The existence of neon gas in our Moon’s ultra-thin atmosphere has been confirmed for the first time, by NASA’s Lunar Atmosphere and Dust Environment Explorer (LADEE) spacecraft. Its presence had been theorized for decades, but has now finally been confirmed and found to be relatively abundant, even though it’s not nearly enough for the Moon to actually glow like a neon sign.
“The presence of neon in the exosphere of the moon has been a subject of speculation since the Apollo missions, but no credible detections were made,” said Mehdi Benna of NASA’s Goddard Space Flight Center in Greenbelt, Md., and the University of Maryland, Baltimore County. “We were very pleased to not only finally confirm its presence, but to show that it is relatively abundant.” Benna is the lead author of a new paper about observations from LADEE’s Neutral Mass Spectrometer (NMS) instrument, which was published May 28 in Geophysical Research Letters.
Even though LADEE is no longer active, scientists are still busy pouring over the data it returned during its mission.
The Moon is typically thought to be airless, but it actually does have an atmosphere—of sorts—or, more accurately, an exosphere, which is extremely tenuous—about 100 trillion times less dense than Earth’s atmosphere at sea level. Therefore, there is no neon glow visible, even though it is the same neon gas used in bright electric signs. Such thin exospheres are the most common type of atmosphere in the Solar System, since thicker atmospheres require objects massive enough to hold onto them and keep the gasses from escaping back into space. Exospheres are so thin that the atoms in them rarely collide with each other. The closeness of the Moon gives scientists a chance to study one in detail, but there is the danger that rocket exhaust and outgassing from spacecraft could affect it and change its composition. Therefore, scientists want to study it as much as they can while they have the opportunity.
“It’s critical to learn about the lunar exosphere before sustained human exploration substantially alters it,” Benna said.
The NMS instrument found that the Moon’s exosphere is composed primarily of helium, argon, and neon. Their relative abundance varies with the time of day on the Moon; the amount of argon peaks at sunrise, neon at 4 a.m., and helium at 1 a.m. NMS measured the gasses over a period of seven months, so scientists were able to monitor the cycle of being supplied to the exosphere and then lost again (known as sources and sinks).
There were some other unexpected findings as well. NMS showed that while most of the gasses come from the solar wind, some also originate in rocks on the Moon’s surface, including argon-40.
“We were also surprised to find that argon-40 creates a local bulge above an unusual part of the moon’s surface, the region containing Mare Imbrium and Oceanus Procellarum,” said Benna. “One could not help to notice that this region happens to be the place where potassium-40 is most abundant on the surface. So there may be a connection between the atmospheric argon, the surface potassium and deep interior sources.”
The solar wind, a thin stream of electrically conducting gas blown from the surface of the Sun into space, is mostly composed of hydrogen and helium, but also contains trace amounts of other gasses such as neon.
LADEE found that the overall amount of argon was not constant over time during the LADEE mission, but instead increased and then later decreased by about 25 percent. The evidence suggests that this may be the result of enhanced outgassing from the surface which is triggered by tidal stress on the Moon.
LADEE also discovered an unexpected source of some of the helium in the exosphere, which is being produced at a rate equivalent to seven liters per second at standard atmospheric pressure.
“About 20 percent of the helium is coming from the moon itself, most likely as the result from the decay of radioactive thorium and uranium, also found in lunar rocks,” said Benna.
The new findings will help to greatly increase scientists’ knowledge about the lunar exosphere, as well as other ones in the Solar System, such as those around smaller moons or dwarf planets.
“The data collected by the NMS addresses the long-standing questions related to the sources and sinks of exospheric helium and argon that have remained unanswered for four decades,” said Benna. “These discoveries highlight the limitations of current exospheric models, and the need for more sophisticated ones in the future.”
LADEE was launched in September 2013 from NASA’s Wallops Flight Facility in Virginia and started orbiting on Oct. 6. It began its primary science orbit on Nov. 20, which lasted 100 days. The mission was extended in March 2014, but later ran out of fuel, and, as planned, was sent crashing into the lunar surface on Apr. 17, 2014. While the mission wasn’t a long one overall, it did successfully return a wealth of data for scientists to study.
It will also be interesting to compare the Moon’s exosphere with the ultra-thin atmospheres of other Solar System bodies, such as Pluto and Europa for example.
LADEE’s science instruments included an Ultraviolet and Visible Light Spectrometer (UVS), Neutral Mass Spectrometer (NMS) and Lunar Dust Experiment (LDEX). Together, they gathered detailed information about the lunar atmosphere, conditions near the surface, and environmental influences on lunar dust. Also included was a technology demonstration, the Lunar Laser Communications Demonstration (LLCD), which used lasers instead of radio waves to achieve broadband speeds to communicate with Earth.
More information about the LADEE mission is available on the LADEE website.
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