A. R. Poppe

Pluto’s interaction with its space environment: Solar wind, energetic particles, and dust

New Horizons unveils the Pluto system In July 2015, the New Horizons spacecraft flew through the Pluto system at high speed, humanitys first close look at this enigmatic system on the outskirts of our solar system. In a series of papers, the New Horizons team present their analysis of the encounter data downloaded so far: Moore et al. present the complex surface features and geology of Pluto and its large moon Charon, including evidence of tectonics, glacial flow, and possible cryovolcanoes. Grundy et al. analyzed the colors and chemical compositions of their surfaces, with ices of H2O, CH4, CO, N2, and NH3 and a reddish material which may be tholins. Gladstone et al. investigated the atmosphere of Pluto, which is colder and more compact than expected and hosts numerous extensive layers of haze. Weaver et al. examined the small moons Styx, Nix, Kerberos, and Hydra, which are irregularly shaped, fast-rotating, and have bright surfaces. Bagenal et al. report how Pluto modifies its space environment, including interactions with the solar wind and a lack of dust in the system. Together, these findings massively increase our understanding of the bodies in the outer solar system. They will underpin the analysis of New Horizons data, which will continue for years to come. Science, this issue pp. 1284, 10.1126/science.aad9189, 10.1126/science.aad8866, 10.1126/science.aae0030, & 10.1126/science.aad9045 Pluto modifies its space environment, interacting with the solar wind plasma and energetic particles. INTRODUCTION The scientific objectives of NASA’s New Horizons mission include quantifying the rate at which atmospheric gases are escaping Pluto and describing its interaction with the surrounding space environment. The two New Horizons instruments that measure charged particles are the Solar Wind Around Pluto (SWAP) instrument and the Pluto Energetic Particle Spectrometer Science Investigation (PEPSSI) instrument. The Venetia Burney Student Dust Counter (SDC) counts the micrometer-sized dust grains that hit the detectors mounted on the ram direction of the spacecraft. This paper describes preliminary results from these three instruments when New Horizons flew past Pluto in July 2015 at a distance of 32.9 astronomical units (AU) from the Sun. RATIONALE Initial studies of the solar wind interaction with Pluto’s atmosphere suggested that the extent of the interaction depends on whether the atmospheric escape flux is strong (producing a comet-like interaction, where the interaction region is dominated by ion pick-up and is many times larger than the object) or weak (producing a Mars-like interaction dominated by ionospheric currents with limited upstream pick-up and where the scale size is comparable to the object). Before the New Horizons flyby, the estimates of the atmospheric escape rate ranged from as low as 1.5 × 1025 molecules s–1 to as high as 2 × 1028 molecules s–1. Combining these wide-ranging predictions of atmospheric escape rates with Voyager and New Horizons observations of extensive variability of the solar wind at 33 AU produced estimates of the scale of the interaction region that spanned all the way from 7 to 1000 Pluto radii (RP). RESULTS At the time of the flyby, SWAP measured the solar wind conditions near Pluto to be nearly constant and stronger than usual. The abnormally high solar wind density and associated pressures for this distance are likely due to a relatively strong traveling interplanetary shock that passed over the spacecraft 5 days earlier. Heavy ions picked up sunward from Pluto should mass-load and slow the solar wind. However, there is no evidence of such solar wind slowing in the SWAP data taken as near as ~20 RP inbound, which suggests that very few atmospheric molecules are escaping upstream and becoming ionized. The reorientation of the spacecraft to enable imaging of the Pluto system meant that both the SWAP and PEPSSI instruments were turned away from the solar direction, thus complicating our analysis of the particle data. Nevertheless, when the spacecraft was ~10 RP from Pluto, SWAP data indicated that the solar wind had slowed by ~20%. We use these measurements to estimate a distance of ~6 RP for the 20% slowing location directly upstream of Pluto. At this time, PEPSSI detected an enhancement of ions with energies in the kilo–electron volt range. The SDC, which measures grains with radii >1.4 µm, detected one candidate impact in ±5 days around its closest approach, indicating a dust density estimate of n = 1.2 km–3, with a 90% confidence level range of 0.6 < n < 4.6 km–3. CONCLUSION New Horizons’s particle instruments revealed an interaction region confined sunward of Pluto to within ~6 RP. The surprisingly small size is consistent with a reduced atmospheric escape rate of 6 × 1025 CH4 molecules s–1, as well as a particularly high solar wind flux due to a passing compression region. This region is similar in scale to the solar wind interaction with Mars’s escaping atmosphere. Beyond Pluto, the disturbance persists to distances greater than 400 RP downstream. Interaction of the solar wind with Pluto’s extended atmosphere. Protons and electrons streaming from the Sun at ~400 km s–1 are slowed and deflected around Pluto because of a combination of ionization of Pluto’s atmosphere and electrical currents induced in Pluto’s ionosphere. CREDIT: STEVE BARTLETT AND NASA’S SCIENTIFIC VISUALIZATION STUDIO The New Horizons spacecraft carried three instruments that measured the space environment near Pluto as it flew by on 14 July 2015. The Solar Wind Around Pluto (SWAP) instrument revealed an interaction region confined sunward of Pluto to within about 6 Pluto radii. The region’s surprisingly small size is consistent with a reduced atmospheric escape rate, as well as a particularly high solar wind flux. Observations from the Pluto Energetic Particle Spectrometer Science Investigation (PEPSSI) instrument suggest that ions are accelerated and/or deflected around Pluto. In the wake of the interaction region, PEPSSI observed suprathermal particle fluxes equal to about 1/10 of the flux in the interplanetary medium and increasing with distance downstream. The Venetia Burney Student Dust Counter, which measures grains with radii larger than 1.4 micrometers, detected one candidate impact in ±5 days around New Horizons’ closest approach, indicating an upper limit of <4.6 kilometers–3 for the dust density in the Pluto system.

