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].|
Elphic, Hine, Delory Salute and Noble, et. al. - The Lunar Atmosphere and Dust Environment Explorer (LADEE): Initial Science Results,
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.
Glenar, Stubbs and Elphic - LADEE Search for a Dust Exosphere: A Historical Perspective,
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.
Horanyi, Gagnard, Gathright, Gruen and James, et al. - The Dust Environment of the Moon as Seen by the Lunar Dust Experiment (LDEX)
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.
|Figure 2. Summary of LADEE/LDEX activities through January 3, 2014. The number of recorded events (noise and dust impacts) sharply increased following LADEE orbit lowering maneuvers. An unusually large burst of events was observed November 12, most likely related to the Taurids meteor stream, and the following intense period, starting December 13, coinciding with the Geminids and the landing of Chang’e-3.|
Kempf, Grün, Horanyi, James and Lankton, et al. - Observations of the Lunar Dust Exosphere with LDEX
This talk will report about first insights into the properties of the lunar dust exosphere based on a preliminary analysis of the LDEX data. The transmitted data set is already larger than any other existing observation of a dust exosphere by orders of magnitudes and deepened our insight into the physics of this important phenomenon. This talk will report about first insights into the properties of the Lunar dust exosphere based on a preliminary analysis of the LDEX data.
Stubbs, Glenar, Wang, Hermalyn and Sarantos, et al. - The Impact of Meteoroid Streams on the Lunar Atmosphere and Dust Environment During the LADEE Mission
We describe the 18 annual meteoroid streams predicted to encounter the Moon during the LADEE mission, and discuss the implications for the lunar environment.
|Figure 1. Locations of radiants for 18 annual meteor streams at the time of peak activity plotted as Selenographic Solar Ecliptic (SSE) latitude and local time. The points are color-coded to show the Zenith Hourly Rate (ZHR) at the peak in shower activity. (ZHR is the hourly rate of meteors seen by standard a observer on the Earth under optimum viewing conditions.) For our purposes ZHR serves as rough guide to meteoroid flux rates incident at the Moon. Only six of the 18 streams have peak ZHR exceeding background HR so it is reasonable to assume these streams will likely have the most noticeable effects on the lunar environment compared with typical conditions. (Gray shading covers the latitudinal range of the LADEE orbit.)|
Halekas, Poppe, Delory, Elphic and Angelopoulos, et al. - ARTEMIS Observations and Data-Based Modeling in Support of LADEE
|Relative positions of the LADEE with the ARTEMIS P1 and P2 spacecraft early on December 23, 2013 [NASA/JPL-CalTech].|
The goal of the LADEE mission is to understand the exosphere of the Moon, including its structure, temporal variability, and sources and sinks. To achieve this goal, we must determine how exospheric dust and neutrals behave, not in isolation, but as part of an inextricably coupled system that includes the surface and the plasma environment.
|A schematic of the very dynamic lunar ionosphere put together originally by Jasper Halekas, now an investigator with the ARTEMIS program.|
Charged particles, their interactions with the surface, and the electric and magnetic fields that they produce and respond to, contribute to source and sink processes for both lofted dust and exospheric gases, and therefore to their spatial and temporal structure and variability. The LADEE payload does not include plasma instrumentation; instead, we utilize a combination of ARTEMIS measurements of upstream plasma parameters and data-based modeling to determine plasma quantities around the Moon, including at the LADEE orbit and in the regions covered by UVS scans and those providing the source populations ultimately measured by LDEX and NMS.
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 two ARTEMIS probes currently reside in stable near-equatorial elliptical orbits. An orbital design targeting periapses near the lunar dawn terminator.
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..
Szalay, Horanyi, Poppe and Halekas - LDEX Observations and Correlations with ARTEMIS Measurements
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.|
Poppe, Halekas, Szalay, Horanyi and Delory - Model-Data Comparisons of LADEE/LDEX Observations of Low-Energy Lunar Dayside Ions,
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 model the response of the LADEE/LDEX instrument to low-energy lunar dayside ions and discuss implications for the lunar exosphere and ionosphere.
