Planetary Ions Research Articles

Reduced Atmospheric Ion Escape Above Martian Crustal Magnetic Fields

AbstractMartian crustal fields were considered as too weak to have a distinctive effect on global escape rates of Martian heavy ions. However, new observations by the Mars Atmosphere and Volatile Evolution mission reveal a more precise result and show a notably lower atmospheric ion escape region above the area of the strongest crustal fields. A comparison between the fluxes of high and low energy O+ ions suggests that the strongest crustal fields may trap low energy ions and reduce the solar wind pick‐up efficiency while high energy ions form a flux depletion above the crustal field. Statistical results indicate a maximum reduction of the global escape flux by nearly 35% when the strongest crustal field region is oriented sunward. This is the first time that the protective effect of the crustal fields on heavy planetary ions has been observed and it might indicate a more effective protection of atmospheres by stronger magnetic fields like at Earth.

Heavy Ion Composition of Mercury's Magnetosphere

AbstractWe modeled the exospheric densities for sputtering and thermal desorption in detail for the time period of the first MESSENGER flyby of Mercury. From the exospheric densities we calculate ion production rates. These ions will be transported to the location of MESSENGER if they are produced on magnetic field lines connecting the cusp with the downwind side. From these ions we produce mass spectra that we compared with the Fast Imaging Plasma Spectrometer measurements performed during this flyby. We find good qualitative agreement between the modeled and the measured ion mass spectrum. We find that sputtering is a major process to contribute to the population of planetary ions in the magnetosphere because of the large scale height of the exospheric particles and the resulting long flight times. In addition, thermal desorption of Na contributes significant amounts to the magnetospheric ion population. From the volatile species we can identify He, OH, H2O, and Ne in the measured mass spectrum. However, for most of the volatile species the reported upper limits must be reduced by 2–3 orders of magnitude to be compatible to the measured ion spectrum.

Measurement of plasma channels in the Venus wake

Plasma channels have been suggested to account for the observation of ionospheric holes in the Venus nightside ionosphere. They are produced by the erosion of the solar wind on the magnetic polar regions of the Venus ionosphere, and are related to a sharp plasma transition that extends along the flanks of the wake and that is located downstream from the bow shock. The sharp plasma (intermediate) transition is a feature that has been detected with instruments onboard all the main spacecraft that have probed the Venus wake. From the early flyby transit of the Mariner 5 spacecraft to the later orbit crossings of the Venera 10, the Pioneer Venus and more recently the Venus Express there has been accumulated evidence of the presence of that transition by the flanks of the Venus wake. We report here measurements made with the ASPERA-4 instrument of the Venus Express that were conducted in orbit crossings by the midnight plane over a 4 year period (between 2006 and 2009), and that show different positions of the intermediate transition in the Venus wake at locations closer to the sun-Venus axis near solar minimum conditions by 2009. Measurements conducted in this latter time period also indicate that the intermediate transition by the midnight plane derives from the plasma channels and that is replaced by a broad velocity layer in orbits that probed farther away from that plane. It is argued that changes in the position of the intermediate transition with respect to the plasma channels are related to processes produced by the transport of solar wind momentum to the Venus polar ionosphere. The energy spectra of planetary ions measured in VEX orbits that probed by the midnight plane show momentum transport from the solar wind protons. This result is contrary to the conclusions reported by Collinson et al. (2014) in the sense that there is ¨no difference in the peak energy of the ionospheric outflow¨ when measured inside and outside an ionospheric hole.

Effects of a Weak Intrinsic Magnetic Field on Atmospheric Escape From Mars

AbstractThe magnetization of a planet significantly changes the nature of atmospheric escape, and the question of whether a planetary field shields atmosphere from erosion by the solar wind or not remains open. Ion escape processes from Mars are investigated under two hypothetical conditions, namely, no intrinsic magnetic field and weak intrinsic dipole field cases under present solar wind conditions based on multispecies magnetohydrodynamics simulations. The existence of a weak dipole field results in an enhancement of the tailward flux of planetary ions through the four escape channels. Two channels are associated with the open fields at the poles, and the others are caused by magnetic reconnection between the planetary and solar wind magnetic fields at the flank magnetopause. The ion escape rate with the weak dipole is greater than in the no‐dipole case. The enhancement is significant for O2+ and CO2+, suggesting ion escape from the low‐altitude ionosphere.

