اشترك بالحزمة الذهبية واحصل على وصول غير محدود شمرا أكاديميا
تسجيل مستخدم جديدSalts and radiation products on the surface of Europa
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The surface of Europa could contain the compositional imprint of a underlying interior ocean, but competing hypotheses differ over whether spectral observations from the Galileo spacecraft show the signature of ocean evaporates or simply surface radiation products unrelated to the interior. Using adaptive optics at the W.M. Keck Observatory, we have obtained spatially resolved spectra of most of the disk of Europa at a spectral resolution ~40 times higher than seen by the Galileo spacecraft. These spectra show a previously undetected distinct signature of magnesium sulfate salts on Europa, but the magnesium sulfate is confined to the trailing hemisphere and spatially correlated with the presence of radiation products like sulfuric acid and SO2. On the leading, less irradiated, hemisphere, our observations rule out the presence of many of the proposed sulfate salts, but do show the presence of distorted water ice bands. Based on the association of the potential MgSO4, detection on the trailing side with other radiation products, we conclude that MgSO4 is also a radiation product, rather than a constituent of a Europa ocean brine. Based on ocean chemistry models, we hypothesize that, prior to irradiation, magnesium is primarily in the form of MgCl2, and we predict that NaCl and KCl are even more abundant, and, in fact, dominate the non-ice component of the leading hemisphere. We propose observational tests of this new hypothesis.
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Seismic data will be a vital geophysical constraint on internal structure of Europa if we land instruments on the surface. Quantifying expected seismic activity on Europa both in terms of large, recognizable signals and ambient background noise is im
portant for understanding dynamics of the moon, as well as interpretation of potential future data. Seismic energy sources will likely include cracking in the ice shell and turbulent motion in the oceans. We define a range of models of seismic activity in Europas ice shell by assuming each model follows a Gutenberg-Richter relationship with varying parameters. A range of cumulative seismic moment release between $10^{16}$ and $10^{18}$ Nm/yr is defined by scaling tidal dissipation energy to tectonic events on the Earths moon. Random catalogs are generated and used to create synthetic continuous noise records through numerical wave propagation in thermodynamically self-consistent models of the interior structure of Europa. Spectral characteristics of the noise are calculated by determining probabilistic power spectral densities of the synthetic records. While the range of seismicity models predicts noise levels that vary by 80 dB, we show that most noise estimates are below the self-noise floor of high-frequency geophones, but may be recorded by more sensitive instruments. The largest expected signals exceed background noise by $sim$50 dB. Noise records may allow for constraints on interior structure through autocorrelation. Models of seismic noise generated by pressure variations at the base of the ice shell due to turbulent motions in the subsurface ocean may also generate observable seismic noise.
Roth et al (2014a) reported evidence for plumes of water venting from a southern high latitude region on Europa - spectroscopic detection of off-limb line emission from the dissociation products of water. Here, we present Hubble Space Telescope (HST)
direct images of Europa in the far ultraviolet (FUV) as it transited the smooth face of Jupiter, in order to measure absorption from gas or aerosols beyond the Europa limb. Out of ten observations we found three in which plume activity could be implicated. Two show statistically significant features at latitudes similar to Roth et al, and the third, at a more equatorial location. We consider potential systematic effects that might influence the statistical analysis and create artifacts, and are unable to find any that can definitively explain the features, although there are reasons to be cautious. If the apparent absorption features are real, the magnitude of implied outgassing is similar to that of the Roth et al feature, however the apparent activity appears more frequently in our data.
We present hemispherically resolved spectra of the surface of Europa from ~3.1--4.13 microns, which we obtained using the near infrared spectrometer NIRSPEC on the Keck II telescope. These include the first high-quality L-band spectra of the surface
to extend beyond 4 microns. In our data we identify a previously unseen spectral feature at 3.78 microns on the trailing hemisphere. The longitudinal distribution of the feature is consistent with that of a radiolytic product created by electron or Iogenic ion bombardment. This feature is coincident with an absorption feature of SO2 frost seen in both laboratory spectra and spectra of Io. However, the corresponding, typically stronger 4.07 micron feature of SO2 frost is absent from our data. This result is contrary to the suggested detection of SO2 at 4.05 microns in Galileo NIMS data of the trailing hemisphere, which was severely affected by radiation noise. We use simple spectral modeling to argue that the 3.78 micron feature is not easily explained by the presence of SO2 frost on the surface. We explore alternative explanations and discuss other potential candidate species.
With the adaptive optics (AO) system on the 10 m Keck-II telescope, we acquired a high quality set of 84 images at 14 epochs of asteroid (52) Europa on 2005 January 20. The epochs covered its rotation period and, by following its changing shape and o
rientation on the plane of sky, we obtained its triaxial ellipsoid dimensions and spin pole location. An independent determination from images at three epochs obtained in 2007 is in good agreement with these results. By combining these two data sets, along with a single epoch data set obtained in 2003, we have derived a global fit for (52) Europa of diameters (379x330x249) +/- (16x8x10) km, yielding a volume-equivalent spherical-diameter of 315 +/- 7 km, and a rotational pole within 7 deg of [RA; Dec] = [257,+12] in an Equatorial J2000 reference frame (ECJ2000: 255,+35). Using the average of all mass determinations available forEuropa, we derive a density of 1.5 +/- 0.4, typical of C-type asteroids. Comparing our images with the shape model of Michalowski et al. (A&A 416, 2004), derived from optical lightcurves, illustrates excellent agreement, although several edge features visible in the images are not rendered by the model. We therefore derived a complete 3-D description of Europas shape using the KOALA algorithm by combining our imaging epochs with 4 stellar occultations and 49 lightcurves. We use this 3-D shape model to assess these departures from ellipsoidal shape. Flat facets (possible giant craters) appear to be less distinct on (52) Europa than on other C-types that have been imaged in detail. We show that fewer giant craters, or smaller craters, is consistent with its expected impact history. Overall, asteroid (52) Europa is still well modeled as a smooth triaxial ellipsoid with dimensions constrained by observations obtained over several apparitions.
We present mid-infrared SOFIA/EXES spectroscopy of Europa, seeking direct evidence of the presence of water vapor arising from plumes venting from the surface of Europa. We place quantitatively useful upper limits on the strength of water vibrational
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