Europa's Interior Research Articles

Variable Salinity and Hydrogen Production in Europa's Ocean

AbstractEuropa's long‐term habitability depends on thermodynamics—modulated by tidal evolution—and geochemical pathways that define chemical and energy inventories for use by putative biology. Hydrogen produced from the reaction of water and rock during serpentinization is one potential source of metabolic energy. Here, we demonstrate how thermally modulated volumetric changes to Europa's ice shell, ocean, and fluid‐accessible rocky mantle influence oceanic salinity and serpentinization rates in Europa's interior. During periods of steady cooling, hydrogen flux into the ocean remains relatively constant because of the continuous propagation of thermal cracking into the rocky mantle regardless of ocean composition. However, during periods of fluctuating heat production driven by changes in orbital eccentricity, modulation of the ice shell volume, and consequently the salinity and water activity of the ocean, alters the serpentinization rate. In parallel, the extent of fracturing within the seafloor varies, changing the volume of reactable rock available. Our results show that the stability of serpentinization reactions on Europa over time highly depends on the thermal‐orbital evolution and oceanic salt content. The variability in the hydrogen production rate is significant across potential orbital histories, varying by up to 15 orders of magnitude for the lowest salinity and most kosmotropic ocean conditions, such as dilute MgSO4 compositions. We also show that this variability decreases with increasing ocean salinity and more chaotropic compositions, such as a concentrated NaCl ocean. These results suggest new ways that ocean composition and water activity are essential to stable habitable conditions in ocean worlds.

Slow evolution of Europa's interior: metamorphic ocean origin, delayed metallic core formation, and limited seafloor volcanism.

Europa's ocean lies atop an interior made of metal and silicates. On the basis of gravity data from the Galileo mission, many argued that Europa's interior, like Earth, is differentiated into a metallic core and a mantle composed of anhydrous silicates. Some studies further assumed that Europa differentiated while (or soon after) it accreted, also like Earth. However, Europa probably formed at much colder temperatures, meaning that Europa plausibly ended accretion as a mixture containing water-ice and/or hydrated silicates. Here, we use numerical models to describe the thermal evolution of Europa's interior assuming low initial temperatures (~200 to 300 kelvin). We find that silicate dehydration can produce Europa's current ocean and icy shell. Rocks below the seafloor may remain cool and hydrated today. Europa's metallic core, if it exists, may have formed billions of years after accretion. Ultimately, we expect the chemistry of Europa's ocean to reflect protracted heating of the interior.

Turbulent Drag at the Ice‐Ocean Interface of Europa in Simulations of Rotating Convection: Implications for Nonsynchronous Rotation of the Ice Shell

AbstractEuropa's geologically scarred surface shows significant evidence that the ice shell may have rotated nonsynchronously in the past. The long‐term spin state of the ice shell is controlled by the time‐mean torques acting upon it. A torque that has not been previously considered is exerted due to drag from oceanic currents beneath the ice. We estimate this torque for the first time by performing global, nonhydrostatic, three‐dimensional simulations of Europa's ocean, including nonlinear turbulent boundary layer drag at the seafloor and ice‐ocean interface. Our simulations show that ocean dynamics, which manifest in alternating east‐west jets, result in a net torque on the ice shell. The torque can act to either spin up or spin down the ice shell depending on the strength of convection, suggesting that a torque reversal can occur as Europa's interior thermally evolves. Scaling analysis indicates that an average jet speed of at least ∼1 cm s−1 is required for the ice‐ocean torque to be comparable to the tidal torque acting to spin up the ice shell. Our results suggest that ocean currents may contribute to any nonsynchronous rotation of the ice shell. Consequently, Europa's present‐day spin state may hold information about the dynamics of its subsurface ocean.

