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.

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.

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.

A Classification of Martian Gullies

A Classification of Martian Gullies from HiRISE Imagery

Authors:

Auld et al

Abstract:

Due to the large and varied population of gullies observed in Mars High Resolution Imaging Science Experiment (HiRISE) imagery that exhibit diverse characteristics, this paper develops a classification of martian gullies based on their morphological characteristics. This provides a firmer foundation for future investigation of the genesis of different gully morphologies. The Mars Reconnaissance Orbiter (MRO) image catalog on the HiRISE website was examined and 869 images, chosen from the first 25,000 orbits, show a wide variety of gully morphologies. The images were analyzed using ENVI 4.4 and ENVI Zoom and the gullies were cataloged and divided into groups based on the presence and character of the dominant morphological components of alcove, channel, and apron associated with each gully. 7519 gullies were identified in the images and the length and width of the components were measured in ENVI to facilitate classification. Seven classes were developed based on morphology.

Did Mercury's Volcanic Activity End 3.5 Billion Years ago?

New research from North Carolina State University finds that major volcanic activity on the planet Mercury most likely ended about 3.5 billion years ago. These findings add insight into the geological evolution of Mercury in particular, and what happens when rocky planets cool and contract in general.

There are two types of volcanic activity: effusive and explosive. Explosive volcanism is often a violent event that results in large ash and debris eruptions, such as the Mount Saint Helens eruption in 1980. Effusive volcanism refers to widespread lava flows that slowly pour out over the landscape -- believed to be a key process by which planets form their crusts.

Determining the ages of effusive volcanic deposits can give researchers a handle on a planet's geological history. For example, effusive volcanism was active a few hundred million years ago on Venus, a few million years ago on Mars, and it still takes place on Earth today. Until now, the duration of effusive volcanic activity on Mercury, made of the same materials as these other planets, had not been known.

NC State assistant professor and planetary geologist Paul Byrne and colleagues determined when the bulk of Mercury's crust-forming volcanism ended by using photographs of the surface imaged by NASA's MESSENGER mission. Because there are no physical samples from the planet that could be used for radiometric dating, the researchers used crater size-frequency analysis, in which the number and size of craters on the planet's surface are placed into established mathematical models, to calculate absolute ages for effusive volcanic deposits on Mercury.

According to their results, major volcanism on Mercury stopped at around 3.5 billion years ago, in stark contrast to the volcanic ages found for Venus, Mars and Earth.

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Do Sand Dune Shapes Suggest Mars had a Thin Atmosphere by the end of the Noachian 3.7 Billion Years ago?

Large wind ripples on Mars: A record of atmospheric evolution

Authors:

Lapotre et al

Abstract:

Wind blowing over sand on Earth produces decimeter-wavelength ripples and hundred-meter– to kilometer-wavelength dunes: bedforms of two distinct size modes. Observations from the Mars Science Laboratory Curiosity rover and the Mars Reconnaissance Orbiter reveal that Mars hosts a third stable wind-driven bedform, with meter-scale wavelengths. These bedforms are spatially uniform in size and typically have asymmetric profiles with angle-of-repose lee slopes and sinuous crest lines, making them unlike terrestrial wind ripples. Rather, these structures resemble fluid-drag ripples, which on Earth include water-worked current ripples, but on Mars instead form by wind because of the higher kinematic viscosity of the low-density atmosphere. A reevaluation of the wind-deposited strata in the Burns formation (about 3.7 billion years old or younger) identifies potential wind-drag ripple stratification formed under a thin atmosphere.

No Statistic Evidence of Lightning on Venus

No statistical evidence of lightning in Venus night-side atmosphere from VIRTIS-Venus Express Visible observations

Authors:

Cardesín Moinelo et al

Abstract:

In this study we describe a dedicated analysis of luminous transient events on Venus night side atmosphere with the visible channel of the VIRTIS instrument (280–1100 nm), this being the most comprehensive search of lightning conducted so far with Venus Express data. Our search results in thousands of signal detections, but unfortunately they can be all explained by cosmic rays impinging on the detector, and further statistical analysis shows that all of the events are randomly distributed along the spectral dimension, therefore not showing any clear evidence of signal coming from lightning emission in the Venus atmosphere. This does not exclude the existence of lightning, but imposes some constraints on their occurrence that are important for future research.