Detections of lunar exospheric ions by the LADEE neutral mass spectrometer

The Lunar Atmosphere and Dust Environment Explorer (LADEE) Neutral Mass Spectrometer (NMS), operating in ion mode, provides sensitive detections of ions from the lunar exosphere. By analyzing ion-mode data from the entire mission, utilizing Acceleration, Reconnection, Turbulence, and Electrodynamics of the Moons Interaction with the Sun (ARTEMIS) plasma and magnetic field measurements to organize NMS data and eliminate background sources, we identify highly significant detections of lunar ions at mass per charge of 2, 4, 12, 20, 28, 39, and 40, moderately significant detections at 14 and 23, and weak detections at 24, 25, and 36. Unlike many previous observations of Moon-derived ions, an outward pointing viewing geometry ensures that these ions originate from the exosphere, rather than directly from the surface. For species with known neutral distributions, inferred ion production rates appear consistent with expectations for both magnitude and spatial distribution, assuming photoionization as the predominant source mechanism. Unexpected signals at mass per charge 12 and 28 suggest the presence of a significant exospheric population of carbon-bearing molecules.

Solar wind plasma interaction with Gerasimovich lunar magnetic anomaly

We present the results of the first local hybrid simulations (particle ions and fluid electrons) for the solar wind plasma interaction with realistic lunar crustal fields. We use a three-dimensiona ...

Experimental study of a photoelectron sheath

We describe a set of laboratory experiments to reproduce and investigate the photoelectron layer that occurs above UV-illuminated surfaces in space. The experiments are done in vacuum with UV illumination at 172 nm that is sufficiently intense for the creation of a photoelectron layer above a large, planar metal surface with a Debye shielding distance of ∼7 centimeters, small in comparison with the scale of the experiment. The emitting surface electrically floats to a potential approximately 1.5 V more positive than a nearby equipotential surface. Retarding potential analysis of the energy distribution of the electrons emitted from the electrically floating surface, as well as Langmuir probe data, show an effective electron temperature of 1.4 (±0.3) eV and a density of 4×1010 m−3. Langmuir probe measurements are taken throughout the photoelectron sheath to determine the electron density, which show good agreement with results from a 1-D particle-in-cell simulation. These experiments enable the better unde...

Evidence for small‐scale collisionless shocks at the Moon from ARTEMIS

ARTEMIS observes structures near the Moon that display many properties commonly associated with collisionless shocks, including a discontinuity with downstream compression of magnetic field and density, heating and wave activity, and velocity deflections away from the Moon. The two-probe ARTEMIS measurements show that these features do not exist in the pristine solar wind and thus must result from lunar influences. Discontinuity analyses indicate mass flux and heating across the boundary, with the normal velocity dropping from supermagnetosonic to submagnetosonic across the discontinuity. The shock location with respect to crustal magnetic fields suggests a causal relationship, implying that solar wind protons reflected from crustal fields may produce the observed structures. These observations may indicate some of the smallest shocks in the solar system (in terms of plasma scales), driven by solar wind interaction with magnetic fields on the order of the ion gyroradius and inertial length.

Anisotropic solar wind sputtering of the lunar surface induced by crustal magnetic anomalies

The lunar exosphere is generated by several processes each of which generates neutral distributions with different spatial and temporal variability. Solar wind sputtering of the lunar surface is a major process for many regolith-derived species and typically generates neutral distributions with a cosine dependence on solar zenith angle. Complicating this picture are remanent crustal magnetic anomalies on the lunar surface, which decelerate and partially reflect the solar wind before it strikes the surface. We use Kaguya maps of solar wind reflection efficiencies, Lunar Prospector maps of crustal field strengths, and published neutral sputtering yields to calculate anisotropic solar wind sputtering maps. We feed these maps to a Monte Carlo neutral exospheric model to explore three-dimensional exospheric anisotropies and find that significant anisotropies should be present in the neutral exosphere depending on selenographic location and solar wind conditions. Better understanding of solar wind/crustal anomaly interactions could potentially improve our results.

