Timothy J. Stubbs

A dynamic fountain model for lunar dust

There is much evidence to show that lunar “horizon glow” and “streamers” observed at the terminator are caused by sunlight scattered by dust grains originating fr om the surface. The dust grains and lunar surface are electrostatically charged by the Moon’s i nteraction with the local plasma environment and the photoemission of electrons due to solar UV and X-rays. This ef fect causes the like-charged surface and dust particles to repel each other, and crea tes a near-surface electric field. Previous models have explained micron-sized dust observed at ~10 cm a bove the surface, by suggesting that charged grains “levitate” in the local elect ric field; however this cannot account for observations of 0.1 m-scale grains at ~100 km altitude. In order to explain the high-altitude dust observations, we propose a dynamic “fountain” model in which cha rged dust grains follow ballistic trajectories, subsequent to being accelerate d upward through a narrow sheath region by the surface electric field. These dust grains could affe ct the optical quality of the lunar environment for astronomical observations and interfere with exploration a ctivities.

Interplanetary magnetic field control of the location of substorm onset and auroral features in the conjugate hemispheres

[1] During 2001 and 2002, when the Imager for Magnetopause-to-Aurora Global Exploration (IMAGE) satellite had its apogee in the Northern Hemisphere and the Polar spacecraft, owing to the apsidal precession of its orbit, reached higher altitudes in the Southern Hemisphere, the two spacecraft offered a unique opportunity to study the aurora in the conjugate hemispheres simultaneously. Owing to the large fields of view of the Polar Visible Imaging System (VIS) Earth camera and the IMAGE-FUV instruments, substorms and auroral features were imaged on a global scale in both hemispheres. We have identified five substorm onsets and several auroral features that can be unambiguously identified and compared in the two hemispheres. When mapped onto apex coordinates in the two hemispheres, we find that substorm onset locations and auroral features are usually not symmetric. The longitudinal displacement in one hemisphere compared with the other can be as much as 1.5 hours of local time (∼1500 km). For southward interplanetary magnetic field (IMF) the hemispherical asymmetry (AMLT) is strongly correlated with the IMF clock angle (θ C ) and a linear fit, ΔMLT = -0.017c C + 3.44, gives a correlation coefficient of 0.83 with a mean deviation of 0.4ΔMLT. These findings are interpreted as the magnetic tensions force acting on open magnetic field lines before reconnecting in the magnetotail. This can also be thought of as the IMF penetrating the magnetosphere.

The high‐altitude cusps: HEOS 2

Magnetic field data from the HEOS 2 spacecraft are presented for magnetopause crossings during the summers of 1972-1974. The orbit of HEOS 2 at this time was optimal for studying the high-altitude cusp. Magnetopause crossings were categorized as being either tailward or on the dayside of the cusp, or through the cusp itself. The tailward crossings form a relatively distinct set, but the dayside and cusp crossings are mixed, indicating either considerable motion or distension of the cusp. The cusp crossings extend from 5.5 to 7.5 R E in the X GSM direction and from 4 to 7 R E in the Z GSM direction (30% of all crossings). A more detailed survey of five crossings representing each category is presented. Most of these illustrate that the cusp is a very dynamic region, with magnetic field fluctuations occurring on short timescales. Although useful for the above classification, in general there is little agreement with the current generation of magnetospheric field models. The gross character and location relative to the model field geometry, however, can be identified, given the prevailing magnetospheric activity.

How surface composition and meteoroid impacts mediate sodium and potassium in the lunar exosphere

The Moons time-variable exosphere Earths Moon does not have a conventional gaseous atmosphere, but instead an “exosphere” of particles ejected from the surface. Colaprete et al. have used NASAs LADEE orbiter to investigate how the exosphere varies over time, by using the glow from sodium and potassium atoms as a probe (see the Perspective by Dukes and Hurley). The exosphere composition varies by a factor of 2 to 3 over the course of a month, as different parts of the Moon are exposed to sunlight. There are also increases shortly after the Moon passes through streams of meteoroids. Science, this issue p. 249; see also p. 230 The Moon’s tenuous atmosphere varies over each month and after meteoroid streams. [Also see Perspective by Dukes and Hurley] Despite being trace constituents of the lunar exosphere, sodium and potassium are the most readily observed species due to their bright line emission. Measurements of these species by the Ultraviolet and Visible Spectrometer (UVS) on the Lunar Atmosphere and Dust Environment Explorer (LADEE) have revealed unambiguous temporal and spatial variations indicative of a strong role for meteoroid bombardment and surface composition in determining the composition and local time dependence of the Moon’s exosphere. Observations show distinct lunar day (monthly) cycles for both species as well as an annual cycle for sodium. The first continuous measurements for potassium show a more repeatable variation across lunations and an enhancement over KREEP (Potassium Rare Earth Elements and Phosphorus) surface regions, revealing a strong dependence on surface composition.

Search for a high‐altitude lunar dust exosphere using Clementine navigational star tracker measurements

During the 1994 Clementine lunar mapping mission, portions of 25 orbits were dedicated to a search for lunar horizon glow (LHG) using the spacecraft star tracker navigation cameras. Previous putative detections of LHG were believed to result from forward scattering of sunlight by exospheric dust grains with radii ≈ 0.1 µm, observable above the limb from within the shadow of the Moon near orbital sunrise or sunset. We have examined star tracker image sequences from five Clementine orbits in which the limb occulted the Sun, and was at least partially shadowed from earthshine, minimizing the chance of stray light contamination. No LHG appears in the image data, or in any of the net brightness images, after subtraction of a reference zodiacal light model. However, some of the images display faint excess limb brightness that appears to be solar streamer structure. Therefore, we derive upper limits for the amount of dust in the lunar exosphere that could be hidden by these brightness fluctuations using a dust-scattering simulation code and simple exponential dust profiles defined by surface concentration n0 and scale height H. Simulations using grains of radius 0.1 µm show that fluctuations in the observed excess brightness can be matched by a dust exosphere with a vertical column abundance n0H of 5–30 cm−2 and overlying mass <10−12 g cm−2. These dust upper limit estimates are highly dependent on assumed grain size due to the rapid increase in per-grain brightness with grain radius.

