Near-Infrared Mapping Spectrometer Data Research Articles

Cautionary Analysis of Spectral Radiance from Io’s Active Volcanoes Derived from Galileo Near-Infrared Mapping Spectrometer Data

Abstract Between 1996 and 2001, the Galileo Near-Infrared Mapping Spectrometer (NIMS) obtained 190 observations of the volcanic Jovian moon Io. Rathbun et al. (2018) [Astron. J., 156, 207] published a list of 287 measurements of 3.5 μm spectral radiance from some of Io’s active volcanoes, derived from a subset of the NIMS data. However, the spectral radiances reported by Rathbun et al. are lower, in some cases by multiple orders of magnitude, than other analyses of the same observations and spectral radiances derived from contemporaneous ground-based data. In many cases, the Rathbun et al. hot-spot radiances are underreported by a factor of π, likely due to a mistake in unit conversion. For a small number of powerful hot spots, additional discrepancies appear to be the result of poor fits to data limited in wavelength range by NIMS detector saturation and a methodology that discards short-wavelength NIMS data that otherwise would have provided more robust temperature model fits.

Magnetospheric ion sputtering and water ice grain size at Europa

We present the first calculation of Europa's sputtering (ion erosion) rate as a function of position on Europa's surface. We find a global sputtering rate of 2×1027H2O s−1, some of which leaves the surface in the form of O2 and H2. The calculated O2 production rate is 1×1026O2 s−1, H2 production is twice that value. The total sputtering rate (including all species) peaks at the trailing hemisphere apex and decreases to about 1/3rd of the peak value at the leading hemisphere apex. O2 and H2 sputtering, by contrast, is confined almost entirely to the trailing hemisphere. Most sputtering is done by energetic sulfur ions (100s of keV to MeV), but most of the O2 and H2 production is done by cold oxygen ions (temperature ∼ 100 eV, total energy ∼ 500 eV). As a part of the sputtering rate calculation we compared experimental sputtering yields with analytic estimates. We found that the experimental data are well approximated by the expressions of Famá et al. for ions with energies less than 100keV (Famá, M., Shi, J., Baragiola, R.A., 2008. Sputtering of ice by low-energy ions. Surf. Sci. 602, 156–161), while the expressions from Johnson et al. fit the data best at higher energies (Johnson, R.E., Burger, M.H., Cassidy, T.A., Leblanc, F., Marconi, M., Smyth, W.H., 2009. Composition and Detection of Europa's Sputter-Induced Atmosphere, in: Pappalardo, R.T., McKinnon, W.B., Khurana, K.K. (Eds.), Europa. University of Arizona Press, Tucson.). We compare the calculated sputtering rate with estimates of water ice regolith grain size as estimated from Galileo Near-Infrared Mapping Spectrometer (NIMS) data, and find that they are strongly correlated as previously suggested by Clark et al. (Clark, R.N., Fanale, F.P., Zent, A.P., 1983. Frost grain size metamorphism: Implications for remote sensing of planetary surfaces. Icarus 56, 233–245.). The mechanism responsible for the sputtering rate/grain size link is uncertain. We also report a surface composition estimate using NIMS data from an area on the trailing hemisphere apex. We find a high abundance of sulfuric acid hydrate and radiation-resistant hydrated salts along with large water ice regolith grains, all of which are consistent with the high levels of magnetospheric bombardment at the trailing apex.

Hydrated minerals on Europa’s surface: An improved look from the Galileo NIMS investigation

The surface composition of Europa is of great importance for understanding both the internal evolution of Europa and its putative ocean. The Near Infrared Mapping Spectrometer (NIMS) investigation on Galileo observed Europa and the other Galilean satellites from 0.7 to 5.2 μm with spatial resolution down to a few kilometers during flybys by the spacecraft as it orbited Jupiter. These data have been analyzed and results published over the life of the Galileo mission and afterward. One result was the discovery of hydrated minerals at some locations on Europa and Ganymede. The data are noisy, especially for Europa, due to radiation affecting the NIMS electronics and detectors, and other artifacts are also present. The NIMS data are now being reprocessed using the accumulated knowledge gained over the entire missions to remove noise spikes and compensate for some other defects in the data. We are analyzing these reprocessed data in an attempt to defined better the nature of the hydrate spectral features and improve their interpretation. We report here on analyses of two NIMS reprocessed observations for the 0.7–3-μm region. A revised hydrate spectrum is calculated and mapped in detail across two lineaments. The spectrum shows the expected distorted water features but little or no spectral structure in these features. A narrow, weak spectral feature appears at 1.344 μm, which is weakly correlated with lower albedo. Several other weak features may be present but are difficult to confirm in these limited data sets. The hydrate signature shows the greatest strength within and toward the center of the lineaments, confirming and strengthening the association of the hydrate with these endogenic features. This trend may indicate that the material in the lineaments is youngest toward the center and has more water frost coverage toward the edge. A small, visually dark, circular feature has a spectrum that shows both hydrate and crystalline water ice features and perhaps contains a hydrate different in spectral characteristics and perhaps composition than found in the lineament.

