IMPACT SPHERULES AS A SOURCE OF SAND ON AMAZONIAN MARS

Aeolian erosion and deposition is a major landscape modification process on Mars today, but it is unclear what processes generate the sand grains that drive this sediment cycle. Here we synthesize several studies investigating the origin of modern martian sands based on their mineralogy, as detected via visible/near-infrared spectra from the CRISM orbital imaging spectrometer.

A global survey of the mineralogy of 45 non-polar dune fields on Mars reveals that most dune fields (50%) are mixtures, including olivine, pyroxene, and glass. However, a large fraction of dune fields (20%) exhibit absorption bands uniquely consistent with high quantities of glass, as well as strong blue spectral slopes. The remainder of the dune fields (30%) exhibit only these strong blue spectral slopes, with weak/no absorption bands, even though they appear to be dark, active, and dust-free. In many locations, a strong correlation between glass absorptions and this spectral slope suggests that this effect is due to weathering rinds on glass, but elsewhere, may be more consistent with aphanitic basalt (e.g., ash). Similar spectra are observed in the north polar region, where dunes are either glass-rich or a glass/pyroxene mixture, dark sediments that may be sourced from young latitude-dependent mantles also exhibit strong blue spectral slopes without absorption bands, and glass-rich sediments are currently eroding out of the young icy polar layered deposits. Thus, glass appears to be a common component of young aeolian sediments on Mars.

Explosive volcanic eruptions generate glass, but only at high abundances when water quenches the magma during phreatomagmatic eruptions. Impacts also produce glass in many forms, but disperse glass-rich sediments when melt is ejected into space, quenched, and reenters to form sand-sized impact spherules. However, volcanic sand will only be deposited in close proximity to the edifice, while impact spherules can be dispersed in strewn fields across the planet, including the north pole. Thus, we propose that much of the glass in martian dunes and northern plains may be due to accumulation of impact spherules during the Amazonian. If true, then impact melt trapped in the NPLD and latitude-dependent mantles could be used to date emplacement of these young deposits and constrain the recent impact history of Mars.