South Polar Terrain Research Articles

A lander mission to probe subglacial water on Saturn׳s moon Enceladus for life

The plumes discovered by the Cassini mission emanating from the south pole of Saturn׳s moon Enceladus and the unique chemistry found in them have fueled speculations that Enceladus may harbor life. The presumed aquiferous fractures from which the plumes emanate would make a prime target in the search for extraterrestrial life and would be more easily accessible than the moon׳s subglacial ocean.A lander mission that is equipped with a subsurface maneuverable ice melting probe will be most suitable to assess the existence of life on Enceladus. A lander would have to land at a safe distance away from a plume source and melt its way to the inner wall of the fracture to analyze the plume subsurface liquids before potential biosignatures are degraded or destroyed by exposure to the vacuum of space. A possible approach for the in situ detection of biosignatures in such samples can be based on the hypothesis of universal evolutionary convergence, meaning that the independent and repeated emergence of life and certain adaptive traits is wide-spread throughout the cosmos. We thus present a hypothetical evolutionary trajectory leading towards the emergence of methanogenic chemoautotrophic microorganisms as the baseline for putative biological complexity on Enceladus. To detect their presence, several instruments are proposed that may be taken aboard a future subglacial melting probe.The “Enceladus Explorer” (EnEx) project funded by the German Space Administration (DLR), aims to develop a terrestrial navigation system for a subglacial research probe and eventually test it under realistic conditions in Antarctica using the EnEx-IceMole, a novel maneuverable subsurface ice melting probe for clean sampling and in situ analysis of ice and subglacial liquids. As part of the EnEx project, an initial concept study is foreseen for a lander mission to Enceladus to deploy the IceMole near one of the active water plumes on the moon׳s South-Polar Terrain, where it will search for signatures of life.The general mission concept is to place the Lander at a safe distance from an active plume. The IceMole would then be deployed to melt its way through the ice crust to an aquiferous fracture at a depth of 100m or more for an in situ examination for the presence of microorganisms.The driving requirement for the mission is the high energy demand by the IceMole to melt through the cold Enceladan ices. This requirement is met by a nuclear reactor providing 5kW of electrical power. The nuclear reactor and the IceMole are placed on a pallet lander platform. An Orbiter element is also foreseen, with the main function of acting as a communications relay between Lander and Earth.After launch, the Lander and Orbiter will perform the interplanetary transfer to Saturn together, using the on-board nuclear reactor to power electric thrusters. After Saturn orbit insertion, the Combined Spacecraft will continue using Nuclear Electric Propulsion to reach the orbit of Enceladus. After orbit insertion at Enceladus, the Orbiter will perform a detailed reconnaissance of the South-Polar Terrain. At the end of the reconnaissance phase, the Lander will separate from the Orbiter and an autonomously guided landing sequence will place it near one of the active vapor plumes. Once landed, the IceMole will be deployed and start melting through the ice, while navigating around hazards and towards a target subglacial aquiferous fracture.An initial estimation of the mission׳s cost is given, as well as recommendations on the further development of enabling technologies. The planetary protection challenges posed by such a mission are also addressed.

HOW THE GEYSERS, TIDAL STRESSES, AND THERMAL EMISSION ACROSS THE SOUTH POLAR TERRAIN OF ENCELADUS ARE RELATED

We present the first comprehensive examination of the geysering, tidal stresses, and anomalous thermal emission across the south pole of Enceladus and discuss the implications for the moon's thermal history and interior structure. A 6.5 yr survey of the moon's south polar terrain (SPT) by the Cassini imaging experiment has located ~100 jets or geysers erupting from four prominent fractures crossing the region. Comparing these results with predictions of diurnally varying tidal stresses and with Cassini low resolution thermal maps shows that all three phenomena are spatially correlated. The coincidence of individual jets with very small (~10 m) hot spots detected in high resolution Cassini VIMS data strongly suggests that the heat accompanying the geysers is not produced by shearing in the upper brittle layer but rather is transported, in the form of latent heat, from a sub-ice-shell sea of liquid water, with vapor condensing on the near-surface walls of the fractures. Normal stresses modulate the geysering activity, as shown in the accompanying paper; we demonstrate here they are capable of opening water-filled cracks all the way down to the sea. If Enceladus' eccentricity and heat production are in steady state today, the currently erupting material and anomalous heat must have been produced in an earlier epoch. If regional tidal heating is occurring today, it may be responsible for some of the erupting water and heat. Future Cassini observations may settle the question.

