauber et al., 2005), although it has had a long history of magmatic, tectonic and water-related interactions (e.g., Fassett & Head, 2006, 2007; Gulick & Baker, 1989, 1990; Hauber et al., 2005; Kangi, 2007; Mouginis-Mark, Head, & Wilson, 1982, 1984; and others summarized in de Pablo, 2009).

As mentioned above, glacial features have been described previously on the flanks of Hecates Tholus. Although a possible ice cap covering the highest part of the edifice has been proposed (Fassett & Head, 2006, 2007), most of the glacial features are located on the NW flank of the edifice (e.g., Hauber et al., 2005; Helbert et al., 2005; Neukum et al., 2004), and mostly on the lower part, related to a nested depression. The main area with glacial elements are, first, an elliptical depression at the base of the volcano (40 × 30 km, SW-NE oriented), and, second, a smaller depression (15 km in diameter) on the middle part of the flank. Following the recent terminology proposed by Head et al. (2010), Lineated Valley Fill and Lobate Debris Aprons have been observed in THEMIS-Vis (18 m/pixel), HRSC (12 m/pixel) and MOC (2 m/pixel) images, as described and mapped previously (e.g., Hauber et al., 2005). However, the newly available high-resolution images from CTX and HiRISE instruments on board the NASA's Mars Reconnaissance Orbiter (MRO) now provide sufficient resolution (6 m/pixel and 0.25 m/pixel, respectively) to produce a detailed geomorphological map of those glacial features that could help to understand the glacial and geological history of this area.

The main target of our research was to produce a detailed (1:100,000 scale) geomorphological map of a selected area (Figure 2) of the Hecates Tholus volcanic edifice where glacial features are particularly abundant, the lower NW flank (31.8-33.0°N, 148.37-149.3°E), using CTX images as the base map, since these provide sufficient spatial coverage and resolution to enhance the study reported here. This map adds new information to the previous regional thematic maps of Mars which include this volcano (e.g., Hauber et al., 2005; Tanaka, Chapman, & Scott, 1992; Tanaka, Skinner, & Hare, 2005). This map also provides a basis for future detailed geological and geomorphological studies focused on understanding the geological and climatic history of this region of the planet and the possible relief related to magma, water and ice interactions, with the aim of expanding on the results of previous extensive studies (e.g., Carr & Head, 2010; Mouginis-Mark et al., 1982, 1984; Mouginis-Mark, 1985; Mustard, Cooper, & Rifkin, 2001; Mustard, 2003, among others).

2. Data, methods and software

Mapping the geomorphological features of the lower NW flank of the Hecates Tholus volcano of Mars was made possible through the analysis of high-resolution images from the CTX instrument on board the MRO spacecraft (NASA: 2006–present). These images were acquired in the visible to near infrared spectrum range (0.5–0.8 µm), with a resolution of about 6 meters per pixel. We used a mosaic of two CTX images: B06_011957_2127_XI_32N211W and B04_011324_2128_XI_32N211W. Although other CTX images covering the study area are available we selected these two because of their high quality (without errors or gaps). These images were obtained through the NASA Planetary Data System (PDS: http://pds.nasa.gov/) and the Mars Space Flight Facility of Arizona State University (ASU-MSFF: http://www.mars.asu.edu/).

During the mapping process, we also used other accessory data to improve our understanding of the geological and geomorphological processes which have affected the volcano. First, a nadir image acquired by the High-Resolution Stereo Camera (HRSC) instrument on board the Mars Express (MEX) spacecraft (ESA: 2003 – present) was used as the basis for co-registration of the CTX images, with a spatial error of 6 meters. Image H2907_0008 (50 m/pixel) was not used for mapping due to its lower spatial resolution compared to the CTX images. We also used an HRSC – derived Digital Elevation Model (DEM), which provides topographic information at 50 m/pixel in resolution (H1262_da4), although it does not cover all of the study area. All these data were obtained through the ESA Planetary Science Archive (ESA-PSA: http://www.rssd.esa.int) and the HRSC Team at the Free University of Berlin (HRSC/FUB: http://hrscview.fu-berlin.de).

The base HRSC image (PDS-IMG in format) was processed using the ESA Mars Express HRSC data to GIS converter (HRSC2GIS tool), developed by J. Oosthoek from the Geological Survey of the Netherlands. Topographic data derived from the HRSC images were also processed using the same tool. Then, the resulting projected HRSC image and HRSC-derived DEM data were incorporated into a Geographic Information System developed for the study area through the use of ArcGIS 9.3 (manufactured by ESRI company). We used this software to derive shaded relief, slope and aspect maps, as well as a contour line map (equidistance of 100 m), useful both in the mapping process, and in the interpretation of the different mapped geomorphological features.

With the GIS, we used the Geographic Coordinate System Mars 2000 spheroid (Semi-major axis: 3396190.0 m; Semi-minor axis: 3376200.0 m; Inverse flattening: 169.894) and the Equidistant Cylindrical projection system with central meridian at 180°. Once the coordinate and projection systems had been established, we included our selected data: the HRSC image, HRSC-derived DEM, and CTX images. The HRSC image was used to manually co-register the CTX image (JPG2000 in format) through the use of ArcGIS 9.3 tools, by including more than 400 control points for co-registration of both images.

