Search

Europa is an unusual world, and its surface features provide clues as to how it has evolved. We mapped, characterized, and analyzed small (≤100 km2 in area) endogenic features on Europa's surface in order to identify patterns among and within different types of features and provide observational tests for formation models. To create a fully digitized and validated data set of these microfeatures, we merged data sets obtained by previous mapping studies that focused on small chaos, domes, pits, spots, and hybrid features in moderate resolution (~230 m/pix) images of Europa, including the four main regional map areas. We expanded upon the definition of a recently-identified feature definition type, here called hybrids, and suggest more quantitative metrics for describing pits, spots, domes, and microchaos. Across all four regions mapped, we find that microchaos are the most numerous, followed by pits and domes, respectively. Spots are the least common features, and the smallest, which might indicate an observational bias, as they may contain disruptions smaller than what is visible at this scale. Microchaos features are, on average, larger and darker than all other microfeature types. We compare our observations to the findings and predictions of previous numerical modeling studies, and suggest that the observations are most consistent with cryovolcanism and liquid water sill models. While no one model is able to explain all the observations, there are elements of each that should be considered in future modeling studies.

The orbits of Saturn’s inner mid-sized moons (Mimas, Enceladus, Tethys, Dione and Rhea) have been notably difficult to reconcile with their geology. Here we present numerical simulations coupling thermal, geophysical and simplified orbital evolution for 4.5 billion years that reproduce the observed characteristics of their orbits and interiors, provided that the outer four moons are old. Tidal dissipation within Saturn expands the moons’ orbits over time. Dissipation within the moons decreases their eccentricities, which are episodically increased by moon−moon interactions, causing past or present oceans to exist in the interiors of Enceladus, Dione and Tethys. In contrast, Mimas’s proximity to Saturn’s rings generates interactions that cause such rapid orbital expansion that Mimas must have formed only 0.1−1 billion years ago if it postdates the rings. The resulting lack of radionuclides keeps it geologically inactive. These simulations explain the Mimas−Enceladus dichotomy, reconcile the moons’ orbital properties and geological diversity, and self-consistently produce a recent ocean on Enceladus. A coupled thermal, geophysical and dynamical simulation covering 4.5 Gyr of evolution of Saturn’s inner mid-sized moons shows that, with the possible exception of Mimas, they formed early in Saturn’s history. A complex game of resonances has impacted the four older moons, shaping their geology and interior.

This white paper advocates for the inclusion of small, captured Outer Solar system objects, found in the Ice Giant region in the next Decadal Survey. These objects include the Trojans and Irregular satellite populations of Uranus and Neptune. The captured small bodies provide vital clues as to the formation of our Solar system. They have unique dynamical situations, which any model of Solar system formation needs to explain. The major issue is that so few of these objects have been discovered, with very little information known about them. The purpose of this document is to prioritize further discovery and characterization of these objects. This will require the use of NASA and NSF facilities over the 2023 2032 decade, including additional support for analysis. This is in preparation for potential future insitu missions in the following decades., Comment: Community Science White Paper for the Planetary and Astrobiology Decadal Survey, 2023-2032

Tidal interactions between planets or stars and the bodies that orbit them dissipate energy in their interiors. The dissipated energy heats the interior and a fraction of that energy will be released as seismic energy. Here we formalize a model to describe the tidally-driven seismic activity on planetary bodies based on tidal dissipation. To constrain the parameters of our model we use the seismic activity of the Moon, driven by tidal dissipation from the Earth-Moon interactions. We then apply this model to predict the amount of seismic energy release and largest seismic events on other moons in our Solar System and exoplanetary bodies. We find that many moons in the Solar System should be more seismically active than the Earth's Moon and many exoplanets should exhibit more seismic activity than the Earth. Finally, we examine how temporal-spatial variations in tidal dissipation manifest as variations in the locations and timing of seismic events on these bodies.

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.

