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.

Did the Early Earth Have Multiple "North & South" Magnetic Poles?

New work from Carnegie's Peter Driscoll suggests Earth's ancient magnetic field was significantly different than the present day field, originating from several poles rather than the familiar two. It is published in Geophysical Research Letters.

Earth generates a strong magnetic field extending from the core out into space that shields the atmosphere and deflects harmful high-energy particles from the Sun and the cosmos. Without it, our planet would be bombarded by cosmic radiation, and life on Earth's surface might not exist. The motion of liquid iron in Earth's outer core drives a phenomenon called the geodynamo, which creates Earth's magnetic field. This motion is driven by the loss of heat from the core and the solidification of the inner core.

But the planet's inner core was not always solid. What effect did the initial solidification of the inner core have on the magnetic field? Figuring out when it happened and how the field responded has created a particularly vexing and elusive problem for those trying to understand our planet's geologic evolution, a problem that Driscoll set out to resolve.

Here's the issue: Scientists are able to reconstruct the planet's magnetic record through analysis of ancient rocks that still bear a signature of the magnetic polarity of the era in which they were formed. This record suggests that the field has been active and dipolar--having two poles--through much of our planet's history. The geological record also doesn't show much evidence for major changes in the intensity of the ancient magnetic field over the past 4 billion years. A critical exception is in the Neoproterozoic Era, 0.5 to 1 billion years ago, where gaps in the intensity record and anomalous directions exist. Could this exception be explained by a major event like the solidification of the planet's inner core?

link.

The Contraction/Expansion History of Charon with implication for its Planetary Scale Tectonic Belt

The Contraction/Expansion History of Charon with implication for its Planetary Scale Tectonic Belt

Authors:

Malamud et al

Abstract:

The New-Horizons mission to the Kuiper Belt has recently revealed intriguing features on the surface of Charon, including a network of chasmata, cutting across or around a series of high topography features, conjoining to form a belt. It is proposed that this tectonic belt is a consequence of contraction/expansion episodes in the moon's evolution associated particularly with compaction, differentiation and geophysical reactions of the interior. The proposed scenario involves no need for solidification of a vast subsurface ocean and/or a warm initial state. This scenario is based on a new, detailed thermo-physical evolution model of Charon that includes multiple processes. According to the model, Charon experiences two contraction/expansion episodes in its history that may provide the proper environment for the formation of the tectonic belt. This outcome remains qualitatively the same even if we assume a different initial composition and mass. Two alternative explanations for the precise localization and orientation of Charon's tectonic belt are speculated.

Neptune's Internal Temperature NOT Caused by Centaur Impacts

Assessing the contribution of centaur impacts to ice giant luminosities

Author:

Dodson-Robinson et al

Abstract:

Voyager 2 observations revealed that Neptune’s internal luminosity is an order of magnitude higher than that of Uranus. If the two planets have similar interior structures and cooling histories, Neptune’s luminosity can only be explained by invoking some energy source beyond gravitational contraction. This paper investigates whether centaur impacts could provide the energy necessary to produce Neptune’s luminosity. The major findings are (1) that impacts on both Uranus and Neptune are too infrequent to provide luminosities of order Neptune’s observed value, even for optimistic impact-rate estimates and (2) that Uranus and Neptune rarely have significantly different impact-generated luminosities at any given time. Uranus and Neptune most likely have structural differences that force them to cool and contract at different rates.

Encledadus' Irregularly Shaped Core Caused by Very large Impacts AFTER the Late Heavy Bombardment

Consequences of large impacts on Enceladus’ core shape

Authors:

Monteux et al

Abstract:

The intense activity on Enceladus suggests a differentiated interior consisting of a rocky core, an internal ocean and an icy mantle. However, topography and gravity data suggests large heterogeneity in the interior, possibly including significant core topography. In the present study, we investigated the consequences of collisions with large impactors on the core shape. We performed impact simulations using the code iSALE2D considering large differentiated impactors with radius ranging between 25 and 100 km and impact velocities ranging between 0.24 and 2.4 km/s. Our simulations showed that the main controlling parameters for the post-impact shape of Enceladus’ rock core are the impactor radius and velocity and to a lesser extent the presence of an internal water ocean and the porosity and strength of the rock core. For low energy impacts, the impactors do not pass completely through the icy mantle. Subsequent sinking and spreading of the impactor rock core lead to a positive core topographic anomaly. For moderately energetic impacts, the impactors completely penetrate through the icy mantle, inducing a negative core topography surrounded by a positive anomaly of smaller amplitude. The depth and lateral extent of the excavated area is mostly determined by the impactor radius and velocity. For highly energetic impacts, the rocky core is strongly deformed, and the full body is likely to be disrupted. Explaining the long-wavelength irregular shape of Enceladus’ core by impacts would imply multiple low velocity (less than 2.4 km/s) collisions with deca-kilometric differentiated impactors, which is possible only after the LHB period.

