Home South pole ice New analysis indicates comets are a source of near-surface ice at the moon’s south pole

New analysis indicates comets are a source of near-surface ice at the moon’s south pole


More than a decade ago, a NASA spacecraft intentionally cratered the surface of the moon, throwing a cloud of ice and volatiles into space that had likely been trapped there for billions of years. Now, a new analysis of the elemental abundances of this cloud suggests that impacting comets deposited these volatiles from less than 1 billion years ago until 3.5 billion years ago. It adds another piece to the moon’s (and Earth’s) history puzzle and highlights how lunar ices and volatiles can shed light on the past.

The study, led by planetary scientist Kathleen Mandt of Johns Hopkins Applied Physics Laboratory, was published Feb. 8 in the journal Nature Communication.

“For decades, we thought there were no volatile substances on the Moon, because it was very dry,” said Olivier Mousis, planetary scientist from the University of Aix-Marseille in France and co-author. of the study.

The prevailing theory that the moon formed from a Mars-sized object that hit primordial Earth suggests that most of the water should have burned off as a result of the collision. The dehydrated moon rocks returned by the Apollo missions set the stage for this idea, Mousis explained.

“Understanding the impact history of the moon – and the delivery of volatiles onto it – helps infer what the Earth has been through for the past 1-2 billion years.”

Kathleen Mandt

Planetary Scientist, Johns Hopkins Applied Physics Laboratory

Recent investigations have shown that the rocks of Apollo actually contain a small amount of water within them. However, sightings of water ice in craters at the Moon’s poles that never receive direct sunlight, called permanently shadowed regions or PSRs, have been more impactful.

First discovered in 1994 by NASA’s Clementine mission and later confirmed by several other missions, this water ice is found in the regolith, or earth, that covers the moon’s surface at depths of several meters. . But how it and other volatile molecules got there – whether through external sources, such as asteroids and comets, or internal such as outgassing from volcanoes – has remained an open question.

Mandt saw a way to probe the question using data collected by NASA’s Lunar Crater Observation and Sensing Satellite (LCROSS) mission, which intentionally created a crater in a PSR in 2009.

The 2.2-ton upper stage of the Atlas V rocket that launched the LCROSS slammed about 62 miles from the lunar south pole into the moon’s Cabeus crater at 5,600 miles per hour, creating a plume that even telescopes earthly could see. Above, NASA’s Lunar Reconnaissance Orbiter (LRO), which was already orbiting the Moon, hovered just 30 seconds after impact, collecting data to determine the molecular composition of the resulting plume. LCROSS, on the other hand, was on the same collision course, flying head-on into the plume and sampling chemicals for four minutes to determine the plume’s composition until it finally crashed to the surface.

LCROSS and LRO together found molecular hydrogen and the first molecules containing carbon, nitrogen and sulfur, as well as confirmed water ice just below the surface. But because the molecules measured by LCROSS and LRO were different from the molecules that came from the sources of the volatiles, it was not possible to deduce the source of the volatiles.

So Mandt and a team of collaborators decided to try a new approach to solving the problem. They compared the elemental composition of the LCROSS plume with five potential sources: volcanoes, comets, asteroids, micrometeoroids, and the Sun’s ubiquitous solar wind composed of charged particles, which can react chemically to create water on the surface. from the moon.

Using a computer model, the team assessed whether a combination of sources would potentially match LCROSS observations of lunar volatiles. But after 164,000 different combinations failed to yield the measured LCROSS quantities detected, the team had to consider processes that would subdivide the elements from the time of their delivery to the moon and their eventual trapping in the PSRs. The result was just one source that best matched the data: comets – the water-rich icy balls of earth from the lower regions of the solar system.

This result presents an interesting piece of history not only of the moon but also of the Earth.

“Understanding the impact history of the moon — and the delivery of volatiles onto it — helps infer what Earth has experienced over the past 1 to 2 billion years,” Mandt said. Since the Earth’s surface is constantly being recycled by plate tectonics and weathering, much of this ancient history is either rare or completely lost. But on the moon, where the volatiles stored in the regolith can remain unchanged for billions of years, this history is still preserved.

The lunar volatiles look a lot like the Arctic and Greenland ice sheets, which capture the history of Earth’s water and atmosphere, Mandt added. “The birds in the permanently shaded regions preserve a history of the Earth-Moon system that will be lost if we use them for human exploration without first characterizing them.”

With increased interest in establishing permanent human bases on the Moon (as early as 2027, in the case of China) and possibly using the lunar surface as a launchpad to even more distant destinations such as Mars, the The Moon’s ice and volatiles could provide an important resource for rocket fuel, industry, and life support for resident astronauts.

“I’m not against exploration, but I also don’t want to miss this single opportunity to study this ancient ice,” Mandt said. “We need scientists to work alongside people developing the instrument and tools for using ice. This will allow us to strike a balance between scientific feedback and human exploration of the future.”