Geology topics

J.R. Spencer

Publications and source records attributed to J.R. Spencer.

6 recordsLinked to original sources

USGS2024

Planned geological investigations of the Europa Clipper mission

Geological investigations planned for the Europa Clipper mission will examine the formation, evolution, and expression of geomorphic structures found on the surface. Understanding geologic features, their formation, and any recent activity are key inputs in constraining Europa’s potential for habitability. In addition to providing information about the moon’s habitability, the geologic study of Europa is compelling in and of itself. Here we provide a high-level, cross-instrument, and cross-discipline overview of the geologic investigations planned within the Europa Clipper mission. Europa’s fascinating collection of ice-focused geology provides an unparalleled opportunity to investigate the dynamics of icy shells, ice-ocean exchange processes, and global-scale tectonic and tidal stresses. We present an overview of what is currently known about the geology of Europa, from global to local scales, highlighting outstanding issues and open questions, and detailing how the Europa Clipper mission will address them. We describe the mission’s strategy for searching for and characterizing current activity in the form of possible active plumes, thermal anomalies, evidence for surface changes, and extremely fresh surface exposures. The complementary and synergistic nature of the data sets from the various instruments and their integration will be key to significantly advancing our understanding of Europa’s geology.

Space Science Reviews↗

USGS1999

Active Volcanism on Io: Global Distribution and Variations in Activity

Io's volcanic activity has been monitored by instruments aboard the Galileo spacecraft since June 28, 1996. We present results from observations by the near-infrared mapping spectrometer (NIMS) for the first 10 orbits of Galileo, correlate them with results from the Solid State Imaging System (SSI) and from groundbased observations, and compare them to what was known about Io's volcanic activity from observations made during the two Voyager flybys in 1979. A total of 61 active volcanic centers have been identified from Voyager, groundbased, and Galileo observations. Of these, 41 are hot spots detected by NIMS and/or SSI. Another 25 locations were identified as possible active volcanic centers, mostly on the basis of observed surface changes. Hot spots are correlated with surface colors, particularly dark and red deposits, and generally anti-correlated with white, SO2-rich areas. Surface features corresponding to the hot spots, mostly calderas or flows, were identified from Galileo and Voyager images. Hot spot temperatures obtained from both NIMS and SSI are consistent with silicate volcanism, which appears to be widespread on Io. Two types of hot spot activity are present: persistent-type activity, lasting from months to years, and sporadic events, which may represent either short-lived activity or low-level activity that occasionally flares up. Sporadic events are not often detected, but may make an important contribution to Io's heat flow and resurfacing. The distribution of active volcanic centers on the surface does not show any clear correlation with latitude, longitude, Voyager-derived global topography, or heat flow patterns predicted by the asthenosphere and deep mantle tidal dissipation models. However, persistent hot spots and active plumes are concentrated toward lower latitudes, and this distribution favors the asthenosphere rather than the deep mantle tidal dissipation model. ?? 1999 Academic Press.

Icarus↗

USGS1998

High-temperature silicate volcanism on Jupiter's moon Io

Infrared wavelength observations of Io by the Galileo spacecraft show that at last 12 different vents are erupting lavas that are probably hotter than the highest temperature basaltic eruptions on Earth today. In at least one case, the eruption near Pillan Patea, two independent instruments on Galileo show that the lava temperature must have exceeded 1700 kelvin and may have reached 2000 kelvin. The most likely explanation is that these lavas are ultramafic (magnesium-rich) silicates, and this idea is supported by the tentative identification of magnesium-rich orthopyroxene in lava flows associated with thse high-temperature hot spots.

Science↗

USGS1997

Volcanic resurfacing of Io: Post-repair HST imaging

In March 1994, we used the newly refurbished Hubble Space Telescope (HST) to obtain global imaging of Io at five wavelengths between 0.34 and 1.02 μm, with a spatial resolution of 160 km. The images provided the clearest view of Io since Voyager and the first systematic observations in the wavelength range 0.7–1.0 μm. We have produced absolutely calibrated global mosaics of Io's reflectance in all our five wavelengths. The near-infrared images reveal that the 0.55- to 0.7-μm absorption edge seen in Io's disk-integrated spectrum has a very different spatial distribution from the better-known 0.40- to 0.50-μm absorption edge studied by Voyager, and must be generated by a different chemical species. The 0.55- to 0.7-μm absorption edge is strongly concentrated in the pyroclastic ejecta blanket of the volcano Pele, at a few much smaller discrete spots, and probably also in the polar regions. The Pele ejecta spectrum is consistent with the idea that S 2 O, partially decomposed to S 4 (and probably S 3 ), may be the species responsible for the 0.55- to 0.7-μm absorption edge at Pele and elsewhere on Io, though S 4 generated by other processes may also be a possibility. S 2 O can be produced by high-temperature decomposition of SO 2 gas, and the high temperature of the Pele volcano may account for its concentration there. Spectral anomalies of comparable size and prominence are not seen around the other “Pele-type” volcanos Surt and Aten (A. S. McEwen and L. A. Soderblom, 1983, Icarus 55, 191–217), suggesting that these volcanos, if chemically similar to Pele, are much less active. The spectrum of high-latitude regions is similar to that of quenched red sulfur glass, and if this similarity is not coincidental, the glass may be preserved here by the low polar surface temperatures. Alternatively, the low polar temperatures may preserve sulfur that has been reddened by radiation. There are many changes in albedo patterns in the 15 years between Voyager and these HST observations, but these are generally subtle at HST resolution and are not strongly concentrated in longitude; however there was a major brightening of a 400-km-diameter region centered on Ra Patera between March 1994 and repeat HST observations in July 1995, which was a larger albedo change than any seen in the previous 15 years. This was presumably due to a large eruption at Ra Patera, as confirmed by Galileo images. Long-exposure eclipse images of Io at 1.02 μm on March 6, 1994, place strong limits on the area of exposed silicate magma on Io at the time of the observations.

Icarus↗

USGS1997

Latitudinal distribution of O2on ganymede: Observations with the hubble space telescope

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 ??m, 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 ??m. 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. ?? 1997 Academic Press.

Icarus↗

USGS1996

O2 on ganymede: Spectral characteristics and plasma formation mechanisms

Weak absorption features in the visible reflectance spectrum of Jupiter's satellite Ganymede have been correlated to those observed in the spectrum of molecular oxygen. We examine the spectral characteristics of these absorption features in all phases of O 2 and conclude that the molecular oxygen is most likely present at densities similar to the liquid or solid γ-phase. The contribution of O 2 to spectral features observed on Ganymede in the near-infrared wavelength region affects the previous estimates of photon pathlength in ice. The concentration of the visible absorption features on the trailing hemisphere of Ganymede suggests an origin due to bombardment by magnetospheric ions. We derive an approximate O 2 formation rate from this mechanism and consider the state of O 2 within the surface.

Geophysical Research Letters↗