UV Imaging of the Moon from the Hubble Space Telescope

Pioneering telescopic measurements of the Moon in the near UV (360 nm) and Visible (610 nm) wavelengths first indicated that compositional units on the Moon might be mapped through remote sensing techniques [1]. From these data it was noted that mare units exhibited a wide range of color contrasts and it was initially hypothesized that these differences might in part be due to varying abundances of ilmenite in lunar soils [2]. However, due to absorption in the Earth’s atmosphere, ground-based telescopic observations of lunar UV reflectance below about 360 nm are not possible. Furthermore no spacecraft to date has acquired high resolution spectral observations across a broad range of the UV, thus the full potential of lunar compositional mapping in the UV has not been realized. The Hubble Space Telescope (HST) provides the unique ability to acquire multi-spectral (UV through near-infrared) lunar observations at scales approaching ~100 m/pixel (and down to ~ 60 m/pixel in the Visible). On the basis of the rapid relative velocities between the HST and the Moon, such observations are technically challenging in terms of pointing accuracy and stability, and had not been successfully achieved until August of 2005. The new HST lunar data consist of ACS/HRC 4 filter observations (F250W, F344N, F502N, F658N) of three high priority targets: Apollo 17 landing site, Apollo 15 landing site, and the Aristarchus crater and plateau. The successfully-acquired HST observations had two primary objectives: (1) to assess the magnitude and spatial relations of contrast variations in the UV; and (2) to assess the ability of UV reflectance spectroscopy to quantitatively measure TiO2 content within mare soils as one approach for identifying potential lunar resources in support of human exploration. The Apollo sites were chosen to allow calibration of the HST measurements in association with known compositions of lunar samples. The Aristarchus region was chosen for its diversity of composition and geologic processes, and to facilitate UV-based assessment of materials associated with a large, Copernican-age impact crater and adjacent pyroclastic deposits. Figure 1. (A) HST 250 nm reflectance in region of Apollo 17 (LM indicated with arrow). (B) HST 502/250 nm ratio (same area as in A). Width is 32.4 km, North is up (see also [3]).