Constraining Europa's ice shell thickness with fundamental mode surface wave dispersion

Determining the thickness of Europa's outer ice shell is a key factor for understanding Europa's internal dynamics, evolution, and potential habitability. As such, one of the primary goals of any future lander mission to Europa would be to constrain the thickness of the ice shell and confirm the presence of a global subsurface ocean. Tidally induced ice fracturing events provide a natural source of seismic energy to illuminate the subsurface, thus a seismic instrument onboard a future lander mission could provide a promising means to probe ice shell thickness. A variety of seismic techniques could be used to constrain Europa's interior structure and dynamics, including body wave, surface wave, and normal mode seismology. Here, we use numerical simulations of seismic wave propagation on Europa in order to investigate the potential of using long period dispersion measurements of Rayleigh and flexural waves to constrain the ice shell thickness. Since the sensitivity kernels of group velocity dispersion measurements depend strongly on the structure of the ice shell, inverting for ice shell thickness is a non-linear problem.To address this, we use either a grid search or Markov chain Monte Carlo inversion approach, and test the method on a variety of plausible models of Europa's interior. Additionally, we demonstrate our approach in a “blind” inversion using the 1 week long synthetic catalogs of Europa's seismicity from Panning et al. (2018). We find that under most scenarios, group velocity dispersion measurements between periods of 25–250 s can constrain Europa's ice shell thickness to within several km uncertainty, although the method becomes increasingly inaccurate for thicker ice shells and at large epicentral distances. Our results, which suggest that surface waves from naturally occurring ice fracturing events on Europa can be used to help determine ice shell thickness, may help set instrument requirements for spaceflight capable seismometers aimed at exploring icy ocean worlds.

A magnetically driven equatorial jet in Europa’s ocean

During recent decades, data from space missions have provided strong evidence of deep liquid oceans underneath a thin outer icy crust on several moons of Jupiter, particularly Europa. But these observations have also raised many unanswered questions regarding the oceanic motions generated under the ice, or the mechanisms leading to the geological features observed on Europa. By means of direct numerical simulations of Europa's interior, we show here that Jupiter's magnetic field generates a retrograde oceanic jet at the equator, which may influence the global dynamics of Europa's ocean and contribute to the formation of some of its surface features by applying a unidirectional torque on Europa's ice shell.

Assessing the potential for measuring Europa's tidal Love number h2 using radar sounder and topographic imager data

The tidal Love number h2 is a key geophysical measurement for the characterization of Europa's interior, especially of its outer ice shell if a subsurface ocean is present. We performed numerical simulations to assess the potential for estimating h2 using altimetric measurements with a combination of radar sounding and stereo imaging data. The measurement principle exploits both delay and Doppler information in the radar surface return in combination with topography from a digital terrain model (DTM). The resulting radar range measurements at cross-over locations can be used in combination with radio science Doppler data for an improved trajectory solution and for estimating the h2 Love number. Our simulation results suggest that the absolute accuracy of h2 from the joint analysis of REASON (Radar for Europa Assessment and Sounding: Ocean to Near-surface) surface return and EIS (Europa Imaging System) DTM data will be in the range of 0.04–0.17 assuming full radio link coverage. The error is controlled by the SNR budget and DTM quality, both dependent on the surface properties of Europa. We estimate that this would unambiguously confirm (or reject) the global ocean hypothesis and, in combination with a nominal radio-science based measurement of the tidal Love number k2, constrain the thickness of Europa's outer ice shell to up to ±15 km.

Surface composition of pull-apart bands in Argadnel Regio, Europa: Evidence of localized cryovolcanic resurfacing during basin formation

