Cryomagma on Europa

Cryomagma ascent on Europa

Authors:

Lesage et al

Abstract:

Europa's surface exhibits morphological features associated with a low craters density that demonstrate a recent internal activity. In particular, the morphology of the smooth plains covering parts of the surface, and their relationship to the surrounding terrains suggest that they result from viscous liquid extrusions. Furthermore, recent literature explains the emplacement of liquid-related features, such as double ridges, lenticulae and chaos by the presence of liquid reservoirs beneath the surface. We model the ascent of liquid water through a dike or a pipe-like conduit, from a sub-surface reservoir to Europa's surface and derive the eruption time-scale and the total volume extruded at the end of the eruption, depending on the chamber volume and depth. We also estimate the freezing time of the sub-surface reservoir necessary to trigger an eruption. Considering available data for density and eutectic temperature of salt impurities recently proposed for Europa, we discuss their effect on the cryomagma freezing time and ascent. For plausible volumes and depths varying between 0.1 km3 ≤ V ≤ 10 km3 and 100 m ≤ H ≤ 10 km, the total extruded cryolava volume ranges from 105 to 108 m3 and the time scale of the eruptions varies from few minutes to few tens of hours. The freezing time-scale of the cryomagma pocket varies with the cryomagma composition: it varies between 102 to 103 years for a pure water cryomagma and from 102 to 104 years for a briny cryomagma.

Sputtering and detection of large organic molecules from Europa

Sputtering and detection of large organic molecules from Europa 

Authors: 

Johnson et al 

Abstract:
Mass spectroscopy of bio-molecules by heavy ion induced sputtering, which became a practical laboratory procedure, was also suggested as a potential tool for spacecraft studies of targets of interest in astrobiology. With the planning of new missions to Europa, there is renewed interest in the possibility of detecting organic molecules that might have originated in its subsurface ocean and can be sputtered from its surface often intact by impacting energetic heavy ions trapped in Jupiter's magnetosphere. Here we review the laboratory data and modeling bearing on this issue. We then give estimates of the ejection into the gas-phase of trace organic species embedded in an ice matrix on Europa's surface and their possible detection during a flyby mission.

Emily Lakadwalla on Titan @ LPSC

The Lunar and Planetary Science Conference (LPSC) is always madness. Once upon a time, I was able to dash off numerous blog posts in teeny bits of time snatched between sessions, but that was a younger me. I think it was also a less busy time in planetary science, with fewer missions. Now there’s an embarrassment of riches and I hardly know where to begin. So I’ll spin the wheel of possible LPSC session topics, and: Titan! I’ll talk about Saturn’s moon Titan.

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Modeling the Mantles of Icy Moons

Heat transport in the high-pressure ice mantle of large icy moons

Authors:

Choblet et al

Abstract:

While the existence of a buried ocean sandwiched between surface ice and high-pressure (HP) polymorphs of ice emerges as the most plausible structure for the hundreds-of-kilometers thick hydrospheres within large icy moons of the Solar System (Ganymede, Callisto, Titan), little is known about the thermal structure of the deep HP ice mantle and its dynamics, possibly involving melt production and extraction. This has major implications for the thermal history of these objects as well as on the habitability of their ocean as the HP ice mantle is presumed to limit chemical transport from the rock component to the ocean. Here, we describe 3D spherical simulations of subsolidus thermal convection tailored to the specific structure of the HP ice mantle of large icy moons. Melt production is monitored and melt transport is simplified by assuming instantaneous extraction to the ocean above. The two controlling parameters for these models are the rheology of ice VI and the heat flux from the rock core. Reasonable end-members are considered for both parameters as disagreement remains on the former (especially the pressure effect on viscosity) and as the latter is expected to vary significantly during the moon’s history. We show that the heat power produced by radioactive decay within the rock core is mainly transported through the HP ice mantle by melt extraction to the ocean, with most of the melt produced directly above the rock/water interface. While the average temperature in the bulk of the HP ice mantle is always relatively cool when compared to the value at the interface with the rock core (∼ 5 K above the value at the surface of the HP ice mantle), maximum temperatures at all depths are close to the melting point, often leading to the interconnection of a melt path via hot convective plume conduits throughout the HP ice mantle. Overall, we predict long periods of time during these moons’ history where water generated in contact with the rock core is transported to the above ocean.