Enceladus' Plumes Explained?

Sustained eruptions on Enceladus explained by turbulent dissipation in tiger stripes

Authors:

Kite et al

Abstract:

Spacecraft observations suggest that the plumes of Saturn's moon Enceladus draw water from a subsurface ocean, but the sustainability of conduits linking ocean and surface is not understood. Observations show sustained (though tidally modulated) fissure eruptions throughout each orbit, and since the 2005 discovery of the plumes. Peak plume flux lags peak tidal extension by ∼1 radian, suggestive of resonance. Here we show that a model of the tiger stripes as tidally-flexed slots that puncture the ice shell can simultaneously explain the persistence of the eruptions through the tidal cycle, the phase lag, and the total power output of the tiger stripe terrain, while suggesting that the eruptions are maintained over geological timescales. The delay associated with flushing and refilling of \emph{O}(1) m-wide slots with ocean water causes erupted flux to lag tidal forcing and helps to buttress slots against closure, while tidally pumped in-slot flow leads to heating and mechanical disruption that staves off slot freeze-out. Much narrower and much wider slots cannot be sustained. In the presence of long-lived slots, the 106-yr average power output of the tiger stripes is buffered by a feedback between ice melt-back and subsidence to \emph{O}(1010) W, which is similar to the observed power output, suggesting long-term stability. Turbulent dissipation makes testable predictions for the final flybys of Enceladus by the \emph{Cassini} spacecraft. Our model shows how open connections to an ocean can be reconciled with, and sustain, long-lived eruptions. Turbulent dissipation in long-lived slots helps maintain the ocean against freezing, maintains access by future Enceladus missions to ocean materials, and is plausibly the major energy source for tiger stripe activity.

Recent volcanic resurfacing of Venusian craters

Recent volcanic resurfacing of Venusian craters

Authors:

Whitten et al

Abstract:

Ejecta from impact craters on Venus are a major source of fine-grained materials across the planet, and crater spatial distribution has been studied as a guide to the relative age and resurfacing rates of large regions. Of particular interest is the potential intersection of distal crater deposits and tesserae, highly deformed landforms that may be the oldest materials on Venus. The composition of tesserae is unknown, but is key to understanding whether water played a role in crustal differentiation. Thus, tesserae are ideal sites for future landed missions to identify possible felsic materials, but the short lifespan of surface landers means that efforts must be made to avoid contaminating surface materials from craters. Here we develop a method to detect distal crater ejecta on tessera terrain across Venus using NASA Magellan radar data. Our results show that fine-grained ejecta are unevenly distributed in the tesserae with respect to nearby craters. Many tesserae within a few hundred kilometers of plains craters do not have evidence for thick (greater than 5–10 cm) mantling material, indicating that eolian or mass-wasting processes have moved the debris off the highland ridge slopes. At Sudenitsa Tesserae, within the young Beta-Atla-Themis region, we observe a radar signature of mantling debris, but there is no apparent source crater to which this material can be traced. We infer that the source crater has been resurfaced by volcanic activity within the past 80 m.y., and suggest that similar fine-grained ejecta deposits may have built up over time in other tesserae across Venus.

Atmospheric nitrogen evolution on Earth and Venus

Atmospheric nitrogen evolution on Earth and Venus

Author:

Wordsworth et al

Abstract:

Nitrogen is the most common element in Earth's atmosphere and also appears to be present in significant amounts in the mantle. However, its long-term cycling between these two reservoirs remains poorly understood. Here a range of biotic and abiotic mechanisms are evaluated that could have caused nitrogen exchange between Earth's surface and interior over time. In the Archean, biological nitrogen fixation was likely strongly limited by nutrient and/or electron acceptor constraints. Abiotic fixation of dinitrogen becomes efficient in strongly reducing atmospheres, but only once temperatures exceed around 1000 K. Hence if atmospheric N2 levels really were as low as they are today 3.0 - 3.5 Ga, the bulk of Earth's mantle nitrogen must have been emplaced in the Hadean, most likely at a time when the surface was molten. The elevated atmospheric N content on Venus compared to Earth can be explained abiotically by a water loss redox pump mechanism, where oxygen liberated from H2O photolysis and subsequent H loss to space oxidises the mantle, causing enhanced outgassing of nitrogen. This mechanism has implications for understanding the partitioning of other Venusian volatiles and atmospheric evolution on exoplanets.

