The Diviner Lunar Radiometer Experiment is mapping the temperature of the surface of the Moon in unprecedented detail. Diviner is one of seven instruments aboard NASA’s Lunar Reconnaissance Orbiter (LRO) http://lunar.gsfc.nasa.gov/ which launched June 18, 2009.
The Moon’s surface temperatures are among the most extreme of any planetary body in the solar system. Noontime surface temperatures near the lunar equator are hotter than boiling water, whereas nighttime surface temperatures on the Moon are almost as cold as liquid oxygen. It has been estimated that near the lunar poles, in areas that never receive direct sunlight, temperatures can dip to within a few tens of degrees of absolute zero. During the course of LRO’s mapping mission, Diviner will map the entire surface of the Moon at a resolution of better than 500 meters to create the first global picture of the current thermal state of the moon and its daily and seasonal variability.
The Moon’s extreme temperature environment is of interest to future human and robotic explorers, especially if they plan on visiting the Moon for extended periods. Detailed thermal maps of the Moon can also yield information regarding the locations of rocky areas that may be hazardous to landing vehicles, and for mapping compositional variations in lunar rocks and soils. In the Moon’s polar regions, temperature maps also point to the locations of cold traps where water ice and other volatile materials may have accumulated. Mapping the locations of these lunar cold traps and searching for the presence of frozen water is one of the main goals of the LRO mission.
Diviner is designed, built, and operated by the California Institute of Technology Jet Propulsion Laboratory (JPL) in Pasadena, CA. Prof. David A. Paige of the University of California, Los Angeles (UCLA) is the Principal Investigator.
How Diviner Works
Diviner determines the temperature of the Moon by measuring the intensity of infrared radiation emitted by the lunar surface. The hotter the surface, the greater the intensity of emitted infrared radiation. Diviner measures infrared radiation in seven infrared channels (Channels 3-9) that cover a wavelength range from 7.6 to 400 microns. Diviner is the first instrument designed to measure the full range of lunar surface temperatures, from the hottest to the coldest. Diviner also includes two solar channels (Channels 1-2) that measure the intensity of reflected solar radiation.
As LRO orbits the Moon every two hours, Diviner maps a nearly continuous ~3.5 km-wide swath on the lunar surface. Diviner’s swath samples the full range of lunar longitudes once per month to create thermal maps. Diviner will acquire 24 thermal maps of the Moon over the course of a year - 12 daytime maps and 12 nighttime maps, each covering a different range of lunar local times.
Diviner Commissioning Orbit Operations
Diviner has been mapping the Moon continuously during the LRO commissioning phase. Since the instrument was first activated on July 5, 2009, it has acquired over 8 billion calibrated radiometric measurements, and has mapped almost 50% of the surface area of the Moon. “The performance of the instrument has been excellent, and closely matches our predictions” quotes Instrument Engineer Marc Foote at JPL. Says Principal Investigator David Paige of UCLA, “We have already accumulated an enormous amount of high-quality data”.
Diviner has obtained enough data during the LRO commissioning phase to characterize many aspects of the Moon’s current thermal environment. There are large gaps between Diviner’s individual ground tracks at the equator, but in the polar regions, the ground tracks overlap to create continuous high-resolution maps. The plane of the LRO orbit sampled from 5:40 am to 5:40 pm lunar local time at the start of commissioning and gradually drifted to sampling from 1:10 am to 1:10 pm by the end of commissioning. It will take about six months for LRO’s orbit to sample the full range of lunar local times.
In addition to mapping the Moon, Diviner executed a series of specialized calibration sequences to during the commissioning phase. These included scans of the limb of the Moon to better define the instruments fields of view, an infrared panorama of a portion of the LRO spacecraft (See Figures 1a-c), as well as infrared scans of the Earth from lunar orbit, which are presently being analyzed. “Diviner’s operations have run very smoothly” says JPL scientist and lead observational sequence designer Benjamin Greenhagen, “Diviner has been put through her paces and has executed our commands brilliantly”.
First Global Temperature Maps
Figures 2a-b show the first global daytime and nighttime thermal maps of the Moon, created using Diviner data accumulated during August and the first-half of September, 2009. The maps show Channel 8 (50-100 micron wavelength range) brightness temperatures which approximate actual surface physical temperatures. Equatorial and mid-latitude daytime temperatures are close to 380K (224° F), and then decrease sharply poleward of 70° north latitude. Equatorial and mid-latitude nighttime temperatures are close to 95K (-298° F) and then decrease poleward of 80° north latitude. At low and mid-latitudes, there are isolated warmer regions with nighttime temperatures of 140K (-208° F). These correspond to the locations of larger fresh impact craters that have excavated rocky material that remains significantly warmer than the surrounding lunar soil throughout the long lunar night. The thermal behavior at high latitudes contrasts sharply with that of the equatorial and mid-latitudes. Close to the poles, both daytime and nighttime temperatures are strongly influenced by local topography and the thermal outlines of many partially illuminated impact craters are apparent. “Getting a look at the first global thermal maps of the lunar surface has been very exciting” says David Paige, “it’s a whole new way of seeing the Moon”.
High-Resolution North and South Polar Thermal Maps
Figures 3a-b, 4a-b and 5a-b show Diviner high-resolution daytime thermal maps of the north and south polar regions acquired between July 17 and August 14, which corresponds to 5 pm and 3 pm lunar local time. Because of the eccentricity of the LRO commissioning orbit, the north polar maps have a spatial resolution of 700 m and the south polar maps have a spatial resolution of 250 m. Figures 3a and 4a show Diviner Channel 3 (8 micron wavelength) brightness temperatures. Channel 3 is Diviner’s shortest wavelength infrared channel and is insensitive to extremely low temperatures in unilluminated areas. This property makes it useful for mapping the extent of shadowed regions, which appear black just as in a visible image. Figures 3b and 4b show Diviner Channel 8 (50-100 micron wavelength) brightness temperatures. Channel 8 is one of Diviner’s longest wavelength infrared channels and it has excellent capability to measure extremely low temperatures in unilluminated areas.
The Channel 8 maps reveal richly detailed thermal behavior throughout the north and south polar regions that extends to the limit of Diviner’s spatial resolution (see Figures 5 and 6). Most notable are the measurements of extremely cold temperatures within the permanently shadowed regions of large polar impact craters in the south polar region. Diviner has recorded minimum daytime brightness temperatures in portions of these craters of less than 35K (-397° F) in the coldest areas. These are to our knowledge, these super-cold brightness temperatures are among the lowest that have been measured anywhere in the solar system, including the surface of Pluto. According to science team member Ashwin Vasavada of JPL, “After decades of speculation, Diviner has given us the first confirmation that these strange, permanently dark and extremely cold places actually exist on our Moon. Their presence greatly increases the likelihood that water or other compounds are frozen there. Diviner has lived up to its name.”
The Diviner commissioning phase observations provide a snapshot in time of current polar temperatures that will evolve with the lunar seasons. However, it is safe to conclude that the temperatures in these super-cold regions are definitely low enough to cold-trap water ice, as well as other more volatile compounds for extended periods. The existence of such cold traps has been predicted theoretically for almost 50 years. Diviner is now providing detailed information regarding their spatial distribution and temperatures. Diviner’s thermal observations represent one component of LRO’s strategy for determining the nature and distribution of cold-trapped water ice in the lunar polar regions. Future comparisons between Diviner data, physical models, and other polar datasets may provide important scientific conclusions regarding the nature and history of the Moon’s polar cold traps, and any cold-trapped volatile materials they contain.