The Response of the Venusian Plasma Environment to the Passage of an ICME: Hybrid Simulation Results and Venus Express Observations

AbstractOwing to the heritage of previous missions such as the Pioneer Venus Orbiter and Venus Express, the typical global plasma environment of Venus is relatively well understood. On the other hand, this is not true for more extreme driving conditions such as during passages of interplanetary coronal mass ejections (ICMEs). One of the outstanding questions is how do ICMEs, either the ejecta or sheath portions, impact (1) the Venusian magnetic topology and (2) escape rates of planetary ions? One of the main issues encountered when addressing these problems is the difficulty of inferring global dynamics from single spacecraft obits; this is where the benefits of simulations become apparent. In the present study, we present a detailed case study of an ICME interaction with Venus on 5 November 2011 in which the magnetic barrier reached over 250 nT. We use both Venus Express observations and hybrid simulation runs to study the impact on the field draping pattern and the escape rates of planetary O+ ions. The simulation showed that the magnetic field line draping pattern around Venus during the ICME is similar to that during typical solar wind conditions and that O+ ion escape rates are increased by approximately 30% due to the ICME. Moreover, the atypically large magnetic barrier appears to manifest from a number of factors such as the flux pileup, dayside compression, and the driving time from the ICME ejecta.

The Structure of Martian Magnetosphere at the Dayside Terminator Region as Observed on MAVEN Spacecraft

AbstractWe analyzed 44 passes of the Mars Atmosphere and Volatile EvolutioN mission (MAVEN) spacecraft through the magnetosphere, arranged by the angle between electric field vector and the projection of spacecraft position radius vector in the plane perpendicular to the Mars‐Sun line (θE). All passes were divided into three angular sectors near 0°, 90°, and 180° θE angles in order to estimate the role of the interplanetary magnetic field direction in plasma and magnetic properties of dayside Martian magnetosphere. The time interval chosen was from 17 January to 4 February 2016 when MAVEN was crossing the dayside magnetosphere at solar zenith angle ~70°. Magnetosphere as the region with prevailing energetic planetary ions is always found between the magnetosheath and the ionosphere. The analysis of dayside interaction region showed that for each angular sector with different orientation of the solar wind electric field vector E = −1/c V × B one can find specific profiles of the magnetosheath, the magnetic barrier (Michel, 1971, https://doi.org/10.1029/RG009i002p00427; Zhang et al., 1991, https://doi.org/10.1029/91JA00088), and the magnetosphere. Magnetic barrier forms in front of the magnetosphere, and relative magnetic field magnitudes in these two domains vary. The average height of the boundary with ionosphere is ~530 km, and the average height of the magnetopause is ~730 km. We discuss the implications of the observed magnetosphere structure to the planetary ions loss mechanism.

Sodium Ion Dynamics in the Magnetospheric Flanks of Mercury

AbstractWe investigate the transport of planetary ions in the magnetospheric flanks of Mercury. In situ measurements from the MErcury Surface, Space ENvironment, GEochemistry, and Ranging spacecraft show evidences of Kelvin‐Helmholtz instability development in this region of space, due to the velocity shear between the downtail streaming flow of solar wind originating protons in the magnetosheath and the magnetospheric populations. Ions that originate from the planet exosphere and that gain access to this region of space may be transported across the magnetopause along meandering orbits. We examine this transport using single‐particle trajectory calculations in model Magnetohydrodynamics simulations of the Kelvin‐Helmholtz instability. We show that heavy ions of planetary origin such as Na+ may experience prominent nonadiabatic energization as they E × B drift across large‐scale rolled up vortices. This energization is controlled by the characteristics of the electric field burst encountered along the particle path, the net energy change realized corresponding to the maximum E × B drift energy. This nonadiabatic energization also is responsible for prominent scattering of the particles toward the direction perpendicular to the magnetic field.

