Polar frost formation on Ganymede

doi:10.1016/0019-1035(85)90130-7

Icarus

Volume 62, Issue 2, May 1985, Pages 344-347

https://doi.org/10.1016/0019-1035(85)90130-7 Get rights and content

Abstract

The suggested models of polar frost formation on Ganymede are reviewed. A model in which plasma bombardment changes the reflectance characteristics of the icy surface is proposed.

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Cited by (25)

New constraints on Ganymede's hydrogen corona: Analysis of Lyman-α emissions observed by HST/STIS between 1998 and 2014
2017, Planetary and Space Science
Citation Excerpt :
The inversion may also result from differential space weathering, possibly due to radiolysis of the surface materials. Space weathering could cause an increase in defects and voids in the surface layer (Johnson, 1985) or the production of nano phase iron (Pieters et al., 2000), resulting in an upturn of the spectrum in the FUV. Since the UV is particularly sensitive to even small amounts of weathering (Hapke, 2001; Hendrix et al., 2003), an area with slightly enhanced charged particle bombardment may result in an increase in the UV albedo (Hendrix et al., 2005), as observed in Ganymede and Europa.
Far-ultraviolet observations of Ganymede's atmospheric emissions were obtained with the Space Telescope Imaging Spectrograph (STIS) onboard of the Hubble Space Telescope (HST) on several occasions between 1998 and 2014. We analyze the Lyman-α emission from four HST campaigns in order to constrain the abundance and variation of atomic hydrogen in Ganymede's atmosphere. We apply a forward model that estimates surface reflection and resonant scattering in an escaping corona of the solar Lyman-α flux, taking into account the effects of the hydrogen in the interplanetary medium. The atmospheric emissions around Ganymede's disk derived for the observations taken between 1998 and 2011 are consistent with a hydrogen corona in the density range of (5–8) × 10³ cm⁻³ at the surface. The hydrogen density appears to be generally stable in that period. In 2014, Ganymede's corona brightness is approximately 3 times lower during two observations of Ganymede's trailing hemisphere and hardly detectable at all during two observations of the leading hemisphere. We also investigate extinction of Ganymede's coronal emissions in the Earth's upper atmosphere or geocorona. For small Doppler shifts, resonant scattering in the geocorona of the moon corona emissions can effectively reduce the brightness observed by HST. In the case of the 2014 leading hemisphere observations, an estimated extinction of 80% might explain the non-detection of Ganymede's hydrogen corona. Geocoronal extinction might also explain a previously detected hemispheric difference from Callisto's hydrogen corona.
Energetic charged particle weathering of Saturns inner satellites
2012, Planetary and Space Science
Citation Excerpt :
Recent examples in Solar System contexts support the importance of charged particle weathering in modifying the optical surface. For example, Khurana et al. (2007) have explained the so-called bright polar cap feature of Ganymede (e.g., Johnson, 1985) by mapping the edge of the bright region to the open/closed field line boundary of the internally generated magnetic field of that moon. Open and closed Ganymede field lines have been observed to have different levels of charged particle flux (Williams et al., 1998).
We characterize the relative importance of energetic electrons and protons to the weathering of five of the inner satellites of Saturn. To do this, we present data from the Magnetospheric Imaging Instrument on the Cassini spacecraft, some of which is averaged over the whole mission to date. We also compute averaged proton and electron energy spectra relevant to the distances of these inner satellites. Where data are available, we estimate the power per unit area into a satellite's surface. For electron energy deposition into satellite leading hemispheres, we find the power per unit area is greatest at Mimas and falls off with distance from Saturn. Using fluxes of 1–50 MeV protons detected within the sweeping corridors of Mimas and Enceladus, we find the corresponding deposition would be about 2×10⁸ and 3.7×10⁷ eV/cm² s.
The origin of Ganymede's polar caps
2007, Icarus
Citation Excerpt :
Thus, in the polar regions where thermal sublimation is minimal, plasma sputtering might maintain a bright frost layer, whereas in regions of darker surface (mainly at low latitudes), frost formation is inhibited by sublimation. Johnson (1985) later shelved the idea of direct frost formation from sputtered molecules, noting that in laboratory experiments even a rapid deposition of water molecules on an extremely cold surface leads to a relatively clear (non-scattering) surface. He instead proposed that changes in reflectance properties were caused by an increase in defects and voids and growth of dendritic crystals in the bombarded surficial ice layer.
