accurate, long-term observations made with adequate spatial and temporal resolution in a synoptic context. From lunar vantage points, it would be possible to sample the outgoing energy from virtually an entire hemisphere of Earth at once with high temporal and spatial resolution. At present this is only partially possible by combining data from LEO and geostationary orbit (GEO) satellites into an asynoptic composite of hundreds of thousands of pixels—rather like assembling an enormous jigsaw puzzle. Because of the integral view of the planet’s hemispheres continuously in the infrared and periodically at visible wavelengths, the observations will simultaneously overlap the observations of every LEO and GEO satellite in existence, making possible a unique synergy with great potential benefits for Earth sciences. One of the major potential benefits is the enabling of calibration sharing with all satellites, thus allowing the integration of all Earth-observing satellites in a single, integrated Earth-observing system that would provide self-consistent enriched data sets for Earth sciences. Such synergism would undoubtedly represent a major advance in Earth sciences and a greatly enhanced return for the nation’s investment in space and in particular for its investment in the exploration of the Moon.

Spectral observations from the Moon would allow for the first time continuous hemispheric synoptic retrievals of fundamental climate parameters, as well as data on cloud cover, aerosols, water vapor, and other atmospheric constituents, thus enabling unique observations of the diurnal cycle and its effects on the atmosphere. Note that observations in the IR are possible from all lunar orbital positions, while observations at visible wavelengths will be somewhat restricted.

Examples of possible instruments for observing Earth from the Moon are as follows:

  • Telescopes as a front end for spectroscopes to provide high spatial resolution imaging,

  • High-spectral-resolution spectrometers and/or interferometers covering the ultraviolet-visible-infrared spectrum, and

  • Reference radiometers.

The lunar view is nicely balanced in that it shows Earth rotating as well as going through its day-night phase alternation every 27 Earth days. Thus, both visual near-infrared observations that show solar reflected light and mid-IR observations that show Earth temperature could be obtained and would give a complete view to reveal changes such as El Niño–La Niña alternations and longer-term changes, and to reveal changes in the albedo of clouds with time.

Observations of Earth at relatively fine scale are possible with existing instrumentation. For example, a modest 2048 × 2048 pixel coverage of Earth has a pixel width of about 6.2 km. Yet the angular resolution required for this is possible with a 4 cm aperture at visible wavelengths. Even a resolution 10 times coarser would be adequate to put ~75 pixels across the United States and to watch weather patterns moving across Earth. Such observations would usefully allow cross calibration of the various geosynchronous orbiting weather satellite observations at 0.1 percent precision and so permit issues of the phase functions for observations to be resolved without recourse to theory.

In Situ Observations of the Variable Sun

Lunar exploration will provide opportunities to make use of the Moon itself to help develop a unified understanding of the radiative variability of the Sun (e.g., the constancy of the “solar constant”) on timescales of centuries. Drilling into the Moon’s regolith, if extended to a depth of 10 meters, will enable a measurement of the borehole temperature profile. The intent of the borehole heat flux experiment is to derive the history of the solar constant from the present-day temperature profile in the borehole and to gain knowledge of the thermal diffusivity of the regolith. The physics here involves the analysis of the transport of heat from the lunar surface down into the regolith. The effect of the constant heat flow from the lunar interior is removed by looking at deviations from the linear profile. This experiment was already attempted by Apollo 15 and Apollo 17, which succeeded in measuring the diffusivity but could only measure temperature to a depth of 2.7 m. Interpretation of a lunar borehole thermal profile is straightforward, providing a measure of variations in the solar constant extending back to the time of Galileo’s original 1610 mapping of sunspot areas, across the Maunder Minimum of sunspot numbers and



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