Cryovolcanic emplacement of domes on Europa

Cryovolcanic emplacement of domes on Europa

Authors:

Quick et al

Abstract:

Here we explore the hypothesis that certain domes on Europa may have been produced by the extrusion of viscous cryolavas. A new mathematical method for the emplacement and relaxation of viscous lava domes is presented and applied to putative cryovolcanic domes on Europa. A similarity solution approach is applied to the governing equation for fluid flow in a cylindrical geometry, and dome relaxation is explored assuming a volume of cryolava has been rapidly emplaced onto the surface. Nonphysical singularities inherent in previous models for dome relaxation have been eliminated, and cryolava cooling is represented by a time-variable viscosity. We find that at the onset of relaxation, bulk kinematic viscosities may lie in the range between 103 and 106 m2/s, while the actual fluid lava viscosity may be much lower. Plausible relaxation times to form the domes, which are linked to bulk cryolava rheology, are found to range from 3.6 days to 7.5 years. We find that cooling of the cryolava, while dominated by conduction through an icy skin, should not prevent fluids from advancing and relaxing to form domes within the timescales considered. Determining the range of emplacement conditions for putative cryolava domes will shed light on Europa's resurfacing history. In addition, the rheologies and compositions of erupted cryolavas have implications for subsurface cryomagma ascent and local surface stress conditions on Europa.

Evidence of Cryovolcanism on Europa in the Argadnel Regio

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

Authors:

Prockter et al

Abstract:

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.

Hunting for Nonice on Europa's Surface

SPATIALLY RESOLVED SPECTROSCOPY OF EUROPA'S LARGE-SCALE COMPOSITIONAL UNITS AT 3–4 μm WITH KECK NIRSPEC

Authors:

Fischer et al

Abstract:

We present spatially resolved spectroscopic observations of Europa's surface at 3–4 μm obtained with the near-infrared spectrograph and adaptive optics system on the Keck II telescope. These are the highest quality spatially resolved reflectance spectra of Europa's surface at 3–4 μm. The observations spatially resolve Europa's large-scale compositional units at a resolution of several hundred kilometers. The spectra show distinct features and geographic variations associated with known compositional units; in particular, large-scale leading hemisphere chaos shows a characteristic longward shift in peak reflectance near 3.7 μm compared to icy regions. These observations complement previous spectra of large-scale chaos, and can aid efforts to identify the endogenous non-ice species.

Enceladus is too hot

Interior thermal state of Enceladus inferred from the viscoelastic state of the ice shell

Author:

Karmata

Abstract:

Recent geodetic measurements for Enceladus suggest a global subsurface ocean that is thicker beneath the south pole. In order to maintain such an ocean, viscous relaxation of topography at the base of the ice shell and melting of ice need to be balanced. In this study, we investigate the interior thermal state that can lead to the relaxation timescale being comparable to the melting timescale. Our results indicate that a basal heat flux about ten times higher than that due to radiogenic heating, or an ice shell tidal heating rate about ten times higher than the conventional estimate of 1.1 GW is necessary if the ice shell is in thermal equilibrium. These requirements are concordant with recent astrometric studies.

How to Explore Ocean Worlds

Ocean worlds exploration

Author:

Lunine

Abstract:

Ocean worlds is the label given to objects in the solar system that host stable, globe-girdling bodies of liquid water—“oceans”. Of these, the Earth is the only one to support its oceans on the surface, making it a model for habitable planets around other stars but not for habitable worlds elsewhere in the solar system. Elsewhere in the solar system, three objects—Jupiter's moon Europa, and Saturn's moons Enceladus and Titan—have subsurface oceans whose existence has been detected or inferred by two independent spacecraft techniques. A host of other bodies in the outer solar system are inferred by a single type of observation or by theoretical modeling to have subsurface oceans. This paper focusses on the three best-documented water oceans beyond Earth: those within Europa, Titan and Enceladus. Of these, Europa's is closest to the surface (less than 10 km and possibly less than 1 km in places), and hence potentially best suited for eventual direct exploration. Enceladus’ ocean is deeper—5–40 km below its surface—but fractures beneath the south pole of this moon allow ice and gas from the ocean to escape to space where it has been sampled by mass spectrometers aboard the Cassini Saturn Orbiter. Titan's ocean is the deepest—perhaps 50–100 km—and no evidence for plumes or ice volcanism exist on the surface. In terms of the search for evidence of life within these oceans, the plume of ice and gas emanating from Enceladus makes this the moon of choice for a fast-track program to search for life. If plumes exist on Europa—yet to be confirmed—or places can be located where ocean water is extruded onto the surface, then the search for life on this lunar-sized body can also be accomplished quickly by the standards of outer solar system exploration.