The effect of an asymmetric core on convection in Enceladus' ice shell: Implications for south polar tectonics and heat flux

Enceladus exhibits a strong tectonic contrast between its south polar terrain (SPT), which is young and geologically active, and its northern hemisphere, which is relatively ancient and stable. Previous global three‒dimensional (3‒D) spherical models of convection exhibit patterns that are symmetrical around the equator and fail to explain the formation of a hemispheric dichotomy. Here we present global 3‒D spherical models of convection in Enceladus' ice shell to show that convection in Enceladus' ice shell with plasticity and irregular core geometry can self‒consistently generate a hemispheric dichotomy in tectonics and heat flux. With a spherical core, convection produces global overturning, which cannot explain the regional confinement of Enceladus' current tectonic activity to the SPT. Models with appropriate nonspherical core geometry and plasticity tend to produce overturning confined to the SPT or regional overturning in different regions at different times, which can explain the tectonic dichotomy and local age differences on Enceladus. Our models predict heat flows up to 5–10 gigawattts (GW) during active episodes, consistent with Cassini observations.

New images support proposal for water ice on Mercury

Two decades ago, radio telescope observations showed radar‐bright features in Mercury's polar regions. The radar characteristics are similar to those of the icy satellites of Jupiter and at the south polar ice cap on Mars. The radarbright spots on Mercury were therefore believed to be water ice, and images from MESSENGER provide new support for this idea. MESSENGER, the first spacecraft to orbit Mercury, has provided a view of Mercury's south polar terrain over a full Mercury day that makes it possible to identify areas in permanent shadow. Chabot et al. compared the MESSENGER images of craters in the south pole region with the distribution of radarbright features from previous radio telescope studies.

On the application of simple rift basin models to the south polar region of Enceladus

The south polar terrain (SPT) of Saturn's moon Enceladus is a mysteriously active region that exhibits intriguing tectonic signatures and widespread fracturing. The central region of the nearly‐circular SPT is depressed into the surface by a few hundred meters and bounded by a ring of cliffs roughly 1 km high. In this study, we investigate whether this depression and surrounding mountainous uplift is consistent with the morphology of terrestrial rift basins and the possibility that the SPT could have formed during a tectonic event analogous to those of such rift basins on Earth. Using three mechanical models of basin formation, we compare our predicted topography of the SPT with observed topography of the region. The first of three models we consider assumes crustal stretching by factor β, and predicts a basin depth of roughly 600 m, closely matching previously published estimates of the depth at the SPT. Models of extension and compression, assuming an elastic response in the ice crust, predict best‐fit mountain uplift of roughly 1820 m and 1130 m, respectively. Our preferred model suggests that the icy shell in the SPT has been stretched, but the extension is (partially) balanced by compression along the edges of the basin leading to the uplift of the mountains along the boundary, thereby implying that the SPT may have a tectonic origin analogous to that of a terrestrial basin.

