¹Senthil Kumar Perumal, ¹J. Prasanna Lakshmi Kopisetti, ¹Krishna Nagula, ¹Rajeev Menon, ¹Sruthi Uppalapati, ²Keerthi Vallabaneni, ²Senthil Kumar Arumugam, ¹Mysaiah Dasari, ¹Seshunarayana, ¹Tangirala, Mrinal K. Sen
¹National Geophysical Research Institute, Council of Scientific & Industrial Research, Hyderabad, India
²National Remote Sensing Centre, Indian Space Research Organization, Hyderabad, India

Impact fragmentation is an energetic process that has affected all planetary bodies. To understand its effects in basalt, we studied Lonar Crater, which is a rare terrestrial simple impact crater in basalt and analogues to km-scale simple craters on Mars. The Lonar ejecta consists of basaltic fragments with sizes ranging from silt to boulder. The cumulative size and mass frequency distributions of these fragments show variation of power index for different size ranges, suggesting simple and complex fragmentation. The general shape of the fragments is compact, platy, bladed and elongated with an average edge angle of 100 degrees. The size distribution of cobble- to boulder-sized fragments is similar to the fracture spacing distribution in the upper crater wall, indicating the provenance of those large fragments. Its consistency with a theoretical spallation model suggests that the large fragments were ejected from near surface of the target, whereas the small fragments from deeper level. The petrophysical properties of the ejecta fragments reflect the geophysical heterogeneity in the target basalt that significantly reduced the original size of spall fragments. The presence of Fe/Mg phyllosilicates (smectites) both in the ejecta and wall indicates the role of impact in excavating the phyllosilicates from the interior of basaltic target affected by aqueous alteration. The seismic images reveal a thickness variation in the ejecta blanket, segregation of boulders, fractures and faults in the bedrock beneath the crater rim. The fracturing, fragmentation and fluvial degradation of Lonar Crater has important implications for Mars.

Reference
Perumal SK, Kopisetti JPL, Nagula K, Menon R, Uppalapati S, Vallabaneni K, Arumugam SK, Dasari M, Tangirala S, Sen MK (2014) Impact fragmentation of Lonar Crater, India: Implications for impact cratering processes in basalt. Journal of Geophysical Research: Planets (in Press)
Link to Article [DOI: 10.1002/2013JE004543]

Published by arrangement with John Wiley&Sons

¹D. Polishook, ^1,2N. Moskovitz, ^1,3F.E. DeMeo, ¹R.P. Binzel
¹Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
²Lowell Observatory, 1400 West Mars Hill Road, Flagstaff, AZ 86001, USA
³Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA

The fission of an asteroid due to fast rotation can expose sub-surface material that was never previously exposed to any space weathering process. We examine the spectral properties of asteroid pairs that were disrupted in the last 2 million years to examine whether the site of the fission can be revealed. We studied the possibility that the sub-surface material, perhaps on one hemisphere, has spectral characteristics differing from the original weathered surface. This was achieved by performing rotationally-resolved spectroscopic observations to look for local variations as the asteroid rotates.
We spectrally observed 11 asteroids in pairs in the near-IR and visible wavelength range. Photometric observations were also conducted to derive the asteroid lightcurves and to determine the rotational phases of the spectral observations. We do not detect any rotational spectral variations within the signal-to-noise of our measurements, which allows us to tightly constrain the extent of any existing surface heterogeneity.
For each observed spectrum of a longitudinal segment of our measured asteroids, we estimate the maximal size of an un-detected “spot” with a spectral signature different than the average. For five asteroids the maximal diameter of such a “spot” is smaller by a factor of two than the diameter of the secondary member of the asteroid. Therefore, the site of the fission is larger than any area with a unique spectral parameters. This means the site of the fission does not have a unique spectrum. In the case of an ordinary chondrite asteroid (S-complex), where the site of fission is expected to present non-weathered spectra, a lack of a fission “spot” (detectable spectroscopically) can be explained if the rotational-fission process is followed by the spread of dust that re-accumulates on the primary asteroid and covers it homogeneously. This is demonstrated for the young asteroid 6070 that presents an Sq-type spectrum while its inner material, that is presumably revealed on the surface of its secondary member, 54827, has a non-weathered, Q-type spectrum. The spread of dust observed in the disintegration event of the asteroid P/2013 R3, might be an example of such a process and an indication that P/2013 R3 was indeed formed in a rotational-fission event.

Reference
Polishook D, Moskovitz FE, DeMeo RP, Binzel RP (2014) Rotationally Resolved Spectroscopy of Asteroid Pairs: No Spectral Variation Suggests Fission is followed by Settling of Dust. Icarus (in Press)
Link to Article [DOI: 10.1016/j.icarus.2014.08.010]

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