45th Lunar and Planetary Science Conference
Lunar Dust and Exosphere -
The First Results from LADEE
Lunar Dust and Exosphere -
The First Results from LADEE
The Woodlands, Texas
Tuesday morning, March 18, 2014
Tuesday morning, March 18, 2014
Richard Elphic and Andrew Poppe, Chairs
Special Session on Lunar Dust and Exosphere Featuring the First Results from LADEE: This session will address new results concerning the lunar exosphere, the mystery of electrostatically lofted dust, and other new research concerning the exotic phenomena surrounding the nearest example of a surface boundary exosphere. The focus will be on results from the Lunar Atmosphere and Dust Environment Explorer (LADEE) mission, but will also incorporate relevant Lunar Reconnaissance Orbiter (LRO) exosphere/dust measurements and ARTEMIS observations related to LADEE science.
|Notional view of the LADEE spacecraft superimposed on Clementine star-tracker imagery from 1994 [NASA/DOD/JPL-Caltech].
LADEE is making measurements of the tenuous lunar exosphere and the dust cloud from meteoroid impacts. The talks presented in this special session will highlight LADEE’s preliminary science results. These include initial observations of argon, neon and helium exospheres, and their diurnal variations; the lunar micrometeoroid impact ejecta cloud and its variations; spatial and temporal variations of the sodium exosphere; and observations of sunlight extinction caused by dust, as well as other topics.
8:45 a.m. Glenar, Stubbs and Elphic - LADEE Search for a Dust Exosphere: A Historical Perspective, #2640
The LADEE search for a dust exosphere is discussed in the context of recent dust upper-limit measurements. In general the detection of a small-grain dust population consistent with the low Clementine and LAMP upper limit estimates will be a challenge for the LADEE mission. On the other hand, these prior measurements represent only a small part of the LADEE search space, and none coincide with the occurance of major meteor streams. The LADEE dust search is sure to produce surprises.
9:00 a.m. Horanyi, Gagnard, Gathright, Gruen and James, et al. - The Dust Environment of the Moon as Seen by the Lunar Dust Experiment (LDEX), #1303
The Lunar Dust Experiment (LDEX) onboard the LADEE mission continues to make observations in lunar orbit since its cover was deployed on October 13, 2013.
9:15 a.m. Kempf, Grün, Horanyi, James and Lankton, et al. - Observations of the Lunar Dust Exosphere with LDEX, #1389
9:30 a.m. Stubbs, Glenar, Wang, Hermalyn and Sarantos, et al. - The Impact of Meteoroid Streams on the Lunar Atmosphere and Dust Environment During the LADEE Mission, #2705
9:45 a.m. Halekas, Poppe, Delory, Elphic and Angelopoulos, et al. - ARTEMIS Observations and Data-Based Modeling in Support of LADEE, #1548
|Relative positions of the LADEE with the ARTEMIS P1 and P2 spacecraft early on December 23, 2013 [NASA/JPL-CalTech].
|A schematic of the very dynamic lunar ionosphere put together originally by Jasper Halekas, now an investigator with the ARTEMIS program.
The ARTEMIS (Acceleration, Reconnection, Turbulence, & Electrodynamics of Moon’s Interaction with the Sun) mission consists of two probes from the heliospheric constellation THEMIS retasked to the Moon in 2011. ARTEMIS provides comprehensive measurements of charged particles and magnetic and electric fields around the Moon from a two-point vantage that enables continuous upstream plasma monitoring.
|Early representation from the highly innovative proposal to gradually reposition two of the five member constellation of THEMIS probes into lunar orbit, where today they operate as ARTEMIS P1 and P2. On the way to being re-tasked, the vehicles became the first to orbit the Earth Moon Lagrange points L1 and L2. Their functions are presently an important part of the LADEE mission.
The ambient plasma, directly and through its control of the nearsurface electrostatic environment, contributes significantly to the exospheric cycle. By comparing various measurements, and utilizing ARTEMIS-measured fields to determine their trajectories and likely sources, we can obtain additional constraints on the sources, composition, and dynamics of the tenuous lunar atmosphere..
10:00 a.m. Szalay, Horanyi, Poppe and Halekas - LDEX Observations and Correlations with ARTEMIS Measurements, #1500
ARTEMIS provides a variety of plasma measurements which show a high degree of correlation with LDEX current measurements. There are two regions in the LDEX data that show consistent correlation: 1) the subsolar point and 2) the time LADEE crosses into umbral shadow.
Shown in Figure 3 are the average LDEX current for 15 minutes from subsolar and shadow crossing time respectively. Additionally, the component of the convection electric field parallel to the LDEX boresight appears to be significant in LDEX’s current measurements and will be discussed in this presentation.
This talk will focus on the correlations between LDEX and ARTEMIS data.
|Figure 2. - LDEX current measurements. Above: Orbit with high LDEX current variability, and bottom: Quiet period in LDEX current data.
