S. A. Stern

Southwest Research Institute, San Antonio, Texas, United States

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Publications (457)1644.02 Total impact

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    ABSTRACT: Lunar swirls – the enigmatic, magnetically-anomalous regions – are observed for the first time at far-UV (FUV) wavelengths using LRO/LAMP. Swirls in both highlands and mare regions are spectrally relatively red (or less blue) than surrounding terrains, indicating a difference in weathering and/or composition in the swirls vs. non-swirl regions.
    No preview · Article · Feb 2016 · Icarus
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    ABSTRACT: The Alice far-ultraviolet spectrograph in operation around the comet 67P/Churyumov-Gerasimenko on the Rosetta spacecraft experiences an anomalistic feature (AF) that is ubiquitous at comet separations less than 450 km. This feature is highly temporally variable and displays no relation to any studied parameters with the exception of comet separation. This paper tests several possible causes with simulations, but still finds no definitive source of the AF.
    No preview · Article · Jan 2016
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    Full-text · Dataset · Nov 2015
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    Full-text · Dataset · Nov 2015
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    ABSTRACT: We present results from Lunar Reconnaissance Orbiter’s (LRO) UV spectrograph LAMP (Lyman-Alpha Mapping Project) campaign to study the lunar atmosphere. Several off-nadir maneuvers (lateral rolls) were performed to search for resonantly scattering species, increasing the illuminated line-of-sight (and hence the signal from atoms resonantly scattering the solar photons) compared to previously reported LAMP’s “twilight observations” (Cook, J.C., Stern, S.A. [2014]. Icarus 236, 48–55). Helium was the only element distinguishable on a daily basis, and we present latitudinal profiles of its line-of-sight column density in December 2013. We compared the helium line-of-sight column densities with solar wind alpha particle fluxes measured from the ARTEMIS (Acceleration, Reconnection, Turbulence, & Electrodynamics of Moon’s Interaction with the Sun) twin spacecraft. Our data show a correlation with the solar wind alpha particle flux, confirming that the solar wind is the main source of the lunar helium. We also support the finding by Benna et al. (Benna, M. et al. [2015]. Geophys. Res. Lett. 42, 3723–3729) and Hurley et al. (Hurley, D.M. et al. [2015]. Icarus, this issue), that a non-zero contribution from endogenic helium, coming from radioactive decay of 232Th and 238U, is present. Moreover, our results suggest that not all of the incident alpha particles are converted to thermalized helium, allowing for a non-negligible fraction to escape as suprathermal helium or simply backscattered from the lunar surface. We compare LAMP-derived helium surface density with the one recorded by the mass spectrometer LACE (Lunar Atmospheric Composition Experiment) deployed on the lunar surface during the Apollo 17 mission, finding good agreement between the two measurements. The LRO/LAMP roll observations presented here are in agreement with the most recent lunar exospheric helium model (Hurley, D.M. et al. [2015]. Icarus, this issue) around mid- to high-latitudes (50–70°) regardless of the local solar time, while there is an underestimation of the model around the low- to mid-latitudes (10–30°), especially around the dawn terminator. The LRO/LAMP roll observations presented here provide unique coverage of local solar time and latitude of the lunar exospheric helium, filling a gap in the knowledge of the structure of the lunar exosphere as a whole. These observations will inform future models of transport of volatiles, since at the terminator the analytic expressions for the surface temperature, essential to determine the energy distribution, the residence time, and the hop length of the particles, is least accurate.
    No preview · Article · Nov 2015 · Icarus
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    ABSTRACT: The Pluto system was recently explored by NASA’s New Horizons spacecraft, making closest approach on 14 July 2015. Pluto’s surface displays diverse landforms, terrain ages, albedos, colors, and composition gradients. Evidence is found for a water-ice crust, geologically young surface units, surface ice convection, wind streaks, volatile transport, and glacial flow. Pluto’s atmosphere is highly extended, with trace hydrocarbons, a global haze layer, and a surface pressure near 10 microbars. Pluto’s diverse surface geology and long-term activity raise fundamental questions about how small planets remain active many billions of years after formation. Pluto’s large moon Charon displays tectonics and evidence for a heterogeneous crustal composition; its north pole displays puzzling dark terrain. Small satellites Hydra and Nix have higher albedos than expected.