Energy conversion through mass loading of escaping ionospheric ions for different Kp values

Abstract. By conserving momentum during the mixing of fast solar wind flow and slow planetary ion flow in an inelastic way, mass loading converts kinetic energy to other forms – e.g. first to electrical energy through charge separation and then to thermal energy (randomness) through gyromotion of the newly born cold ions for the comet and Mars cases. Here, we consider the Earth's exterior cusp and plasma mantle, where the ionospheric origin escaping ions with finite temperatures are loaded into the decelerated solar wind flow. Due to direct connectivity to the ionosphere through the geomagnetic field, a large part of this electrical energy is consumed to maintain field-aligned currents (FACs) toward the ionosphere, in a similar manner as the solar wind-driven ionospheric convection in the open geomagnetic field region. We show that the energy extraction rate by the mass loading of escaping ions (ΔK) is sufficient to explain the cusp FACs, and that ΔK depends only on the solar wind velocity accessing the mass-loading region (usw) and the total mass flux of the escaping ions into this region (mloadFload), as ΔK ∼ −mloadFloadu2sw∕4. The expected distribution of the separated charges by this process also predicts the observed flowing directions of the cusp FACs for different interplanetary magnetic field (IMF) orientations if we include the deflection of the solar wind flow directions in the exterior cusp. Using empirical relations of u0 ∝ Kp + 1.2 and Fload ∝ exp(0.45Kp) for Kp = 1–7, where u0 is the solar wind velocity upstream of the bow shock, ΔK becomes a simple function of Kp as log10(ΔK) = 0.2 ⋅ Kp + 2 ⋅ log10(Kp + 1.2) + constant. The major contribution of this nearly linear increase is the Fload term, i.e. positive feedback between the increase of ion escaping rate Fload through the increased energy consumption in the ionosphere for high Kp, and subsequent extraction of more kinetic energy ΔK from the solar wind to the current system by the increased Fload. Since Fload significantly increases for increased flux of extreme ultraviolet (EUV) radiation, high EUV flux may significantly enhance this positive feedback. Therefore, the ion escape rate and the energy extraction by mass loading during ancient Earth, when the Sun is believed to have emitted much higher EUV flux than at present, could have been even higher than the currently available highest values based on Kp = 9. This raises a possibility that the ion escape has substantially contributed to the evolution of the Earth's atmosphere.

Flows, Fields, and Forces in the Mars‐Solar Wind Interaction

AbstractWe utilize suprathermal ion and magnetic field measurements from the Mars Atmosphere and Volatile EvolutioN (MAVEN) mission, organized by the upstream magnetic field, to investigate the morphology and variability of flows, fields, and forces in the Mars‐solar wind interaction. We employ a combination of case studies and statistical investigations to characterize the interaction in both quasi‐parallel and quasi‐perpendicular regions and under high and low solar wind Mach number conditions. For the first time, we include a detailed investigation of suprathermal ion temperature and anisotropy. We find that the observed magnetic fields and suprathermal ion moments in the magnetosheath, bow shock, and upstream regions have observable asymmetries controlled by the interplanetary magnetic field, with particularly large asymmetries found in the ion parallel temperature and anisotropy. The greatest temperature anisotropies occur in quasi‐perpendicular regions of the magnetosheath and under low Mach number conditions. These results have implications for the growth and evolution of wave‐particle instabilities and their role in energy transport and dissipation. We utilize the measured parameters to estimate the average ion pressure gradient, J × B, and v × B macroscopic force terms. The pressure gradient force maintains nearly cylindrical symmetry, while the J × B force has larger asymmetries and varies in magnitude in comparison to the pressure gradient force. The v × B force felt by newly produced planetary ions exceeds the other forces in magnitude in the magnetosheath and upstream regions for all solar wind conditions.

Analysis of dayside magnetosphere of Mars: High mass loading case as observed on MAVEN spacecraft