The Closest Close up of Pluto yet From New Horizons

Crustal failure on icy Moons from a strong tidal encounter

Crustal failure on icy Moons from a strong tidal encounter

Authors:

Quillen et al

Abstract:

Close tidal encounters among large planetesimals and Moons should have been more common than grazing or normal impacts. Using a mass spring model within an N-body simulation, we simulate the deformation of the surface of an elastic spherical body caused by a close parabolic tidal encounter with a body that has similar mass as that of the primary body. Such an encounter can induce sufficient stress on the surface to cause brittle failure of an icy crust and simulated fractures can extend a large fraction of the radius of body. Strong tidal encounters may be responsible for the formation of long graben complexes and chasmata in ancient terrain of icy Moons such as Dione, Tethys, Ariel and Charon.

Titan's Kraken Mare Should NOT be There

The influence of subsurface flow on lake formation and north polar lake distribution on Titan

Authors:

Horvath et al

Abstract:

Observations of lakes, fluvial dissection of the surface, rapid variations in cloud cover, and lake shoreline changes indicate that Saturn's moon Titan is hydrologically active, with a hydrocarbon-based hydrological cycle dominated by liquid methane. Here we use a numerical model to investigate the Titan hydrological cycle – including surface, subsurface, and atmospheric components – in order to investigate the underlying causes of the observed distribution and sizes of lakes in the north polar region. The hydrocarbon-based hydrological cycle is modeled using a numerical subsurface flow model and analytical runoff scheme, driven by a general circulation model with an active methane-cycle. This model is run on synthetically generated topography that matches the fractal character of the observed topography, without explicit representation of the effects of erosion and deposition. At the scale of individual basins, intermediate to high permeability (10−8–10−6 cm2) aquifers are required to reproduce the observed large stable lakes. However, at the scale of the entire north polar lake district, a high permeability aquifer results in the rapid flushing of methane through the aquifer from high polar latitudes to dry lower polar latitudes, where methane is removed by evaporation, preventing large lakes from forming. In contrast, an intermediate permeability aquifer slows the subsurface flow from high polar latitudes, allowing greater lake areas. The observed distribution of lakes is best matched by either a uniform intermediate permeability aquifer, or a combination of a high permeability cap at high latitudes surrounded by an intermediate permeability aquifer at lower latitudes, as could arise due to karstic processes at the north pole. The stability of Kraken Mare further requires reduction of the evaporation rate over the sea to 1% of the value predicted by the general circulation model, likely as a result of dissolved ethane, nitrogen, or organic solutes, and/or a climatic lake effect. These results reveal that subsurface flow through aquifers plays an important role in Titan's hydrological cycle, and exerts a strong influence over the distribution, size, and volatile budgets of Titan's lakes.

Venus was Recently Resurfaced

Mantle potential temperature estimates of basalt from the surface of Venus

Author:

Shellnutt

Abstract:

The crater density and distribution of Venus indicates the average surface age is younger (≤1 Ga) than most terrestrial planets and satellites in the Solar System. The type and rate (i.e. equilibrium, catastrophic or differential) of volcanism associated with the stagnant lid tectonic system of Venus is a first order problem that has yet to be resolved but is directly related to the thermal conditions of the mantle. The calculated primary melt composition of basalt at the Venera 14 landing site is high-Mg basalt to picrite with a mantle potential temperature estimate similar to terrestrial ambient mantle (1370 ± 70 °C). The calculated accumulated fractional melting curves indicate the olivine compositions from the melt have Mg# of 89–91. The results show that the thermal regime required to generate the primary melt composition of the Venera 14 basalt was not anomalously high (i.e. mantle-plume system) but rather consistent with a lithospheric tensional rift system. The juxtaposition of high thermal regime structures (e.g. Beta Regio) and ‘ambient’ mantle potential temperature estimates of the Venera 14 basalt suggests that the relatively young surface of Venus is the result of volcanism from a combination of thermal systems that resurfaced the planet at variable rates.

