Dawn Over Bhabha Crater

Center peak of Bhabha crater from shadows of dawn — Central peak complex of Bhabha crater (70 kilometer diameter) rising from the shadows of dawn, image snapped on 28 August 2019 from an altitude of 73 kiolmeters. View is seen from east-to-the west, north is to the right, visible portion of central peak complex is about 14 kilometers wide, NAC M1321101374LR [NASA/GSFC/Arizona State University].

Bhabha crater is located on the Moon's farside, deep within the South Pole–Aitken (SPA) basin. At 80 km in diameter, Bhabha is approximately the same size as Tycho crater, though it lacks Tycho's spectacular rays and crisp features because it is substantially older. But Bhabha has other aspects going for it that make the crater important in its own way. Its location within SPA means that the impact event exposed material that originally resided deep within the Moon, but was excavated and melted by the giant SPA imact event. Reflectance spectroscopy has shown that the central peaks of Bhabha contain pyroxenes that are rich in magnesium but poor in calcium, typical of many craters within SPA. This material may represent the melt sheet of SPA. However the walls of Bhabha have pyroxenes that are rich in calcium, more typical of volcanic material. A mound just to the southeast of Bhabha (termed "mafic mound") also shares this composition, and has been proposed to be a volcanic feature that formed from extruded SPA melt, or melting of the underlying mantle (Moriarty and Pieters, 2015).

Full panorama of Bhabha oblique — Sunrise at Bhabha crater, where sunlight hasn't yet reached the crater floor after the two-week night. Note that a portion of the eastern rim (bottom of image) is still in shadow due to highstanding terrain just outside of the scene (see topographic profile below) NAC M1321101374LR [NASA/GSFC/Arizona State University].

In the profile below, note the crater's unusual topography, where outside of the eastern rim of the crater the ground continues to rise. Was this mound outside of the eastern rim there before the crater formed? The high-calcium pyroxene composition of Bhabha's walls suggest that the Bhabha impactor happened upon on another region of non-mare volcanism – perhaps this mound is a remnant of that volcanic history as well?

Topographic profiles of east half of Bhabha crater — Topographic profiles of the eastern half of Bhabha crater, note the large topogrpahic rise on the eastern flank of the crater. Was that peak there before the Bhabha event, or did ejecta pile up more on the eastern side than elsewhere around the crater? Use the topographic profile tool in Quickmap (magnifying glass icon) to better explore the landscape.

A site like Bhabha crater would make an excellent spot to explore the unusual geologic history of the SPA basin and how the surface and subsurface evolved after the formation of that basin. Samples obtained from this area would allow us to test theories about the timing of the formation of the basin, learn whether there was a large spike in impact events around 3.9 billion years ago, better understand the composition of the Moon's interior, and learn about non-mare volcanism. Scroll around the full-resolution image of Bhabha below, and imagine the sunrise you would see as you explored this magnificent crater.

Published by Mark Robinson on 22 October 2019