MAVEN spacecraft provides new opportunities for analysis of Martian environment and physical process in near-Mars space. One of interesting regions of near-Mars space is the Martian magnetosphere that is formed from mass-loaded magnetic flux tubes. There is quite detailed knowledge of the night-side magnetosphere of Mars, however the number of publications on the dayside magnetosphere are quite limited. We analyze the plasma and magnetic structure and properties of Martian magnetosphere at strong mass-loading conditions as observed on MAVEN at Mars at the solar-zenith angle of ∼80° on January 4, 2015. This strong mass loading of upstream flow was apparently associated with plume ions ejected from upper part of Martian magnetosphere by the solar wind motional electric field. The magnetosphere is defined by two current layers separating it from the magnetosheath at higher altitudes and from ionosphere plasma at lower altitudes. It is characterized by dominance of planetary ions which number density increases by two orders of magnitude from upper boundary to lower one. There is approximate equipartition between magnetic, ion thermal and kinetic energies through magnetosphere. The data suggest that the boundary of the magnetosphere is in pressure equilibrium with magnetosheath flow. The total energy of ion flow above (in the magnetosheath) and below (in the region of accelerated ionospheric ions) magnetosphere exceeds the magnetic energy. The upper boundary of magnetosphere was located at the place where the ratio of heavy ions and protons number densities reached ∼0.4. Within magnetosphere this ratio continued to rise and increased by about 2 orders of magnitude at the inner boundary of magnetosphere. The heavy ion number density profile within magnetosphere suggests that it was formed by the solar wind magnetic flux tubes that reached Mars in a narrow region near the subsolar point, and then drifted around Mars to the terminator region, mass-loaded by UV-ionized upper atmosphere neutrals during this drift.

Coupling between Mercury and its nightside magnetosphere: Cross‐tail current sheet asymmetry and substorm current wedge formation

AbstractWe analyzed MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) magnetic field and plasma measurements taken during 319 crossings of Mercury's cross‐tail current sheet. We found that the measured BZ in the current sheet is higher on the dawnside than the duskside by a factor of ≈3 and the asymmetry decreases with downtail distance. This result is consistent with expectations based upon MHD stress balance. The magnetic fields threading the more stretched current sheet in the duskside have a higher plasma beta than those on the dawnside, where they are less stretched. This asymmetric behavior is confirmed by mean current sheet thickness being greatest on the dawnside. We propose that heavy planetary ion (e.g., Na+) enhancements in the duskside current sheet provides the most likely explanation for the dawn‐dusk current sheet asymmetries. We also report the direct measurement of Mercury's substorm current wedge (SCW) formation and estimate the total current due to pileup of magnetic flux to be ≈11 kA. The conductance at the foot of the field lines required to close the SCW current is found to be ≈1.2 S, which is similar to earlier results derived from modeling of Mercury's Region 1 field‐aligned currents. Hence, Mercury's regolith is sufficiently conductive for the current to flow radially then across the surface of Mercury's highly conductive iron core. Mercury appears to be closely coupled to its nightside magnetosphere by mass loading of upward flowing heavy planetary ions and electrodynamically by field‐aligned currents that transfer momentum and energy to the nightside auroral oval crust and interior. Heavy planetary ion enhancements in Mercury's duskside current sheet provide explanation for cross‐tail asymmetries found in this study. The total current due to the pileup of magnetic flux and conductance required to close the SCW current is found to be ≈11 kA and 1.2 S. Mercury is coupled to magnetotail by mass loading of heavy ions and field‐aligned currents driven by reconnection‐related fast plasma flow.

Seasonal variability of Martian ion escape through the plume and tail from MAVEN observations

AbstractWe study the Mars Atmosphere and Volatile Evolution spacecraft observations of Martian planetary ion escape during two time periods: 11 November 2014 to 19 March 2015 and 4 June 2015 to 24 October 2015, with the focus on understanding the seasonal variability of Martian ion escape in response to the solar extreme ultraviolet (EUV) flux. We organize the >6 eV O+ ion data by the upstream electric field direction to estimate the escape rates through the plume and tail. To investigate the ion escape dependence on the solar EUV flux, we constrain the solar wind dynamic pressure and interplanetary magnetic filed strength and compare the ion escape rates through the plume and tail in different energy ranges under high and low EUV conditions. We found that the total >6 eV O+ escape rate increases from 2 to 3 × 1024 s−1 as the EUV irradiance increases by almost the same factor, mostly on the <1 keV tailward escape. The plume escape rate does not vary significantly with EUV. The relative contribution from the plume to the total escape varies between ~30% and ~20% from low to high EUV. Our results suggest that the Martian ion escape is sensitive to the seasonal EUV variation, and the contribution from plume escape becomes more important under low EUV conditions.