Europa is a compelling target because of its unique geology and potential for habitability. Efforts to map the small-scale global geology of the moon have been hampered by the availability of suitable imaging (≤ 230 m/pix), which covers only 15% of the surface. New data on the abundance of microfeatures (features ≤100 km2 in area) on a global scale and their locations would help put better constraints on the geophysics of the icy world. We mapped microfeatures in low-resolution images of the E15RegMap01 region of Europa and compared our low-resolution dataset to a validated dataset collected using regional mosaic imaging. We evaluated how many features were missed in low-resolution mapping, the feature types to which they belonged, how often misclassifications were made, and how many false-positive features there were in the low-resolution dataset. We found that microchaos, hybrids, and spots were found at relatively high completeness rates when compared to the regional mosaic dataset, but domes and especially pits were almost always missed. Microchaos, hybrids, and spots also had the highest rates of consistent classification between the low-resolution and regional mosaic datasets. Finally, 36% of our total potential feature count from the low-resolution dataset were revealed to be false positives, mostly caused by ridges and shadows that were mistaken for features in low-resolution imaging. These findings quantitatively estimate the likely errors in existing and future global geologic maps that note the presence of microfeatures, offer remedies to minimize these errors, and provide guidance for future Europa mappers.

We investigate whether a present-day global ocean within Mimas is compatible with the lack of tectonic activity on its surface by computing tidal stresses for ocean-bearing interior structure models derived from observed librations. We find that, for the suite of compatible rheological models, peak surface tidal stresses caused by Mimas’ high eccentricity would range from a factor of two smaller to an order of magnitude larger than those on tidally-active Europa. Thermal stresses from a freezing ocean, or a past higher eccentricity, would enhance present-day tidal stresses, exceeding the magnitudes associated with Europa's ubiquitous tidally-driven fractures and, in some cases, the failure strength of ice in laboratory studies. Therefore, in order for Mimas to have an ocean, its ice shell cannot fail at the stress values implied for Europa. Furthermore, if Mimas’ ocean is freezing out, the ice shell must also be able to withstand thermal stresses that could be an order of magnitude higher than the failure strength of laboratory ice samples. In light of these challenges, we consider an ocean-free Mimas to be the most straightforward model, best supported by our tidal stress analysis.

In this article, we summarize the work of the NASA Outer Planets Assessment Group (OPAG) Roadmaps to Ocean Worlds (ROW) group. The aim of this group is to assemble the scientific framework that will guide the exploration of ocean worlds, and to identify and prioritize science objectives for ocean worlds over the next several decades. The overarching goal of an Ocean Worlds exploration program as defined by ROW is to “identify ocean worlds, characterize their oceans, evaluate their habitability, search for life, and ultimately understand any life we find.” The ROW team supports the creation of an exploration program that studies the full spectrum of ocean worlds, that is, not just the exploration of known ocean worlds such as Europa but candidate ocean worlds such as Triton as well. The ROW team finds that the confirmed ocean worlds Enceladus, Titan, and Europa are the highest priority bodies to target in the near term to address ROW goals. Triton is the highest priority candidate ocean world to target in the near term. A major finding of this study is that, to map out a coherent Ocean Worlds Program, significant input is required from studies here on Earth; rigorous Research and Analysis studies are called for to enable some future ocean worlds missions to be thoughtfully planned and undertaken. A second finding is that progress needs to be made in the area of collaborations between Earth ocean scientists and extraterrestrial ocean scientists.

Phobos, the innermost satellite of Mars, displays an extensive system of grooves that are mostly symmetric about its sub-Mars point. Phobos is steadily spiraling inward due to the tides it raises on Mars lagging behind Phobos' orbital position and will suffer tidal disruption before colliding with Mars in a few tens of millions of years. We calculate the surface stress field of the deorbiting satellite and show that the first signs of tidal disruption are already present on its surface. Most of Phobos' prominent grooves have an excellent correlation with computed stress orientations. The model requires a weak interior that has very low rigidity on the tidal evolution time scale, overlain by an approximately 10-100 m exterior shell that has elastic properties similar to lunar regolith as described by Horvath et al. (1980).

