Sharon Wilson, Smithsonian Institution

Alan Howard, University of Virginia

Jeff Moore, NASA Ames Research Center

Terby CraterTerby CraterFirst 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 (~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

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 floormoat

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

3/421

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 Process Problem

Volcanic flows or intrusions

Fine grained, repetitive nature, erodable X

Mass wasting Fine grained, repetitive nature, lack of source X

Volcanic AirfallRepetitive 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

FluvialGeometry 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

LoessFine-grained, terrain conforming and cliff-forming, need upslope winds, might be rhythmically layered

O

LacustrineNature, 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, 2001

-3.1 km

-4.5km

-5.8km

-6.9km

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

0.00E+00

1.00E+05

2.00E+05

3.00E+05

4.00E+05

5.00E+05

6.00E+05

7.00E+05

8.00E+05

-8000 -7000 -6000 -5000 -4000 -3000 -2000 -1000 0 1000 2000 3000 4000

Elevation (meters)

-3.1 km-4.5 km -2.1 km

-0.7 km

0.6 km

Histogram of Elevation Around Hellas

-5.8 km

-6.9 kmMoore 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?

- 0.7 km

+0.6 km

Elevations correlate to well-developed, inward-facing scarps

ENGINEERING PARAMETER REQUIREMENT

Latitude 60N 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 surface Not dominated by dust

EDL windsSteady 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