On June 15th, 2011, sky-watchers across much of the world will be treated to one of nature’s most impressive spectacles – a total lunar eclipse. The eclipse is set to be one of the longest and darkest of the 21st century, lasting just over 100 minutes.
Although it will not be visible in North America, the eclipse provides a rare opportunity for the Diviner Lunar Radiometer to gather valuable information about the lunar surface.
Figure 1. Projection of the lunar north pole showing solar illumination and predicted orbit tracks of NASA's Lunar Reconnaissance Orbiter during the eclipse on June 15, 2011.
Figure 2. Projection of the lunar south pole showing solar illumination and predicted orbit tracks of NASA's Lunar Reconnaissance Orbiter during the eclipse on June 15, 2011.
Lunar eclipses occur when the Sun, Moon and Earth are aligned so that the Earth is positioned between the Sun and Moon in such a way that it blocks the Sun’s rays, casting a shadow over the Moon. In the outer part of the shadow or ‘penumbra’ the Sun’s rays are only partially blocked, but in the center of the shadow, the ‘umbra’, there is no direct sunlight at all. A total lunar eclipse occurs when the Moon travels completely into the umbra.
Figure 3 shows the Moon’s near-central path through the penumbra and umbra on June 15. The times of each stage of the eclipse are displayed in the upper right-hand corner. The penumbral eclipse (“P1”) begins when the Moon first enters the Earth’s penumbra. This is followed by a partial eclipse (“U1”), as the Moon first enters the umbra. The total eclipse (“U2”) will begin as soon as the Moon travels completely into the umbra, and the point of greatest eclipse will occur when the Moon reaches the center of this darkest region. U3, U4 and P4 indicate the ends of the total, partial and penumbral eclipse, respectively. SOURCE: http://eclipse.gsfc.nasa.gov/eclipse.html
As the Moon passes into Earth’s shadow on June 15th, its dayside will be plunged into darkness, resulting in a rapid cooling of its surface. However, not all parts of the Moon’s surface will cool equally; rocks and boulders for example, because of their smaller surface area relative to their mass, will cool more slowly than fine-grained dust and sand.
The Diviner Lunar Radiometer, which will be measuring lunar surface temperatures continuously throughout the entire eclipse period, will be able to observe this difference in cooling rate, allowing scientists to deduce what lies just beneath the surface.
Animation showing predicted lunar surface temperatures during the June 15th eclipse
“This is an unprecedented opportunity to learn more about the uppermost few millimeters of the Moon,” says David Paige, principal investigator for Diviner. “Unlike on Earth, which takes 24 hours to rotate through one full day, the typical day-night cycle on the Moon lasts around 29 Earth days, so lunar dusk and dawn usually extend over a number of days.”
“During a total lunar eclipse, the Moon is essentially experiencing a day-night cycle that lasts only a matter of hours, so it’s going to be interesting to see how the surface responds.”
“You might imagine how it will respond if you’ve ever walked into the shadow of a building on a warm, sunny day - as you walk into the shadow, you are effectively walking into an eclipse, and as the building blocks the Sun’s rays you can usually notice a marked drop in temperature.”
“This effect is much more pronounced on the Moon, where there is no atmosphere to insulate the surface, so heat is lost much more rapidly into space. And of course, unlike in the case of the building on a sunny day, during a real eclipse there is no surrounding sunlit area to act as a heat source.”
During the eclipse, Diviner will target ten specific regions on the lunar surface. These regions were chosen because they represent a wide range of lunar terrains, from seemingly smooth areas, to regions that appear to be relatively rocky; and from dark, iron-rich locations in the lunar maria to light, iron-poor locales in the lunar highlands. The rest of the instruments on LRO will be switched off to conserve energy.