We combine Galileo Solid State Imager (SSI) and Near-Infrared Mapping Spectrometer (NIMS) data to investigate the composition of pull-apart bands in Europa's Argadnel Regio. Using spectral linear mixture modeling employing cryogenic laboratory reference spectra, we find that bands of intermediate age (“grey” bands) are compositionally distinct from bands that are stratigraphically younger (“dark” bands). The grey bands have higher abundances of larger ice grains and lower abundances of hydrated salts than the dark bands; both of these tendencies are statistically significant at the 1% level. The grey and dark bands have similar abundances of hexahydrite, a material which is relatively stable under irradiation; however, the derived abundances of frozen magnesium sulfate brine and of mirabilite, which are more susceptible to fragmentation by radiation, are significantly higher in the dark bands than in the grey bands. These results are consistent with a physical model in which the differences in composition and in ice grain sizes are linked to space weathering and radiolytic processing levels; the grey bands have presumably undergone higher levels of processing, due to being exposed on Europa's surface for a longer period of time. One prominent wedge-shaped band exhibits an anomalous albedo variation across its northern portion, appearing dark in its top third, and grey in its southernmost two-thirds. We find that the dark part of the band has a modeled composition that is in-family with other dark bands, while the grey portion has a modeled composition that is indistinguishable from other grey bands in the study area. Because these variations cannot easily be attributed to the band's formation mechanism (bands open sequentially along a central axis), we surmise that the northern part has been resurfaced, probably in response to the formation of a large topographic basin that cuts through the band. Faulting accompanying basin formation may provide conduits allowing transport to the surface of materials from Europa's interior. We hypothesize that the formation of the basin resulted in fresh cryovolcanic material being deposited across the northern portion of the band, effectively “resetting” its surface age. If, as has been suggested, the giant arcuate basins resulted from an episode of true polar wander, our study may help to more tightly constrain the age of that event within Europa's geologic column.

Improved detection of tides at Europa with radiometric and optical tracking during flybys

Due to its eccentric orbit about Jupiter, Europa experiences periodic tidal deformation, which causes changes in its gravitational field and induces both radial and transverse displacements of the surface. The amplitude and phase of these tidal changes are diagnostic of internal structure, and can be measured with sufficient radiometric and optical tracking of a spacecraft during a series of flyby encounters with Europa. This paper presents results of the simulated accuracy for recovery of the tides of Europa through measuring the second-degree tidal Love numbers k2, h2, and l2. A reference trajectory, which consists of a total of 45 close flybys, was considered and a detailed covariance analysis was performed. The study was based on Earth-based Doppler tracking during ±2h of each periapsis passage and surface imaging data taken below 500km altitude. The result shows that the formal uncertainty of the second-degree tidal Love numbers can be estimated to be σk2=0.01, σh2=0.02, and σl2=0.01, which is sufficient to constrain the global ice thickness to about 10km under reasonable assumptions. Moreover, the forced librations of Europa can be measured to 0.3″ accuracy, which can further constrain Europa's interior structure.

Measuring tidal deformations at Europa’s surface

Abstract Key information on Europa’s interior can be gained by monitoring tidally-induced surface deformations from orbiting and landed spacecraft. Such observations would provide constraints on the thickness and rheology of Europa’s ice and liquid water layer, being thus an important tool to characterize basic physical properties of the satellite’s putative subsurface water ocean. Focusing on the outer ice-I layer we will present relations between the interior of Europa and key tidal parameters that can be retrieved from an instrument suite monitoring tidally-induced changes of local gravity, tilt, latitude and strain at the surface. A most promising approach would involve laser altimetry and gravitational field observations from an orbiting spacecraft combined with monitoring of tidally-induced gravity and tilt changes at the surface. However, tidal measurements at the surface may be significantly impeded by instrumental drift, instrument coupling to the surface, local sources of noise and the presumably short life-time of the instruments due to the harsh radiation environment.