The impact of a weak south pole on thermal convection in Enceladus’ ice shell

Enceladus exhibits a strong hemispheric dichotomy of tectonism and heat flux, with geologically young, heavily tectonized terrains and a high heat flux in the South Polar Terrain (SPT) and relatively ancient terrains with presumably lower heat fluxes over the rest of the satellite. To understand the convective pattern and its relationship with surface tectonics, we present three-dimensional numerical models of convection in Enceladus’ ice shell including basal heating and tidal heating. Our thermal boundary conditions exhibit no north–south asymmetries, but because the tectonism at the SPT may weaken the ice there, we impose a mechanically weak lithosphere within the SPT. The weakening is parameterized by adopting a reduced viscosity contrast within the SPT. Without such a weak zone, convection (if any) resides in stagnant-lid mode and exhibits no hemispheric dichotomy. In the presence of such an SPT weak zone, however, we find vigorous convection in the ice underneath the SPT, with convective plumes rising close to the surface. In contrast, only stagnant lid convection, or no convection at all, occurs elsewhere over the satellite. Away from the SPT, the heat flux in our models is small (5–10mWm−2) and the surface strains are small enough to imply surface ages >109years. Within the SPT, however, our models yield peak heat fluxes of ∼70–200mWm−2, implying heat flows integrated across the SPT of up to 5GW, similar to that inferred from Cassini thermal observations. The surface strains in our models are high enough near the south pole to cause intense tectonism and imply surface ages of ∼106–107years, consistent with age estimates of the SPT.

A fracture history on Enceladus provides evidence for a global ocean

[1] The region surrounding the south pole of Saturn's moon Enceladus shows a young, pervasively fractured surface that emanates enough heat to be detected by the Cassini spacecraft. To explain the elevated heat and eruptive icy plumes originating from large cracks (informally called “tiger stripes”) in the surface, many models implicitly assume a global liquid ocean beneath the surface. Here we show that the fracture patterns in the south-polar terrain (SPT) of Enceladus are inconsistent with contemporary stress fields, but instead formed in a temporally varying global stress field related to nonsynchronous rotation of a floating ice shell above a global liquid ocean. This finding increase to at least three the number of outer planet satellites likely to possess a subsurface liquid water layer.

Joule heating of the south polar terrain on Enceladus

[1] We report that Joule heating in Enceladus, resulting from the interaction of Enceladus with Saturn's magnetic field, may account for 150 kW to 52 MW of power through Enceladus. Electric currents passing through subsurface channels of low salinity and just a few kilometers in depth could supply a source of power to the south polar terrain, providing a small but previously unaccounted for contribution to the observed heat flux and plume activity. Studies of the electrical heating of Jupiter's moon Europa have concluded that electricity is a negligible heating source since no connection between the conductive subsurface and Alfven currents has been observed. Here we show that, contrary to results for the Jupiter system, electrical heating may be a source of internal energy for Enceladus, contributing to localized heating, production of water vapor, and the persistence of the “tiger stripes.” This contribution is of order 0.001–0.25% of the total observed heat flux, and thus, Joule heating cannot explain the total south polar terrain heat anomaly. The exclusion of salt ions during refreezing serves to enhance volumetric Joule heating and could extend the lifetime of liquid water fractures in the south polar terrain.

High heat flow from Enceladus' south polar region measured using 10–600 cm−1Cassini/CIRS data

Analysis of 2008 Cassini Composite Infrared Spectrometer (CIRS) 10 to 600/cm thermal emission spectra of Enceladus shows that for reasonable assumptions about the spatial distribution of the emission and the thermophysical properties of the solar-heated background surface, which are supported by CIRS observations of background temperatures at the edge of the active region, the endogenic power of Enceladus' south polar terrain is 15.8 +/- 3.1 GW. This is significantly higher than the previous estimate of 5.8 +/- 1.9 GW. The new value represents an improvement over the previous one, which was derived from higher wave number data (600 to 1100/cm-I) and was thus only sensitive to high-temperature emission. The mechanism capable of producing such a high endogenic power remains a mystery and challenges the current models of proposed heat production.

On the origin of south polar folds on Enceladus

Recent high-resolution Cassini images of the south polar terrain of Enceladus reveal regions of short-wavelength deformation, inferred to be compressional folds between the Baghdad and Damascus tiger stripes (Spencer, J.R., Barr, A.C., Esposito, L.W., Helfenstein, P., Ingersoll, A.P., Jaumann, R., McKay, C.P., Nimmo, F., Waite, J.H. [2009a]. Enceladus: An active cryovolcanic satellite. In: Saturn after Cassini–Huygens. Springer, New York, pp. 683–722). Here, we use Fourier analysis of the bright/dark variations to show that the folds have a dominant wavelength of 1.1 ± 0.4 km. We use the simple model of lava flow folding from Fink (Fink, J. [1980]. Geology 8, 250–254) to show that the folds could form in an ice shell with an upper high-viscosity boundary layer of thickness <400 m, with a driving stress of 40–80 kPa, and strain rate between 10 −14 s −1 and 10 −12 s −1. Such deformation rates imply resurfacing of the SPT in 0.05–5 Myr, consistent with its estimated surface age. Measurements of fold topography and more sophisticated numerical modeling can narrow down the conditions of fold formation and provide valuable constraints on the thermal structure of the ice shell on Enceladus.

