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B.J. Thomson et al., 2011; via Ben Tweed

Constraints on the origin and evolution of the layered mound in Gale Crater, Mars using Mars Reconnaissance Orbiter data. B.J. Thomson et al., 2011; via Ben Tweed. Gale Crater: What ‘n’ where. Curiosity landing site 152 km diameter Quite large, as craters go 5.3 deg S, 222.3 deg W

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B.J. Thomson et al., 2011; via Ben Tweed

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  1. Constraints on the origin and evolution of the layered mound in Gale Crater, Mars using Mars Reconnaissance Orbiter data B.J. Thomson et al., 2011; via Ben Tweed

  2. Gale Crater: What ‘n’ where • Curiosity landing site • 152 km diameter • Quite large, as craters go • 5.3 deg S, 222.3 deg W • Straddles dichotomy boundary • Heavily dissected and several km shorter north rim • Local fretted terrain unique • No ice-facilitated mass wasting indicators • Lineated valley fill • Lobate debris aprons • And then there’s the mound…

  3. Gale Crater: The Mound ~1.7x10^4 km^3 Average height of 3.8 km, max relief of about 5.2 km • Shorter than southern rim in places, taller than northern rim in places • Possible indication that the whole crater used to be buried So what’s the big deal, anyway?

  4. The Mound: The Big Deal Other similar mounds Reuyl (86 km dia, 9.8 deg S, 193.2 deg W) Nicholson (103 km dia, 0.2 deg N, 164.6 deg W) Similar spectra, similar surface geometry (yardangsetc) => larger MFF? How’d they get there anyway? Low thermal inertia: Wind Volcanic ash Dust + ice Southern rim channels, layering and terraces, other channels: Water Lakes No terraces, deltas or fans: Hot spring Buried evaporite=>thermal anomalies=>sustained hydrologic activity

  5. The Mound: A Lady Never Tells Her Age… • Scott & Carr, 1978 • Mariner 9 map: Entire crater + mound labeled Noachian cratered plateau material • Eastern plains interpreted as signs of wind erosion/exhumation • Greeley and Guest, 1987 • Viking orbiter map: Mound and some plains placed in Hesperian • Scott & Chapman, 1995 / Cabrol et al., 1999 • Possibly as young as late Amazonian • Malin & Edgett, 2000 • Widespread crater exhumation evidence indicates mound from Noachian • Irwin et al., 2005 • Gale is so fresh-looking it must have come after the widespread crater degradation period but before all the terrain-fretting => Noachian/Hesperian boundary Mya

  6. But They All Made A Big Mistake, Which Was • that none of them really bothered with doing crater counts of both the mound and the rest of the crater separately. • => separate crater size/freq plots => better idea of separate ages • Cabrol et al., 1999 used Viking images to do this for the whole thing • Only marginally useful; mound and crater crater densities were averaged together, resulting in a lower crater density and therefore a lower inferred age • This paper does that, as well as a much more thorough examination of the various materials comprising the mound, in order to arrive at a super concrete age estimate

  7. MRO • Launched 8/12/05, Martian orbital insertion 3/10/06 • Intended to be MGS 2.0 plus a sort of Martian TDRS • Instruments:HiRISE, CTX, MOLA, CRISM, SHARAD, MARCI, MCS

  8. And Hey, It Can Do This Too

  9. High Resolution Imaging Science Experiment (HiRISE) • Operates in visible and near-IR • 400-600, 550-850, 800-1000 nm • 0.3m spatial resolution • Stereo topographical accuracy of about 0.25 m

  10. Context Camera (CTX) • Pretty much what it says on the tin – designed to provide context for the other, finer instruments with their smaller FOVs • 8 meters/pixel • Mapped 50% of Mars by 2010

  11. Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) • 18 meter spatial resolution • 370-3920 nm, in 544 channels

  12. Anyway, the map. It Looks Like This: Divided into upper/lower mound, then given additional morphological descriptors

  13. How’d they make it? • CTX & HiRISE images map traceable extent of units • Slope assessed using Mars Orbiter Laser Altimeter (MOLA) data • Footprint ~168m dia., spaced every ~300m along spacecraft ground track • HiRISE stereo also played a big role, which I shall now explain

  14. HiRISE Stereo

  15. MOLA Fun Times

  16. More MOLA Fun Times

  17. Map Details • 22 distinct units • Divided between upper and lower mound • Lower units: layered strata are more horizontal to sub-horizontal • Upper units: more finely layered, higher-angle bounding surfaces • Evidence for aqueous activity in lowers • Polygonal ridge networks from beneath fractured, light surfaces • Subsurface fluid flow, preferential cementation along fractures • Channels, both inverted and otherwise

  18. Hydrated sulfates/clays? Survey says…

  19. (…Yes)

  20. NIR alteration mineralogy: some details • CRISM spectra of lower formation reveal alteration minerals • Sulfate-bearing rocks • Smectite and/or olivine • Sulfates appear to be Mg-bearing

  21. MIR Results • Detailed MIR imaging of Gale from Mars Odyssey Thermal Emission Imaging System (THEMIS) • Measure of both temperature and thermal inertia

  22. Crater Size-Frequency distribution

  23. Crater Size-Frequency distribution

  24. Crater Size-Frequency distribution

  25. Minimum Possible age: Floor Units and Valley Deposits • Crater floor units embay, overlap lower layered ones • => they are younger • => finding an age for the floor means we’ve got a minimum for the mound, unless water can time-travel • Earlier CSF dib gives min age of Early Hesperian

  26. Maximum: gale ejecta craters • R_ce=(2.348+/-0.5)R^1.006 (Moore et al., 1974) • 2 largest size bins => Late Noachian • Smaller => LN/Early Hesperian • Possible some craters older than Gale may have been included

  27. So The Takeaway is…? • Upper, Lower mound have distinctmineralogic characteristics • Perhaps formed by distinct processes • Non-horizontal erosion surface =?> wind and/or landslide based erosion • Upward transition in lower mound from phyllosilicate-bearing layers to sulfate-bearing ones means water

  28. So the takeaway is…? • A lot of mound craters are exhumed => the CSF age is likely resurfacing rather than formation • When the overlying surface was removed, any craters it had obviously went with it =>you can’t say how old that surface was or how long it was in place very accurately • Still the matter of the unknown origin of the Upper/Lower mound boundary • Distinct change in material properties • Lower: lots of exhumed craters • Upper: doesn’t seem to be as good at crater retention • Valley networks around crater match floor units age • => Layered deposits likely between Late Noachian, Early Hesperian • Data suggest a global transition to sulfate formation from phyllosilicate formation • Lots of sulfur plus a rapid global pressure drop and water loss? • Well, that or they could be codeposited like on Earth

  29. …This: • Lacustrine/aquatic origin • Alteration minerals • Ridge networks • Relief channels • Mound height greater than nearly all the rim => fill must have been complete => surrounding terrain also inundated • Aeolian origin • Not much evidence, save that the alteration minerals could have formed elsewhere and been blown to Gale (à la Atacama); also, how did they get lithified if no water was around? • In either case, the sharpness of the upper/lower boundary indicates that Gale is a snapshot of an important transition in the Martian climate/mineralogy at the N/H boundary.

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