ARTEMIS observations of extreme diamagnetic fields in the lunar wake

We present two Acceleration, Reconnection, Turbulence, and Electrodynamics of the Moons Interaction with the Sun (ARTEMIS) observations of diamagnetic fields in the lunar wake at strengths exceeding twice the ambient magnetic field during high plasma beta conditions. The first observation was 350 km from the lunar surface while the Moon was located in the terrestrial magnetosheath with elevated particle temperatures. The second observation was in the solar wind ranging from 500 to 2000 km downstream, with a relatively low magnetic field strength of approximately 1.6 nT. In both cases, the plasma beta exceeded 10. We discuss the observations and compare the data to hybrid plasma simulations in order to validate the model under such extreme conditions and to elucidate the global structure of the lunar wake during these observations. The extreme nature of the diamagnetic field in the lunar wake provides an important end-member test case for theoretical and modeling studies of the various plasma processes operating in the lunar wake.

Statistical characterization of the foremoon particle and wave morphology: ARTEMIS observations

Although the zeroth-order picture of the Moon-solar wind interaction involves no upstream perturbation, the presence of the Moon does affect the upstream plasma in a variety of ways. In this paper, a large volume of data obtained by the dual-probe Acceleration, Reconnection, Turbulence and Electrodynamics of the Moons Interaction with the Sun (ARTEMIS) mission are used to characterize the large-scale morphology of the “foremoon,” which is defined as the region upstream of the Moon and its wake that contains Moon-related particles and waves. Solar wind ions reflected from the unshielded surface and by crustal magnetic fields, together with heavy ions of lunar surface/exospheric origin, are picked up by the solar wind magnetic and electric fields. Partially coinciding with populations of these Moon-related ions, ∼0.01 Hz and ∼1 Hz magnetic field fluctuations are observed. The morphology of the Moon-related ion and wave distributions is well organized by the upstream magnetic field direction. In addition, the low-frequency wave distributions depend on the upstream Alfven Mach numbers, suggesting that propagation effects also play a role in determining the wave foremoon morphology. Occurrence of modified electron velocity distributions and higher-frequency, electromagnetic, and electrostatic waves is primarily controlled by magnetic connection to the Moon and its wake. These statistical results observationally demonstrate the large-scale properties of the foremoon and upstream-parameter control thereof.

Dust ablation on the giant planets: Consequences for stratospheric photochemistry

Ablation of interplanetary dust supplies oxygen to the upper atmospheres of Jupiter, Saturn, Uranus, and Neptune. Using recent dynamical model predictions for the dust influx rates to the giant planets (Poppe, A.R. et al. [2016], Icarus 264, 369), we calculate the ablation profiles and investigate the subsequent coupled oxygen-hydrocarbon neutral photochemistry in the stratospheres of these planets. We find that dust grains from the Edgeworth-Kuiper Belt, Jupiter-family comets, and Oort-cloud comets supply an effective oxygen influx rate of 1.0 - 0.7 + 2.2 × 10 7 O atoms cm-2 s-1 to Jupiter, 7.4 - 5.1 + 16 × 10 4 cm-2 s-1 to Saturn, 8.9 - 6.1 + 19 × 10 4 cm-2 s-1 to Uranus, and 7.5 - 5.1 + 16 × 10 5 cm-2 s-1 to Neptune. The fate of the ablated oxygen depends in part on the molecular/atomic form of the initially delivered products, and on the altitude at which it was deposited. The dominant stratospheric products are CO, H2O, and CO2, which are relatively stable photochemically. Model-data comparisons suggest that interplanetary dust grains deliver an important component of the external oxygen to Jupiter and Uranus but fall far short of the amount needed to explain the CO abundance currently seen in the middle stratospheres of Saturn and Neptune. Our results are consistent with the theory that all of the giant planets have experienced large cometary impacts within the last few hundred years. Our results also suggest that the low background H2O abundance in Jupiters stratosphere is indicative of effective conversion of meteoric oxygen to CO during or immediately after the ablation process - photochemistry alone cannot efficiently convert the H2O into CO on the giant planets.

Martian planetary heavy ion sputtering of Phobos

The Martian moons, Phobos and Deimos, have long been suspected to be the sources of tenuous neutral gas tori encircling Mars. While direct outgassing has been ruled out as a strong source, micrometeoroid impact vaporization and charged particle sputtering must operate based on observations at other airless bodies. Previous models have addressed solar wind sputtering of Phobos; however, Phobos and Deimos are also subject to a significant, yet temporally variable, flux of heavy planetary ions escaping from Mars. In this report, we use a combination MHD/test-particle model to calculate the planetary heavy ion flux to Phobos and the ensuing neutral sputtered flux. Depending on ambient solar wind conditions and the location of Phobos, heavy ion sputtering of Phobos generates neutral fluxes up to and exceeding that from solar wind sputtering. We model pickup ions from the Phobos torus itself with applications for observations by the upcoming Mars Atmospheric and Volatile Evolution mission.