Benna, Mahaffy and Hodges - Early Results from Exospheric Observations by the Neutral Mass Spectrometer (NMS)
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.
Early Results From NMS Observations
|Figure 3. Evolution of Helium abundance (in instrument counting units) as a function of solar local time observed during the LADEE commissioning phase.|
: Owing to its very low chemical background the NMS instrument was successful at detecting and mapping exospheric Helium during the high altitude (250 km) commissioning phase. This early mapping campaign provided the unique opportunity to observe variation of Helium at a constant altitude, for nearly a full lunation. Figure 3 shows results of Helium spatial and temporal variability as the Moon moves in and out of the Earth’s magnetotail.
|LADEE Neutral Mass Spectrometer (NMS)|
At the end of the commissioning phase and after the orbit’s periselene
was lowered to approximately 50 km, the NMS was able to detect Argon and Neon.
Colaprete, Elphic, Landis, Karcz and Shirley, et al. - Overview of the LADEE Ultraviolet-Visible Spectrometer: Design, Operations, and Initial Results,
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)|
Limb observations, using the UVS three-inch telescope, have been made on a routine basis, with limb “stares” at 20 km at the terminators, and 40 km at around local noon time. At the terminators the spacecraft “nods” the telescope between the surface and about 50 km. At noon it was found that near-surface scatter precluded observations below about 30 km, so nods are not performed then. There have been a mix of both “backward” looks (stares that point in the anti-
velocity direction of the spacecraft), and “forward” looks (which flip the spacecraft to allow UVS to look in the velocity direction). This permits observations both in and away from the direction of the sun.
|LPSC-2014, #2518 - Figure 1. Schematic of a LADEE “nod” operation to detect dust and gases at the lunar limb with the UVS. Nod begins in the stare orientation, scans down to lunar surface, up to higher altitudes, then back to stare. This activity provides science data at many altitudes down to lunar surface. The telescope can also be pointed 180° opposite from normal Limb stare orientation to allow for observations in forward scatter at sunset terminator.|
Occultations have been performed about once every two days, tracking the sun as it sets behind the sunrise terminator between about 35 km and the surface. In this mode UVS has gathered spectra at a rate of either 15 msec or 26 msec, corresponding to a spatial sampling of less than 1 km with very high SNR (typically less than 500 for a single scan).
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.
Hermalyn, Colaprete, Elphic, Landis and Karcz, et al. - Impact Lofted Ejecta Contribution to the Lunar Exosphere: Experiments and Results from the LADEE Ultraviolet Visible Spectrometer
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.
Wooden, Cook, Colaprete, Shirley and Vargo, et al. - LADEE UVS Observation of Solar Occultation by Exospheric Dust Above the Lunar Limb
LADEE UVS solar occultation measurements (40–0 km altitudes) reveal spectral signatures of forward scattering and absorption by dust in the lunar exosphere.
|Figure 1. (a) UVS solar viewer foreoptics, showing six sequential baffles, which define the field of view and minimize the contribution from off-axis scattered light. The solar diffuser is shown just before the fiber optic input, which leads to the spectrometer. (b) Schematic of an UVS occultation session showing the solar viewer field of view cone, grazing the lunar exosphere and subsequently the lunar limb, as LADEE’s orbit progresses through local sunrise.|
|Figure 2. A light curve from an example occultation activity, annotated with cartoons of the Sun as seen from the UVS solar viewer. The yellow line around the Sun in each cartoon marks the solar viewer field of view. Each view corresponds to a different section of the solar occultation light curve.|
Our preliminary results indicate wavelength dependent extinction as a function of altitude. We attribute the detected spectral color changes to the presence of sub-micron sized dust grains in the lunar exosphere. Details of these results will be presented, and compared to previous models of the lunar dust exosphere.
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
The native lunar exosphere is sparse. The estimated total mass of the lunar exosphere is ~107 g, with a source rate of ~10 g/s. Landing a vehicle such as the Chinese Chang’e-3 lander on the surface of the Moon would require burning an estimated 106 g of rocket fuel over ~12 minutes.
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.
. To be Announced.