Sharon Wilson, Smithsonian Institution Alan Howard, University of Virginia Jeff Moore, NASA Ames Research Center Terby Crater Terby Crater First MSL Landing Site Workshop Pasadena, CA May 31 – June 2, 2006

D ~170 km Noachian [Leonard and Tanaka, 2001] 28°S, 287°W Diverse suite of landforms –Indicative of varying geologic processes throughout martian history Terby Terby Crater

Justification: Interior Deposits Light-toned layers –Ridges (>2.5 km) –Trough floors –Crater floor: exposed on scarps of the moat deposit (~400m) and crater floor Troughs Moat-like depression Flat Crater Floor Viscous flow features Fan, channels, depressions, scoured caprock, landslides

Possible Fluvial Evidence In the Landing Ellipse ridges Wall rock?

1 km Sinuous ridges –fluvial or glacial activity related to formation of moat?

Smooth, flat, dust free surface with intermediate TI (THEMIS NIR) Ellipse: LDs and sinuous ridges on moat floor Drive to sites in moat: –layered deposits in main ridges and crater floor (moat scarp) –fluvial features in trough related to erosion of LDs –access ancient wall rock (?) Scientific Objectives:

Layered Deposits Indurated, fine-grained sediment based on cliff-forming nature, TI and faults Sub-horizontal bedding units [Ansan and Mangold, 2004; Ansan et al., 2005, 2006], conformable with regional slope (1.5 degree dip) Laterally continuous on km scale Beds are massive and scalloped –no fine-scale interbedding at MOC scale Hydrated mineral signature (clay or sulfates) [Ansan et al., 2005; Bibring et al., 2006] Late Noachian/Hesperian [Ansan et al., 2005; Millochau Crater by Mest and Crown]

Western Ridge –2.5 km thick layered sequence in N. Terby –Layered mounds on trough floor –Layered sequence with 4 units identified in each ridge 5km

Layered sequence of 4 units recognized in both ridges

Characteristics of Units 1 and 3 ~800 m thick Dark toned layers (~ m) interbedded with light-toned layers (10-25 m) Low albedo layers – residual mantle or compositional difference? Very regular thickness of beds Laterally continuous

Unit 2 Light-toned Some horiz bedding Discontinuous, curved laminations and small-scale folding Change in depositional environment Soft sediment deformation? Surge deposits? Tectonic activity? Aeolian processes? Unique to this unit Need more data!

Unit 4 Caps layered ridges Intermediate-toned, massive layer “sandwiched” between distinctive thin, dark- toned layers Dark layers weather non- uniformly into a small- scale knobby surface Dark layers more indurated and are either: –coherent beds that break down along widely (multi- meter) spaced fractures or –they occur as beds of multi- meter scale clasts

N Northern Terby rim crater floor moat Projected elevation profile of Terby’s southern rim Flat mesa surface Northern rim of unnamed crater 20 km Origin of the Layered Deposits Original Depositional Geometry

3/4 2 1 Terby Rim N 20 km No evidence of thinning, pinching out or steepening of layers No evidence for past lateral obstruction Possible layered ridges across moat depression Layers in ridges likely extended out past the center of the crater Possible Scenario: Layers in crater floor only correlate to lower unit in layered ridge Mechanism to erode back layers and form moat? 3 km N Original Depositional Geometry

Possible Origins of the Layered Deposits ProcessProblem Volcanic flows or intrusions Fine grained, repetitive nature, erodableX Mass wastingFine grained, repetitive nature, lack of sourceX Volcanic Airfall Repetitive nature, consistent thickness, induration of layers, lack of obvious proximal volcanic source. X Glacial Faults, absence of glacial flow and internal collapse features, layers of regular thickness X Fluvial Geometry not consistent with prograding fan, lack of course grained material, consistent thickness and no obvious source X Aeolian DunesFine grained, lack of cross-beddingX Loess Fine-grained, terrain conforming and cliff-forming, need upslope winds, might be rhythmically layered O Lacustrine Nature, geometry and hydrated signature consistent with deposition in fluid moderated by an environmental cycle such as climate or seasons O

Terby is special, but not unique! Similar morphology in other craters around Hellas Important to discern regional history of deposition and erosion Is Terby One-Of-A-Kind? 20 km

Craters in Circum Hellas with Pits and Layers Moore and Howard, 2005

Moore and Wilhelms, km -4.5km -5.8km -6.9km Possible stands of ice covered lakes based on topographic, morphologic and stratigraphic evidence

Histogram of Elevation Around Hellas -5.8 km -6.9 km Moore and Wilhelms

-2.1 km, -3.1 km and -4.5 km High elevation related to deposition Low elevation related to erosion? Possible Water Stands

Hellas and surrounding region under water? km +0.6 km Elevations correlate to well-developed, inward-facing scarps

ENGINEERING PARAMETERREQUIREMENT Latitude60N to 60S Altitude≤ 2 km Landing ellipse radius≤ 10 km Slopes ≤ 3° (2 to 5 km length scale) ≤ 5° (200 to 500 m length scale) ≤ 15° (20 to 40 m length scale) ≤ 15° (5 m length scale) Rock Height≤ 0.6 m Load bearing surfaceNot dominated by dust EDL winds Steady state horizontal ≤ 30 m/s Steady state vertical ≤ 10 m/s EDL wind gusts/variability Radar Reflectivity Surface winds<15 m/s (steady); <30 m/s (gusts) Summary

Size and age of Terby represent a long period of martian history Geologic history is complex, but perhaps more well- constrained than other craters with ILDs and relevant to greater Hellas region Climate-related landforms in Terby indicative of potentially hospitable environments– making it an excellent candidate for MSL –Layered deposits consistent with lacustrine deposition –Presence of hydrated minerals (clay?) might indicate an ideal environment for preserving organic material –Fluvial Features (sinuous ridges, fan, flow features) also accessible Summary