New Light on the Lunar Poles
A new image released this week by the Lunar Reconnaissance Orbiter Camera Team shows the lighting conditions of the south pole of the Moon. This new data supports the conclusions of many previous studies that areas exist on the Moon that are illuminated by the sun for more than one-half the lunar day (the time it takes the Moon to rotate once on its axis, a bit more than 29 Earth days or about 708 hours).
Why do such areas exist and why are they important? Most locations on the Moon experience a day/night cycle, albeit one of an Earth month duration. But unlike the Earth, the spin axis of the Moon is nearly perpendicular (off from the vertical by 1.5°) to the plane of its orbit around the Sun (the Moon orbits the Earth, but as the Earth orbits the Sun, the Moon can be said to do the same). This means that at the poles, the Sun is always close to the horizon. As the Moon slowly rotates during the course of a lunar day, the Sun tracks a 360° circle around the pole, sometimes just above the horizon, sometimes dipping just below it.
Or rather, it would do that if the Moon were a smooth sphere. But as we all know, the Moon is not smooth – deep craters and basin make rims, peaks and holes that complicate the picture. The deep interiors of craters may never see any sunlight at all. These areas are extremely cold; we’ve learned from new orbital data that some of these cold traps are only a couple of tens of degrees above absolute zero. It is for this reason that we find water ice and other volatiles near the poles – they are stable in the permanently dark, cold areas here.
On the other hand, if some bit of terrain near the pole is topographically high, it may stick up into the sunlight for a much longer time than other spots on the Moon. This concept was first postulated in 1837 by German astronomers Wilhelm Beer and Johann Mädler and popularized in 1879 by French astronomer Camille Flammarion, who dubbed these areas pics de lumière éternelle (peaks of eternal light). If such an area could be found near one of the lunar poles, the only time it would not be in sunlight would be during a lunar eclipse, which occur infrequently and last only a few hours.
We got our first good look at the lunar poles in 1994 with the global mapping obtained by the Clementine spacecraft. Although Clementine only orbited the Moon for 71 days, we were able to determine that no peaks of “eternal light” existed at the south pole. However, we did find small areas near the south pole that are lit more than 70% of the lunar day, and this was during the southern “winter” season (the 1.5° obliquity of the Moon provides some small seasonal variation). We also found locations that are lit 100% of the day at the north pole. These images were taken during mid-summer, when the north pole receives maximum solar illumination.
Lighting at the poles is primarily dependent on local topographic relief. Because Clementine did not get laser topography for latitudes greater than 70°, we had a poor understanding of polar topography until the Japanese Kaguya mission flew in 2008. The Kaguya spacecraft made a detailed laser altimetry map of the entire Moon, including both poles. From this precision topographic data, we made a simulated relief model of the poles and illuminated it as the real Moon would be illuminated by the Sun over the course of a year. Our new results suggest at least four areas near the south pole are in sunlight for large fractions of the lunar day. One location (B) is illuminated more than 82% of the lunar day and is only 10 km from another point (A) that is lit 81% of the day. Moreover, these two points are complementary in that the dark times at one corresponds to sunlit times at the other. The four topographically high sunlight points are collectively illuminated 100% of the time during the lunar seasons.
The new composite image from LROC confirms the inferences from the illumination model we devised from the Kaguya altimetry. The four high points (A-D) correspond to bright zones on the illumination map (see image above), indicating that they are sunlit most of the time. These areas of “quasi-permanent” sunlight are the closest things we have found to correspond to Flammarion’s imagined pics de lumière éternelle. Although not “eternal” in the original sense, they are sunlit for extended periods, well beyond the typical lunar day-night cycle.
What is the significance of such features? Permanently lit areas of the Moon are important for future habitation and use of the Moon for two principal reasons. First, these sunlit areas are prime locations for the establishment of solar photovoltaic arrays. The constant sunlight here means continuous generation of electrical power using solar panels. This solves one of the most difficult problems of lunar habitation, survival during the 354-hour lunar night. Prior to the discovery of the quasi-permanently lit areas, we imagined that the only feasible power source to survive this long night was nuclear reactors. Such a power system does not exist and would require several tens of billions of dollars to develop. So sunlit zones allow us to go to the Moon and stay there without this expense and technology development.
The second advantage of a sunlit area is that it is thermally benign. The surface temperatures at the lunar equator and mid-latitudes depend almost entirely upon incident solar illumination and range from less than -150° to over 100° C, a 250° temperature-swing over the course of a day. In contrast, the surface temperature of these quasi-permanent lit areas is nearly constant – a nice, toasty -50° ± 10° C. This simplifies the thermal design of surface habitats and equipment and greatly relieves the energy required for thermal control at an outpost.
The sunlit areas of the poles occur in close proximity to high concentrations of water ice and other volatiles at the poles of the Moon. Their presence indicates the lunar poles are the best places we have found off-planet for human habitation. Constant sunlight, benign temperatures, near the water and a great view – that’s prime real estate.