arXiv subjects

Amy C. Barr

Publications and source records attributed to Amy C. Barr.

15 recordsLinked to original sources

arXiv2023

Water content of rocky exoplanets in the habitable zone

In this study we investigated the interiors of rocky exoplanets in order to identify those that may have large quantities of water. We modelled the interiors of 28 rocky exoplanets, assuming four different layers - an iron core, a rock mantle, a high-pressure ice layer, and a surface ice/water layer. Due to observational bias, our study is limited to habitable zone exoplanets. We determined a range of possible water mass fractions for each planet consistent with the modelled planetary structures. We calculated the tidal heating experienced by these exoplanets through gravitational interactions with their host stars, assuming a temperature- and composition dependent Maxwell viscoelastic rheology. Assuming radioactive elemental abundances observed in Solar System meteorites, we also calculated the radiogenic heat flux inside the planets. We estimated the probability of the presence of a thick ocean layer in these planets, taking into account the effect of both tidal and radiogenic heating. Our results showed that Proxima Centauri b, Ross 128 b, Teegarden's b and c, GJ 1061 c and d, and TRAPPIST-1 e may have an extended liquid water reservoir. Furthermore, extremely high H2O-content of the exoplanets Kepler-62 f, Kepler-1652 b, Kepler-452 b, and Kepler-442 b suggests that these planets may maintain a water vapour atmosphere and may in fact be examples of larger ocean worlds. Upon the discovery of new rocky exoplanets beyond the habitable zone, our study can be extended to icy worlds.

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arXiv2019

The mass and density of the dwarf planet (225088) 2007 OR10

The satellite of (225088) 2007 OR10 was discovered on archival Hubble Space Telescope images and along with new observations with the WFC3 camera in late 2017 we have been able to determine the orbit. The orbit's notable eccentricity, e$\approx$0.3, may be a consequence of an intrinsically eccentric orbit and slow tidal evolution, but may also be caused by the Kozai mechanism. Dynamical considerations also suggest that the moon is small, D$_{eff}$ $<$ 100 km. Based on the newly determined system mass of 1.75x10$^{21}$ kg, 2007 OR10 is the fifth most massive dwarf planet after Eris, Pluto, Haumea and Makemake. The newly determined orbit has also been considered as an additional option in our radiometric analysis, provided that the moon orbits in the equatorial plane of the primary. Assuming a spherical shape for the primary this approach provides a size of 1230$\pm$50 km, with a slight dependence on the satellite orbit orientation and primary rotation rate chosen, and a bulk density of 1.75$\pm$0.07 g cm$^{-3}$ for the primary. A previous size estimate that assumed an equator-on configuration (1535$^{+75}_{-225}$ km) would provide a density of 0.92$^{+0.46}_{-0.14}$ g cm$^{-3}$, unexpectedly low for a 1000 km-sized dwarf planet.

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arXiv2019

Tidal Heating and the Habitability of the TRAPPIST-1 Exoplanets

Context. New estimates of the masses and radii of the seven planets orbiting the ultracool M-dwarf TRAPPIST-1 star permit improved modelling of their compositions, heating by tidal dissipation, and removal of tidal heat by solid-state convection. Aims. Here, we compute the heat flux due to insolation and tidal heating for the inner four planets. Methods. We apply a Maxwell viscoelastic rheology to compute the tidal response of the planets using the volume-weighted average of the viscosities and rigidities of the metal, rock, high-pressure ice and liquid water/ice I layers. Results. We show that TRAPPIST-1d and e can avoid entering a runaway greenhouse state. Planet e is the most likely to support a habitable environment, with Earth-like surface temperatures and possibly liquid water oceans. Planet d also avoids a runaway greenhouse, if its surface reflectance is at least as high as that of the Earth. Planets b and c, closer to the star, have heat fluxes high enough to trigger a runaway greenhouse and support volcanism on the surfaces of their rock layers, rendering them too warm for life. Planets f, g, and h are too far from the star to experience significant tidal heating, and likely have solid ice surfaces with possible subsurface liquid water oceans.

