S. A. Stern

Southwest Research Institute, San Antonio, Texas, United States

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Publications (433)1406.18 Total impact

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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.
    12/2014;
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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.
    12/2014;
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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.
    12/2014;
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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.
    12/2014;
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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.
    SPIE Astronomical Telescopes + Instrumentation; 07/2014
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    ABSTRACT: We report on Hubble Space Telescope (HST) observations of three Kuiper Belt Objects (KBOs), discovered in our dedicated ground-based search campaign, that are candidates for long-range observations from the New Horizons spacecraft: 2011 JY31, 2011 HZ102, and 2013 LU35. Astrometry with HST enables both current and future critical accuracy improvements for orbit precision, required for possible New Horizons observations, beyond what can be obtained from the ground. Photometric colors of all three objects are red, typical of the Cold Classical dynamical population within which they reside; they are also the faintest KBOs to have had their colors measured. None are observed to be binary with HST above separations of ~0.02 arcsec (~700 km at 44 AU) and {\Delta}m less than or equal to 0.5.
    05/2014;
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    ABSTRACT: We observed the 2600-3200 Å (hereafter, mid-UV) reflectance of two Kuiper Belt Objects (KBOs), two KBO satellites, and a Centaur, using the Hubble Space Telescope (HST) Cosmic Origins Spectrograph (COS). Other than measurements of the Pluto system, these constitute the first UV measurements obtained of KBOs, and KBO satellites, and new HST UV measurements of the Centaur 2060 Chiron. We find significant differences among these objects, constrain the sizes and densities of Haumea's satellites, and report the detection of a possible spectral absorption band in Haumea's spectrum near 3050 Å. Comparisons of these objects to previously published UV reflectance measurements of Pluto and Charon are also made here.
    04/2014; 147(5).
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    ABSTRACT: Far ultraviolet albedo maps obtained using the Lunar Reconnaissance Orbiter (LRO) Lyman Alpha Mapping Project (LAMP) uniquely address lunar volatile processes.
    02/2014;
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    ABSTRACT: We present results from planetary science experiment payloads flight tested on a zero-gravity research flight.
    02/2014;
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    ABSTRACT: We will present results of simulations of argon in the lunar exosphere in order to investigate cold trapping in the permanently shaded regions.
    02/2014;
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    ABSTRACT: We will discuss the possible detection of Ar as observed by LRO's LAMP instrument.
    02/2014;
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    ABSTRACT: SIRSE is a next generation spectral imaging instrument, based on New Horizons Ralph, with improved capability to meet Europa Clipper science objectives.
    02/2014;
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    ABSTRACT: We discuss FUV results of the Compton-Belkovich region using the LRO UV spectrograph. This region displays redder FUV spectra than anywhere else on the Moon.
    02/2014;
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    ABSTRACT: Charon has suffered thousands of impacts by comets; these import significant supervolatile inventories that create tenuous transient atmospheres there.
    02/2014;
  • Jason C. Cook, S. Alan Stern
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    ABSTRACT: We report on a multi-year dataset of daily averaged observations of He in the lunar atmosphere by the LAMP UV spectrograph on NASA’s Lunar Reconnaissance Orbiter (LRO). We examine data obtained from the start of the LRO orbital tour in September 2009 to March 2013. We find that the maximum He number density occurs about two hours after local midnight, which is consistent with earlier measurements by the Apollo ALSEP LACE mass spectrometer. However, our measured maximum He density is 2–3 times lower than that of LACE. We also observed several instances where the surface He number density rapidly increased to higher than normal values and then declined for several days. We term these events “He flares”. We examined several plausible causes of these events, and found two plausible mechanisms that could be responsible for generating them. One is that the He may be generated by strong, coincident bursts of αα particles in the solar wind. To do so, we compare our observations with solar wind αα particle observations by ARTEMIS (Acceleration, Reconnection, Turbulence and Electrodynamics of the Moon’s Interaction with the Sun). Another plausible cause we discuss is that the He in the flares may be released from the Moon itself via moonquakes. Determining which is actually the cause requires further work and new measurements.
    Icarus 01/2014; 236:48–55. · 3.16 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    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 (Lyαα) 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 Lyαα lines from the Sun and the interplanetary medium (IPM). The IPM Lyαα signal at Earth is very much less than direct solar Lyαα, but IPM Lyαα falls off much more slowly than r-2r-2, so that at Pluto’s distance from the Sun the two sources are of comparable strength. Detailed simulations of its Lyαα 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 Lyαα brightness of the disk of Pluto is expected to be ∼30 R, against an IPM background of ∼90 R.
    Icarus 01/2014; · 3.16 Impact Factor
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    ABSTRACT: Since early 2012, the Lyman-Alpha Mapping Project (LAMP) far-ultraviolet spectrograph on the Lunar Reconnaissance Orbiter (LRO) spacecraft has carried out a series of limb observations from within lunar shadow to search for the presence of a high altitude dust exosphere via forward-scattering of sunlight from dust grains. Bright “horizon-glow” was observed from orbit during several Apollo missions and interpreted in terms of dust at altitudes of several km and higher. However, no confirmation of such an exosphere has been made since that time. This raises basic questions about the source(s) of excess brightness in the early measurements and also the conditions for producing observable dust concentrations at km altitudes and higher. Far-ultraviolet measurements between 170 and 190 nm, near the LAMP long wavelength cutoff, are especially sensitive to scattering by small (0.1–0.2 μm radius) dust grains, since the scattering cross-section is near-maximum, and the solar flux is rising rapidly with wavelength. An additional advantage of ultraviolet measurements is the lack of interference by background zodiacal light which must be taken into account at longer wavelengths. As of July 2013, LAMP has completed several limb-observing sequences dedicated to the search for horizon glow, but no clear evidence of dust scattering has yet been obtained. Upper limits for vertical dust column abundance have been estimated at less than 10 grains cm−2 (0.1 μm grain radius), by comparing the measured noise-equivalent brightness with the results of Mie scattering simulations for the same observing geometries. These results indicate that Lunar Atmosphere Dust Environment Explorer (LADEE) UVS lunar dust observations will be considerably more challenging than planned.
    Icarus 01/2014; 233:106–113. · 3.16 Impact Factor
  • Jason Cook, S. Alan Stern
    Icarus 11/2013; 226:1210. · 3.16 Impact Factor
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    ABSTRACT: While Pluto and Charon are easily resolvable in some space-based telescopes, ground-based imaging of Pluto and Charon can yield separate PSF photometry in excellent seeing. We present B and Sloan g', r', i', and z' separate photometry of Pluto and Charon taken at the Magellan Clay telescope using LDSS-3. In 2011, observations were made on 7, 8, 9, 19, and 20 March, at 9:00 UT, covering sub-Earth longitudes 130°, 74°, 17°, 175° and 118°. The solar phase angle ranged from 1.66-1.68° to 1.76-1.77°. In 2012, observations were made on February 28, 29 and March 1 at 9:00 UT covering longitudes 342°, 110° and 53° and on May 30 and 31 at 9:30 UT and 7:00 UT, covering longitudes 358° and 272°. Solar phase angles were 1.53-1.56° and 0.89°-0.90° degrees. All longitudes use the convention of zero at the sub-Charon longitude and decrease in time. Seeing ranged from 0.46 to 1.26 arcsecond. We find that the mean rotationally-averaged Charon-to-Pluto light ratio is 0.142±0.003 for Sloan r',i' and z'. Charon is brighter in B and g', with a light ratio of 0.182±0.003 and 0.178±0.002 respectively. Additionally, we present separate PSF photometry of Pluto and Charon from New Horizons images taken by the LORRI instrument on 1 and 3 July 2013 at 17:00 UT and 23:00 UT, sub-Earth longitude 251° and 125°. We find that the rotation-dependent variations in the light ratio are consistent with earlier estimates such as those from Buie et al. 2010, AJ 139, 1117-1127. However, at a solar phase angle of 10.9°, Charon appears 0.25 magnitudes fainter relative to Pluto at the same rotational phase than measurements from the ground with the largest possible solar phase angle. Thus we provide the first estimate of a Pluto phase curve beyond 2°. These results represent some of the first Pluto science from New Horizons. This work has been funded in part by NASA Planetary Astronomy Grant NNX10AB27G and NSF Award 0707609 to MIT and by NASA's New Horizons mission to Pluto.
    10/2013;
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    ABSTRACT: The Lyman Alpha Mapping Project (LAMP) UV spectrograph on the Lunar Reconnaissance Orbiter (LRO) was positioned to directly view the expanding gas plumes from the two GRAIL spacecraft impacts on 17 December 2012. LAMP detected resonantly scattered emissions from Hg and H atoms in the sunlit regions of these plumes. The spectral, spatial, and light-curve analyses used in these gas detections are consistent with previous LAMP observations of the LCROSS impact into the permanently shadowed region of Cabeus crater. LAMP's detection of atomic H by Lyman-α emission at the Moon (a first) was facilitated by pointing at the nightside surface to eliminate sky background noise. Volatile transport of Hg and H species is known to concentrate them near the poles, and in the context of LRO-Diviner temperature measurements of these high-latitude (75.6° N) impact sites the LAMP detections address this process.
    10/2013;