The nature of the volcanic activity at Loki: Insights from Galileo NIMS and PPR data

Loki is the largest patera and the most energetic hotspot on Jupiter's moon Io, in turn the most volcanically active body in the Solar System, but the nature of the activity remains enigmatic. We present detailed analysis of Galileo Near-Infrared Mapping Spectrometer (NIMS) and PhotoPolarimeter/Radiometer (PPR) observations covering the 1.5–100 μm wavelength range during the I24, I27, and I32 flybys. The general pattern of activity during these flybys is consistent with previously proposed models of a resurfacing wave periodically crossing a silicate lava lake. In particular our analysis of the I32 NIMS observations shows, over much of the observed patera, surface temperatures and implied ages closely matching those expected for a wave advancing counterclockwise at 0.94–1.38 km/day. The age pattern is different than other published analyses which do not show as clearly this azimuthal pattern. Our analysis also shows two additional distinctly different patera surfaces. The first is located along the inner and outer margins where components with a 3.00–4.70-μm color temperature of 425 K exist. The second is located at the southwestern margin where components with a 550-K color temperature exist. Although the high temperatures could be caused by disruption of a lava lake crust, some additional mechanism is required to explain why the southwest margin is different from the inner or outer ones. Finally, analysis of the temperature profiles across the patera reveal a smoothness that is difficult to explain by simple lava cooling models. Paradoxically, at a subpixel level, wide temperature distributions exist which may be difficult to explain by just the presence of hot cracks in the lava crust. The resurfacing wave and lava cooling models explain well the overall characteristics of the observations. However, additional physical processes, perhaps involving heat transport by volatiles, are needed to explain the more subtle features.

Volcanism on Io: Estimation of eruption parameters from Galileo NIMS data

Galileo Near Infrared Mapping Spectrometer (NIMS) data of volcanic thermal emission are analyzed to determine the power output of a number of Ionian volcanoes, from which are calculated volumetric eruption rates. A two‐temperature model is used to determine surface temperatures and power output. Portions of Prometheus and six other volcanoes are found to be in excess of 1100 K: areas of Prometheus are at temperatures close to 1500 K. These are minimum eruption temperatures and are consistent with basaltic silicate volcanism. For the 14 volcanoes in G1INNSPEC01, volumetric eruption rates (E) are constrained between 3 and 300 m3/s for basaltic and ultramafic compositions. Actual E values lie between these limits. The small crack fractions (hot area to warm area ratio) implied from these fits are indicative of a relatively quiescent eruption style, such as the emplacement of a pahoehoe or a‐a flow field. The volumetric eruption rates derived from the NIMS data are greater than those observed in similar styles of volcanism on Earth. It is apparent that for similar eruption styles, the areal extent of activity is greater on Io than on Earth. Derived flow thicknesses range from 0.1 to 13 m.

Silicate Cooling Model Fits to Galileo NIMS Data of Volcanism on Io

The Near Infrared Mapping Spectrometer (NIMS) has obtained spectra of volcanoes on the surface of the jovian satellite Io. Fits to data using a silicate cooling model allow us to constrain lava eruption rates and eruption age. The thermal signatures of the hot spots are indicative of active and cooling silicate lava flows. For large, active hot spots maximum ages of flow surfaces detected by NIMS typically range from days to months, although in one case it is nearly 30 years. Mass eruption rates for the main hot spots are in the range 7 m 3 s −1 to 79 m 3 s −1, using an average flow thickness of 1 m. These mass eruption rates are orders of magnitude less than those implied for large thermal outbursts on Io: the eruptions observed during June 1996 are most likely of a different eruption style, on a much smaller scale. The observation analyzed here may be representative of the current background level of volcanic activity on the anti-jove hemisphere of Io. The global mass eruption rate from these “non-outburst” hot spots is calculated to be 43 km 3/year, equivalent to a global resurfacing rate of about 1 mm/year, about 10% of the minimum required global resurfacing volume. The total observed energy output from the 14 hot spots analyzed is 3.6×10 12 W. Normalized globally, these hot spots contribute approximately 10% of Io's radiometric heat flux.