Lorenz Roth

Transient Water Vapor at Europa’s South Pole

Europas Plumes Jupiters moon Europa has a subsurface ocean and a relatively young icy surface. Roth et al. (p. 171, published online 12 December 2013; see the Perspective by Spencer) analyzed spectral images taken by the Hubble Space Telescope that show ultraviolet emissions from the moons atmosphere, and report a statistically significant emission signal extending above the satellites southern hemisphere. This emission is consistent with two 200-km-high plumes of water vapor. Tidal stresses likely play a role in opening and closing fractures at the surface. Hubble Space Telescope images of Jupiter’s moon Europa reveal emission consistent with transient water vapor plumes. [Also see Perspective by Spencer] In November and December 2012, the Hubble Space Telescope (HST) imaged Europa’s ultraviolet emissions in the search for vapor plume activity. We report statistically significant coincident surpluses of hydrogen Lyman-α and oxygen OI 130.4-nanometer emissions above the southern hemisphere in December 2012. These emissions were persistently found in the same area over the 7 hours of the observation, suggesting atmospheric inhomogeneity; they are consistent with two 200-km-high plumes of water vapor with line-of-sight column densities of about 1020 per square meter. Nondetection in November 2012 and in previous HST images from 1999 suggests varying plume activity that might depend on changing surface stresses based on Europa’s orbital phases. The plume was present when Europa was near apocenter and was not detected close to its pericenter, in agreement with tidal modeling predictions.

The search for a subsurface ocean in Ganymede with Hubble Space Telescope observations of its auroral ovals

We present a new approach to search for a subsurface ocean within Ganymede through observations and modeling of the dynamics of its auroral ovals. The locations of the auroral ovals oscillate due to Jupiters time-varying magnetospheric field seen in the rest frame of Ganymede. If an electrically conductive ocean is present, the external time-varying magnetic field is reduced due to induction within the ocean and the oscillation amplitude of the ovals decreases. Hubble Space Telescope (HST) observations show that the locations of the ovals oscillate on average by 2.0 ◦ ± 1.3 ◦ . Our model calculations predict a significantly stronger oscillation by 5.8 ◦ ± 1.3 ◦ without ocean compared to 2.2 ◦ ± 1.3 ◦ if an ocean is present. Because the ocean and the no-ocean hypotheses cannot be separated by simple visual inspection of individual HST images, we apply a statistical analysis including a Monte Carlo test to also address the uncertainty caused by the patchiness of observed emissions. The observations require a minimum electrical conductivity of 0.09 S/m for an ocean assumed to be located between 150 km and 250 km depth or alternatively a maximum depth of the top of the ocean at 330 km. Our analysis implies that Ganymedes dynamo possesses an outstandingly low quadrupole-to-dipole moment ratio. The new technique applied here is suited to probe the interior of other planetary bodies by monitoring their auroral response to time-varying magnetic fields.

Orbital apocenter is not a sufficient condition for HST/STIS detection of Europa’s water vapor aurora

Significance Images of Europa’s UV aurora taken by the Hubble Space Telescope in December 2012 have revealed local hydrogen and oxygen emissions in intensity ratios that identify the source as electron impact excitation of water molecules. The existence of water vapor plumes as a source for the detected localized water vapor and the possible accessibility of subsurface liquid water reservoirs at these locations have important implications for the exploration of Europa’s potentially habitable environments. The observations reported here tested whether orbital position near the apocenter is an essential requirement for plume activity. Only an upper limit on the amount of water vapor was obtained. Orbital position is therefore not a sufficient condition for detecting plumes and they may be episodic events. We report far-ultraviolet observations of Jupiter’s moon Europa taken by Space Telescope Imaging Spectrograph (STIS) of the Hubble Space Telescope (HST) in January and February 2014 to test the hypothesis that the discovery of a water vapor aurora in December 2012 by local hydrogen (H) and oxygen (O) emissions with the STIS originated from plume activity possibly correlated with Europa’s distance from Jupiter through tidal stress variations. The 2014 observations were scheduled with Europa near the apocenter similar to the orbital position of its previous detection. Tensile stresses on south polar fractures are expected to be highest in this orbital phase, potentially maximizing the probability for plume activity. No local H and O emissions were detected in the new STIS images. In the south polar region where the emission surpluses were observed in 2012, the brightnesses are sufficiently low in the 2014 images to be consistent with any H2O abundance from (0–5)×1015 cm−2. Large high-latitude plumes should have been detectable by the STIS, independent of the observing conditions and geometry. Because electron excitation of water vapor remains the only viable explanation for the 2012 detection, the new observations indicate that although the same orbital position of Europa for plume activity may be a necessary condition, it is not a sufficient condition. However, the December 2012 detection of coincident HI Lyman-α and OI 1304-Å emission surpluses in an ∼200-km high region well separated above Europa’s limb is a firm result and not invalidated by our 2014 STIS observations.