The lunar exosphere is a tenuous, collisionless combination of various neutral species derived from a variety of sources, including charged particle sputtering, micrometeoroid impact vaporization, internal gas release, and photon-, electron-, and thermally-stimulated desorption. Solar irradiation will photoionize these neutrals which are in turn picked up by the ambient interplanetary magnetic and electric fields and lost to interplanetary space.
The Lunar Dust EXperiment (LDEX) onboard the Lunar Atmospheric and Dust Environment Explorer (LADEE) is currently searching for the signature of charged, sub-micron sized dust grains lofted to kilometer altitudes above the lunar surface, but such measurements are also sensitive to ambient, low-energy ions including those of lunar exospheric origin.
|The LADEE Lunar Dust Experiment (LDEX)
We present early observations of He, Ar, and Ne observations from the LADEE NMS in lunar orbit. The Neutral Mass Spectrometer (NMS) of the Lunar Atmosphere and Dust Environment Explorer (LADEE) Mission is designed to meas-ure the composition and variability of the tenuous lunar atmosphere. The NMS complements two other instru-ments on the LADEE spacecraft designed to secure spectroscopic measurements of lunar composition and in situ measurement of lunar dust over the course of a 100-day mission in order to sample multiple lunation periods.
Instrument activities are designed and scheduled to provide time resolved measurements of Helium and Argon and determine abundance or upper limits for many other species either sputtered or thermally evolved from the lunar surface.
|Figure 3. Evolution of Helium abundance (in instrument counting units) as a function of solar local time observed during the LADEE commissioning phase.
|LADEE Neutral Mass Spectrometer (NMS)
This talk will overview the design, performance, and initial results of the LADEE UVS instrument.
UVS deployed its limb-viewing telescope door on October 17 and began a series commissioning activities, including pointing, wavelength and preliminary radiometric calibrations. UVS made its first lunar limb observations on October 23, 2013.
UVS has been routinely monitoring two previously measured atmospheric species, potassium and sodium, and has been making observations to search for other, previously sought species including OH, H2O, Si, Al, Mg, Ca, Ti, and Fe. UVS is also able to detect the scattered light from lofted dust between the altitudes of a few km up to 50 km using its limb telescope, as well as search for dust very near the surface using solar occultation measurements. The UVS instrument operates between 230 – 810 nm with a spectral resolution of less than 1 nm. The spectrometer has been operating nominally.
|LADEE ultra-violet - visible light (UVVIS) spectrometer (UVS)
Sodium and potassium are regularly measured in all activities, except for occultations. Trends in these measurements are made both spatially and temporally, and associations are with specific events, such as meteor streams, and surface composition are examined.
This study presents preliminary results of lunar limb observations from the UVS on LADEE toward understanding the impact contribution to the dust exosphere.
“Larger” impact events (e.g., impactor size>>target grain size), which are expected to form sporadically over the course of a year, are capable of lofting a considerable amount of material for a measureable period of time. As these ejecta return to the surface (or encounter local topography), they impact at hundreds of meters per second or higher, thereby “scouring” the surface with low-mass oblique dust impacts. While these high-speed ejecta represent only a small fraction of the total ejected mass, the lofting and subsequent ballistic return of this dust has a high potential for mobilization.
The actual visibility of these ejecta clouds diminishes with height and time as the particles spread ballistically.
Given the amount of material expected to be lofted above 5km, these results indicate that there is a significant chance of LADEE observations of a primary ejecta cloud even from relatively small impacts- if they occur at the right time and place (e.g., at a location and recently enough to enter the fields of view of the instruments before spreading too much).
The chances of observing such an event grow significantly higher during a meteor shower, which have been observed to cause very frequent impact flashes on the moon. around the moon for durations of several hours. These smaller, more frequent, craters can loft diminished (but measurable with the UVS & LDEX instruments) amounts of regolith for tens of minutes.
11:30 a.m. Hurley, Benna, Mahaffy, Elphic and Colaprete, et al. - Upper Limits on the Propagation of Constituents of the Chang’e-3 Exhaust Plume from LADEE Observations, #2160
Thus, the introduction of vapor into the lunar environment via rocket exhaust during a soft lunar landing constitutes a 100 times temporary enhancement to the source rate to the lunar exosphere and an increase in the total mass of 10%. Whereas the native lunar exosphere is comprised primarily of helium and argon; the rocket exhaust comprises water, carbon dioxide, ammonia, and other HCNO products.
The distribution of particles in the lunar exosphere is largely controlled by the interactions between the particles and the lunar surface. For example, helium does not stick to lunar regolith grains, thus follows an inverse relationship between the density and the surface temperature. In contrast, argon does stick to the surface at colder, nightside temperatures. Argon density is observed to peak at the terminators.
Thus, if the propagation of the exhaust vapors can be monitored, it can reveal previously unknown properties of the gas-surface interaction with the lunar regolith. We model the release and propagation of the exhaust gases on the Moon and compare to observations in orbit around the Moon from the Lunar Atmosphere and Dust Environment Explorer (LADEE) spacecraft.
12:00 p.m. To be Announced.
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