Peak of eternal light
A peak of eternal light (PEL) is a point on a body within the Solar System which is always in sunlight. Such a peak must have high altitude and be on a body with very small axial tilt. The existence of such peaks was first postulated by Beer and Mädler in 1837. The pair said of the lunar polar mountains, "...many of these peaks have (with the exception of eclipses caused by the Earth) eternal sunshine." These polar peaks were later mentioned by Camille Flammarion in 1879, who speculated that there may exist pics de lumière éternelle at the poles of the Moon. PELs 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 or day of the year, and the relatively stable temperature range.
On the Moon
The ESA SMART-1, NASA Clementine and JAXA SELENE spacecrafts made maps of the lunar poles that have been used to identify sites receiving eternal light. The orbiting craft monitored the lighting at the poles and looked for seasonal variations as well as mapping the relief so that peaks of eternal light could be identified topographically.
Peaks of eternal light on the Moon would not be perfectly "eternal", since sunlight would still be cut off occasionally by the Earth's shadow during a lunar eclipse (which can last up to six hours) and by the shadows of other mountains and plateaus. The term "peak of eternal light" for the Moon is commonly used in the technical literature and newspaper articles as a popularization and is surprisingly applied even though the duration of illumination is not permanent, excluding the lunar eclipses. No peaks of eternal light have been positively identified on the moon, but many peaks have been detected that, via simulations based on imaging and laser and radar topography, appear to be illuminated for greater than 80% of a lunar year. Of course the concept of "eternity" in this case lasts only during the lifetime of the Sun (roughly 10 billion years) and will end when the Sun runs out of fuel and becomes a red giant, which will likely absorb and destroy the Moon, or else end when the Sun becomes a white dwarf and no longer produces enough light to directly illuminate the lunar surface.
Lunar north pole
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 candidates for peaks of eternal light. This crater lies near the north pole of the Moon. Clementine's images were taken during the northern summer and were unable to confirm whether these four peaks would be shaded at any point during the winter. Further data from the SELENE spaceprobe determined that one peak at Peary Crater receives sunlight for 89% of a lunar year, the highest level of illumination predicted for any peak of eternal light on the moon.
Lunar south pole
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 the ESA revealed a small number of illuminated ridges within 15 km of 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 (for ~70–90% of the lunar winter, and likely all of the lunar summer).
The Malapert Mountain region, on the rim of the Malapert crater 122 km from the lunar south pole on the Earth-facing side, may also have high levels of illumination. One study estimates the Malapert Mountain region to receive less than full sunlight 11% of the time. Sunlight exposure varies by year due to the Moon's orbit being only 1.5 degree off plan with the sun. 2005 estimates of sunlight coverage of the Malapert Mountain region found only six partially lit or unlit events that year: 0–159 hours of complete sunset per event and 41–199 hours of complete or partial sunset per event. A Later study using a combination of both Clementine imaging and SELENE topographical data estimated only 74% of full sunlight for the year 2020. This study found that two points only ~8 km from each other along a straight ridge extending from Shackleton Crater at the Lunar South Pole are illuminated a combined ~94% of a lunar year. This is because both points cast shadows upon each other during different times of the lunar year and only a few times of darkness occur when further peaks throw shadows over both of these points simultaneously.
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 data from the orbiter MESSENGER is fully analyzed. Such peaks would not even suffer the sporadic shadow of an eclipse, as Mercury has no known moons.
- Beer, Wilhelm,and Johann Heinrich Mädler: Der Mond nach seinen kosmischen und individuellen Verhältissen oder allgemeine vergleichende Selenographie. Berlin, Simon Schropp and Co., 1837. (German)
- Flammarion, Camille. Astronomie Populaire, description générale du ciel. Paris, 1879. (French)
- Nodac et al. Illumination conditions at the lunar polar regions by KAGUYA(SELENE) laser altimeter. 30 DEC 2008 DOI: 10.1029/2008GL035692
- 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.
- B. Sharpe and Schrunk, D. (2002), Malapert Mountain Revisited. Space 2002 and Robotics 2002: pp. 129-135. doi: 10.1061/40625(203)18
- Busseya et al. Illumination conditions of the south pole of the Moon derived using Kaguya topography, Icarus Volume 208, Issue 2, August 2010, Pages 558–564
- Koschny, Detlef; Grieger, Björn. "Taking a SMART sidelong look at Peak of Eternal Light". Europlanet Research Infrastructure. Retrieved 2009-09-24.
- The Peak of Eternal Light video by the European Space Agency.
- Babak Shakouri Hassanabadi ," A legal regime for lunar peaks of eternal light ".