    Full-text · Article · Oct 2015 · Science
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    ABSTRACT: Aims. The Alice far-ultraviolet (FUV) spectrograph onboard Rosetta has, for the first time, imaged the surface of a comet, 67P/Churyumov-Gerasimenko (67P), in the FUV. With spatially resolved data, the nucleus properties are characterized in the FUV, including phase dependence, albedo, and spectral slope. Regional measurements across the nucleus are compared to discern any compositional variations. Methods. Hapke theory was utilized to model the phase dependence of the material on the surface of 67P. The phase dependence of 67P was derived from a subset of data acquired at various phase angles in November 2014, within 50 km of the comet such that the nucleus was spatially resolved. The derived photometric correction was then applied to a different subset of spatially resolved data sampling several distinct geographical regions on the nucleus acquired in August-November 2014 under similar viewing geometries. Results. In the FUV, the surface of 67P is dark, blue sloped, has an average geometric albedo of 0.054±0.008 at 1475 Å near the center of the Alice bandpass, and is mostly uniform from region to region, with the exception of the Hatmehit region, which is slightly more reflective. These results are consistent with the suggestion made by the Rosetta OSIRIS and VIRTIS teams that the surface of 67P is covered with a homogeneous layer of material and that surface ice is not ubiquitous in large abundances. The modeled Hapke parameters, specifically the single scattering albedo (w) and the asymmetry factor (ζ), are determined to be 0.031 ± 0.003 and -0.530 ± 0.025 near the center of the Alice bandpass at 1475 Å. These parameters are consistent with measurements of other comet nuclei that have been observed by flyby missions in the visible and the near-infrared regimes.
    No preview · Article · Oct 2015 · Astronomy and Astrophysics
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    ABSTRACT: Simultaneous measurements of helium in the exosphere of the Moon are made from the Lunar Reconnaissance Orbiter (LRO) Lyman Alpha Mapping Project (LAMP) and the Lunar Atmosphere and Dust Environment Explorer (LADEE) Neutral Mass Spectrometer (NMS) through the entire 5-month span of the LADEE mission. In addition, the ARTEMIS mission monitored the solar wind alpha particle flux to the Moon. Modeling the lunar helium exosphere, we relate the LAMP polar observations to the LADEE equatorial observations. Further, using the ARTEMIS alpha flux in the Monte Carlo model reproduces the temporal variations in helium density. Comparing the LAMP data to the LADEE data shows excellent agreement. Comparing those with the ARTEMIS data reveals that the solar wind alpha flux is the primary driver to variability in the helium exosphere throughout the LADEE mission. Using a decay time for exospheric helium of 5. days, we determine that the solar wind contributes 64. ±. 5% of the helium to the lunar exosphere. The remaining 36. ±. 5% is presumed to come from outgassing of radiogenic helium from the interior of the Moon. Furthermore, the model reproduces the measurements if 63. ±. 6% of the incident alpha particles are converted to thermalized helium atoms through the interaction between the alphas and the lunar surface. However, these values are dependent on both inferred source rates from LAMP and LADEE observations and on the assumed time constant of the exospheric decay rate.
    No preview · Article · Sep 2015 · Icarus
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    ABSTRACT: The New Horizons mission, the first mission in NASA’s New Frontiers Program, is also the first mission with primary science objectives to explore the Pluto/Charon system. After launch in January 2006 and an interplanetary cruise of more than 9.5 years, New Horizons has completed the approach and flyby of Pluto. This paper presents an overview of the analysis and operational constraints that led to the navigation strategy used. Also presented are operational results for that strategy during this final phase of the prime mission.
