THE THRILLING IMPACT OF CRATERS IN HIGH RESOLUTION

In 2006, the exciting discovery of new impact craters forming on Mars in the present day was announced. This was a revolution in a field where most features studied are millions of years old. Since then, Reconnaissance Orbiter (MRO) has discovered ~1,000 of these new, dated craters using the Context (CTX) camera for initial detection and the High Resolution Imaging Science Experiment (HiRISE) to follow up and resolve diameters. Similar techniques were automated to find >500 resolved craters and 120,000 splotches (small surface disturbances) in Lunar Reconnaissance Orbiter Camera (LROC) images. These rich databases of fresh impacts on the Moon and Mars, along with the vast collection of high-resolution HiRISE images, have opened up many avenues of investigation to researchers in the planetary cratering community:

• The current Martian cratering rate appears to be lower than predicted by standard production functions at small sizes.
• The current lunar cratering rate is higher than predicted, resulting in a much faster overturn of the top centimeters of regolith than previously thought.
• Water ice exposed in the Martian mid-latitudes indicates past atmospheric water vapor content higher than present values. The ice requires years to fade, indicating a low lithic content.
• New craters have a depth to diameter ratio of ~0.2, as expected for primary craters and distinct from secondary craters.
• Dark albedo patterns around new impacts on Mars fade on timescales of decades, except at high latitudes where seasonal processes erase them in their first winter. The size of halos is has a nonlinear relationship with impact energy.
• Half of new Martian craters are clusters, which reveal new information about atmospheric fragmentation processes.
• Layered bedrock and megabreccia exposed in the central uplifts and pits of Martian craters have shown that target layering is an important factor in their formation.
• Impact melt ponds and flows with dense clusters of pits due to devolatilization have been recognized on Mars, and then by extension on volatile-rich bodies including Vesta and Ceres.
• Seismic detectability of impacts is an ongoing area of study.

These and other discoveries in impact cratering are due to the increased resolution, coverage, and long-baseline change detection afforded by orbiting cameras over the last decade.