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July 27, 2022

On May 16, 2022, sky watchers were not the only ones observing the Moon during the longest total lunar eclipse in three decades. Two satellites that keep near-constant watch on Earth also turned to look. In doing so, they observed dramatic and uneven temperature changes across our closest, but still sometimes surprising, celestial neighbor.

Imagery of the Moon via Landsat
Two Earth-observing satellites turned to watch a lunar eclipse, revealing dramatic and uneven temperature changes across our closest celestial neighbor. 

The changes are shown this series of images, derived from the Thermal Infrared Sensor (TIRS) on the Landsat 8 and Landsat 9. The images have a resolution of roughly 60 kilometers per pixel. Note that the contrast in each image has been adjusted based on the minimum and maximum temperatures of the lunar surface in each scene.

Over the course of the four-hour eclipse, the TIRS sensors observed the intensity of infrared light radiating from the Moon’s surface. From those radiances, scientists calculated the temperatures required to generate them. This “brightness temperature” of the lunar surface changed as Earth’s shadow marched across the lunar landscape. Brighter areas in the images are warmer than darker, cooler areas.

Dennis Reuter, a TIRS instrument scientist, pointed out two notable features revealed by the images. “Look at how quickly the surface cools down when the solar illumination is removed,” he said. “It drops at a rate of more than 100 Kelvin (100°C/180°F) per hour when the full eclipse begins.” Full eclipse—the period during which Earth’s shadow falls across the entire lunar surface—lasted from 03:29 to 04:53 Universal Time on May 16. Notice the quickly darkening (cooling) surface across the middle row of images.

After the removal of solar heating, the most obvious reason for the swift drop in temperature is because the Moon lacks a thick heat-trapping atmosphere. But physical characteristics of the lunar surface are also a factor, Reuter explained. Meteoroids have been pounding the Moon for billions of years, and they have left the entire surface covered with a layer of fine rocky dust, or “regolith.” The tiny size of each particle, and the low-density nature of the loose dust, helps the lunar surface quickly lose heat.

“This feeds into the second interesting point: the craters cool down slower than the material surrounding them,” Reuter said. For example, notice that Tycho Crater appears to stay relatively bright (warm) as the eclipse progresses. The chart below depicts this difference between Tycho Crater and a small region of surrounding terrain over the course of the day. The crater starts out cooler than its surroundings, but it loses heat much more slowly. By the time the crater is in full shadow (last-third of the chart), it is clearly warmer than the surface around it.

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Brightness Temperature v. Time chart for TIRS


Originally published by NASA Earth Observatory on July 5, 2022.