    Full-text · Conference Paper · Aug 2015

  • No preview · Article · Aug 2015
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    ABSTRACT: Argon is one of the few known constituents of the lunar exosphere. The surface-based mass spectrometer Lunar Atmosphere Composition Experiment (LACE) deployed during the Apollo 17 mission first detected argon, and its study is among the subjects of the Lunar Reconnaissance Orbiter (LRO) Lyman Alpha Mapping Project (LAMP) and Lunar Atmospheric and Dust Environment Explorer (LADEE) mission investigations. We performed a detailed Monte Carlo simulation of neutral atomic argon that we use to better understand its transport and storage across the lunar surface. We took into account several loss processes: ionization by solar photons, charge-exchange with solar protons, and cold trapping as computed by recent LRO/Lunar Orbiter Laser Altimeter (LOLA) mapping of Permanently Shaded Regions (PSRs). Recycling of photo-ions and solar radiation acceleration are also considered. We report that (i) contrary to previous assumptions, charge exchange is a loss process as efficient as photo-ionization, (ii) the PSR cold-trapping flux is comparable to the ionization flux (photo-ionization and charge-exchange), and (iii) solar radiation pressure has negligible effect on the argon density, as expected. We determine that the release of 2.6 × 1028 atoms on top of a pre-existing argon exosphere is required to explain the maximum amount of argon measured by LACE. The total number of atoms (1.0 × 1029) corresponds to ∼6700 kg of argon, 30% of which (∼1900 kg) may be stored in the cold traps after 120 days in the absence of space weathering processes. The required population is consistent with the amount of argon that can be released during a High Frequency Teleseismic (HFT) Event, i.e. a big, rare and localized moonquake, although we show that LACE could not distinguish between a localized and a global event. The density of argon measured at the time of LACE appears to have originated from no less than four such episodic events. Finally, we show that the extent of the PSRs that trap argon, 0.007% of the total lunar surface, is consistent with the presence of adsorbed water in such PSRs.
    No preview · Article · Jul 2015 · Icarus
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    ABSTRACT: Aims. The Alice far-ultraviolet spectrograph onboard Rosetta is designed to observe emissions from various atomic and molecular species from within the coma of comet 67P/ Churyumov-Gerasimenko and to determine their spatial distribution and evolution with time and heliocentric distance. Methods. Following orbit insertion in August 2014, Alice made observations of the inner coma above the limbs of the nucleus of the comet from cometocentric distances varying between 10 and 80 km. Depending on the position and orientation of the slit relative to the nucleus, emissions of atomic hydrogen and oxygen were initially detected. These emissions are spatially localized close to the nucleus and spatially variable with a strong enhancement above the comet's neck at northern latitudes. Weaker emission from atomic carbon and CO were subsequently detected. Results. Analysis of the relative line intensities suggests photoelectron impact dissociation of H2O vapor as the source of the observed H I and O I emissions. The electrons are produced by photoionization of H2O. The observed C I emissions are also attributed to electron impact dissociation, of CO2, and their relative brightness to H I reflects the variation of CO2 to H2O column abundance in the coma.
    No preview · Article · Jun 2015 · Astronomy and Astrophysics
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    ABSTRACT: We used the Alice spectrograph onboard the Rosetta comet orbiter spacecraft to observe the surface of comet 67P/Churyumov-Gerasimenko in the extreme and far ultraviolet (EUV/FUV) from 700-2050 Å in mid-August 2014. These observations were before significant EUV/FUV coma signatures were observed by Alice. The resulting coadded spectrum has high signal to noise and reveals: (1) a very FUV-dark surface with (2) a blue spectral slope and (3) no evidence of significant H2O ice absorption in the FUV. We fit the measured reflectance spectrum with a model including 99.5% tholins, 0.5% H2O-ice, and a neutral darkening agent. Since we could not find any natural material with sufficiently low EUV/FUV reflectance, we interpret the low I/F as evidence of a fluffy, light-trapping surface. We interpret the blue spectral slope as consistent with a surface consisting primarily of tholins, though it may alternatively be the result of Rayleigh scattering by fine particles in the regolith.
    No preview · Article · Apr 2015 · Icarus
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    ABSTRACT: LRO-LAMP far-UV albedo maps show global spectral evidence for surficial water frost/hydration, and probe PSRs using an innovative nightside observing technique.
    No preview · Article · Feb 2015
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    ABSTRACT: The cameras of New Horizons will provide robust data sets that should be imminently amenable to geological analysis of the Pluto system's landscapes. In this paper, we begin with a brief discussion of the planned observations by the New Horizons cameras that will bear most directly on geological interpretability. Then we broadly review the major geological processes that could potentially operate on the surfaces of Pluto and its major moon Charon. We first survey exogenic processes (i.e. those for which energy for surface modification is supplied externally to the planetary surface): impact cratering, sedimentary processes (including volatile migration), and the work of wind. We conclude with an assessment of the prospects for endogenic activity in the form of tectonics and cryovolcanism.
