Arya Udry^a, Nicole G. Lunning^a, Harry Y. McSween JR.^a and Robert J. Bodnar^b

^aPlanetary Geosciences Institute, Department of Earth and Planetary Sciences, University of Tennessee, Knoxville, Tennessee 37996, TN, USA
^bDepartment of Geosciences, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061, VA, USA

Northwest Africa (NWA) 7034 and its paired meteorites NWA 7533 and NWA 7475 are the first recognized martian polymict breccia samples. An unusual, large, subrounded clast in NWA 7034 shows a vitrophyric texture, consisting of skeletal pyroxene and olivine with mesostasis. This lithology has not been observed in the paired meteorites. It crystallized under disequilibrium conditions as indicated by its olivine and pyroxeneK_D^Fe/Mg partitioning values, as well as reversed order of crystallization and mineral compositions relative to those predicted by MELTS. We report the highest bulk Ni value (1020 ppm) measured in any known martian meteorite or martian igneous rock, suggesting an impact melt origin for the vitrophyre. Addition of 5.3-7.7% chondritic material to the target rock would account for the Ni enrichment. The bulk major and trace element abundances of the vitrophyre indicate that the protolith was not the host breccia nor any other martian meteorites. However, the clast is compositionally similar to Humphrey rock in Gusev crater analyzed by the Spirit rover and to a texturally distinct group of clasts in the paired meteorite NWA 7533. Thus, we propose that the target rock was an igneous lithology similar to Gusev basalts, which was subsequently contaminated by a chondritic impactor.

Reference
Udry A, Lunning NG, McSween JR. HY and Bodnar RJ (in press) Petrogenesis of a vitrophyre in the martian meteorite breccia NWA 7034. Geochimica et Cosmochimica Acta
[doi:10.1016/j.gca.2014.06.026]
Copyright Elsevier

Link to Article

Davide Farnocchia¹, Steven R. Chesley¹, Paul W. Chodas¹, Pasquale Tricarico², Michael S. P. Kelley³ and Tony L. Farnham³

¹Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA
²Planetary Science Institute, Tucson, AZ 85719, USA
³Department of Astronomy, University of Maryland, College Park, MD 20742, USA

Comet C/2013 A1 (Siding Spring) will experience a high velocity encounter with Mars on 2014 October 19 at a distance of 135,000 km ± 5000 km from the planet center. We present a comprehensive analysis of the trajectory of both the comet nucleus and the dust tail. The nucleus of C/2013 A1 cannot impact on Mars even in the case of unexpectedly large nongravitational perturbations. Furthermore, we compute the required ejection velocities for the dust grains of the tail to reach Mars as a function of particle radius and density and heliocentric distance of the ejection. A comparison between our results and the most current modeling of the ejection velocities suggests that impacts are possible only for millimeter to centimeter size particles released more than 13 AU from the Sun. However, this level of cometary activity that far from the Sun is considered extremely unlikely. The arrival time of these particles spans a 20-minute time interval centered at 2014 October 19 at 20:09 TDB, i.e., around the time that Mars crosses the orbital plane of C/2013 A1. Ejection velocities larger than currently estimated by a factor >2 would allow impacts for smaller particles ejected as close as 3 AU from the Sun. These particles would reach Mars from 19:13 TDB to 20:40 TDB.

Reference
Farnocchia D, Chesley SR, Chodas PW, Tricarico P, Kelley MSP and Farnham TL (2014) Trajectory Analysis for the Nucleus and Dust of Comet C/2013 A1 (Siding Spring). The Astrophysical Journal 790:114.
[doi:10.1088/0004-637X/790/2/114]

Link to Article