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Mapping Impact Melt around a Young Lunar Crater 


Impact melt is an important part of a fundamental process on terrestrial bodies. Impact melt is also poorly understood. I propose to map impact melt around the young lunar crater Giordano Bruno in order to identify where impact melt facies occur around a lunar crater. This crater has excellent exposures of impact melt, making it a great site for this endeavor. I will map the region at a 1:20,000 scale, which allows me to include fine detail near the limit of resolution. I will use LROC, KAGUYA, and Mini-RF to constrain the locations of impact melt around Giordano Bruno.

Impact cratering is a fundamental surface process on terrestrial bodies in the Solar System (Melosh, 1989). However, one of the least understood aspects of impact cratering involves the emplacement of impact melt, which is the portion of the target that is liquefied by the energy of the impact (Melosh, 1989). Impact melt can fill the floors of impact craters, flow down crater rims, or be ballistically ejected (Osinski et al., 2011; 2018; Bray et al., 2013; Bandfield et al., 2017). Melt produced by a shock event is not isothermal, but rather includes material initially ranging in temperature from just above the solidus to just below its vaporization temperature. Impact melts therefore have a wide range in viscosities (Melosh, 1989; Denevi et al., 2012).

The Moon provides one of the best places to study impact melt emplacement processes because Copernican-age lunar craters are generally well preserved (Bray et al., 2010; Neish et al., 2017). Lunar studies are also enabled by excellent sources of data, such as NASA’s Lunar Reconnaissance Orbiter Camera (LROC) and JAXA’s SELenological and ENgineering Explorer (SELENE) “KAGUYA” Terrain Camera (TC), both of which have sufficiently high resolution (LROC: 1 m/pixel; TC: 10 m/pixel) to allow for the creation of incredibly detailed geologic maps (Robinson et al., 2010; Kato et al., 2010).

In this work, I am geologically mapping impact melt and the units associated with a young impact crater. I aim to quantify the environments and facies types associated with impact melts (Öhman and Kring, 2012). Facies in this context refers to a region with similar attributes that exists within a more broadly defined lithostratigraphic unit (i.e., an impact melt). In other words, facies mapping provides a method of subdividing a geologic unit into regions that may all be associated with the same event, but each exhibit distinct physical appearances and settings that are related to different emplacement conditions. These facies include veneers, ponds, and flows, which may either be channelized or lobate (Bray et al., 2018). Veneers are thin coats of melt that tend to occur near their parent impact crater, and typically on steep slopes. Channelized flows are places where the impact melt creates a leveed path, or channel, through which melt flowed. In contrast, lobate flows tend to occur closer to distal margin, or toe, of a flow where the melt is expected to be cooler and more viscous. Ponding can occur anywhere that melt accumulates within closed topographic depressions. I will map the impact melt around the young luanr crater Giordano Bruno, because it has excellent exposures of each of these facies types.

Giordano Bruno is a 22-kilometer-diameter crater on the farside of the Moon. It has impact melt ponds, veneers, lobate flows, and channelized flows. With a wide variety of impact melt facies, Giordano Bruno is an excellent case study. The total area I will map is approximately 3000 square km, approximately a rectangle with sides 50 km by 60 km, and the resulting geologic map will have a resolution of 1:20,000, near the limit of resolution. I chose this resolution based on the size of resolvable features, and inclusion of important details.

I will map this crater using data from the LROC, Mini-RF, and KAGUYA (Robinson et al., 2010; Nozette et al., 2010; Kato et al., 2010). LROC has high-resolution imagery (1 m/pixel) that will allow for detailed mapping of the region. Radar imagery reveals varying textures of impact melt and can be used to identify flow because the circular polarization ratio of the S-band data is higher for impact melt deposits than the surrounding material (Neish et al., 2014). Therefore, I will use the S-band (12.6 cm) data to help reveal locations of impact melt (Neish et al. 2014, 2017) and characterize deposits that have roughness that this length-scale (Neish et al., 2014). 

"You develop an instant global consciousness, a people orientation, an intense dissatisfaction with the state of the world, and a compulsion to do something about it. From out there on the moon, international politics look so petty." – Edgar Mitchell