    No preview · Article · Dec 2014 · Icarus
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    ABSTRACT: We compare Mid-Ultraviolet (MUV) spectra of Pluto taken over a period of 20 years by the International Ultraviolet Explorer, the HST-Faint Object Spectrograph, and the HST-Cosmic Origins Spectrograph. We extract Pluto-only spectra from the IUE data and associate them with corrected longitudes when necessary. Comparing them with HST spectra provides further evidence of temporal changes in Pluto's geometric albedo between 2000 and 3200 Å. These various spectra are used to explore the contributions of atmospheric or surface changes to Pluto's reflectance. We also provide predictions for the Far-Ultraviolet (FUV) surface reflectance and atmospheric emission spectra of Pluto that will be measured by the Alice spectrograph (Stern, S.A. et al. [2008]. Space Sci. Rev. 140, 155-187) during the New Horizons flyby of Pluto in 2015. FUV surface reflectance predictions are also made for Charon, Hydra, and Nix.
    Preview · Article · Dec 2014 · Icarus
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    ABSTRACT: The surface of Pluto as it is understood on the eve of the encounter of the New Horizons spacecraft (mid-2015) consists of a spatially heterogeneous mix of solid N2, CH4, CO, C2H6, and an additional component that imparts color, and may not be an ice. The known molecular ices are detected by near-infrared spectroscopy. The N2 ice occurs in the hexagonal crystalline β-phase, stable at T > 35.6 K. Spectroscopic evidence for wavelength shifts in the CH4 bands attests to the complex mixing of CH4 and N2 in the solid state, in accordance with the phase diagram for N2 + CH4. Spectra obtained at several aspects of Pluto's surface as the planet rotates over its 6.4-day period show variability in the distribution of CH4 and N2 ices, with stronger CH4 absorption bands associated with regions of higher albedo, in correlation with the visible rotational light curve. CO and N2 ice absorptions are also strongly modulated by the rotation period; the bands are strongest on the anti-Charon hemisphere of Pluto. Longer term changes in the strengths of Pluto's absorption bands occur as the viewing geometry changes on seasonal time-scales, although a complete cycle has not been observed. The non-ice component of Pluto's surface may be a relatively refractory material produced by the UV and cosmic-ray irradiation of the surface ices and gases in the atmosphere, although UV does not generally penetrate the atmospheric CH4 to interact with the surface. Laboratory simulations indicate that a rich chemistry ensues by the irradiation of mixtures of the ices known to occur on Pluto, but specific compounds have not yet been identified in spectra of the planet. Charon's surface is characterized by spectral bands of crystalline H2O ice, and a band attributed to one or more hydrates of NH3. Amorphous H2O ice may also be present; the balance between the amorphization and crystallization processes on Charon remains to be clarified. The albedo of Charon and its generally spatially uniform neutral color indicate that a component, not yet identified, is mixed in some way with the H2O and NH3·nH2O ices. Among the many known small bodies in the transneptunian region, several share characteristics with Pluto and Charon, including the presence of CH4, N2, C2H6, H2O ices, as well as components that yield a wide variety of surface albedo and color. The New Horizons investigation of the Pluto-Charon system will generate new insight into the physical properties of the broader transneptunian population, and eventually to the corresponding bodies expected in the numerous planetary systems currently being discovered elsewhere in the Galaxy.
    No preview · Article · Dec 2014 · Icarus
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    G. Randall Gladstone · Wayne R. Pryor · S. Alan Stern
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    ABSTRACT: The Alice instrument on New Horizons will perform several observations of Pluto's far-ultraviolet (FUV) airglow emissions during its July 2015 flyby. While Pluto's atmosphere is dominated by N2, simulations suggest that the brightest airglow signal at Pluto will actually be due to Lyman alpha (Ly α) emissions of atomic hydrogen. This is because H atoms, produced at lower altitudes due to the photolysis of CH4 and other hydrocarbons, rise up above the homopause to become an important constituent of the atmosphere at high altitudes, and are able to scatter the very bright Ly α lines from the Sun and the interplanetary medium (IPM). The IPM Ly α signal at Earth is very much less than direct solar Ly α , but IPM Ly α falls off much more slowly than r-2 , so that at Pluto's distance from the Sun the two sources are of comparable strength. Detailed simulations of its Ly α emissions indicate that Pluto will appear dark against the IPM background, but that enough contrast exists for the useful extraction of H densities from the Alice observations. As viewed on approach (or from the inner solar system), the Ly α brightness of the disk of Pluto is expected to be ∼30 R, against an IPM background of ∼90 R.