Scale Model of Titan Maritime Explorer (TiME) Tested in Chile

Onboard Science Insights and Vehicle Dynamics from Scale-Model Trials of the Titan Mare Explorer (TiME) Capsule at Laguna Negra, Chile

Author:

Lorenz

Abstract:

A scale model of the proposed Titan Mare Explorer capsule was deployed at the Planetary Lake Lander field site at Laguna Negra, Chile. The tests served to calibrate models of wind-driven drift of the capsule and to understand its attitude motion in the wave field, as well as to identify dynamic and acoustic signatures of shoreline approach. This information enables formulation of onboard trigger criteria for near-shore science data acquisition.

GeoMapping Titan's Polar Terrain Reveals Potential Sedimentary Deposits

Geomorphologic mapping of titan's polar terrains: Constraining surface processes and landscape evolution

Authors:

Birch et al

Abstract:

We present a geomorphologic map of Titan's polar terrains. The map was generated from a combination of Cassini Synthetic Aperture Radar (SAR) and Imaging Science Subsystem imaging products, as well as altimetry, SARTopo and radargrammetry topographic datasets. In combining imagery with topographic data, our geomorphologic map reveals a stratigraphic sequence from which we infer process interactions between units. In mapping both polar regions with the same geomorphologic units, we conclude that processes that formed the terrains of the north polar region also acted to form the landscape we observe at the south. Uniform, SAR-dark plains are interpreted as sedimentary deposits, and are bounded by moderately dissected uplands. These plains contain the highest density of filled and empty lake depressions, and canyons. These units unconformably overlay a basement rock that outcrops as mountains and SAR-bright dissected terrains at various elevations across both poles. All these units are then superposed by surficial units that slope towards the seas, suggestive of subsequent overland transport of sediment. From estimates of the depths of the embedded empty depressions and canyons that drain into the seas, the SAR-dark plains must be >600 m thick in places, though the thickness may vary across the poles. At the lowest elevations of each polar region, there are large seas, which are currently liquid methane/ethane filled at the north and empty at the south. The large plains deposits and the surrounding hillslopes may represent remnant landforms that are a result of previously vast polar oceans, where larger liquid bodies may have allowed for a sustained accumulation of soluble and insoluble sediments, potentially forming layered sedimentary deposits. Coupled with vertical crustal movements, the resulting layers would be of varying solubilities and erosional resistances, allowing formation of the complex landscape that we observe today.

Methane Humidity Near TItan's Surface

The near-surface methane humidity on Titan

Authors:

Lora et al

Abstract:

We retrieve vertical and meridional variations of methane mole fraction in Titan's lower troposphere by re-analyzing near-infrared ground-based observations from 17 July 2014 UT (Adamkovics et al., 2016). We generate synthetic spectra using atmospheric methane profiles that do not contain supersaturation or discontinuities to fit the observations, and thereby retrieve minimum saturation altitudes and corresponding specific humidities in the boundary layer. We relate these in turn to surface-level relative humidities using independent surface temperature measurements. We also compare our results with general circulation model simulations to interpret and constrain the relationship between humidities and surface liquids. The results show that Titan's lower troposphere is undersaturated at latitudes south of 60N, consistent with a dry surface there, but increases in humidity toward the north pole indicate appreciable surface liquid coverage. While our observations are consistent with considerably more liquid methane existing at the north pole than is present in observed lakes, a degeneracy between low-level methane and haze leads to substantial uncertainty in determining the extent of the source region.

