Peak of Eternal Light

Peak of Eternal Light (PEL) describes a point on a body within the Solar System which is eternally bathed in sunlight. This is due to both the bodies' rotation and the point's altitude. The existence of such peaks was first proposed by Camille Flammarion in 1879, who speculated that there may exist "pics de lumière éternelle" at the poles of the Moon. Peaks of Eternal Light would be advantageous for space exploration and colonization due to the ability of an electrical device located there to receive solar power regardless of the time of day, and the relatively stable temperature range.

On the Moon

Based on images from the Clementine lunar mission, a team from Johns Hopkins University determined that four locations along the rim of the Peary crater are Peaks of Eternal Light. This crater lies near the north pole of the Moon. Clementine's images were taken during the northern Lunar hemisphere's summer season and were unable to confirm whether these four mountains were shaded at any point during their local winter season. The Lunar south pole is situated in a huge depression (leading to 16 km altitude differences over the region). Careful analysis of imagery and topographic conditions on the lunar South Pole by teams from NASA and Europe revealed a small number of illuminated ridges within 15 km from the pole, each of them much like an island of no more than a few hundred meters across in an ocean of eternal darkness, where a lander could receive near-permanent lighting (~70-90% of time in Lunar winter, likely 100% in Lunar summer). It is interesting to note that these regions themselves appear rather dark in the imagery due to a.o. the long shadows of local terrain features. The Malapert Mountain region, on the rim of the Malapert crater 122 km from the lunar south pole on the Earth-facing side, also may have high levels of illumination, but available data is not sufficient to determine this. Considering its distance from the pole it can be calculated that its peak is unlikely to be permanently lit.

From 2005 to 2006 the ESA SMART-1 spacecraft made a systematic search of the lunar poles to more concisely identify sites receiving eternal light. The orbiting craft monitored the lighting at the poles and looked for seasonal variations, confirming that they remained illuminated during their Lunar winter. Confirmed sites may serve as targets for future landing craft that will utilize the steady lighting and temperature conditions for long-duration missions.

Peaks of Eternal Light on the Moon would not be perfectly "eternal", since sunlight would still be cut off occasionally by Earth's shadow during a Lunar eclipse (which can last up to 6 hours).

On Mercury

The existence of peaks of eternal light on Mercury has also been theorized, but due to the lack of detailed mapping, no Mercurian peaks have been positively confirmed or ruled out as such. This may change when the orbiter MESSENGER arrives. Such peaks would not even suffer the sporadic shadow of an eclipse as Mercury has no moons.

References

* Kruijff, M., "The Peaks of Eternal Light on the Lunar South Pole: How they were found and what they look like", 4th International Conference on Exploration and Utilization of the Moon (ICEUM4), ESA/ESTEC, SP-462, September 2000.