Publication Stats

3k Citations
1,406.18 Total Impact Points

Institutions

  • 1992–2014
    • Southwest Research Institute
      • • Space Science and Engineering Division
      • • Space Science Department
      San Antonio, Texas, United States
  • 1997–2011
    • Johns Hopkins University
      • • Applied Physics Laboratory
      • • Department of Physics and Astronomy
      Baltimore, Maryland, United States
    • Vanderbilt University
      Nashville, Michigan, United States
  • 2010
    • Nebraska Wesleyan University
      • Physics
      Baltimore, Maryland, United States
    • Lowell Observatory
      Flagstaff, Arizona, United States
  • 2009
    • University of California, Los Angeles
      • Institute of Geophysics and Planetary Physics
      Los Angeles, CA, United States
  • 2007
    • NASA
      Washington, West Virginia, United States
  • 2005
    • Cornell University
      • Center for Radiophysics and Space Research (CRSR)
      Ithaca, NY, United States
  • 2002
    • United States Geological Survey
      • Astrogeology Science Center
      Reston, Virginia, United States
  • 2001
    • Massachusetts Institute of Technology
      • Department of Earth Atmospheric and Planetary Sciences
      Cambridge, MA, United States
  • 2000
    • Space Studies Institute
      Mojave, California, United States
  • 1997–1998
    • St. Cloud State University
      Saint Cloud, Minnesota, United States
  • 1988–1997
    • University of California, Berkeley
      • Space Sciences Laboratory
      Berkeley, California, United States
  • 1994
    • Trinity University
      San Antonio, Texas, United States
  • 1988–1992
    • University of Texas at Austin
      • Department of Astronomy
      Austin, Texas, United States
  • 1986–1992
    • University of Colorado at Boulder
      • • Center for Astrophysics and Space Astronomy
      • • Laboratory for Atmospheric and Space Physics (LASP)
      Boulder, CO, United States
  • 1989–1990
    • University of Colorado
      Denver, Colorado, United States