Enceladus has a young, tectonically active south polar region, which is erupting material from a prominent set of fractures called Tiger Stripes. No comparable activity is observed at the north pole, which is heavily cratered with limited tectonism. Given the many lines of evidence supporting a global ocean under Enceladus' icy shell, the reason for the dichotomy in geologic activity is unclear. We model the formation of the Tiger Stripes as tidally-driven fractures and examine the magnitudes of tidal stresses with different ice shell structures in order to explore whether and how tidal stress might explain Enceladus' distribution of tectonic activity. We find that eccentricity-driven tidal stresses would produce fractures of nearly identical orientations to the observed Tiger Stripe Fractures and that a 10-km difference in ice shell thickness between the north and south poles can result in substantially different tidal stress magnitudes, providing a natural explanation for the hemispheric dichotomy in tectonic activity on Enceladus. Finally, we synthesize these results with Enceladus' global geologic record to offer insight into the evolution of this enigmatic moon.

International audience; Ice-covered ocean worlds possess diverse energy sources and associated mechanisms that are capable of driving significant seismic activity, but to date no measurements of their seismic activity have been obtained. Such investigations could reveal the transport properties and radial structures, with possibilities for locating and characterizing trapped liquids that may host life and yielding critical constraints on redox fluxes and thus on habitability. Modeling efforts have examined seismic sources from tectonic fracturing and impacts. Here, we describe other possible seismic sources, their associations with science questions constraining habitability, and the feasibility of implementing such investigations. We argue, by analogy with the Moon, that detectable seismic activity should occur frequently on tidally flexed ocean worlds. Their ices fracture more easily than rocks and dissipate more tidal energy than the

Observations of the south pole of the Saturnian moon Enceladus revealed large rifts in the terrain that were found to be the sources of the observed jets of water vapour; now it is shown that much of the eruptive activity can be explained by broad, curtain-like eruptions, many of which were probably misinterpreted previously as discrete jets. Images taken by the Cassini probe have revealed large fractures bounded by rifts towards the south pole of Saturn's moon Enceladus. These features, popularly known as 'tiger stripes', reach higher temperatures than their surroundings and are thought to be the sources of observed jets of water vapour and icy particles. Joseph Spitale et al. compare Cassini images with simulated curtains of material erupting from Enceladus' south-polar terrain to produce detailed maps of the emissions at various times. Much of the eruptive activity can be explained by broad, curtain-like eruptions, many of which were probably misinterpreted previously as discrete jets. Phantom jets in the synthesized curtains correspond closely to regions of enhanced brightness in the Cassini images. Observations of the south pole of the Saturnian moon Enceladus revealed large rifts in the south-polar terrain, informally called ‘tiger stripes’, named Alexandria, Baghdad, Cairo and Damascus Sulci. These fractures have been shown to be the sources of the observed jets of water vapour and icy particles1,2,3,4 and to exhibit higher temperatures than the surrounding terrain5,6. Subsequent observations have focused on obtaining close-up imaging of this region to better characterize these emissions. Recent work7 examined those newer data sets and used triangulation of discrete jets3 to produce maps of jetting activity at various times. Here we show that much of the eruptive activity can be explained by broad, curtain-like eruptions. Optical illusions in the curtain eruptions resulting from a combination of viewing direction and local fracture geometry produce image features that were probably misinterpreted previously as discrete jets. We present maps of the total emission along the fractures, rather than just the jet-like component, for five times during an approximately one-year period in 2009 and 2010. An accurate picture of the style, timing and spatial distribution of the south-polar eruptions is crucial to evaluating theories for the mechanism controlling the eruptions.

