LADEE Project Scientist Update: December 2014
It’s been about a year since LADEE started its Science Mission Phase on Nov. 21, 2013, and successfully completed it on March 1, 2014. LADEE then went on to acquire low altitude data for another month and a half, before impacting the lunar surface as planned on April 18, 2014. With the science data acquisition completed, the science team worked hard over spring and summer to ready the instrument measurements and background information for submission to the Planetary Data System (PDS). The instrument teams finished off this task in September, 2014, a year after launch. Now planetary scientists everywhere can delve into the LADEE exosphere and dust datasets!
The LADEE science team also has been busy analyzing the returned data, figuring out how the exosphere breathes and changes, and how the moon’s tenuous dust shroud varies in time and space.
The Lunar Dust Experiment (LDEX) discovered a low-density cloud of small dust particles over the part of the moon that faces the more-or-less steady rain of micrometeoroid particles onto the lunar surface. The Earth/moon system orbits the sun with an average speed of 30 km/sec (67,000 mph), and like bugs on a car windshield, the interplanetary micrometeoroid materials smack into the “upstream” side of the Earth and moon. On Earth these cause meteors, which burn up in the atmosphere, but with the almost negligible atmosphere on the moon, these particles smash into the surface with tremendous speed. Each particle impact sends a spray of ejecta up into the lunar sky; this process is continuous but really increases when the moon encounters a micrometeoroid stream. The flux of incoming particles can increase by factors of up to ten times the normal rates – we see these as meteor showers on Earth. On the moon, it’s a very heavy rain of tiny, tiny rocks, and the spray of ejecta increases accordingly.
LADEE saw these meteor shower dust particles several times during the mission. LADEE also looked for mysterious “levitated” dust, thought to be suspended by electric forces over the sunrise and sunset terminators. This mystery was prompted in part by astronaut sightings of a horizon glow seen from orbit. However, the spacecraft’s instruments found no evidence of anything that could be seen by the human eye.
However, LADEE’s Ultraviolet/Visible Spectrometer (UVS) has detected a very weak glow in the anti-sunward direction, possibly due to a population of very small grains of lunar ejecta. Picture the moon as an extremely weak comet, throwing off this very tenuous cloud of tiny particles into a kind of tail stretching out behind, away from the sun.
We have learned that the major gas species in the thin lunar atmosphere are three noble gases: helium, neon and argon. LADEE’s neutral mass spectrometer (NMS) systematically measured these species, and we now know that helium and neon are supplied by the solar wind. The flux of doubly-ionized solar wind helium went up and down from day to day (as measured by LADEE’s lunar companions, the two ARTEMIS spacecraft), and the NMS helium measurements tracked those changes. In fact, for helium it’s easy come, easy go! The dayside lunar surface gets very hot, over 240 degrees Fahrenheit (117 degrees Celsius) at the equator near noon, and the helium atoms pick up this heat as they bounce across the surface. Some helium atoms move so fast as a result that they escape the moon completely and are lost. On daily time scales, the supply of helium provided by the solar wind is balanced by the loss of the escaping portion of helium. But when the moon’s phase is full, it is located inside a protected region, the Earth’s geomagnetic tail, where it is shielded from the solar wind. Here, the supply of solar wind helium is cut off. But the hot lunar dayside surface continues to drive the escape of some of the resident helium. Because of this ongoing loss process, and a temporary cutoff in supply, we see the density decrease with time. When the moon re-emerges into the solar wind, the supply is restored and the exosphere quickly recovers to equilibrium.
Neon, delivered by the solar wind in very small quantities, generally cannot escape like helium does. It’s too heavy and doesn’t travel fast enough to leave the moon. And once there, like an unwelcome house guest, it sticks around. And sticks around. In fact, about the only way the neon leaves is when the sun’s ultraviolet radiation causes photoionization: the neon atom loses an electron, and the resulting positively charged ion is swept away by the solar wind. But photoionization of neon takes a long time – over 200 days. So even though neon is a very minor solar wind constituent, the slow loss rate means it can build up to levels comparable to helium in the moon’s atmosphere.
The third lunar noble gas is argon, specifically, argon-40. This isotope comes from the decay of naturally occurring radioactive potassium-40, found in the rocks of all the terrestrial planets as a leftover from formation. As potassium-40 in the moon decays, the argon-40 product is able to diffuse and percolate up to the lunar surface, where it becomes part of the tenuous atmosphere. Lunar argon behaves differently from helium and neon; it condenses on the moon’s cold nightside where temperatures drop below -280 degrees Fahrenheit (-173 Celsius). As the moon slowly rotates, and the condensed argon sees sunrise, temperatures rise and the atoms jump off the surface into the exosphere again. Some of these jumping argon atoms leap back into the cold nightside, and are re-trapped on the cold surface until sunrise occurs for that patch of real estate. But the moon’s argon exhibits other interesting behavior. LADEE discovered that argon-40 creates a local bulge above an unusual part of the moon’s surface, the region containing Mare Imbrium and Oceanus Procellarum. This happens to be the place where potassium-40 is most abundant on the surface, and there may be a connection between the atmospheric argon, the surface potassium and deep interior sources.
UVS also monitored the two minor species sodium and potassium over the course of LADEE’s mission. These observations show that sodium follows a monthly cycle, responding to passages through the geomagnetic tail. This suggests that the solar wind is one influence on releasing sodium from the surface into the exosphere, possibly through a process called “sputtering.” Sputtering occurs when a solar wind protons slam into lunar surface materials and deposit their energy there. The energy deposition process can knock other atoms, including sodium, out of these materials. UVS also found that exospheric sodium increases in density in response to meteoroid showers, possibly because these micrometeoroids are vaporizing lunar regolith grains. Sodium is one of the easiest species to liberate. Potassium exhibits a monthly variation as well, though slightly different from that of sodium. Other metal species may also be showing up in UVS data. Several external stimuli can affect the thin lunar atmosphere, among them solar wind sputtering, desorption by extreme ultraviolet photons, and micrometeoroid impact vaporization. It can be challenging to untangle cause and effect!
LADEE’s science team is working to share this and more fascinating in-depth research results with the public and the scientific community. Several LADEE presentations will be given at the Fall Meeting of the American Geophysical Union this week in San Francisco, including: new studies by the LDEX team on the micrometeoroid streams that increase the dust exosphere; NMS identification of water and carbon dioxide in the lunar atmosphere; a study of how rocket exhaust from Chang’e 3’s landing and LADEE’s own maneuvers interacts with the lunar surface; a model simulation of how the sodium exosphere varies over a lunar month. Look for more presentations and papers in the coming months!
Posted by: Soderman/SSERVI Staff
Source: Rick Elphic/NASA Ames Research Center