    Full-text · Article · Dec 2014 · Icarus
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    S. Alan Stern · Simon Porter · Amanda Zangari
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    ABSTRACT: Pluto and its satellites will be the most distant objects ever reconnoitered when NASA's New Horizons spacecraft conducts its intensive flyby of this system in 2015. The size-frequency distribution (SFD) of craters on the surfaces in the Pluto system have long been expected to provide a useful measure of the size distribution of Kuiper Belt Objects (KBOs) down to much smaller size scales than presently observed. However, currently predicted escape rates of Pluto's atmosphere suggest that of order one-half to several kilometers of nitrogen ice has been removed from Pluto's surface over geologic time. Because this range of depths is comparable to or greater than most expected crater depths on Pluto, one might expect that many craters on Pluto's surface may have been removed or degraded by this process, biasing the observed crater SFD relative to the production-function crater SFD. Further, if Pluto's surface volatile layer is comparable to or deeper than crater depths, and if the viscosity of this layer surface ice is low like the viscosity of pure N2 ice at Pluto's measured 35 K surface temperature (or as low as the viscosity of CH4 ice at warmer but plausible temperatures on isolated pure-CH4 surfaces on Pluto), then craters on Pluto may also have significantly viscously relaxed, also potentially biasing the observed crater SFD and surface crater retention age. Here we make a first exploration of how these processes can affect the displayed cratering record on Pluto. We find that Pluto's surface may appear to be younger owing to these effects than it actually is. We also find that by comparing Pluto's cratering record to Charon's, it may be possible to estimate the total loss depth of material from Pluto's surface over geologic time, and therefore to estimate Pluto's time-averaged escape rate.
    Preview · Article · Dec 2014 · Icarus
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    ABSTRACT: Four compact planetary ultraviolet spectrographs have been built by Southwest Research Institute and successfully operated on different planetary missions. These spectrographs underwent a series of ground radiometric calibrations before delivery to their respective spacecraft. In three of the four cases, the in-flight measured sensitivity was approximately 50% lower than the ground measurement. Recent tests in the Southwest Research Institute Ultraviolet Radiometric Calibration Facility (UV-RCF) explain the discrepancy between ground and flight results. Revised ground calibration results are presented for the Rosetta-Alice, New Horizons-Alice, the Lunar Reconnaissance Orbiter Lyman- Alpha Mapping Project, and Juno-Ultraviolet Spectrograph (UVS) and are then compared to the original ground and flight calibrations. The improved understanding of the calibration system reported here will result in improved ground calibration of the upcoming Jupiter Icy Moons Explorer (JUICE)-UVS.
    No preview · Conference Paper · Jul 2014

Publication Stats

5k Citations
1,644.02 Total Impact Points


  • 1992-2015
    • Southwest Research Institute
      • • Space Science and Engineering Division
      • • Space Science Department
      San Antonio, Texas, United States
  • 2010
    • Nebraska Wesleyan University
      • Physics
      Baltimore, Maryland, United States
  • 2009
    • Planetary Science Institute
      Houston, Texas, United States
  • 2002-2007
    • NASA
      Вашингтон, West Virginia, United States
    • United States Geological Survey
      • Astrogeology Science Center
      Reston, Virginia, United States
  • 2006
    • Massachusetts Institute of Technology
      • Department of Earth Atmospheric and Planetary Sciences
      Cambridge, Massachusetts, United States
  • 2005
    • Johns Hopkins University
      Baltimore, Maryland, United States
  • 2000
    • Space Studies Institute
      Mojave, California, United States
  • 1998
    • St. Cloud State University
      Saint Cloud, Minnesota, United States
  • 1996
    • University of Wyoming
      Ларами, Wyoming, United States
  • 1992-1996
    • University of Colorado
      Denver, Colorado, United States
  • 1995
    • The Scotch Whisky Research Institute
      Edinburgh, Scotland, United Kingdom
  • 1994
    • Harvard University
      Cambridge, Massachusetts, United States
    • Trinity University
      San Antonio, Texas, United States
  • 1986-1991
    • University of Colorado at Boulder
      • • Center for Astrophysics and Space Astronomy
      • • Laboratory for Atmospheric and Space Physics (LASP)
      • • Department of Astrophysical and Planetary Sciences
      Boulder, Colorado, United States