Dielectric breakdown weathering of the Moon's polar regolith

Galactic cosmic rays and solar energetic particles (SEPs) can charge the Moons subsurface, a process expected to be particularly important in the polar regions. Experiments have shown that sufficient fluences (i.e., time-integrated fluxes) of energetic charged particles can cause dielectric breakdown, in which the electric field rapidly vaporizes small, filamentary channels within a dielectric. Lunar regolith has both the characteristics and, in some polar locations, the environment needed to make breakdown likely. We combine the Jet Propulsion Laboratory proton fluence model with temperature measurements from the Lunar Reconnaissance Orbiters (LROs) Diviner instrument and related temperature modeling to estimate how often breakdown occurs in the polar regions. We find that all gardened regolith within permanently shadowed regions (PSRs) has likely experienced up to 2×106 SEP events capable of causing breakdown, while the warmest polar regions have experienced about 2 orders of magnitude fewer events. We also use measurements from the Cosmic Ray Telescope for the Effects of Radiation on LRO to show that at least two breakdown-inducing events may have occurred since LRO arrived at the Moon in 2009. Finally, we discuss how such “breakdown weathering” may increase the percentage of fine and monomineralic grains within PSRs; explain the presence of so-called “fairy castle” regolith structures; and contribute to other low-albedo features detected by LROs Lyman Alpha Mapping Project, possibly establishing a correlation between these features and the average temperatures within craters that are only partly in permanent shadow.

Deep dielectric charging of regolith within the Moon's permanently shadowed regions

Energetic charged particles, such as galactic cosmic rays (GCRs) and solar energetic particles (SEPs), can penetrate deep within the lunar surface, resulting in deep dielectric charging. This charging process depends on the GCR and SEP currents, as well as on the regoliths electrical conductivity and permittivity. In permanently shadowed regions (PSRs) near the lunar poles, the discharging timescales are on the order of a lunation (∼20 days). We present the first predictions for deep dielectric charging of lunar regolith. To estimate the resulting subsurface electric fields, we develop a data-driven, one-dimensional, time-dependent model. For model inputs, we use GCR data from the Cosmic Ray Telescope for the Effects of Radiation on board the Lunar Reconnaissance Orbiter and SEP data from the Electron, Proton, and Alpha Monitor on the Advanced Composition Explorer. We find that during the recent solar minimum, GCRs create persistent electric fields up to ∼700 V/m. We also find that large SEP events create transient but strong electric fields (≥106 V/m) that may induce dielectric breakdown. Such breakdown would likely result in significant modifications to the physical and chemical properties of the lunar regolith within PSRs.

Lunar and Martian Dust: Evaluation and Mitigation

Dust is a ubiquitous phenomenon which must be explicitly addressed during upcoming robotic and human planetary exploration missions. The near term plans to revisit the moon as a stepping stone to further exploration of Mars and beyond brings places a primary emphasis on evaluation and mitigation of lunar dust. Comprised of regolith particles ranging in size from tens of nanometers to microns, lunar dust is a manifestation of the complex interaction of the lunar soil with multiple mechanical, electrical, and gravitational effects. Charged dust particles could levitate in the solar wind plasma environment, and may mediate significant differential charging effects with potential harmful consequences, as well as pose toxicological health problems when inhaled. This work outlines the scientific basis for lunar dust behavior, it’s characteristics and potential effects, and surveys several potential strategies for its control and mitigation both for surface operations and inside the habitable working volumes of a lunar outpost. This paper presents a preliminary analysis of dust as a component of the lunar environment, an assessment of it’s potential impacts on lunar exploration, as well as a perspective on lessons learned and information still to be gained from prior exploration. Also presented are the current perspective and planning for dust management activities within NASA’s Exploration Technology Development Program.

The Lunar X-ray Observatory (LXO)

X-ray emission from charge exchange recombination between the highly ionized solar wind and neutral material in Earths magnetosheath has complicated x-ray observations of celestial objects with x-ray observatories including ROSAT, Chandra, XMM-Newton, and Suzaku. However, the charge-exchange emission can also be used as an important diagnostic of the solar-wind interacting with the magnetosheath. Soft x-ray observations from low-earth orbit or even the highly eccentric orbits of Chandra and XMM-Newton are likely superpositions of the celestial object of interest, the true extra-solar soft x-ray background, geospheric charge exchange, and heliospheric charge exchange. We show that with a small x-ray telescope placed either on the moon, in a similar vein as the Apollo ALSEP instruments, or in a stable orbit at a similar distance from the earth, we can begin to disentangle the complicated emission structure in the soft x-ray band. Here we present initial results of a feasibility study recently funded by NASA to place a small x-ray telescope on the lunar surface. The telescope operates during lunar night to observe charge exchange interactions between the solar wind and magnetosphic neutrals, between the solar wind and the lunar atmosphere, and an unobstructed view of the soft x-ray background without the geospheric component.