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irregular mare patches (imps): 100 ma or 3 ga?IMPs exhibit a perplexing combination of characteristics that are consistent with either an approximately 100 Ma or 3 Ga formation. Dozens of small-area IMPs have crisp morphologies and crater size-frequency distributions (SFDs) that denote relatively recent geologic activity (less than 100 Ma); however, the apparently well-developed regolith on portions of the IMPs are in conflict with such a young age [1]. To test possible formation hypotheses (e.g., [1-5]), which range from ancient volcanism to contemporary outgassing, we examined IMP morphology at the meter-scale with LROC NAC images and derived elevation models. We focused on the largest IMPs (Ina, Sosigenes, Cauchy, Maskelyne, and Nubium), where contacts between deposits are best developed. Most of our observations are consistent with multiple generations of inflation and breakouts (or squeeze-ups) of basaltic lavas that were affected by local slopes. Some of the extrusions coalesced into larger mounds or filled pre-existing craters. We did not observe evidence of large-scale void space (e.g., fissures, fractures, linear depressions, or pits) within or beneath the mounds or rougher deposits (e.g., [5]). But, small-scale voids may be signified by isolated pitted textures. We also did not detect evidence of the cooling fractures or lava plates expected in young lava flows and observed in lunar impact melt deposits. The smooth texture of the mounds is enigmatic. Block-less craters suggest at least 5 m of friable or poorly-cohesive material (such as regolith), yet mound margins exhibit slopes greater than 30 deg requiring significant material strength. Blocks are not common on the mounds, but are sometimes excavated by impacts (usually excavated from beneath the mounds). The uneven deposits are equally enigmatic and texturally varied (blocky, pitted, and crenulated). They are deficient in superposed craters compared to the mounds. If the mounds are indeed of similar age to the rougher units, then their different superposed crater morphologies and SFDs need to be explained by factors other than their ages. Any mounds originally composed of friable surface materials would evolve differently from more coherent deposits (e.g., [6-7]).
Document ID
20160012275
Document Type
Conference Paper
Authors
Stopar, Julie
(Lunar and Planetary Inst. Houston, TX, United States)
Robinson, Mark Southwick
(Arizona State Univ. Tempe, AZ, United States)
van der Bogert, Carolyn H.
(Muenster Univ. Germany)
Giguere, Thomas
(Hawaii Univ. at Manoa Honolulu, HI, United States)
Lawrence, Samuel J.
(NASA Johnson Space Center Houston, TX, United States)
Ostrach, Lillian Rose
(NASA Goddard Space Flight Center Greenbelt, MD, United States)
Clegg-Watkins, Ryan N.
(Washington Univ. Saint Louis, MO, United States)
Date Acquired
October 12, 2016
Publication Date
December 12, 2016
Subject Category
Lunar and Planetary Science and Exploration
Report/Patent Number
JSC-CN-37531
Meeting Information
AGU Fall Meeting(San Francisco, CA)
Distribution Limits
Public
Copyright
Public Use Permitted.

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