The lunar environment and its effect on optical astronomy

Abstract

The Moon's geologic environment features: (1) gravity field one-sixth that of Earth; (2) sidereal rotation period of 27.3 days; (3) surface with greater curvature than Earth's surface (a chord along a 10 km baseline would have a bulge of 7.2 m); (4) seismically and tidally stable platform on which to make astronomical observations (most moonquakes have magnitudes of 1 to 2 on the Richter scale, within the earth's seismic noise, resulting in ground motions only 1 nm); (5) tenuous atmosphere (the total mass at night is only 10(exp 4) kg) that has an optical depth of 10(exp -6) and does not cause wind induced stresses and vibrations on structures; (6) large diurnal temperature variation (100 to 385 K in equatorial regions), which telescopes must be designed to withstand; (7) weak magnetic field, ranging from 3 to 330 x 10(exp -9) T, compared to 3 x 10(exp -5) T on Earth at the equator; (8) surface exposed to radiation, the most dangerous of which are high energy (1 to 100 Mev) particles resulting from solar flares; (9) high flux of micrometeorites which are not slowed down from their cosmic velocities because of the lack of air (data indicate that microcraters greater than 10 microns across will form at the rate of 3000/sq m/yr); (10) regolith 2 to 30 m thick which blankets the entire lunar surface (this layer is fine-grained (average grain sizes range from 40 to 268 microns), has a low density (800 to 1000 kg/cu m in the upper few mm, rising to 1500 to 1800 kg/cu m at depths of 10 to 20 cm), is porous (35 to 45 pct), cohesive (0.1 to 1.0 kN/sq m), and has a low thermal diffusivity (0.7 to 1.0 x 110-8 sq m/sec); about 29 pct of the regolith is less than 20 micron in size (this dust could pose a hazard to optical telescopes); (11) rubbly upper several hundred meters in which intact bedrock is uncommon, especially in the lunar highlands; and (12) craters with diameter-to-depth ratios of 5 if fresh and less than km across (larger and eroded craters have diameter-to-depth ratios greater than 5)