Galileo observations of post‐imbrium lunar craters during the first Eearth‐Moon flyby

Abstract

Copernican‐age craters are among the most conspicuous features seen on the far side and western limb of the Moon in the Galileo multispectral images acquired in December 1990. Among the new morphologic observations of far‐side craters are bright rays, continuous ejecta deposits, and dark rings associated with probable impact‐melt veneers. These observations suggest that the mapped age assignments of several large far‐side craters (Ohm, Robertson, and possibly Lowell and Lenz) need revision. New crater size‐frequency measurements on Lunar Orbiter images suggest the following age reassignments: Hausen (170 km diameter), Pythagoras (120 km), and Bullialdus (61 km) from Eratosthenian to Upper Imbrian, and Carpenter (60 km) and Harpalus (39 km) from Copernican to Eratosthenian. Colors and albedos of craters (away from impact‐melt veneers) are correlated with their geologic emplacement ages as determined from counts of superposed craters; these age‐color relations are used to estimate the emplacement age (time since impact event) for other Copernican‐age craters. These age‐color relations indicate a probable Copernican age for 27 far‐side or western limb craters larger than 10 km diameter that were not previously mapped as Copernican. The apparent deficiency of Copernican craters on the far side compared with the near side in published geologic maps is not present in our data. Age‐color trends differ between mare and highland regions and between the interiors and continuous ejecta of the craters. Similar trends are established for color and albedo versus soil‐maturity indices for the returned lunar samples, with distinct trends for mare and highland soils. However, the mare versus highland offsets are reversed in the two comparisons. These relations can be explained by variations in regolith thicknesses and rates of mixing with relatively fresh, crystalline ejecta. Therefore, the soil‐maturity trends represent longer geologic time periods in regions with thinner regoliths, such as the maria.

Can planetesimals left over from terrestrial planet formation produce the lunar Late Heavy Bombardment?

“…For reference, the postmare cratering rate on the Moon over the last 3 Gy has been ∼3 × 10 −15 km −2 yr −1 for D crater 20 km (e.g., Grieve and Shoemaker, 1994;Shoemaker and Shoemaker, 1996;McEwen et al, 1997;Shoemaker, 1998; see also Bottke et al, 2005aBottke et al, , 2005b. The largest impact structure formed on the Moon during the last ∼3 Gy is Langrenus, a 132 km crater that is too small to qualify for basin status (McEwen et al, 1993). We infer from this data that the early impact history of the Moon was very different from that of the last few Gy.…”

Nonuniform cratering of the terrestrial planets

“…11. We additionally plot the number of Copernican and Eratosthenian craters larger than 25 km on the whole surface of the Moon (127 craters), using data provided by N. Petro, which is a compilation of data from Wilhelms et al (1987), Grier et al (2001), McEwen et al (1993 and McEwen and Robinson (1997). For comparative purposes, we plot our predicted cratering rate using the same crater scaling laws as in Fig.…”

New Views of Lunar Geoscience: An Introduction and Overview

“…During the fi rst fl yby, Galileo imaged the western nearside and parts of the farside that were not illuminated during the Apollo missions, thus becoming the fi rst spacecraft to obtain multispectral images of the Moon since Mariner 10 days . The images from the SSI camera (e.g., Belton et al 1992) led to investigations of the crustal diversity of the western hemisphere, the geology of several lunar impact basins such as the Orientale basin and South Pole-Aitken basin, the western maria and their related deposits, and the post-Imbrium impact craters (e.g., Greeley et al 1993;Head et al 1993;McEwen et al 1993;Pieters et al 1993a). During the second fl yby, Galileo took multispectral images from the north-central nearside (e.g., Belton et al 1994) that allowed detailed studies, for example, of the Humboldtianum basin, a large impact structure only partially visible from Earth.…”