A peak of eternal light is a hypothetical point on the surface of an astronomical body that is always in sunlight. Such a peak must have high latitude, high elevation, 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 about the lunar polar mountains: "...many of these peaks have 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. Detailed lunar topography collected by the Lunar Reconnaissance Orbiter suggest that no points on the Moon receive perpetual light during both the winter and summer, but points on crater rims exist which have very extended periods of sunlight.
On the Moon
Orbital imagery
The ESASMART-1, NASAClementine and JAXASELENE 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. The Lunar Reconnaissance Orbiter then took a data set including 24,000 wide-area camera and 31,500 narrow-angle camera images within 2° of the poles. These were used to construct high-resolution maps that reveal illuminated terrain under varying lighting conditions. 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 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, in 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 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 suggested 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. Clementines images were taken during the northern summer, however, and did not address illumination of these peaks during the winter, when the Sun is below the horizon. Further data from the SELENE spaceprobe determined that one peak at Peary Crater receives sunlight for 89% of a lunar year.
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. 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 1.5° off-plane 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 data set from Lunar Reconnaissance Orbiter shows that some sites on the rim of Shackelton remain illuminated for 94% of a lunar year. The longest all three Shackleton outposts are eclipsed is 43 h.
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. 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.