HUBBLE SPACE TELESCOPE/ADVANCED CAMERA FOR SURVEYS OBSERVATIONS OF EUROPA'S ATMOSPHERIC ULTRAVIOLET EMISSION AT EASTERN ELONGATION

We report results of a Hubble Space Telescope (HST) campaign with the Advanced Camera for Surveys to observe Europa at eastern elongation, i.e., Europa’s leading side, on 2008 June 29. With five consecutive HST orbits, we constrain Europa’s atmospheric Oi 1304 A and Oi 1356 A emissions using the prism PR130L. The total emissions of both oxygen multiplets range between 132 ± 14 and 226 ± 14 Rayleigh. An additional systematic error with values on the same order as the statistical errors may be due to uncertainties in modeling the reflected light from Europa’s surface. The total emission also shows a clear dependence of Europa’s position with respect to Jupiter’s magnetospheric plasma sheet. We derive a lower limit for the O2 column density of 6 × 10 18 m −2 . Previous observations of Europa’s atmosphere with the Space Telescope Imaging Spectrograph in 1999 of Europa’s trailing side show an enigmatic surplus of radiation on the anti-Jovian side within the disk of Europa. With emission from a radially symmetric atmosphere as a reference, we searched for an anti-Jovian versus sub-Jovian asymmetry with respect to the central meridian on the leading side and found none. Likewise, we searched for departures from a radially symmetric atmospheric emission and found an emission surplus centered around 90 ◦ west longitude, for which plausible mechanisms exist. Previous work about the possibility of plumes on Europa due to tidally driven shear heating found longitudes with strongest local strain rates which might be consistent with the longitudes of maximum UV emissions. Alternatively, asymmetries in Europa’s UV emission can also be caused by inhomogeneous surface properties, an optically thick atmospheric contribution of atomic oxygen, and/or by Europa’s complex plasma interaction with Jupiter’s magnetosphere.

Constraints on an exosphere at Ceres from Hubble Space Telescope observations

We report far ultraviolet observations of Ceres obtained with the Cosmic Origin Spectrograph (COS) of the Hubble Space Telescope in the search for atomic emissions from an exosphere. The derived br ...

DETECTION OF A HYDROGEN CORONA IN HST Ly alpha IMAGES OF EUROPA IN TRANSIT OF JUPITER

We report far-ultraviolet observations of Europa in transit of Jupiter obtained with the Space Telescope Imaging Spectrograph of the Hubble Space Telescope on six occasions between 2014 December an ...

Morphology of Ganymede's FUV auroral ovals

We study the morphology of Ganymedes FUV aurora by analyzing spectral images obtained over the past two decades by the Space Telescope Imaging Spectrograph on board the Hubble Space Telescope. The ...

Europa's plasma interaction with an inhomogeneous atmosphere : Development of Alfvén winglets within the Alfvén wings

We apply a three-dimensional magnetohydrodynamic (MHD) model to study the influence of inhomogeneities in Europas atmosphere, as, for example, water vapor plumes, on Europas plasma interaction wi ...

Constraints on Io's interior from auroral spot oscillations

The morphology of Ios aurora is dominated by bright spots near the equator that oscillate up and down in approximate correlation with the oscillating orientation of the Jovian magnetospheric field ...

Detection of a hydrogen corona at Callisto

In December 2001, the Space Telescope Imaging Spectrograph of the Hubble Space Telescope obtained far-ultraviolet spectral images of Jupiters moon Callisto. The leading and trailing hemispheres we ...