over two decades at mono domes: slow changes to potential analogs to martian aeolian bedforms
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Over Two Decades at Mono Domes: Slow Changes to Potential Analogs to Martian Aeolian Bedforms. - PowerPoint PPT PresentationTRANSCRIPT
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Over Two Decades at Mono Domes:Slow Changes to Potential Analogs to Martian
Aeolian Bedforms
Williams, Steven H., Chair, Education Division, National Air and Space Museum, MRC 305, P.O. Box 37012, Washington, DC 20013-7012, [email protected]; Zimbelman, James R., Chair, Center for Earth and Planetary Studies, National Air and Space Museum, MRC 315, Box 37012, Washington, DC 20013-7012, [email protected]; Peterfreund, Alan R., Peterfreund Associates, 30 Boltwood Walk, Amherst, MA 01002; and Greeley, Ronald, Department of Geological Sciences, Arizona State University, Box 871404, Tempe, AZ 85287-1404
NOTE: View this presentation in “Notes Page” mode to see commentary. This is not a transcript of the talk as delivered at GSA 2003 in Seattle, but the Notes comments convey the content and flavor of the remarks as delivered at GSA!
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Over Two Decades at Mono Domes:
Slow Changes in Potential Analogs to Martian Aeolian
BedformsSteven H. Williams
James R. ZimbelmanAlan R. Peterfreund
Ronald Greeley
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Large “Ripples”
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Large Ripples on Mars
• Hinted At in Viking Images
• Common and Widespread
• Overlain by Dunes and Debris
• Some Atop Dunes
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Dunes Override Ripples
Floor of Crater Kaiser
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Ripples Override Dunes
M2000635 excerpt
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?
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Previous Work
Large (“Granule-” / “Deflation-) Ripples:
• Kelso Dunes (California)• Coachella Valley• Rogers Dry Lake @ Edwards AFB• Great Sand Dunes National Monument• Coyote Playa
All involve granules or coarser materials
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Problem!
• Terrestrial Granule Ripples are fragile– Prone to removal– Prone to burial by mobile sand
• BUT: On Mars, at least some large ripples appear to be older and more durable than their terrestrial counterparts
So – do we have the “best” analogs?
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Mars “Ripples” Are Old (?)
• Note debris apron overlies ripples• Implies ripples long-lived• Implies they are more durable than typical terrestrial granule ripple
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Not All Large Ripples Involve Granules!
• What if particles involved are distinguished by density, not by size?
• Candidate examples: Pumice
• Low density allows larger particles to move and create bedforms
• Field Example: Mono Domes, California
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Large ripples in pumice/obsidian
South Mono Dome, California
Minimal changes over ~22+ years
June, 2003
May, 1981
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June, 2003
Bedform Comparison
May, 1981
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June, 2003
May, 1981
Bedform Comparison
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Ripple Crest Alteration
Raked
Stomped
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Typical Bedform Profile
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Sedimentary Characteristics• Bedform Crests (D = 1.2 cm)
– Pumice 66 wt % = 1.8– Obsidian 11 wt % = 2.1
• Windward Slope (D = ~5 mm)
– Pumice 74 wt % = 1.6– Obsidian 11 wt % = 2.1– More poorly sorted; skewed to fines
• Leeward Slope (D = ~5 mm)
– Pumice 26 wt % = 1.6– Obsidian 37 wt % = 2.2
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Conclusions
• Some terrestrial large ripple bedforms are very long-lived
• Larger, but lighter, particles may be more mobile