Since their discovery in Voyager images, the origin of the bright polar caps of Ganymede has intrigued investigators. Some models attributed the polar cap formation to thermal migration of water vapor to higher latitudes, while other models implicated plasma bombardment in brightening ice. Only with the arrival of Galileo at Jupiter was it apparent that Ganymede possesses a strong internal magnetic field, which blocks most of the plasma from bombarding the satellite's equatorial region while funneling plasma onto the polar regions. This discovery provides a plausible explanation for the polar caps as related to differences in plasma-induced brightening in the polar and the equatorial regions. In this context, we analyze global color and high resolution images of Ganymede obtained by Galileo, finding a very close correspondence between the observed polar cap boundary and the open/closed field lines boundary obtained from new modeling of the magnetic field environment. This establishes a clear link between plasma bombardment and polar cap brightening. High resolution images show that bright polar terrain is segregated into bright and dark patches, suggesting sputter-induced redistribution and subsequent cold trapping of water molecules. Minor differences between the location of the open/closed field lines boundary and the observed polar cap boundary may be due to interaction of Ganymede with Jupiter's magnetosphere, and our neglect of higher-order terms in modeling Ganymede's internal field. We postulate that leading-trailing brightness differences in Ganymede's low-latitude surface are due to enhanced plasma flux onto the leading hemisphere, rather than darkening of the trailing hemisphere. In contrast to Ganymede, the entire surface of Europa is bombarded by jovian plasma, suggesting that sputter-induced redistribution of water molecules is a viable means of brightening that satellite's surface.
Energetic Ion and Electron Irradiation of the Icy Galilean Satellites
2001, Icarus
Galileo Orbiter measurements of energetic ions (20 keV to 100 MeV) and electrons (20–700 keV) in Jupiter's magnetosphere are used, in conjunction with the JPL electron model (<40 MeV), to compute irradiation effects in the surface layers of Europa, Ganymede, and Callisto. Significant elemental modifications are produced on unshielded surfaces to approximately centimeter depths in times of ≤10⁶ years, whereas micrometer depths on Europa are fully processed in ∼10 years. Most observations of surface composition are limited to optical depths of ∼1 mm, which are in direct contact with the space environment. Incident flux modeling includes Størmer deflection by the Ganymede dipole magnetic field, likely variable over that satellite's irradiation history. Delivered energy flux of ∼8×10¹⁰ keV (cm²-s)⁻¹ at Europa is comparable to total internal heat flux in the same units from tidal and radiogenic sources, while exceeding that for solar UV energies (>6 eV) relevant to ice chemistry. Particle energy fluxes to Ganymede's equator and Callisto are similar at ∼2–3×10⁸ keV (cm²-s)⁻¹ with 5×10⁹ at Ganymede's polar cap, the latter being comparable to radiogenic energy input. Rates of change in optical reflectance and molecular composition on Europa, and on Ganymede's polar cap, are strongly driven by energy from irradiation, even in relatively young regions. Irradiation of nonice materials can produce SO₂ and CO₂, detected on Callisto and Europa, and simple to complex hydrocarbons. Iogenic neutral atoms and meteoroids deliver negligible energy ∼10^4–5 keV (cm²-s)⁻¹ but impacts of the latter are important for burial or removal of irradiation products. Downward transport of radiation produced oxidants and hydrocarbons could deliver significant chemical energy into the satellite interiors for astrobiological evolution in putative sub-surface oceans.
Dark Terrain on Ganymede: Geological Mapping and Interpretation of Galileo Regio at High Resolution
1998, Icarus
