Texas Tech University

Lunar Warming

Apollo 15 Moonwalk, Aug. 1, 1971, Image Credit: NASA

Apollo 15 Mission Commander David R. Scott leans to put down the Apollo Lunar Surface drill used to take core samples and set up a heat-flow experiment on Aug. 1, 1971. Apollo 15 was the fourth crewed mission to land on the Moon and the first to visit and explore the Moon's Hadley Rille and Apennine Mountains, located on the southeast edge of the Mare Imbrium (Sea of Rains). Also pictured are the Apollo Lunar Surface Experiments Package (ALSEP) Central Station in the foreground, the Passive Seismic Experiment beyond the left side, the Lunar Surface magnetometer in the background near the center, and the Solar Wind Spectrometer in the right foreground . The panorama was taken by Apollo 15 Lunar Module Pilot Jim Irwin. Image Credit: NASA

It's a Dust-Up That a TTU Researcher
Attributes to the NASA Astronauts

Compiled by Toni Salama

With the 50th anniversary of the first manned Moon landing about a year away, a Texas Tech University geophysicist says that human activity has led to higher temperatures at lunar testing sites.

Seiichi Nagihara, Associate Professor of Geophysics in the Department of Geosciences, is lead author on the study of data—both archived and restored by Nagihara's research team—from the Apollo 15 and Apollo 17 heat-flow experiments. The results of their new study, "Examination of the Long-Term Subsurface Warming Observed at the Apollo 15 and 17 Sites Utilizing the Newly Restored Heat Flow Experiment Data From 1975 to 1977," published May 8 in the Journal of Geophysical Research: Planets.

LONG STORY SHORT

The very devices that astronauts deployed to test subsurface temperatures in the lunar crust, or rather, the activities of the astronauts themselves, disrupted the Moon's soil just enough to cause a measurable temperature increase, the study concluded.

"In the process of installing the instruments, you may actually end up disturbing the surface thermal environment of the place where you want to make some measurements," Nagihara was quoted as saying in a Space.com article published June 14. Others on the research team are co-authors W. S. Kiefer of the Lunar and Planetary Institute; P. T. Taylor and D. R. Williams of the Goddard Space Flight Center; and Y. Nakamura of the University of Texas at Austin.

Apollo 17 Moon Landing, Dec. 13, 1971, Image Credit: NASA

Scientist and Astronaut Harrison H. Schmitt stands beside a lunar boulder on Dec. 13, 1972, during the third Apollo 17 extravehicular activity (EVA) at the Taurus-Littrow landing site. The Lunar Roving Vehicle (LRV) appears beyond the boulder to the right. The panorama was taken by Astronaut Eugene A. Cernan. Image Credit: NASA.

PROBING THE DATA

In 1971 and 1972, Apollo astronauts used a rotary-percussive drill to excavate the holes for the heat flow-probes; the bore stems used for drilling were left in place and served as the casing into which the heat-flow probes were slid, according to the new study. Originally, scientists wanted to track the degree to which heat from the Moon's interior made its way to the surface. Early findings should have shown a reduction in temperature as measured from the bottom of the probes to the top. But that's not what happened. Instead, the data from 1971/1972 through 1974 showed temperature increases from the part of the probes nearest the surface.

And it didn't help that much of the measurement data, collected through 1977, had been misplaced over the years. So Nagihara's team tracked down the data from 1975 through 1977, restored it and analyzed it. Their study indicated that rising temperatures nearer the surface meant the warming was coming from above the surface, not below.

But they found more to go on than probe data. The team also examined photos taken by the Lunar Reconnaissance Orbiter Camera. As reported in the June 14 Smithsonian.com article, "Astronauts' Footprints May Have Warmed the Moon," those images showed darker soil wherever the Apollo 15 and Apollo 17 astronauts had walked, worked or driven their moon rover; those activities had dislodged the topmost layer of lighter-colored dust, revealing darker levels beneath. Nagihara and company found that wherever the Moon had been thus disturbed, the darker surfaces absorbed more of the sun's energy than the lighter surfaces, "raising lunar surface temperatures by 1.8 to 3.6 degrees Fahrenheit in areas where the disruptions occurred," according to the Smithsonian.com article.

"You can actually see the astronauts' tracks, where they walked," co-author Kiefer was quoted as telling CBS in the Smithsonian.com article. "And we can see ... where they scuffed dirt up—and what it leaves behind is a darker path."

PINPOINTING THE CAUSE

Over the decades since the Apollo missions, various ideas were put forward to explain the lunar warming. Nagihara's study examined four of them:

  • The Moon's orbital precession.
  • Radiation from the Earth.
  • Darkening (albedo reduction) of the surface soil (regolith) caused by the astronauts' activities.
  • Solar radiation into the bore stems.

The study's conclusion? The temperature-versus-time records from the heat-flow probes clearly indicate that the warming originated from the surface and propagated downward, according to the research paper. Further, only the surface warming brought about by human activity, which darkened the surface, can satisfy the magnitude and the timing of the subsurface warming observed.

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