3. Geomorphological elements and units

The geomorphological map (elements and units) of the lower NW flank of the Hecates Tholus volcanic edifice is based on texture and relief, using terrestrial analogue sites, and previous geomorphological maps of glacial areas (e.g., Evans, Twigg, Rea, & Orton, 2009; Hättestrand & Stroeven, 2002; Kjaer, Korsgaard, & Schomacker, 2008; Smith, Rose, & Booth, 2006). In addition, some of the geomorphological features (such as impact craters, ridges, faults or channels, among others) have already been included in the geological maps of Mars at different scales (e.g., de Hon, Mouginis-Mark, & Brickl, 1999; Tanaka et al., 1992). In recent times, more detailed studies have been conducted of the Martian surface using the high-resolution images provided by the last missions to Mars. These studies have made it possible to identify and define more features, especially those related to glaciers (e.g., Hauber et al., 2005; Helbert et al., 2005; Neukum et al., 2004; Pacifici, Komatsu, & Pondrelli, 2009), and also to define and establish their possible origin (Head & Marchant, 2003; Head et al., 2003, 2010; Head, Marchant et al., 2006a, Head, Nahm et al., 2006b). Consequently, our geomorphological map is based on previous common practices for geomorphologic mapping of Mars, the most recent geomorphological criteria for glacial features, and terrestrial analogues.

We identified 18 geomorphological elements and 18 geomorphological units (Table 1), named in agreement with our interpretation (eskers, moraines, pingos, etc.) and classified according to their origin (glacial, fluvial, impact, etc.). In other words, the geomorphological units mapped here were classified and named according to their origin and main texture (e.g., smooth lava flows, rough slopes, etc.). These units represent terrain sectors characterized by an association of geomorphological elements related to common genetic processes. Units are marked on the map in different colors, with an acronym to identity them clearly on the map.

Table 1. Mapped geomorphological units and elements, and their interpreted origin.

Origin	Units	Elements
Volcanic	Smooth outcrop
	Smooth lava flow
	Dissected lava flow
	Rough lava flow
	Patterned lava flow
Slope	Slope deposit	Scarp
	Creep deposit	Slope base ridge
	Rocky slope
	Rough slope
	Smooth slope
Fluvial	Badland	Channel
	Gulley sedimentary deposit	Shallow channel
		Terrace scarp
Periglacial		Pingo
Glacial	Roche moutonée	Serac
	Glaciar deposit	Flute
		Esker
		Arête
		Morraine
		Glacier erratic
Aeolian		Dune field
Tectonics		Graben
		Alignment
		Ridge
Impact	Ejecta deposit	Crater
	Crater material	Impact crater
Other	Knobby terrain
	Rugged terrain

The map also shows the relationship between units and elements according to their origin (Table 1). However, some elements are only related to specific units with a similar origin (e.g., crevasse element to glacier unit), meanwhile some other elements are widely distributed throughout all of the geomorphological units (e.g., impact crater or channel elements).

The resulting geomorphological map of the lower NW flank of the Hecates Tholus volcano (1:100,000 scale) contains a brief description of each unit, its spatial distribution and geological/geomorphological interpretation, as well as an image of the study area of each unit.

4. Conclusions

The 1:100,000 scale geomorphological map of the lower NW flank of the Hecates Tholus, based on CTX images, shows that the studied area is rich in possible glacial features (landforms, deposits, etc.), including moraines, covered glacial tongues (or possibly, rocky glaciers), lineated deposits, eskers, drumlins, etc. It also reveals the existence of other interesting relief related to several processes, including: tectonics (ridges, faults and graben, or morphological alignments), fluvial and water-related (gullies, channels, or terraces), periglacial (pingos), gravitational (mass wasting deposits or creeping slopes) and impact-related (impact craters or ejecta deposits) forms. All those features suggest a complex hydrological, climatic and geological history for this volcano in very recent times, perhaps as recent as 0.4 Myr (Hauber et al., 2005). A detailed analysis of each feature and its distribution will help to understand the geological evolution of this area and the possible important role of climate, or how changes in climatic conditions may have been responsible for the present surface characteristics.

This map provides a substantial amount of information about the existence and spatial distribution of features not described in previous studies of this area, based on THEMIS-Vis, HRSC (due to their lower resolution), or MOC (due to their low spatial coverage) images.

We hope this geomorphological map will inspire other thematic cartographies of the Martian surface using CTX and HiRISE images, due to their high quality and usefulness for understanding the geological evolution of the Red Planet.

Acknowledgements

The authors thank CTX/MRO/MSSS/NASA and HRSC/MEX/FUB/ESA instrument teams for CTX and HRSC data, respectively. They also thank Dr. Jelmer Oosthoek, from the Geological Survey of the Netherlands, for his tool HRSC2GIS used to process HRSC data.