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

Abstract

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 Jupiter's 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 5.8Because 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 Ganymede's 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.

Pressure-induced ion pairing in MgSO4 solutions: Implications for the oceans of icy worlds

“…3). However, Ganymede (Vance et al, 2014;Saur et al, 2015) and Saturn's moon Titan (Grasset and Pargamin, 2005;Tobie et al, 2012) likely possess deep subsurface oceans with substantial portions of the liquid water column in the LDW-HDW transition region (Fig. 3).…”

“…The salts of planetary oceans are acquired chiefly from sub-seafloor weathering reactions (Zolotov and Shock, 2001). About 75 % of the salts produced by low-temperature aqueous alteration of chondrite under oxidising conditions are magnesium sulphate hydrates (Hogenboom et al, 1995 et al, 2012), Europa (Spohn and Schubert, 2003;Hand and Chyba, 2007;Valenti et al, 2012), Ganymede (Vance et al, 2014;Saur et al, 2015), and Callisto (Kuskov and Kronrod, 2005) with approximate depths of the subsurface ocean boundaries in km. The shadowed band is the pressure range of the LDW-HDW transition at about 20 °C (Fanetti et al, 2014).…”

Planetary space weather: scientific aspects and future perspectives

“…Indeed, such space weather phenomena evidenced in the magnetospheric electron spectrum and spatial distribution could be the determining factors for the observed auroral emissions studied several times in the past (see McGrath et al 2004;Saur et al 2011Saur et al , 2015Roth et al 2014aRoth et al , 2014bRoth et al , 2016. Moreover, the exosphere generation and loss depend on the actual energetic particle and plasma distributions, respectively, around the moon.…”

Corrigendum to “Iron snow dynamo models for Ganymede” [Icarus 247 (2015) 248–259]

“…The dynamo models aim at explaining the properties of Ganymede's magnetic field determined by magnetometer measurements during flybys of the Galileo mission. This is in particular the very low quadrupole moment in relation to the dipole moment, which is unusual in comparison to the ratio at other planetary dynamos (Kivelson et al, 2002;Saur et al, 2015). Technically, the dynamo model consists of two separate shells: a convecting inner shell where the dynamo operates and an outer shell representing the snow layer in which only horizontal fluid motion is possible.…”