¹Selby Cull-Hearth, ¹Alexis van Venrooy, ¹M. Caroline Clark,²Adriana Cvitkovic
¹Department of Geology, Bryn Mawr College, 101 N. Merion Avenue, Bryn Mawr, PA 19123, USA
²Haverford College, Haverford, PA, 19041, USA

Most sulfate minerals form only in specific pH conditions, making them useful markers of past environmental conditions on Mars. However, interpreting past environments requires a full understanding of the suite of minerals present, a task which is complicated by the fact that some minerals can spectrally mask others in the visible- to near-infrared (VNIR, 0.4 to 2.5 μm). Here, we report VNIR spectra of two-phase mineral combinations obtained from the Río Tinto acid mine drainage system of southern Spain. Our results show that in VNIR reflectance spectroscopy: 1) copiapite masks rhomboclase and partially masks melanterite; 2) coquimbite masks copiapite, jarosite, and rhomboclase; 3) at wavelengths 1.2 μm, gypsum masks these minerals; 4) unlike copiapite, jarosite, or melanterite, halotrichite masks gypsum completely; 5) in two-phase mixtures of copiapite and jarosite, both phases are evident. No consistent VNIR relationship is observed in two-phase mixtures of melanterite and halotrichite, suggesting that microtextures are likely more important than optical properties in determining VNIR reflectance. We also show that the shorter wavelengths are more sensitive to the presence of both phases: even in mixtures where one phase is masking another, both phases usually impact absorptions in the 0.75 – 0.95 μm region. This region may therefore be useful in remotely identifying mineral mixtures on Mars. These results have implications for several regions on Mars: most notably, they imply that the jarosite exposures reported at Mawrth Vallis may be jarosite-copiapite mixtures.

Reference
Cull-Hearth S, van Venrooy A, Clark MC, Cvitkovic A (2016) Acid-sulfate mixtures from Río Tinto, Spain: Spectral masking relationships and implications for Mars. Icarus (in Press)
Link to Article [doi:10.1016/j.icarus.2016.01.001]
Copyright Elsevier

^1,2B. Carry, ^3,4E. Solano, ¹S. Eggl, ^5,6F.E. DeMeo
¹IMCCE, Observatoire de Paris, PSL Research University, CNRS, Sorbonne Uni- versit es, UPMC Univ Paris 06, Univ. Lille
^“European Space Astronomy Centre, ESA, P.O. Box 78, 28691 Villanueva de la Cañada, Madrid, Spain
^§Centro de Astrobiologia (INTA-CSIC), Departamento de Astrofisica. P.O. Box 78, E-28691 Villanueva de la Cañada, Madrid, Spain
⁴Spanish Virtual Observatory
⁵Department of Earth, Atmospheric and Planetary Sciences, MIT, 77 Massachusetts Avenue, Cambridge, MA, 02139, USA
⁶Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, MS-16, Cambridge, MA, 02138, USA

The nature and origin of the asteroids orbiting in near-Earth space, including those on a potentially hazardous trajectory, is of both scientific interest and practical importance. We aim here at determining the taxonomy of a large sample of near-Earth and Mars-crosser asteroids and analyze the distribution of these classes with orbit. We use this distribution to identify the source regions of near-Earth objects and to study the strength of planetary encounters to refresh asteroid surfaces. We measure the photometry of these asteroids over four filters at visible wavelengths on images taken by the Sloan Digital Sky Survey (SDSS). These colors are used to classify the asteroids into a taxonomy consistent with the widely used Bus-DeMeo taxonomy (DeMeo et al., Icarus 202, 2009) based on visible and near-infrared spectroscopy. We report here on the taxonomic classification of 206 near-Earth and 776 Mars-crosser asteroids determined from SDSS photometry, representing an increase of 40% and 663% of known taxonomy classifications in these populations. Using the source region mapper by Greenstreet et al. (Icarus, 217, 2012), we compare for the first time the taxonomic distribution among near-Earth and main-belt asteroids of similar diameters. Both distributions agree at the few percent level for the inner part of the Main Belt and we confirm this region as a main source of near-Earth objects. The effect of planetary encounters on asteroid surfaces are also studied by developing a simple model of forces acting on a surface grain during planetary encounter, which provides the minimum distance at which a close approach should occur to trigger resurfacing events. By integrating numerically the orbit of the 519 S-type and 46 Q-type asteroids in our sample back in time for 500,000 years and monitoring their encounter distance with Venus, Earth, Mars, and Jupiter, we seek to understand the conditions for resurfacing events. The population of Q-type is found to present statistically more encounters with Venus and the Earth than S-types, although both S- and Q-types present the same amount of encounters with Mars.

Reference
Carry B, Solano E, Eggl S, DeMeo FE (2016) Spectral properties of near-Earth and Mars-crossing asteroids using Sloan photometry. Icarus (in Press)
Link to Article [doi:10.1016/j.icarus.2015.12.047]
Copyright Elsevier