Mercury's cross‐tail current sheet: Structure, X‐line location and stress balance

AbstractThe structure, X‐line location, and magnetohydrodynamic (MHD) stress balance of Mercury's magnetotail were examined between −2.6 < XMSM < −1.4 RM using MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) measurements from 319 central plasma sheet (CPS) crossings. The mean plasma β in the CPS calculated from MESSENGER data is ~ 6. The CPS magnetic field was southward (i.e., tailward of X‐line) ~ 2–18% of the time. Extrapolation of downtail variations in BZ indicates an average X‐line location at −3 RM. Modeling of magnetic field measurements produced a cross‐tail current sheet (CS) thickness, current density, and inner CS edge location of 0.39 RM, 92 nA/m2 and −1.22 RM, respectively. Application of MHD stress balance suggests that heavy planetary ions may be important in maintaining stress balance within Mercury's CPS. Qualitative similarities between Mercury's and Earth's magnetotail are remarkable given the differences in upstream conditions, internal plasma composition, finite gyro‐radius scaling, and Mercury's lack of ionosphere.

MAVEN observations of a giant ionospheric flux rope near Mars resulting from interaction between the crustal and interplanetary draped magnetic fields

AbstractWe present Mars Atmosphere and Volatile EvolutioN (MAVEN) observations of a giant magnetic flux rope in the Martian dayside ionosphere. The flux rope was observed at an altitude of <300 km, downstream from strong subsolar crustal magnetic fields. The peak field amplitude was ∼200 nT, resulting in the largest difference between the observed magnetic field strength and a model for crustal magnetic fields of the entire MAVEN primary science phase. MAVEN detected planetary ions, including H+, O+, and , across the structure. The axial orientation estimated for the flux rope indicates that it likely formed as a result of interactions between the local crustal and overlaid draped interplanetary magnetic fields. Pitch angle distributions of ionospheric photoelectrons imply that this structure is connected to the Martian upper atmosphere. However, the flux rope is not present in observations at the next commensurable orbit crossing (approximately two Martian days later), implying that it eventually detaches from the atmosphere and is carried downstream. The flux rope observations occurred during an interplanetary coronal mass ejection event at Mars, suggesting that the disturbed upstream state played a role in allowing the interplanetary magnetic field to penetrate deeper into the Martian ionosphere than is typical, allowing the formation of the flux rope.

Pressure and ion composition boundaries at Mars

AbstractThis study analyzes results from a multifluid MHD simulation to investigate the shape and structure of the pressure and composition boundaries at Mars, which can provide physical insight for the observational analysis. These boundaries are examined via the unity contours and gradients of the plasma β, as well as β∗, which includes the dynamic pressure in the numerator, and the ion mass and number density ratios. It is found that unity contours are well aligned with the gradient extrema, indicating that the unity contour is a topological boundary. In addition, these two transitions of pressure and composition are of a thickness of 0.05–0.1 RM near the subsolar region to 1–1.5 RM in the tail. The comparison of the pressure and composition boundaries indicates that the two are very similar and that not only the plasma sheet but also the full volume of the lobes are dominated by planetary ions. It suggests that the tail escape for ions not only concentrates in the central plasma sheet but also the magnetic lobes. It is also worthy pointing out that the ion number density ratio unity contour is found to be systematically smaller than other unity boundaries, which calls for attention when the ion number density is used to identify such boundaries. Finally, the comparison between the boundaries of this study and two analytical fittings is carried out. We found a good agreement with the Vignes fitting, with little flaring in the tail, in contrast to a larger flaring angle from the Trotignon fitting.

Plasma clouds and snowplows: Bulk plasma escape from Mars observed by MAVEN

AbstractWe present initial Mars Atmosphere and Volatile EvolutioN (MAVEN) observations and preliminary interpretation of bulk plasma loss from Mars. MAVEN particle and field measurements show that planetary heavy ions derived from the Martian atmosphere can escape in the form of discrete coherent structures or “clouds.” The ions in these clouds are unmagnetized or weakly magnetized, have velocities well above the escape speed, and lie directly downstream from magnetic field amplifications, suggesting a “snowplow” effect. This postulated escape process, similar to that successfully used to explain the dynamics of active gas releases in the solar wind and terrestrial magnetosheath, relies on momentum transfer from the shocked solar wind protons to the planetary heavy ions, with the electrons and magnetic field acting as intermediaries. Fluxes of planetary ions on the order of 107 cm−2 s−1 can escape by this process, and if it operates regularly, it could contribute 10–20% of the current ion escape from Mars.

