Wednesday, December 16, 2009

Coldest Spot on the Moon

Hermite is foreground for an Earthrise in 2007, witnessed by Japan's Kaguya. Hugging the west side of the Moon's north polar region Hermite is within the Zone of Librations. Though two-thirds of its 114 km-wide interior is technically on the Moon's Far Side it can still be seen from Earth. Not along its southwestern interior, however, in perpetual shadow and where LRO's Diviner has measured the Moon's coldest surface temperatures, temperatures as cold as any now known this side of the Kuiper Belt [JAXA/NHK/SELENE].

Diviner News - The Diviner lunar radiometer has been mapping the temperature of the Moon since July, 2009. During this period, temperatures in the lunar polar regions have changed gradually as the lunar seasons have evolved. The tilt of the moon’s spin axis is only 1.54 degrees and as a consequence, lunar seasons are barely noticeable in most locations on the Moon. However, at the north and south poles, the height of the sun above the horizon varies by more than 3 degrees over the course of the year. This affects the percentage of sunlit regions and surface temperatures at the poles.

During October, 2009, Diviner observed the passage of summer solstice in the southern hemisphere and winter solstice in the northern hemisphere. The LRO launch date was chosen so that its orbital plane passed through the noon to midnight plane in October, allowing Diviner to measure the extremes of polar temperatures. Figure 1 illustrates the configuration of the LRO orbit and the lunar seasons.

Figure 1.
The configuration of the LRO orbit during October 2009 allowed Diviner to measure maximum temperatures near summer solstice in the south polar region, and minimum temperatures near winter solstice in the north polar region. (NASA/GSFC/UCLA)

Figure 2. (BELOW) shows a Diviner Channel 8 thermal image of the south polar region acquired between October 3-30, 2009. The mapping period overlaps with the LCROSS impact on October 9, 2009. Figure 3 shows an annotated version of the image, including the location of the LCROSS impact. The rugged south polar topography makes it one of the most picturesque regions on the planet. Diviner’s thermal measurements allow us to “see” both the warm sunlit and cold shadowed regions in striking clarity and detail. Even at their warmest, the permanently shadowed areas in the south polar region are extremely cold. The coldest areas are located in doubly shadowed regions inside small craters that themselves lie within the permanently shadowed regions of larger craters. Diviner measured minimum channel 9 brightness temperatures as low as 35K (-238C or -397F) in these areas, even at noon on the warmest day of the year.

Read the Diviner News Release HERE.

On the opposite side of the planet, Diviner mapped the north polar region at winter solstice. Figure 4 from Diviner's news release (December 15, 2009) shows a nighttime false-color channel 9 map of the region that reveals the presence of areas with temperatures as low as 25K (-258C or -415F). The coldest spot on the Moon that Diviner has detected thus far is located on the south western edge of the floor of Hermite Crater. There are also regions on the southern edges of the floors of Peary and Bosch Craters that are almost as cold. To put these cold temperatures in perspective, one would have to travel to a distance well beyond the Kuiper belt to find objects with surfaces this cold. Diviner measures the temperature of the top millimeter of the lunar surface. We would expect temperatures below the surface to be warmer due to heat retention from the spring and summer seasons.

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