What Lies Beneath Titan's Seas?

Back when Cassini was approaching Saturn and we all anticipated the arrival of the Huygens payload on the surface, speculation grew that rather than finding a solid surface, Huygens might ‘splash down’ in a hydrocarbon sea. I can remember art to that effect in various Internet venues of the time. In the event, Huygens came down on hard terrain, but since then Cassini’s continuing surveys have shown that seas and lakes do exist on the moon. Over 1.6 million square kilometers (about two percent of the surface of Titan) are covered in liquid.

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Hypotheses for near-surface exchange of methane on Mars

Hypotheses for near-surface exchange of methane on Mars

Authors:

Hu et al

Abstract:

The Curiosity rover recently detected a background of 0.7 ppb and spikes of 7 ppb of methane on Mars. This in situ measurement reorients our understanding of the Martian environment and its potential for life, as the current theories do not entail any geological source or sink of methane that varies sub-annually. In particular, the 10-fold elevation during the southern winter indicates episodic sources of methane that are yet to be discovered. Here we suggest a near-surface reservoir could explain this variability. Using the temperature and humidity measurements from the rover, we find that perchlorate salts in the regolith deliquesce to form liquid solutions, and deliquescence progresses to deeper subsurface in the season of the methane spikes. We therefore formulate the following three testable hypotheses. The first scenario is that the regolith in Gale Crater adsorbs methane when dry and releases this methane to the atmosphere upon deliquescence. The adsorption energy needs to be 36 kJ/mol to explain the magnitude of the methane spikes, higher than existing laboratory measurements. The second scenario is that microorganisms convert organic matter in the soil to methane when they are in liquid solutions. This scenario does not require regolith adsorption, but entails extant life on Mars. The third scenario is that deep subsurface aquifers produce the bursts of methane. Continued in situ measurements of methane and water, as well as laboratory studies of adsorption and deliquescence, will test these hypotheses and inform the existence of the near-surface reservoir and its exchange with the atmosphere.

A Proposed Strategic Roadmap for Exploring Enceladus

Strategic map for exploring the ocean-world Enceladus

Author:

Sherwood

Abstract:

Among the many “ocean worlds” of our solar system, Enceladus appears unique in its combination of astrobiologically relevant and exploration-worthy attributes: extensive liquid-water ocean with active hydrothermal activity, containing salts and organics expressed predictably into space. The Enceladus south polar plume allows direct access to telltale molecules, ions, isotopes, and potential cytofragments in space. Plume mass spectroscopy and sample return, in situ investigation of surface fallback deposits, direct vent exploration, and eventually oceanographic exploration can all be envisioned. However, building consensus to fund such ambitious exploration hinges on acquiring key new data. A roadmap is essential. It could start with cost-capped onramps such as flythrough analysis of the plume, following up on Cassini measurements with modern instruments; and sample return of plume material for analysis on Earth. A methodical mission sequence in which each step depends on emergent results from prior missions would push in situ oceanographic exploration into the second half of this century. Even for this scenario, prioritization by the next planetary Decadal Survey would be pivotal.

The Contribution of Water Filled Cracks to Enceladus' Plumes

Controlled boiling on Enceladus. 2. Model of the liquid-filled cracks

Authors:

Ingersoll et al

Abstract:

Controlled boiling will occur on Enceladus whenever a long, narrow conduit connects liquid water to the vacuum of space. In a companion paper we focus on the upward flow of the vapor and show how it controls the evaporation rate through backpressure, which arises from friction on the walls. In this paper we focus on the liquid and show how it flows through the conduit up to its level of neutral buoyancy. For an ice shell 20 km thick, the liquid water interface could be 2 km below the surface. We find that the evaporating surface can be narrow. There is no need for a large vapor chamber that acts as a plume source. Freezing on the icy walls and the evaporating surface is avoided if the crack width averaged over the length of the tiger stripes is greater than 1 m and the salinity of the liquid is greater than 20 g kg−1. Controlled boiling plays a crucial role in our model, which makes it different from earlier published models. The liquids on Enceladus are boiling because there is no overburden pressure—the saturation vapor pressure is equal to the total pressure. Salinity plays a crucial role in preventing freezing, and we argue that the subsurface oceans of icy satellites can have water vapor plumes only if their salinities are greater than about 20 g kg−1.

