The Diviner Lunar Radiometer Experiment is one of seven instruments aboard NASA’s Lunar Reconnaissance Orbiter, which launched on June 18, 2009. It is the first instrument to create detailed day and night surface temperature maps of the Moon. Data from Diviner has helped identify potential ice deposits in the polar regions, map compositional variations on the surface and derive subsurface temperatures. Since July of 2009, Diviner has operated continuously, acquiring nearly one trillion radiometric measurements to create the most detailed and complete set of thermal measurements of any planet in the solar system.
Why Measure the Moon's Temperature?
There are three key environmental factors that set the Moon apart from Earth: its lower gravity, its virtual lack of an atmosphere, and the extreme temperature fluctuations experienced on its surface. Understanding the fluctuations in lunar surface and subsurface temperature is critical for future human and robotic exploration: whereas the Apollo missions all took place during the lunar day and involved landings in equatorial regions, future missions will cover a much wider range of latitudes, and will take place over longer durations. Diviner is currently mapping surface and subsurface temperatures, providing information about potential ice reservoirs and suitable thermal environments for habitation.
The Lunar Thermal Enviroment
With the exception of Mercury, the Moon has the most extreme surface thermal environment of any planetary body in the solar system. At the lunar equator, mean surface temperatures reach almost 400K (260.6 ºF) at noon and then drop to below 100K (-279.4 ºF) during the night. For comparison, the mean surface temperature on Earth is a temperate 295K (71.6 ºF).
The Earth and Moon each receive the same flux of solar radiation; the important difference is that the Moon doesn't have an atmosphere to insulate its surface. In addition to this the lunar day/night cycle lasts ~1 month (compared to 24 hours on Earth). Both of these factors are key in producing the extreme range of temperatures experienced on the Moon.
Thermal model calculations of monthly and annual lunar surface temperature variations at various latitudes.
Annimation showing measured Diviner-measured monthly global bolometric lunar surface temperatures
The spin axis of the Moon is almost perpendicular to the eclliptic plane. The result is that areas of high and low elevation at the lunar poles experience extremes in illumination, with some crater floors remaining in permanent shadow. Diviner has revealed that temperatures within these permanently-shadowed craters can fall as low as 25K (-414.4 ºF). These cold traps present ideal candidates for potential stores of ice on the lunar surface because although the Moon has no indigenous water, molecules are deposited on its surface from comet and asteroid impacts.
Conversely, some of the crater rims adjacent to these permanently-shadowed regions are high enough that they receive continuous sunlight, and Diviner has observed that temperatures in these regions remain constant at around 220K. This makes them ideal sites for extended surface operations.
Heat flow measurements made during the Apollo 15 and 17 missions revealed that the top 1-2 cm of lunar regolith has extremely low thermal conductivity. The mean temperature measured 35cm below the surface of the Apollo sites was 40-45K warmer than the surface. At a depth of 80cm the day/night temperature variation experienced at the surface was imperceptible. This implies that habitations in the lunar subsurface exist that are not subject to the harsh temperature extremes prevalent on the surface.
The Diviner team will produce and archive a range of data products. These include low-level products derived from instrument telemetry (Level 0); calibrated data with associated geometry (Level 1); and higher-level data products that include gridded temperatures (Level 2); and derived fields such as thermal inertia, rock abundance, and mineralogy that will be created with the aid of topographic data and models (Level 3). Additionaly, the Diviner team will provide specialized data products relating to permanently shadowed regions at the lunar poles (Level 4). These products will be made available to the public online through this web site, and archived through the Geosciences Node of NASA's Planetary Data System.