Pluto and Charon may Have Iron Cores

Internal structure of Pluto and Charon with an iron core

Author:

Aitta

Abstract:

Pluto has been observed by the New Horizons space probe to have some relatively fresh ice on the old ices covering most of the surface. Pluto was thought to consist of only a rocky core below the ice. Here I show that Pluto can have an iron core, as can also its companion Charon, which has recently been modelled to have one. The presence of an iron core means the giant impact origin calculations should be redone to include iron and thus higher temperatures. An iron core leads to the possibility of a different geology. An originally molten core becomes solid later, with contraction and a release of latent heat. The space vacated allows the upper rock layers to flow downwards at some locations at the surface of the core, and some of the ice above the rock to descend, filling the spaces left by the rock motion downwards. These phenomena can lead to the forces recently deforming the icy surface of Pluto, and in a lesser way, of Charon.

Earth's Inner Core Solidified 1.5 Billion Years ago During Calymmian MesoProterozoic

There have been many estimates for when the earth's inner core was formed, but scientists from the University of Liverpool have used new data which indicates that the Earth's inner core was formed 1 - 1.5 billion years ago as it "froze" from the surrounding molten iron outer core.

The inner core is Earth's deepest layer. It is a ball of solid iron just larger than Pluto which is surrounded by a liquid outer core. The inner core is a relatively recent addition to our planet and establishing when it was formed is a topic of vigorous scientific debate with estimates ranging from 0.5 billion to 2 billion years ago

In a new study published in Nature, researchers from the University's School of Environmental Sciences analysed magnetic records from ancient igneous rocks and found that there was a sharp increase in the strength of the Earth's magnetic field between 1 and 1.5 billion years ago.

This increased magnetic field is a likely indication of the first occurrence of solid iron at Earth's centre and the point in Earth's history at which the solid inner core first started to "freeze" out from the cooling molten outer core.

Liverpool palaeomagnetism expert and the study's lead author, Dr Andy Biggin, said: "This finding could change our understanding of the Earth's interior and its history."

link.

Proposed: Titan, Europa, Enceladus, etc. are "Membrane Worlds"

Tidal Love numbers of membrane worlds: Europa, Titan, and Co.

Author:

Beuthe

Abstract:

Under tidal forcing, icy satellites with subsurface oceans deform as if the surface were a membrane stretched around a fluid layer. ‘Membrane worlds’ is thus a fitting name for these bodies and membrane theory provides the perfect toolbox to predict tidal effects. I describe here a new membrane approach to tidal perturbations based on the general theory of viscoelastic–gravitational deformations of spherically symmetric bodies. The massive membrane approach leads to explicit formulas for viscoelastic tidal Love numbers which are exact in the limit of zero crust thickness. Formulas for load Love numbers come as a bonus. The accuracy on k2k2 and h2h2 is better than one percent if the crust thickness is less than five percents of the surface radius, which is probably the case for Europa and Titan. The new approach allows for density differences between crust and ocean and correctly includes crust compressibility. This last feature makes it more accurate than the incompressible propagator matrix method. Membrane formulas factorize shallow and deep interior contributions, the latter affecting Love numbers mainly through density stratification. I show that a screening effect explains why ocean stratification typically increases Love numbers instead of reducing them. For Titan, a thin and dense liquid layer at the bottom of a light ocean can raise k2k2 by more than ten percents. The membrane approach can also deal with dynamical tides in a non-rotating body. I show that a dynamical resonance significantly decreases the tilt factor and may thus lead to underestimating Europa’s crust thickness. Finally, the dynamical resonance increases tidal deformations and tidal heating in the crust if the ocean thickness is of the order of a few hundred meters.

Does Enceladus Have a "Fluffy," Fragmented Core?

The fluffy core of Enceladus

Author:

Roberts

Abstract:

Enceladus is well known for its young south polar terrain, observed by Cassini to emit several GW of heat as well as plumes of vapor and ice. The source of this energy is believed to be tidal dissipation. However, the observed south polar heat flux cannot be sustained over the age of the Solar System. Furthermore, thermal evolution models suggest that any global subsurface ocean should freeze on a timescale of tens to hundreds of My, sharply reducing future tidal heating, unless large amounts of antifreeze are present in the ocean. Here I propose an alternative internal structure for Enceladus, in which the silicate core is fragmented, and that the tidal deformation of the core may be partially controlled by interstitial ice. I find that fragmentation of the core increases tidal dissipation by a factor of 20, consistent with the long-term dynamically sustainable level, even when the interior is completely frozen, but only if the interior starts out warm and tidal heating is strong from the beginning. If this is not the case, radioactive heating will be insufficient to prevent the interior from cooling. Although an ocean need not be present in order for the interior to experience significant tidal heating, all models that dissipate enough heat to prevent runaway cooling are also warm enough to have an ocean. Tidal dissipation in the weak core provides an additional source of heat that may prevent a global subsurface ocean from freezing.