Emission of hydrogen energetic neutral atoms from the Martian subsolar magnetosheath

AbstractWe have simulated the hydrogen energetic neutral atom (ENA) emissions from the subsolar magnetosheath of Mars using a hybrid model of the proton plasma charge exchanging with the Martian exosphere to study statistical features revealed from the observations of the Neutral Particle Detectors on Mars Express. The simulations reproduce well the observed enhancement of the hydrogen ENA emissions from the dayside magnetosheath in directions perpendicular to the Sun‐Mars line. Our results show that the neutralized protons from the shocked solar wind are the dominant ENA population rather than those originating from the pickup planetary ions. The simulation also suggests that the observed stronger ENA emissions in the direction opposite to the solar wind convective electric field result from a stronger proton flux in the same direction at the lower magnetosheath; i.e., the proton fluxes in the magnetosheath are not cylindrically symmetric. We also confirm the observed increasing of the ENA fluxes with the solar wind dynamical pressure in the simulations. This feature is associated with a low altitude of the induced magnetic boundary when the dynamic pressure is high and the magnetosheath protons can reach to a denser exosphere, and thus, the charge exchange rate becomes higher. Overall, the analysis suggests that kinetic effects play an important and pronounced role in the morphology of the hydrogen ENA distribution and the plasma environment at Mars, in general.

Control of Mars global atmospheric loss by the continuous rotation of the crustal magnetic field: A time‐dependent MHD study

AbstractWe present a time‐dependent MHD study of the controlling effects of the Mars crustal field on atmospheric escape. We calculate globally integrated planetary ion loss rates under quiet solar conditions considering the continuous rotation of crustal anomalies with the planet. It is found that the rotating crustal field plays an important role in controlling atmospheric escape. Significant time variation of ∼20% and ∼50% is observed during the entire rotation period for O+ and for and , respectively. The control is exerted mainly through two processes. First, the crustal magnetic pressure over the subsolar regime controls solar wind penetration and mass loading and therefore the escaping planetary ion source. There is a strong negative correlation between the magnetic pressure and ion loss, with a time lag of <1 h for O+ and ∼2.5 h for and . Second, the crustal magnetic pressure near the terminator region controls the cross‐section area between the induced magnetospheric boundary and 100 km altitude at the terminator. The change in day‐night connection regulates the extent to which planetary ions created on the dayside can be ultimately carried away by the solar wind and escape Mars. There is a strong positive correlation between the cross‐section area and ion loss, with no significant time lag. As the planet rotates, the dayside process and the terminator process work together to control the total amount of escaping planetary ions. However, their relative importance changes with the local time of the strong crustal field region.

The spatial distribution of planetary ion fluxes near Mars observed by MAVEN

AbstractWe present the results of an initial effort to statistically map the fluxes of planetary ions on a closed surface around Mars. Choosing a spherical shell ~1000 km above the planet, we map both outgoing and incoming ion fluxes (with energies >25 eV) over a 4 month period. The results show net escape of planetary ions behind Mars and strong fluxes of escaping ions from the northern hemisphere with respect to the solar wind convection electric field. Planetary ions also travel toward the planet, and return fluxes are particularly strong in the southern electric field hemisphere. We obtain a lower bound estimate for planetary ion escape of ~3 × 1024 s−1, accounting for the ~10% of ions that return toward the planet and assuming that the ~70% of the surface covered so far is representative of the regions not yet visited by Mars Atmosphere and Volatile EvolutioN (MAVEN).

Mars heavy ion precipitating flux as measured by Mars Atmosphere and Volatile EvolutioN

AbstractIn the absence of an intrinsic dipole magnetic field, Mars' O+ planetary ions are accelerated by the solar wind. Because of their large gyroradius, a population of these planetary ions can precipitate back into Mars' upper atmosphere with enough energy to eject neutrals into space via collision. This process, referred to as sputtering, may have been a dominant atmospheric loss process during earlier stages of our Sun. Yet until now, a limited number of observations have been possible; Analyzer of Space Plasmas and Energetic Atoms‐3/Mars Express observed such a precipitation only during extreme conditions, suggesting that sputtering might be not as intense as theoretically predicted. Here we describe one example of precipitation of heavy ions during quiet solar conditions. Between November 2014 and April 2015, the average precipitating flux is significant and in agreement with predictions. From these measured precipitating fluxes, we estimate that a maximum of 1.0 × 1024 O/s could have been lost due to sputtering.