The Possible Origin and Persistence of Life on Enceladus and Detection of Biomarkers in the Plume

The jets of icy particles and water vapor issuing from the south pole of Enceladus are evidence for activity driven by some geophysical energy source. The vapor has also been shown to contain simple organic compounds, and the south polar terrain is bathed in excess heat coming from below. The source of the ice and vapor, and the mechanisms that accelerate the material into space, remain obscure. However, it is possible that a liquid water environment exists beneath the south polar cap, which may be conducive to life. Several theories for the origin of life on Earth would apply to Enceladus. These are (1) origin in an organic-rich mixture, (2) origin in the redox gradient of a submarine vent, and (3) panspermia. There are three microbial ecosystems on Earth that do not rely on sunlight, oxygen, or organics produced at the surface and, thus, provide analogues for possible ecologies on Enceladus. Two of these ecosystems are found deep in volcanic rock, and the primary productivity is based on the consumption by methanogens of hydrogen produced by rock reactions with water. The third ecosystem is found deep below the surface in South Africa and is based on sulfur-reducing bacteria consuming hydrogen and sulfate, both of which are ultimately produced by radioactive decay. Methane has been detected in the plume of Enceladus and may be biological in origin. An indicator of biological origin may be the ratio of non-methane hydrocarbons to methane, which is very low (0.001) for biological sources but is higher (0.1-0.01) for nonbiological sources. Thus, Cassini's instruments may detect plausible evidence for life by analysis of hydrocarbons in the plume during close encounters.

Mobile lid convection beneath Enceladus' south polar terrain

Enceladus' south polar region has a large heat flux, 55–110 mW m−2, that is spatially associated with cryovolcanic and tectonic activity. Tidal dissipation and vigorous convection in the underlying ice shell are possible sources of heat, however, prior predictions of the heat flux carried by stagnant lid convection range from Fconv ∼ 15 to 30 mW m−2, too low to explain the observed heat flux. The high heat flux and increased cryovolcanic and tectonic activity suggest that near‐surface ice in the region has become rheologically and mechanically weakened enough to permit convective plumes to reach close to the surface. If the yield strength of Enceladus' lithosphere is less than ∼1–10 kPa, convection may instead occur in the “mobile lid” regime, which is characterized by large heat fluxes and large horizontal velocities in the near‐surface ice. I show that model ice shells with effective surface viscosities between 1016 and 1017 Pa s and basal viscosities between 1013 and 1015 Pa s have convective heat fluxes comparable to that observed by Composite Infrared Spectrometer. If this style of convection is occurring, the south polar terrain should be spreading horizontally with v ∼1–10 mm a−1 (where a is years) and should be resurfaced in ∼0.1–10 Ma. On the basis of Cassini imaging data, the south polar terrain is ∼0.5 Ma old, consistent with the mobile lid hypothesis. Maxwell viscoelastic tidal dissipation in such ice shells is not capable of generating enough heat to balance convective heat transport. However, tidal heat may also be generated in the near‐surface along faults as suggested by Nimmo et al. (2007) and/or viscous dissipation within the ice shell may occur by other processes not accounted for by the canonical Maxwell dissipation model.

