The moon, as it turns out, has clouds — a permanent but asymmetrical mist of dust stirred up from the lunar surface by the impact of thousands of cosmic particles from space.
The finding is in data from NASA's Lunar Atmosphere and Dust Environment Explorer (LADEE), which launched in September 2013 and spent six months orbiting the moon, researchers report in the journal Nature.
During that time, an instrument aboard LADEE, known as the Lunar Dust Experiments (LDEX) – designed and constructed by researchers at the University of Colorado, Boulder – detected more than 140,000 such impacts, the study reported.
The impact of just a single particle from, say, a passing comet can throw thousands of grains from the moon's powdery surface dust into its thin atmosphere, the researchers explained.
UC Boulder physics professor Mihaly Horanyi said he first became interested in the possibility of dust above seemingly airless worlds when he was a co-investigator for a dust detector system on NASA's Galileo mission to Jupiter and its moons in the 1990s.
The Galileo instrument detected fine clouds of dust surrounding the giant gas planet's moons Europa, Ganymede and Callisto.
"The question naturally emerged if our moon also has a dust cloud generated by the impacts of interplanetary dust particles," he said.
LADEE neatly answered the question.
"Identifying this permanent dust cloud engulfing the moon was a nice gift from this mission," said Horanyi, who was the LDEX principal investigator and lead study author. "We can carry these findings over to studies of other airless planetary objects like the moons of other planets and asteroids."
The first suggestion of a dust cloud circling the moon dates back to the late 1960s, when cameras aboard unmanned NASA moon landers collected images of a bright glow during lunar sunsets, Horanyi noted. Although permanent, the dust cloud is not static.
He added that the Geminid meteor shower seen from Earth every December also bombards the moon — kicking up extra surface dust and giving the surrounding dust cloud a boost for a number of days.
Understanding that worlds with almost no atmosphere can still feature a dusty character – and that it's the result of impact from space – could yield practical applications, Horanyi pointed out.
Knowing where dusty environments exist in space, and where those particles are headed, could help reduce hazards to future human space exploration — such as dust impact damaging spacecraft or presenting risks to astronauts.
