Sharon Wilson, Smithsonian Institution

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Terby CraterFirst MSL Landing Site
WorkshopPasadena, CA May 31 June 2, 2006

• Sharon Wilson, Smithsonian Institution
• Alan Howard, University of Virginia
• Jeff Moore, NASA Ames Research Center

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Terby Crater

• D 170 km
• Noachian Leonard and Tanaka, 2001
• 28S, 287W
• Diverse suite of landforms
• Indicative of varying geologic processes
  throughout martian history

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Justification Interior Deposits

• Light-toned layers
• Ridges (gt2.5 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

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Possible Fluvial Evidence In the Landing Ellipse
ridges
Wall rock?
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Sinuous ridges fluvial or glacial activity
related to formation of moat?
1 km
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Scientific Objectives

• 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 (?)

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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

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• 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

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• Layered sequence of 4 units recognized in both
  ridges

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Characteristics of Units 1 and 3

• 800 m thick
• Dark toned layers (50-70 m) interbedded with
  light-toned layers (10-25 m)
• Low albedo layers residual mantle or
  compositional difference?
• Very regular thickness of beds
• Laterally continuous

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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!

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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

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Origin of the Layered Deposits
Original Depositional Geometry
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Original Depositional Geometry
Terby Rim
N
3/4 2 1

• 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?

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Possible Origins of the Layered Deposits
Process Problem
Volcanic flows or intrusions Fine grained, repetitive nature, erodable X
Mass wasting Fine grained, repetitive nature, lack of source X
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 Dunes Fine grained, lack of cross-bedding X
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
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Is Terby One-Of-A-Kind?

• Terby is special, but not unique!
• Similar morphology in other craters around Hellas
• Important to discern regional history of
  deposition and erosion

20 km
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Craters in Circum Hellas with Pits and Layers
Moore and Howard, 2005
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-3.1 km
-6.9km

• Possible stands of ice covered lakes based on
  topographic, morphologic and stratigraphic
  evidence

-5.8km
-4.5km
Moore and Wilhelms, 2001
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Histogram of Elevation Around Hellas
-6.9 km
Moore and Wilhelms
-5.8 km
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Possible Water Stands

• -2.1 km, -3.1 km and -4.5 km
• High elevation related to deposition
• Low elevation related to erosion?

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Hellas and surrounding region under water?
- 0.7 km
0.6 km
Elevations correlate to well-developed,
inward-facing scarps
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Summary
ENGINEERING PARAMETER REQUIREMENT
Latitude 60N to 60S
Altitude 2 km
Landing ellipse radius 10 km
Slopes 3 (2 to 5 km length scale)
Slopes 5 (200 to 500 m length scale)
Slopes 15 (20 to 40 m length scale)
Slopes 15 (5 m length scale)
Rock Height 0.6 m
Load bearing surface Not dominated by dust
EDL winds Steady state horizontal 30 m/s
EDL winds Steady state vertical 10 m/s
EDL wind gusts/variability
Radar Reflectivity
Surface winds lt15 m/s (steady) lt30 m/s (gusts)
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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