Evidence of Water Discharges From Nili Fossae and Syrtis Major on Mars

Extensive aqueous deposits at the base of the dichotomy boundary in Nilosyrtis Mensae, Mars

Authors:

Bandfield et al

Abstract:

Thermal emission imaging system (THEMIS) and Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) spectral datasets were used to identify high bulk SiO2 and hydrated compositions throughout the Nilosyrtis Mensae region. Four isolated locations were identified across the region showing short wavelength silicate absorptions within the 8–12 μm spectral region, indicating surfaces dominated by high Si phases. Much more extensive exposures of hydrated compositions are present throughout the region, indicated by a spectral absorption near 1.9 μm in CRISM data. Although limited in spatial coverage, detailed spectral observations indicate that the hydrated materials contain Fe/Mg-smectites and hydrated silica along with minor exposures of Mg-carbonates and an unidentified hydrated phase. The high SiO2 and hydrated materials are present in layered sediments near the base of topographic scarps at the hemispheric dichotomy boundary, typically near or within low albedo sand deposits. The source of the high SiO2 and hydrated materials appears to be from groundwater discharge from Nili Fossae and Syrtis Major to the south, where there is evidence for extensive aqueous alteration of the subsurface. Although discontinuous, the exposures of high SiO2 and hydrated materials span a wide area and are present in a similar geomorphological context to previously identified deposits in western Hellas Basin. These regional deposits may reflect aqueous conditions and alteration within the adjacent crust of the martian highlands.

Modeling Enceladus' Jet Plumes

Controlled boiling on Enceladus. 1. Model of the vapor-driven jets

Authors:

Nakajima et al

Abstract:

Plumes of water vapor and ice particles have been observed from the so-called tiger stripes at the south polar terrain (SPT) of Saturn’s satellite, Enceladus. The observed high salinity (∼0.5–2%) of the ice particles in the plumes may indicate that the plumes originate from a subsurface liquid ocean. Additionally, the SPT is the source of strong infrared radiation (∼4.2 GW), which is especially intense near (within tens of meters) the tiger stripes. This could indicate that the radiation is associated with plume activity, but the connection remains unclear. Here we investigate the constraints that plume observations place on the widths of the cracks, the depth to the liquid-vapor interface, and the mechanisms controlling plume variability. We solve the fluid dynamics of the flow in the crack and the interaction between the flow and ice walls assuming that the flows of water vapor and ice particles originate from a few kilometers deep liquid ocean. For a crack with a uniform width, we find that our model could explain the observed vapor mass flow rate of the plumes when the crack width is 0.05–0.075 m. A wider crack is not favorable because it would produce a higher vapor mass flow rate than the observed value, but it may be allowed if there are some flows that do not reach the surface of Enceladus due to condensation onto the ice walls or the crack is significantly tortuous. The observed heat flow can be explained if the total crack length is approximately 1.7 × 500 km. A tapering crack (a crack which is ∼1 m wide at the bottom of the flow and sharply becomes 0.05–0.075 m at shallower depths) can also explain the observed vapor mass flow rate and heat flow. Widths of 1 m or more are necessary to avoid freezing at the liquid-vapor interface, as shown in our paired paper (Ingersoll and Nakajima [2016] Icarus). The observed intense heat flow along the tiger stripes can be explained by the latent heat release due to vapor condensation onto the ice walls near the surface. The resulting buildup of ice causes the vents to seal themselves on time scales less than a year. We also find that the ice to vapor ratio of the plumes is sensitive to the ice mass fraction at the bottom of the flow (liquid–vapor interface). We find that the total mass flow rate of the plumes becomes larger when the crack width is larger, which is consistent with the observation that the flow rate increases near the orbital apocenter, where the crack is expected to be widest.