Influence of the internally induced magnetic field on the plasma interaction of Europa

In this paper, we study the influence of electromagnetic induction in Europa's interior on the plasma interaction in the vicinity of Europa. We describe the temporally periodic interaction between Europa and the Jovian magnetosphere with a three‐dimensional time‐dependent model. Our model describes simultaneously the three‐dimensional plasma interaction of Europa's atmosphere with Jupiter's magnetosphere and the electromagnetic induction in a subsurface conducting layer due to time‐varying magnetic fields and plasmas including their mutual feedbacks. We find that the induced magnetic fields cause time varying asymmetries in the current density and the plasma density outside Europa. We show that the Alfvén current system is deformed and displaced because of the induced magnetic fields. Furthermore, the plasma wake of Europa is deformed because of the induction. Remarkably, therefore the influence of the induced fields on the Alfvén wings and the plasma wake may be seen at distances where the induced fields are already negligible. The effects of induction on the wake structure are most clearly visible in the E4 flyby magnetic field and plasma observations, which are in good agreement with our simulation. The resulting structure of Europa's plasma wake could explain why Galileo measurements did not detect high plasma densities along the E4 trajectory.

Near-infrared detection of potential evidence for microscopic organisms on Europa.

The possibility of an ocean within the icy shell of Jupiter's moon Europa has established that world as a primary candidate in the search for extraterrestrial life within our Solar System. This paper evaluates the potential to detect evidence for microbial life by comparing laboratory studies of terrestrial microorganisms with measurements from the Galileo Near Infrared Imaging Spectrometer (NIMS). If the interior of Europa at one time harbored life, some evidence may remain in the surface materials. Examination of laboratory spectra of terrestrial extremophiles measured at cryogenic temperatures reveals distorted, asymmetric nearinfrared absorption features due to water of hydration. The band centers, widths, and shapes of these features closely match those observed in the Europa spectra. These features are strongest in reddish-brown, disrupted terrains such as linea and chaos regions. Narrow spectral features due to amide bonds in the microbe proteins provide a means of constraining the abundances of such materials using the NIMS data. The NIMS data of disrupted terrains exhibit distorted, asymmetric near-infrared absorption features consistent with the presence of water ice, sulfuric acid octahydrate, hydrated salts, and possibly as much as 0.2 mg cm(-3) of carbonaceous material that could be of biological origin. However, inherent noise in the observations and limitations of spectral sampling must be taken into account when discussing these findings.

Probing Europa's interior with natural sound sources

Europa's interior structure may be determined by relatively simple and robust seismo-acoustic echo sounding techniques. The strategy is to use ice cracking events or impacts that are hypothesized to occur regularly on Europa's surface as sources of opportunity. A single passive geophone on Europa's surface may then be used to estimate the thickness of its ice shell and the depth of its ocean by measuring the travel time of seismo-acoustic reflections from the corresponding internal strata. Quantitative analysis is presented with full-field seismo-acoustic modeling of the Europan environment. This includes models for Europan ambient noise and conditions on signal-to-noise ratio necessary for the proposed technique to be feasible. The possibility of determining Europa's ice layer thickness by surface wave and modal analysis with a single geophone is also investigated.

Europa's differentiated internal structure: inferences from four Galileo encounters.

Radio Doppler data from four encounters of the Galileo spacecraft with the jovian moon Europa have been used to refine models of Europa's interior. Europa is most likely differentiated into a metallic core surrounded by a rock mantle and a water ice-liquid outer shell, but the data cannot eliminate the possibility of a uniform mixture of dense silicate and metal beneath the water ice-liquid shell. The size of a metallic core is uncertain because of its unknown composition, but it could be as large as about 50 percent of Europa's radius. The thickness of Europa's outer shell of water ice-liquid must lie in the range of about 80 to 170 kilometers.

Evidence for non-synchronous rotation of Europa. Galileo Imaging Team.