The Intriguing Active Drainage System Into Titan's Ligeia Mare

Liquid-filled canyons on Titan

Authors:

Poggiali et al

Abstract:

In May 2013 the Cassini RADAR altimeter observed channels in Vid Flumina, a drainage network connected to Titan's second largest hydrocarbon sea, Ligeia Mare. Analysis of these altimeter echoes shows that the channels are located in deep (up to ~570 m), steep-sided, canyons and have strong specular surface reflections that indicate they are currently liquid filled. Elevations of the liquid in these channels are at the same level as Ligeia Mare to within a vertical precision of about 0.7 m, consistent with the interpretation of drowned river valleys. Specular reflections are also observed in lower order tributaries elevated above the level of Ligeia Mare, consistent with drainage feeding into the main channel system.

Was Titan's Nitrogen Atmosphere was Originally Ammonia?

Isotopic ratio of nitrogen on Titan: Photochemical interpretation

Author:

Krasnopolsky

Abstract:

Nitrogen isotope fractionation in predissociation of N2 (Liang et al., 2007) is combined with production of N(4S), N(2D), and N+ in dissociation and dissociative ionization by the solar EUV photons, photoelectrons, magnetospheric electrons and protons, and cosmic rays from the photochemical model. The calculated 14N/15N ratio in nitriles is 57, in excellent agreement with the observed ratio in HCN. Loss of nitrogen in condensation and polymerization of nitriles is 392 g cm−2 Byr−1 with nitrogen isotope fractionation factor of 2.8. Loss of nitrogen by sputtering is 57 g cm−2 Byr−1 (De La Haye et al., 2007) with fractionation factor of 0.73 (Mandt et al., 2014). If the current loss was constant throughout the age of the Solar System, then the initial 14N/15N ratio on Titan is 129, similar to 127±32 for ammonia in comets (Rousselot et al., 2014). However, the solar EUV and wind were stronger from the young Sun, and this tends to further reduce the initial 14N/15N ratio. Nevertheless uncertainties of the problem and of the ratio in comets support the idea that nitrogen on Titan appeared as ammonia ice with 14N/15N similar to that in comets.

Europa Almost Assuredly has Watery Plumes

NASA has confirmed “statistically significant evidence” from the Hubble Space Telescope indicating the presence of salty watery ejecta plumes from the Jovian moon Europa. The announcement lends further proof for the existence of a deep, saline ocean under Europa’s icy surface and creates new urgency and objectives for NASA’s upcoming Europa Clipper mission in the mid-2020s.

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The Surface Temperature of Europa

The surface temperature of Europa

Author:

Ashkenazy

Abstract:

Previous estimates of the surface temperature of Jupiter's moon, Europa, neglected the effect of the eccentricity of Jupiter's orbit around the Sun, the effect of the eclipse of Europa (i.e., the relative time that Europa is within the shadow of Jupiter), and the effect of Europa's internal heating. Here we estimate the surface temperature of Europa, when Europa's obliquity, eclipse and internal heating, as well as the eccentricity of Jupiter, are all taken into account. For a typical internal heating rate of 0.05 W/m2 (corresponding to an ice thickness of about 10 kms), the equator, pole, and global mean surface temperatures are 101.7 K, 45.26 K, and 94.75 K, respectively. We found that the temperature at the high latitudes is significantly affected by the internal heating. We also studied the effect of the internal heating on the mean thickness of Europa's icy shell and conclude that the polar region temperature can be used to constrain the internal heating and the depth of the ice. Our approach and formalism can be implemented to estimate the temperature of other moons in the solar system.

Were Titan's Channels Carved by Ethane & Ammonia?

Formation mechanisms of channels on Titan through dissolution by ammonium sulfate and erosion by liquid ammonia and ethane

Authors:

Gilliam et al

Abstract:

Data obtained from the Cassini Visual and Infrared Mapping Spectrometer (VIMS), Imaging Science Subsystem (ISS), and Synthetic Aperture Radar (SAR) instruments have revealed an array of fluvial channels on Titan's surface, often several hundreds of kilometers in length. The paucity of impact craters on Titan's surface suggests a formation by fluvial erosion into the water-ice bedrock. Additionally, at the landing site, the Huygens Probe Descent Imager and Spectral Radiometer (DISR) imaged Earth-like rounded cobbles 0.3–15 cm in diameter composed of water ice, reminiscent of rounded stream clasts on Earth. In this paper we examine different fluvial features on Titan, identified by the Cassini spacecraft, and evaluate the possibilities of channel formation by dissolution of ice by a concentrated solution of ammonium sulfate, and by mechanical erosion by flow of liquid ammonia and liquid ethane. We find that chemical erosion of Titan's channels could be completed in 280 to 1100 years (all units of time in this paper are Terrestrial, not Titanian), much shorter than the period of about 84,000 years that a concentrated (NH4)2SO4-H2O solution could exist as a liquid on the Titan surface. Mechanical erosion of Titan's channels is generally a much slower process, on the order of 102 to 105 years to completion, and is also slower than mechanical erosion of a model river on Earth, averaging 103 to 104 years. The erosional sequence of the channels on Titan may have started after the formation of water-ice on the surface by the process of chemical dissolution by (NH4)2SO4-H2O, overlapping, or followed by, a period of mechanical erosion by liquid NH3. A final stage on the cooling surface of Titan might have been characterized by liquid C2H6 as an agent of mechanical erosion.

