December 15, 2009

Diviner Observes Extreme Polar Temperatures

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.

polar_viewing_geometry_sm

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 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.

 Figure 2. Diviner Channel 8 thermal image of the south polar region.

Figure 2. Diviner Channel 8 thermal image of the south polar region. (NASA/GSFC/UCLA)

Figure 3. Annotated version of Figure 2 including the location of the LCROSS impact.

Figure 3. Annotated version of Figure 2 including the location of the LCROSS impact. (NASA/GSFC/UCLA)

On the opposite side of the planet, Diviner mapped the north polar
region at winter solstice. Figure 4 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.

Figure 4. Diviner channel 9 nighttime brightness temperature map of the north polar region acquired close to winter solstice.

Figure 4. Diviner channel 9 nighttime brightness temperature map of the north polar region acquired close to winter solstice. (NASA/GSFC/UCLA)

Figure 5 shows an animated flyover of the north polar region that
terminates at the coldest measured areas inside of Hermite Crater.

flyover

Figure 5. Animated nighttime flyover of the north polar region during the mid-winter season. (NASA/GSFC/UCLA)

Figure 6 shows a histogram of measured daytime and nighttime Channel 9
brightness temperatures in both polar regions. The results show that
there are large regions at both poles with temperatures colder than
~106K, the temperature necessary to prevent significant loss of water
ice over billion-year timescales.  The Diviner data show that The LCROSS
impact successfully sampled one of the coldest lunar cold traps - a fact
that may help put the results of the LCROSS mission into context.

Figure 6. Histograms of Diviner Channel 9 brightness daytime and nighttime temperatures acquired in in the north and south polar regions (square regions to +/- 80 degrees latitude) during their respective winter and summer solstice seasons. The measured pre-impact daytime and nighttime summer solstice brightness temperatures for LCROSS impact site  are also indicated.

Figure 6. Histograms of Diviner Channel 9 brightness daytime and nighttime temperatures acquired in in the north and south polar regions (square regions to +/- 80 degrees latitude) during their respective winter and summer solstice seasons. The measured pre-impact daytime and nighttime summer solstice brightness temperatures for LCROSS impact site are also indicated. (NASA/GSFC/UCLA)

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