Non-synchronous rotation of Europa was predicted on theoretical grounds, by considering the orbitally averaged torque exerted by Jupiter on the satellite's tidal bulges. If Europa's orbit were circular, or the satellite were comprised of a frictionless fluid without tidal dissipation, this torque would average to zero. However, Europa has a small forced eccentricity e approximately 0.01 , generated by its dynamical interaction with Io and Ganymede, which should cause the equilibrium spin rate of the satellite to be slightly faster than synchronous. Recent gravity data suggest that there may be a permanent asymmetry in Europa's interior mass distribution which is large enough to offset the tidal torque; hence, if non-synchronous rotation is observed, the surface is probably decoupled from the interior by a subsurface layer of liquid or ductile ice. Non-synchronous rotation was invoked to explain Europa's global system of lineaments and an equatorial region of rifting seen in Voyager images. Here we report an analysis of the orientation and distribution of these surface features, based on initial observations made by the Galileo spacecraft. We find evidence that Europa spins faster than the synchronous rate (or did so in the past), consistent with the possibility of a global subsurface ocean.

Visions of Europa

T his 1982 text is science fiction, but the premise may contain a nugget of truth. It's possible-some researchers now say even plausible-that Jupiter's moon Europa has the two ingredients deemed necessary to sustain primitive life: water and volcanic activity. As researchers continue to analyze evidence gathered by the Voyager spacecraft in the late 1970s, they find more and more hints that Europa may harbor an ocean beneath its icy crust. Moreover, a gravitational tug-of-war with Jupiter and two of the giant planet's other large moons, Ganymede and lo, may warm Europa's interior enough to maintain a liquid ocean. The heat may also trigger volcanism. Although the bone-dry channels that crisscross Mars suggest that water once flowed across the Red Planet, if we're looking for liquid water and the possibility of life that exists today, the chances of finding them are just as good, or even more so, on says Torrence V. Johnson of NASA's Jet Propulsion Laboratory in Pasadena, Calif. As project scientist for the Galileo spacecraft, which began its 2-year tour of Jupiter and its moons last December, Johnson has a special interest in Europa. On June 27, a few hours after a flyby of Ganymede, the craft passed close enough to Europa to photograph its surface at a resolution higher than that possible with the Voyager craft. Hampered by a broken main antenna, Galileo transmits data extremely slowly. It won't send a Europa image for several more weeks. Europa aficionados will have to wait even longer for Galileo to take its sharpest images. Subsequent passes late this year and early next are expected to bring the craft within 600 kilometers of Europa, revealing surface features that may be as small as 10 meters across. Although Galileo wasn't designed to search for an ocean and lacks instruments that could peer beneath Europa's icy skin, planetary scientists plan to scrutinize the pictures and other data for indirect signs of a watery reservoir lurking beneath the surface. For example, scientists want to compare the Galileo images with those taken by the Voyager

Crater relaxation as a probe of Europa's interior

Viscous relaxation rates of craters are examined to discriminate among models of Europa's interior. It is shown that the presence of an insulating surface regolith is required for ice lithospheres that do not contain a liquid H2O layer. Without the presence of such an insulating regolith, relaxation times for craters ≈100 km in diameter are excessively long (>1 b.y.) and thus inconsistent with the absence of such features in Voyager images. Finite element calculations indicate that a thick insulating surface frost layer (raising surface temperatures ≳ 40 K above the solar radiation ambient temperature of 92 K) allows a 25‐km‐thick surface viscous layer to relax ≈ 100‐km‐diameter craters on the short timescales (≃100 m.y.) required. If the icy lithosphere is 100 km thick (and without a liquid H2O layer, but containing a subsolidus convecting region), surface temperatures need to be raised 20 K by insulation to allow relaxation of craters in this size range to occur within the required time.

Europan surface phenomena

The interaction of corotating iogenic plasma with the surface of Europa in light of recent ice sputtering, experimental results, and published Voyager data has been examined. It has been found that the residual atmosphere of Europa is made up of sputtered molecular oxygen and is exospheric from the surface outwards. It was also found that if sputtering, redistribution, and escape are considered and the sulfur dioxide/water mixing ratio is held constant over a UV observing depth, the observed sulfur dioxide density on the trailing hemisphere lends support to the hypothesis that liquid water from the interior of Europa is boiling out and being deposited as a frost layer on the surface at the rate of about 0.04 μm/year.