Microbial Morphology and Motility as Biosignatures for Outer Planet Missions

Microbial Morphology and Motility as Biosignatures for Outer Planet Missions

Authors:

Jay et al

Abstract:

Meaningful motion is an unambiguous biosignature, but because life in the Solar System is most likely to be microbial, the question is whether such motion may be detected effectively on the micrometer scale. Recent results on microbial motility in various Earth environments have provided insight into the physics and biology that determine whether and how microorganisms as small as bacteria and archaea swim, under which conditions, and at which speeds. These discoveries have not yet been reviewed in an astrobiological context. This paper discusses these findings in the context of Earth analog environments and environments expected to be encountered in the outer Solar System, particularly the jovian and saturnian moons. We also review the imaging technologies capable of recording motility of submicrometer-sized organisms and discuss how an instrument would interface with several types of sample-collection strategies.

Just what IS Going on at Titan's Polar Regions

Geomorphologic Mapping of Titan's Polar Terrains: Constraining Surface Processes and Landscape Evolution

Authors:

Birch et al

Abstract:

We present a geomorphologic map of Titan's polar terrains. The map was generated from a combination of Cassini Synthetic Aperture Radar (SAR) and Imaging Science Subsystem imaging products, as well as altimetry, SARTopo and radargrammetry topographic datasets. In combining imagery with topographic data, our geomorphologic map reveals a stratigraphic sequence from which we infer process interactions between units. In mapping both polar regions with the same geomorphologic units, we conclude that processes that formed the terrains of the north polar region also acted to form the landscape we observe at the south. Uniform, SAR-dark plains are interpreted as sedimentary deposits, and are bounded by moderately dissected uplands. These plains contain the highest density of filled and empty lake depressions, and canyons. These units unconformably overlay a basement rock that outcrops as mountains and SAR-bright dissected terrains at various elevations across both poles. All these units are then superposed by surficial units that slope towards the seas, suggestive of subsequent overland transport of sediment. From estimates of the depths of the embedded empty depressions and canyons that drain into the seas, the SAR-dark plains must be >600 m thick in places, though the thickness may vary across the poles. At the lowest elevations of each polar region, there are large seas, which are currently liquid methane/ethane filled at the north and empty at the south. The large plains deposits and the surrounding hillslopes may represent remnant landforms that are a result of previously vast polar oceans, where larger liquid bodies may have allowed for a sustained accumulation of soluble and insoluble sediments, potentially forming layered sedimentary deposits. Coupled with vertical crustal movements, the resulting layers would be of varying solubilities and erosional resistances.

NASA Begins Researching Submarine for Icy Moons of the Solar System

One of the most profound and exciting breakthroughs in planetary science in the last two decades has been the discovery of liquid methane lakes on the surface of Saturn’s largest moon Titan, and liquid oceans under the icy surfaces of many of the giant gas planets’ other moons. Thrillingly, these some of these “waters” may actually harbor life.

Unfortunately, we don’t know much about them. Probes such as Juno and Cassini can only get so close. Also, subsurface oceans can only be sensed indirectly. The European Space Agency’s Huygens probe did land on Titan in 2005, but on a solid surface rather than on liquid. So how can we explore these seas?

An exciting idea being explored is developing submarines to send through space to the moons. Over the next two years, NASA is devoting half a million dollars to researching the prospect of sending such a vehicle to Titan. But there are other studies out there, too – with targets including Jupiter’s Europa and Ganymede, and Saturn’s Enceladus. But are such missions actually within our technological reach?

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