Habitability of Enceladus: Planetary Conditions for Life

The prolific activity and presence of a plume on Saturn's tiny moon Enceladus offers us a unique opportunity to sample the interior composition of an icy satellite, and to look for interesting chemistry and possible signs of life. Based on studies of the potential habitability of Jupiter's moon Europa, icy satellite oceans can be habitable if they are chemically mixed with the overlying ice shell on Myr time scales. We hypothesize that Enceladus' plume, tectonic processes, and possible liquid water ocean may create a complete and sustainable geochemical cycle that may allow it to support life. We discuss evidence for surface/ocean material exchange on Enceladus based on the amounts of silicate dust material present in the Enceladus' plume particles. Microphysical cloud modeling of Enceladus' plume shows that the particles originate from a region of Enceladus' near surface where the temperature exceeds 190 K. This could be consistent with a shear-heating origin of Enceladus' tiger stripes, which would indicate extremely high temperatures ( approximately 250-273 K) in the subsurface shear fault zone, leading to the generation of subsurface liquid water, chemical equilibration between surface and subsurface ices, and crustal recycling on a time scale of 1 to 5 Myr. Alternatively, if the tiger stripes form in a mid-ocean-ridge-type mechanism, a half-spreading rate of 1 m/year is consistent with the observed regional heat flux of 250 mW m(-2) and recycling of south polar terrain crust on a 1 to 5 Myr time scale as well.

Association of the jets of Enceladus with the warmest regions on its south-polar fractures

Jets of material have been seen emanating from the south-polar terrain of Saturn's satellite Enceladus. Observations have shown that this region is anomalously warm, with the hottest measured temperatures coinciding with the four 'tiger stripe' fractures, named Alexandria, Cairo, Baghdad and Damascus, that straddle the region. Here we use Cassini images taken from a variety of viewing directions over two years to triangulate the source locations for the most prominent jets, and compare these with the infrared hotspot locations and the predictions from a recent model of tidally induced shear heating within the fractures. We find that the jets emanate from the four tiger stripes, with the strongest sources on Baghdad and Damascus. All the jets from each fracture seem to lie in the same nearly vertical plane. There is a strong spatial coincidence between our geographical sources and the locations of increased temperature revealed by the infrared experiment. Comparison with the shear heating model shows broad agreement; the exception is the prediction that Baghdad is the least active lineament, whereas we find it to be the most active. We predict that several new hotspots remain to be discovered by future thermal observations.

Convection in Enceladus' ice shell: Conditions for initiation

Observations of Enceladus by the Cassini spacecraft indicate that this tiny Saturnian moon is geologically active, with plumes of water vapor and ice particles erupting from its southern polar region. This activity suggests that tidal dissipation has become spatially localized, perhaps due to a compositional, rheological, and/or thermal anomaly in its ice shell. Here we examine the role that solid‐state convection may have played in Enceladus' prolific activity by creating a suitable rheological and thermal anomaly. We find convection can only initiate in the pure water ice I shell of a differentiated Enceladus if the ice grain size is less than 0.3 mm, which is quite small, but may be realistic if non‐water‐ice impurities (and/or tidal stresses) keep grains from growing. This grain‐size restriction becomes more severe for lower basal ice temperatures, which implies that any ammonia present has not become strongly concentrated in a thin basal ocean (while convection occurs). For a maximally thick pure ice shell and underlying ocean, convective heat flows are ∼7–11 mW m−2 for ice grain sizes of 0.1–0.3 mm, compared with the ∼100 mW m−2 measured for Enceladus' south polar terrain from Cassini CIRS observations. Thus whereas solid‐state convection may be a prerequisite for Enceladus' geological activity, the observed heat flow requires strong tidal dissipation within the convecting region, and possibly, that convection reaches the surface.

A Clathrate Reservoir Hypothesis for Enceladus' South Polar Plume

We hypothesize that active tectonic processes in the south polar terrain of Enceladus, the 500-kilometer-diameter moon of Saturn, are creating fractures that cause degassing of a clathrate reservoir to produce the plume documented by the instruments on the Cassini spacecraft. Advection of gas and ice transports energy, supplied at depth as latent heat of clathrate decomposition, to shallower levels, where it reappears as latent heat of condensation of ice. The plume itself, which has a discharge rate comparable to Old Faithful Geyser in Yellowstone National Park, probably represents small leaks from this massive advective system.