Pluto and its largest satellite, Charon, currently orbit in a mutually synchronous state; both bodies continuously show the same face to one another. This orbital configuration is a natural end-state for bodies that have undergone tidal dissipation. In order to achieve this state, both bodies would have experienced tidal heating and stress, with the extent of tidal activity controlled by the orbital evolution of Pluto and Charon and by the interior structure and rheology of each body. As the secondary, Charon would have experienced a larger tidal response than Pluto, which may have manifested as observable tectonism. Unfortunately, there are few constraints on the interiors of Pluto and Charon. In addition, the pathway by which Charon came to occupy its present orbital state is uncertain. If Charon's orbit experienced a high-eccentricity phase, as suggested by some orbital evolution models, tidal effects would have likely been more significant. Therefore, we determine the conditions under which Charon could have experienced tidally-driven geologic activity and the extent to which upcoming New Horizons spacecraft observations could be used to constrain Charon's internal structure and orbital evolution. Using plausible interior structure models that include an ocean layer, we find that tidally-driven tensile fractures would likely have formed on Charon if its eccentricity were on the order of 0.01, especially if Charon were orbiting closer to Pluto than at present. Such fractures could display a variety of azimuths near the equator and near the poles, with the range of azimuths in a given region dependent on longitude; east-west-trending fractures should dominate at mid-latitudes. The fracture patterns we predict indicate that Charon's surface geology could provide constraints on the thickness and viscosity of Charon's ice shell at the time of fracture formation.

Lineaments are thought to form as tensile cracks due to tidal stress, which is driven mainly by Europa’s eccentric orbit. However, this model would not produce the wide range of lineament azimuths observed on Europa unless the stress in a given region, or the conditions for fault failure, change over time. In this work, we test the ability of two mechanisms that would alter the stress field over time to account for the observed lineament azimuths: non-synchronous rotation and spin pole precession. First, we revisit previous analysis of lineaments and find that an underlying assumption used to predict their azimuths was inconsistently applied. After revising these predictions, we incorporate the effects of a non-zero obliquity, which has been shown to influence the formation of other tidal-tectonic features. We then expand our analysis to include the effects of the time-variable phenomena, spin pole precession and non-synchronous rotation of the ice shell. We also consider additional failure assumptions to those used in previous work on lineament azimuths. We test our models against the azimuths of observed lineaments in the Bright Plains region of Europa. Without obliquity, we find that non-synchronous rotation is insufficient to explain the wide range of azimuths observed in this region. In the presence of obliquity, we find that either spin pole precession or non-synchronous rotation could produce wide variations in lineament azimuths. However, neither model can independently account for the observed distribution of azimuths in the Bright Plains region. In fact, a model in which all of the lineaments are assumed to form at random orientations outperforms the non-synchronous rotation model in our statistical tests. The model with the highest likelihood of producing the observations is one in which 45% of the lineaments formed as predicted in the precession model, 55% formed at random orientations, and older lineaments are less represented in the tectonic record. Given the relative timescales expected for spin pole precession and non-synchronous rotation, it makes sense that the signal of precession is more apparent in the tectonic record. These results are in good agreement with assessments of strike-slip faults; modeling of both types of features indicates a large obliquity, the same spin pole direction during the most recent epoch of tectonic activity, a similar percentage of features that are not well-explained by the tidal model, and little evidence of non-synchronous rotation.

We introduce a new mechanical model for producing tidally-driven strike-slip displacement along preexisting faults on Europa, which we call shell tectonics. This model differs from previous models of strike-slip on icy satellites by incorporating a Coulomb failure criterion, approximating a viscoelastic rheology, determining the slip direction based on the gradient of the tidal shear stress rather than its sign, and quantitatively determining the net offset over many orbits. This model allows us to predict the direction of net displacement along faults and determine relative accumulation rate of displacement. To test the shell tectonics model, we generate global predictions of slip direction and compare them with the observed global pattern of strike-slip displacement on Europa in which left-lateral faults dominate far north of the equator, right-lateral faults dominate in the far south, and near-equatorial regions display a mixture of both types of faults. The shell tectonics model reproduces this global pattern. Incorporating a small obliquity into calculations of tidal stresses, which are used as inputs to the shell tectonics model, can also explain regional differences in strike-slip fault populations. We also discuss implications for fault azimuths, fault depth, and Europa’s tectonic history.