The Earth's Core is not Uniform

Seismic waves are helping scientists to plumb the world's deepest mystery: the planet's inner core.

Thanks to a novel application of earthquake-reading technology, a research team at the University of Illinois and colleagues at Nanjing University in China have found that the Earth's inner core has an inner core of its own, which has surprising properties that could reveal information about our planet.

Led by Xiaodong Song, a professor of geology at the U. of I., and visiting postdoctoral researcher Tao Wang, the team published its work in the journal Nature Geoscience on Feb. 9.

"Even though the inner core is small - smaller than the moon - it has some really interesting features," said Song. "It may tell us about how our planet formed, its history, and other dynamic processes of the Earth. It shapes our understanding of what's going on deep inside the Earth."

Researchers use seismic waves from earthquakes to scan below the planet's surface, much like doctors use ultrasound to see inside patients. The team used a technology that gathers data not from the initial shock of an earthquake, but from the waves that resonate in the earthquake's aftermath. The earthquake is like a hammer striking a bell; much like a listener hears the clear tone that resonates after the bell strike, seismic sensors collect a coherent signal in the earthquake's coda.

"It turns out the coherent signal enhanced by the technology is clearer than the ring itself," said Song. "The basic idea of the method has been around for a while, and people have used it for other kinds of studies near the surface. But we are looking all the way through the center of the Earth."

Looking through the core revealed a surprise at the center of the planet - though not of the type envisioned by novelist Jules Verne.

link.

Enceladus Oceanic Structure Picture

Temperature (a) and ice fraction (b) distributions at the end of the 100 Myr cycle for base case. Polar oceans are sustained. Ice thickness over the south polar ocean varies from 20 km to 40 km. The ocean extends from the pole to about 45° latitude.

link.

Modeling Enceladus' Internal & Thermal Structure

Keeping Enceladus warm

Authors:

Travis et al

Abstract:

Despite its small size, Enceladus emits considerable heat, especially at its south pole, even long after simple thermal models predict it should be frozen. Several sources of energy have been proposed as responsible for this heating, such as tidal dissipative heating (TDH), convection and shearing in the ice shell, and exothermic chemical reactions (e.g., serpentine formation). Crater relaxation simulations suggest that episodic heating events have occurred over long stretches of Enceladus’ history. Thermal history and hydrothermal simulations reported here show that a combination of steady plus episodic TDH heating could maintain at least a polar ocean to the present time. Hydrothermal circulation can play a significant role in mining Enceladus’ internal heat, facilitating the persistence of an ocean even to the present by focusing internal heat to the polar regions.

A Collisional Origin to Earth’s Non-chondritic Composition?

A collisional origin to earth’s non-chondritic composition?

Authors:

Bonsor et al

Abstract:

Several lines of evidence indicate a non-chondritic composition for Bulk Earth. If Earth formed from the accretion of chondritic material, its non-chondritic composition, in particular the super-chondritic 142Nd/144Nd142Nd/144Nd and low Mg/Fe ratios, might be explained by the collisional erosion of differentiated planetesimals during its formation. In this work we use an N-body code, that includes a state-of-the-art collision model, to follow the formation of protoplanets, similar to proto-Earth, from differentiated planetesimals (> 100 km) up to isolation mass (> 0.16 M⊕). Collisions between differentiated bodies have the potential to change the core-mantle ratio of the accreted protoplanets. We show that sufficient mantle material can be stripped from the colliding bodies during runaway and oligarchic growth, such that the final protoplanets could have Mg/Fe and Si/Fe ratios similar to that of bulk Earth, but only if Earth is an extreme case and the core is assumed to contain 10% silicon by mass. This may indicate an important role for collisional differentiation during the giant impact phase if Earth formed from chondritic material.

Was Uranus' Moon Miranda Resurfaced by Convection?

Global resurfacing of Uranus's moon Miranda by convection

Authors:

Hammond et al

Abstract:

Miranda, an icy moon of Uranus, is one of the most visually striking and enigmatic bodies in the solar system. Three polygonal-shaped regions of intense deformation, dubbed "coronae," dominate the surface of Miranda. Here we use numerical methods to show that sluggish-lid convection in Miranda's ice shell, powered by tidal heating, can simultaneously match the global distribution of coronae, the concentric deformation pattern, and the estimated heat flow during formation. The expected rheological conditions in Miranda's ice shell lead to the development of low-order convection that produces surface deformation patterns similar to those observed. We find that satellite core size strongly controls convection geometry and that low-order convection patterns are much more stable for core radii less than half the satellite radius.