How Dry the Moon

The Rusty Rock on the surface of the moon.


Whether the lunar interior is wet or dry is not a trivial question. A dry Moon would support the prevalent hypothesis that the Moon was blasted from Earth by a catastrophic impact early in our planet’s history; a wet interior suggests another lunar origin event. 

A lunar rock sample, collected by Apollo 16 astronauts in 1972, has rust on its surface, indicative of the presence of water. A recent chemical analysis, however, reveals information that can account for the rust formation by other means. Several tiny fragments of “Rusty Rock,” officially, 66095, weighing a total of 537 milligrams, were analyzed by a team of researchers led by James Day of the Scripps Institution of Oceanography (SIO) in La Jolla, CA, with colleagues from the University of Paris and the University of New Mexico. 

Electron scan of a grain of the Rusty Rock (BSE), and an X-ray scan showing the presence of zinc (Zn).

James Day, Frédéric Moynier, and Charles Shearer

Rusty Rock was known to contain an especially high amount of volatile, or easily evaporated, elements such as zinc. “At its origin,” said Day in a statement released by SIO, “the Moon was extremely hot, its entire surface essentially an ocean of magma. Volatile elements, including zinc, evaporated, but the rust on 66095 was shown under microscopic analysis to include lighter isotopes of zinc and other volatiles.” Day and his colleagues concluded from their tests that vapor of zinc—which “behaves a bit like water under conditions of Moon formation”—condensed on the rock, imparting its rusty color, after the Moon’s surface had cooled and solidified. 

Their analyses suggest that the lunar interior must be significantly depleted in volatile elements and compounds, confirming that the Moon is indeed dry. As noted by Day, applying modern analysis techniques to old rocks can yield new information and turn them into “gifts that keep on giving.” (Proceedings of the National Academy of Sciences)