Variations in diurnal tidal stress due to Europa's eccentric orbit have been considered as the driver of strike-slip motion along pre-existing faults, but obliquity and physical libration have not been taken into account. The first objective of this work is to examine the effects of obliquity on the predicted global pattern of fault slip directions based on a tidal-tectonic formation model. Our second objective is to test the hypothesis that incorporating obliquity can reconcile theory and observations without requiring polar wander, which was previously invoked to explain the mismatch found between the slip directions of 192 faults on Europa and the global pattern predicted using the eccentricity-only model. We compute predictions for individual, observed faults at their current latitude, longitude, and azimuth with four different tidal models: eccentricity only, eccentricity plus obliquity, eccentricity plus physical libration, and a combination of all three effects. We then determine whether longitude migration, presumably due to non-synchronous rotation, is indicated in observed faults by repeating the comparisons with and without obliquity, this time also allowing longitude translation. We find that a tidal model including an obliquity of 1.2?, along with longitude migration, can predict the slip directions of all observed features in the survey. However, all but four faults can be fit with only 1? of obliquity so the value we find may represent the maximum departure from a lower time-averaged obliquity value. Adding physical libration to the obliquity model improves the accuracy of predictions at the current locations of the faults, but fails to predict the slip directions of six faults and requires additional degrees of freedom. The obliquity model with longitude migration is therefore our preferred model. Although the polar wander interpretation cannot be ruled out from these results alone, the obliquity model accounts for all observations with a value consistent with theoretical expectations and cycloid modeling.

Cycloids, arcuate features observed on Europa’s surface, have been interpreted as tensile cracks that form in response to diurnal tidal stress caused by Europa’s orbital eccentricity. Stress from non-synchronous rotation may also contribute to tidal stress, and its influence on cycloid shapes has been investigated as well. Obliquity, fast precession, and physical libration would contribute to tidal stress but have often been neglected because they were expected to be negligibly small. However, more sophisticated analyses that include the influence of Jupiter’s other large satellites and the state of Europa’s interior indicate that perhaps these rotational parameters are large enough to alter the tidal stress field and the formation of tidally-driven fractures. We test tidal models that include obliquity, fast precession, stress due to non-synchronous rotation, and physical libration by comparing how well each model reproduces observed cycloids. To do this, we have designed and implemented an automated parameter-searching algorithm that relies on a quantitative measure of fit quality, which we use to identify the best fits to observed cycloids. We then apply statistical techniques to determine the tidal model best supported by the data. By incorporating obliquity, fits to observed southern hemisphere cycloids improve, and we can reproduce equatorial and equator-crossing cycloids. Furthermore, we find that obliquity plus physical libration is the tidal model best supported by the data. With this model, the obliquities range from 0.32° to 1.35°. The libration amplitudes are 0.72–2.44°, and the libration phases are −6.04° to 17.72° with one outlier at 84.5°. The variability in obliquity is expected if Europa’s ice shell is mechanically decoupled from the interior, and the libration amplitudes are plausible in the presence of a subsurface ocean. Indeed, the presence of a decoupling ocean may result in feedbacks that cause all of these rotational parameters to become time-variable.

Much of the geologic activity preserved on Europa's icy surface has been attributed to tidal deformation, mainly due to Europa's eccentric orbit. Although the surface is geologically young (30 - 80 Myr), there is little information as to whether tidally-driven surface processes are ongoing. However, a recent detection of water vapor near Europa's south pole suggests that it may be geologically active. Initial observations indicated that Europa's plume eruptions are time-variable and may be linked to its tidal cycle. Saturn's moon, Enceladus, which shares many similar traits with Europa, displays tidally-modulated plume eruptions, which bolstered this interpretation. However, additional observations of Europa at the same time in its orbit failed to yield a plume detection, casting doubt on the tidal control hypothesis. The purpose of this study is to analyze the timing of plume eruptions within the context of Europa's tidal cycle to determine whether such a link exists and examine the inferred similarities and differences between plume activity on Europa and Enceladus., Comment: Accepted for publication in Icarus