During its first two encounters with Ganymede, the Galileo spacecraft obtained images of a 16,500 km²portion of Galileo Regio, a large expanse of dark terrain, at high resolution (76–86 m/pixel). Through mapping of the G1 and G2 target sites within Galileo Regio, we are able to characterize geological units based on their morphology and relative albedo. We find three generally low albedo units: an intermediate albedo plains unit, a lower albedo plains unit, and the lowest albedo unit which is found on furrow and crater floors. We also find high albedo units which include crater rims, furrow rims, and isolated knobs and massifs. Other features include an intermediate albedo lobate feature interpreted to be a palimpsest and a hummocky unit interpreted to be impact ejecta. Several processes are interpreted to have occurred within Galileo Regio. These include tectonic deformation, mass wasting, sublimation, resurfacing by impact ejecta, and possibly cryovolcanism and isostatic adjustment. We observe that the NW–SE trending furrows (Lakhmu Fossae) in Galileo Regio are degraded and are crosscut by the younger N–S trending furrows (Zu Fossae). We also find several other tectonic features which may be minor faults or fractures related to one or other of these systems. Through mapping and crater size–frequency distributions, we are able to propose a stratigraphy for the Galileo Regio target site. The oldest features in the area are high albedo knobs and massifs, which are interpreted to be remnants of early impact-related features and furrow rims. These may have formed at approximately the same time as the intermediate and low albedo plains units and the furrow systems. The lowest albedo unit of furrow floors probably subsequently evolved through sublimation and mass wasting. Much of the northeast portion of the target area was subsequently obscured by one of the youngest units, ejecta from an impact just to the north. We use our mapping of the high-resolution images of Galileo Regio to evaluate three end-member models for the formation of dark terrain: (1) the crust is dark throughout, (2) material on the surface is the result of a low albedo cryovolcanic layer over a higher albedo crust, and (3) dark material is distributed in small quantities throughout the crust, and geological processes have acted to concentrate low albedo material on the surface. Although it is possible that elements of more than one of these models are present within the dark terrain, we find that the third model, that of a thin veneer of low albedo material, best fits observations of Galileo Regio
Latitudinal distribution of O<inf>2</inf>on ganymede: Observations with the hubble space telescope
1997, Icarus
To help constrain the spatial variation of oxygen on Jupiter's satellite Ganymede, and hence have more clues to its mode of production and stability, we have obtained spectral data from the Faint Object Spectrograph (FOS) for a single pole-to-pole latitudinal strip, along with several Wide Field Planetary Camera 2 (WFPC2) images in three narrow band visible filters. All observations were made of the trailing hemisphere. In the FOS data we observe both visible absorptions at 0.577 and 0.627 μm, associated with dense-phase oxygen (compressed gases, liquids, or solids). Filter options limited the WFPC2 observations to wavelengths near the weaker oxygen absorption at 0.627 μm. These observations suggest that the dense-phase or dimer oxygen form is predominantly found in equatorial and mid-latitudes. The spectroscopic absorption feature appears in both bright and dark terrains but may be somewhat weaker in dark regions, which is consistent with the smaller mean photon path length in the surface in darker areas. Therefore, the abundance of oxygen appears more dependent on latitude and longitude constraints than surface albedo. At the highest latitudes, where the ratio spectra have a strong upturn toward the blue, the oxygen bands do not appear. This relation suggests that dimer oxygen and ozone (as seen by Galileo) have opposite trends with latitude. Possible causes include competition or variation in the preferred stable form, which depends on temperature, solar ultraviolet flux, and/or surface age; enhancement of O₃at the poles due to plasma interactions; or viewing geometry effects that reduce the oxygen features at the poles when observed from Earth. The predominantly equatorial feature supports the production of O₂through plasma bombardment and favors defect trapping over physical adsorption of the dimer molecules in the surface. We briefly consider the implications of Ganymede's magnetosphere for our understanding of O₂and O₃distribution on Ganymede.

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NotePolar frost formation on Ganymede

Abstract

Nucl. Instrum. Methods

Surf. Sci.

Icarus

Icarus

Surf. Sci.

Icarus

Icarus

Icarus

Icarus

Sputtering of ice by MeV light ions

Phys. Rev. Lett.

The spectral reflectance of water-mineral mixtures at low temperatures

J. Geophys. Res.

A model for the formation of thin films in dirty ice targets by sputtering: Application to the satellites of Jupiter

Note
Polar frost formation on Ganymede