Light-toned layered deposits in Crater Terby and the Greater Hellas Region, Mars

Sharon Purdy

Mars today is cold and dry and liquid water is not stable at the surface. There are several landforms that suggest the climate on Mars was once warmer and wetter — similar to Earth. Scientists have been intrigued by the discovery of layered deposits on Mars since they were recognized by Mariner 9 and Viking orbiters in the 1970s. Many of the layered deposits on Mars are light-toned, occur in crater interiors, and are characterized by thick, laterally continuous, repetitive sequences. There are many ways in which layered deposits can form on Mars, including volcanic, wind or water-related processes. The physical and chemical nature of the light-toned layered deposits in Terby crater and other craters around the great Hellas impact basin are consistent with deposition of material in a large, standing body of water much earlier in Mars' history1 (Figure 1).

Figure 1. Many craters around the 2,300 km-wide (1,430 mi) Hellas basin, marked with black and white circles, contain interior layered deposits including craters Terby ("T"), Millochau ("M") [Mest and Crown, Icarus, 2005], "A" and "B" [Korteniemi et al., JGR, 2005], Niesten ("N") and the informally named SW crater ("S") [From Wilson et al., 2007].

One of the most spectacular exposures of light-toned layers on Mars occurs in Terby (Figure 2), a large, ancient crater located along the northern rim of Hellas basin in the Martian southern highlands (Figure 1). Over 2 km-thick (1.2 miles) sequences of light-toned layered deposits are exposed in benches (long, nearly planar surfaces dropping off steeply on one or both sides) that extend from the northern crater wall to the center of the crater (Figures 2 and 3). The layers within the benches in Terby are fine-grained, partially indurated (cemented), range in thickness from less than 1 m up to about 10 m and contain hydrated minerals such as phyllosilicates (clay-bearing minerals found on Earth that are associated with water-rich environments). Other craters around Hellas (Figure 1), such as Niesten and "SW" craters, have comparable layered sequences banked along their northern rims at similar elevations, suggesting a similar age and origin1. Through the topographic (elevation), geomorphic (surface texture) and stratigraphic (layer sequence) analyses, we studied the origin of the landforms in Terby and its long geologic history1.

Terby was proposed as a potential landing site for the Mars Science Laboratory (MSL) mission because the thick sequence of layered deposits2 are consistent with deposition in a large ocean that once filled the Hellas, a 2,300 km wide (1,430 mi) impact basin. For more information about the MSL mission visit the Mars Science Laboratory home page and visit MARSOWEB to see what other landing sites were proposed form MSL. Life as we know it on Earth is intimately associated with water, and a deep, long-lived ocean would be a hospitable environment for life to form and be preserved on ancient Mars.

Figure 2. Mars Orbiter Laser Altimeter (MOLA) topography (elevation) of Terby crater. White arrows indicate benches that expose ~2 km thick sequences of layers; the benches extend from the northern rim and terminate in a deep, moat-like depression in the center of the crater (modified from Wilson et al., 2007). White box shows location of Figure 3.

Figure 3. (A) Layered sequence in the western bench (see Figure 2 for context) are ~2.5-km-thick, dashed lines show contacts between sub-units (LD1 – LD4).Hollow white arrow points downslope. HiRISE image 1662–1520. (B) Detail of lowest unit, LD1 in the western bench is characterized by thicker, light-toned (LT) beds interbedded with thin, intermediate-toned layers (black arrows) that are weathering along meter-spaced joints to produce boulders. The boulders accumulate on the slopes and their downslope movement is often recorded by boulder tracks that can be over 1 km long (e.g., white arrow). Although the LT beds are typically heavily mantled, there are hints of finer layers or laminations at HiRISE resolution (28 cm/pixel). Hollow arrow points downslope. Modified from Wilson et al., 2007.

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1. Wilson, S. A., A. D. Howard, J. M. Moore, and J. A. Grant (2007), The geomorphic and stratigraphic analysis of Crater Terby and layered deposits north of Hellas basin, Mars, J. Geophys. Res., 112, E08009, doi:10.1029/2006JE002830.
2. Wilson, S. A., A. D. Howard and J. M. Moore (2006), Terby crater as a potential landing site for the Mars Science Laboratory, First Mars Science Laboratory Landing Site Workshop, May 31–June 2, Pasadena, CA.