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Title: Evidence for a Rapid Turnover of Argon in the Lunar Exosphere
Author: Jacob A. Kegerreis, Vincent R. Eke, Richard J. Massey, Simon K. Beaumont, Rick C. Elphic, Luis F. Teodoro

We have developed a numerical model of the Moon's argon exosphere. The results from our simulations are tested using measurements from the Lunar Atmosphere and Dust Environment Explorer (LADEE) and the Lunar Atmosphere Composition Experiment (LACE). We find that the local time of the near-sunrise peak in exospheric density provides a strong constraint on the nature of the surface interactions of the argon molecules. The time of this peak is the same over both the maria and highlands, and can be reproduced by simple surface interaction models with a single desorption energy of 28 kJ mol^-1. The density at all local times of day is also reproduced to within a factor of 2 with the inclusion of a `squirrelling' process, by which particles build up a subsurface population during the night that reappears the following day. We demonstrate that the persistent enhancement in argon density over the western maria in the LADEE dataset cannot be explained by locally changing the surface interactions over the maria, because this inevitably leads to a decrement at other local times of day, as well as shifting the position of the sunrise peak away from that in the highlands. The only possible explanation for this overdensity appears to be that it is driven by a source coincident with the potassium overabundance in the western maria, and that the average lifetime of argon molecules in the exosphere is very brief (~1.4 lunar days, 41 days). This implies high source and loss rates of 1.1*10^22 argon atoms s^-1 , which could require permanent cold traps comparable with the area of permanently shadowed regions, or a more highly localised source. We demonstrate that the long-term variation in the global argon density seen by LADEE could be explained by the presence of large seasonal cold traps, the required sizes of which depend on the area of the permanent traps.

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Lunar Atmosphere
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Is There an Atmosphere on the Moon?

Until recently, most everyone accepted the conventional wisdom that the moon has virtually no atmosphere. Just as the discovery of water on the moon transformed our textbook knowledge of Earths nearest celestial neighbour, recent studies confirm that our moon does indeed have an atmosphere consisting of some unusual gases, including sodium and potassium, which are not found in the atmospheres of Earth, Mars or Venus. Its an infinitesimal amount of air when compared to Earths atmosphere. At sea level on Earth, we breathe in an atmosphere where each cubic centimetre contains 10,000,000,000,000,000,000 molecules; by comparison the lunar atmosphere has less than 1,000,000 molecules in the same volume. That still sounds like a lot, but it is what we consider to be a very good vacuum on Earth. In fact, the density of the atmosphere at the moons surface is comparable to the density of the outermost fringes of Earths atmosphere where the International Space Station orbits.
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Title: Temporal Variability of Lunar Exospheric Helium During January 2012 from LRO/LAMP
Authors: Paul D. Feldman (1), Dana M. Hurley (2), Kurt D. Retherford (3), G. Randall Gladstone (3), S. Alan Stern (3), Wayne Pryor (4), Joel Wm. Parker (3), David E. Kaufmann (3), Michael W. Davis (3), Maarten Versteeg (3), LAMP team ((1) JHU, (2) JHU/APL, (3) SwRI, (4) Central AZ College)

We report observations of the lunar helium exosphere made between December 29, 2011, and January 26, 2012, with the Lyman Alpha Mapping Project (LAMP) ultraviolet spectrograph on NASA's Lunar Reconnaissance Orbiter Mission (LRO). The observations were made of resonantly scattered He I 584 from illuminated atmosphere against the dark lunar surface on the dawn side of the terminator. We find no or little variation of the derived surface He density with latitude but day-to-day variations that likely reflect variations in the solar wind alpha flux. The 5-day passage of the Moon through the Earth's magnetotail results in a factor of two decrease in surface density, which is well explained by model simulations.

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Lunar atmosphere
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Lunar Reconnaissance Orbiter's LAMP spectrometer detects helium in Moon's atmosphere, raises questions about origin

Scientists using the Lyman Alpha Mapping Project (LAMP) aboard NASA's Lunar Reconnaissance Orbiter have made the first spectroscopic observations of the noble gas helium in the tenuous atmosphere surrounding the Moon. These remote-sensing observations complement in-situ measurements taken in 1972 by the Lunar Atmosphere Composition Experiment (LACE) deployed by Apollo 17.
Although LAMP was designed to map the lunar surface, the team expanded its science investigation to examine the far ultraviolet emissions visible in the tenuous atmosphere above the lunar surface, detecting helium over a campaign spanning more than 50 orbits. Because helium also resides in the interplanetary background, several techniques were applied to remove signal contributions from the background helium and determine the amount of helium native to the Moon. Geophysical Research Letters published a paper on this research in 2012.

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Lunar Ionosphere
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Mystery of the Lunar Ionosphere

How can a world without air have an ionosphere? Somehow the Moon has done it.
Lunar researchers have been struggling with the mystery for years, and they may have finally found a solution.
But first, what is an ionosphere?
Every terrestrial planet with an atmosphere has one.  High above the planets rocky surface where the atmosphere meets the vacuum of space, ultraviolet rays from the sun break apart atoms of air.  This creates a layer of ionised gas--an "ionosphere."

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Lunar exosphere
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Solar wind and extreme ultraviolet modulation of the lunar ionosphere/exosphere

The ALSEP/SIDE detectors routinely monitor the dayside lunar ionosphere. Variations in the ionosphere are found to correlate with both the 2800 MHz radio index which can be related to solar EUV and with the solar wind proton flux. For the solar wind, the ionospheric variation is proportionately greater than that of the solar wind. This suggests an amplification effect on the lunar atmosphere due perhaps to sputtering of the surface or, less probably, an inordinate enhancement of noble gases in the solar wind. The surface neutral number density is calculated under the assumption of neon gas. During a quiet solar wind this number agrees with or is slightly above that expected for neon accreted from the solar wind. During an enhanced solar wind the neutral number density is much higher.
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ScienceCasts: Mystery of the Lunar Ionosphere



How can a world without air have an ionosphere? Somehow the Moon has done it. Lunar researchers have been struggling with this mystery for years, and they may have finally found a solution.



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