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New Analysis Points to Comets as Source of Near-Surface Ices at the Moon’s South Pole
An image of Comet NEAT, captured by Kitt Peak National Observatory in Arizona in 2004. Comets such as this seem to have delivered water and other volatile elements to some of the Moon’s permanently shadowed regions near its poles, a new analysis finds.
Credit: NASA
More than a decade ago, a NASA spacecraft intentionally blasted a crater into the Moon’s surface, throwing a cloud of ice and volatiles that had likely been trapped there for billions of years into space. Now, a new analysis of the elemental abundances of that cloud suggests that impacting comets deposited those volatiles from less than 1 billion years ago to as many as 3.5 billion years ago. It adds another piece to the puzzle of the Moon’s (and Earth’s) history and underscores how lunar ices and volatiles can shed light on the past.
The study, led by planetary scientist Kathleen Mandt at the Johns Hopkins Applied Physics Laboratory (APL) in Laurel, Maryland, was published February 8 in the journal Nature Communications.
“For decades, we thought there were no volatiles on the Moon whatsoever, because it was very dry,” said Olivier Mousis, a planetary scientist from Aix-Marseille University in France and a study co-author.
The prevailing theory that the Moon formed from a Mars-sized object that hit the primordial Earth suggests most water should have burned off from the collision. Dehydrated lunar rocks returned from the Apollo missions set the stage for this idea, Mousis explained.
Recent investigations have shown that the Apollo rocks actually do contain a sparse amount of water within them. However, more hard-hitting have been observations of water ice in craters at the Moon’s poles that never receive direct sunlight, called permanently shaded regions or PSRs.
First discovered in 1994 by NASA’s Clementine mission and later confirmed by several other missions, this water ice lies in the regolith, or dirt, that covers the surface of the Moon to depths of several meters. But how it and other volatiles molecules got there — whether by external sources, such as asteroids and comets, or internal ones such as outgassing from volcanoes — has remained an open question.
Mandt saw a way to probe the question using data collected from NASA’s Lunar Crater Observation and Sensing Satellite (LCROSS) mission, which intentionally created a crater in a PSR back in 2009.
The 2.2-ton upper stage of the Atlas V rocket that launched LCROSS slammed about 62 miles (100 kilometers) from the lunar South Pole into the Moon’s Cabeus crater at 5,600 miles per hour (9,000 kilometers per hour), creating a plume that even Earth-based telescopes could see. High above, NASA’s Lunar Reconnaissance Orbiter (LRO), which was already orbiting the Moon, swooped by just 30 seconds after the impact, collecting data to determine the molecular composition of the resultant plume. LCROSS, on the other hand, was on the same collision course, flying head-on into the plume and sampling the chemicals for four minutes to determine the plume’s composition until finally crashing into the surface.
Artist’s illustration of the LCROSS mission, with a rocket upper stage flying ahead of the LCROSS spacecraft. The mission intentionally created a crater on the Moon to confirm the presence of water ice near its poles.
Credit: NASA
A temperature map (in degrees Kelvin) of the lunar South Pole using data captured by the Diviner instrument on the Lunar Reconnaissance Orbiter. Cabeus Crater (near the top left corner) is a permanently shadowed region and was the destination of NASA’s LCROSS spacecraft, which struck the lunar surface. Its impact location is circled in the image.
Credit: NASA