Eternal light at a lunar pole

A few areas at the Moon's north pole see a midnight Sun — at least in summer.
By | Published: April 20, 2005
April 20, 2005
Just as on Earth, areas around the Moon’s north pole apparently never have darkness during summer, according to research using decade-old images from the Clementine mission. The findings were announced at the March 2004 Lunar and Planetary Science Conference and have been published recently in Nature (April 14, 2005).
Peary Crater
Parts of the northern rim of Peary Crater at the lunar north pole may be
permanently sunlit. The colors indicate the percentage of the lunar
summer day in which sunlight falls on the ground. In the larger version of this image, the scale bar is 9
miles (15 km) long.
B. Bussey, JHUAPL
A team led by Ben Bussey (Johns Hopkins University Applied Physics Laboratory) mosaicked 53 images of the lunar north pole taken in February – March 1994, during the spacecraft’s first month of operations. The researchers used them to map the changing illumination over one lunar month. Several areas on the rim of Peary, a 45-mile (73 kilometer) wide crater, appear to be lit for the entire lunar day.

“Unlike the lunar south pole, which has no ‘mountains of eternal light,” says Bussey, “the north pole has peaks that are constantly illuminated, at least during lunar summer.” Bussey notes that the Clementine images were taken during the Moon’s north polar summer, so the continuous illumination may be a seasonal effect that disappears in winter, for which no data exist. The Moon’s axis, he points out, tilts about 1.5° relative to the plane of Earth’s orbit around the Sun, providing small but detectable seasons.

“A location near the north pole that was constantly lit would have a relatively benign environment,” Bussey says, noting the grazing angle of illumination. “Daily temperature changes would be only about 36° Fahrenheit [20° Celsius], making lunar base operations easier.” In contrast, at the lunar equator the angle of solar illumination can vary from grazing to straight down, producing temperature changes as great as 450° F (250° C) each day.

Bussey’s team also identified regions lying within the small craters Peary B and Peary W that experience permanent shadow. These craters are 10 and 6 miles (12 and 10 km) in diameter, respectively. Bussey notes that the overall region around the lunar north pole consists of highlands terrain with many depressions that contain constantly shadowed ground. These regions may harbor hydrogen-rich material such as water or ground ice.

The team next plans to correlate the topographic results with hydrogen data from the Lunar Prospector mission, in hopes of locating permanently shadowed craters that might contain water or ice in the soil.

Meanwhile, the European Space Agency’s SMART-1 lunar probe is in orbit around the Moon and imaging its high latitudes. The spacecraft’s cameras should be able to settle the question of whether the Moon’s north polar peaks are always sunlit.