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arXiv2018

Compaction and Melt Transport in Ammonia-Rich Ice Shells: Implications for the Evolution of Triton

Ammonia, if present in the ice shells of icy satellites, could lower the temperature for the onset of melting to 176 K and create a large temperature range where partial melt is thermally stable. The evolution of regions of ammonia-rich partial melt could strongly influence the geological and thermal evolution of icy bodies. For melt to be extracted from partially molten regions, the surrounding solid matrix must deform and compact. Whether ammonia-rich melts sink to the subsurface ocean or become frozen into the ice shell depends on the compaction rate and thermal evolution. Here we construct a model for the compaction and thermal evolution of a partially molten, ammonia-rich ice shell in a one-dimensional geometry. We model the thickening of an initially thin ice shell above an ocean with $10\%$ ammonia. We find that ammonia-rich melts can freeze into the upper $5$ to $10$ kilometers of the ice shell, when ice shell thickening is rapid compared to the compaction rate. The trapping of near-surface volatiles suggests that, upon reheating of the ice shell, eutectic melting events are possible. However, as the ice shell thickening rate decreases, ammonia-rich melt is efficiently excluded from the ice shell and the bulk of the ice shell is pure water ice. We apply our results to the thermal evolution of Neptune's moon Triton. As Triton's ice shell thickens, the gradual increase of ammonia concentration in Triton's subsurface ocean helps to prevent freezing and increases the predicted final ocean thickness by up to $50$ km.

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arXiv2018

Experimental Constraints On The Fatigue of Icy Satellite Lithospheres by Tidal Forces

Fatigue can cause materials that undergo cyclic loading to experience brittle failure at much lower stresses than under monotonic loading. We propose that the lithospheres of icy satellites could become fatigued and thus weakened by cyclical tidal stresses. To test this hypothesis, we performed a series of laboratory experiments to measure the fatigue of water ice at temperatures of $198$ K and $233$ K and at a loading frequency of $1$ Hz. We find that ice is \textit{not} susceptible to fatigue at our experimental conditions and that the brittle failure stress does not decrease with increasing number of loading cycles. Even though fatigue was not observed at our experimental conditions, colder temperatures, lower loading frequencies, and impurities in the ice shells of icy satellites may increase the likelihood of fatigue crack growth. We also explore other mechanisms that may explain the weak behavior of the lithospheres of some icy satellites.

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arXiv2018

Interior Structures and Tidal Heating in the TRAPPIST-1 Planets

With seven planets, the TRAPPIST-1 system has the largest number of exoplanets discovered in a single system so far. The system is of astrobiological interest, because three of its planets orbit in the habitable zone of the ultracool M dwarf. Assuming the planets are composed of non-compressible iron, rock, and H$_2$O, we determine possible interior structures for each planet. To determine how much tidal heat may be dissipated within each planet, we construct a tidal heat generation model using a single uniform viscosity and rigidity for each planet based on the planet's composition. With the exception of TRAPPIST-1c, all seven of the planets have densities low enough to indicate the presence of significant H$_2$O in some form. Planets b and c experience enough heating from planetary tides to maintain magma oceans in their rock mantles; planet c may have eruptions of silicate magma on its surface, which may be detectable with next-generation instrumentation. Tidal heat fluxes on planets d, e, and f are lower, but are still twenty times higher than Earth's mean heat flow. Planets d and e are the most likely to be habitable. Planet d avoids the runaway greenhouse state if its albedo is $\gtrsim$ 0.3. Determining the planet's masses within $\sim0.1$ to 0.5 Earth masses would confirm or rule out the presence of H$_2$O and/or iron in each planet, and permit detailed models of heat production and transport in each planet. Understanding the geodynamics of ice-rich planets f, g, and h requires more sophisticated modeling that can self-consistently balance heat production and transport in both rock and ice layers.

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arXiv2017

On the Origin of Earth's Moon

The Giant Impact is currently accepted as the leading theory for the formation of Earth's Moon. Successful scenarios for lunar origin should be able to explain the chemical composition of the Moon (volatile content and stable isotope ratios), the Moon's initial thermal state, and the system's bulk physical and dynamical properties. Hydrocode simulations of the formation of the Moon have long been able to match the bulk properties, but recent, more detailed work on the evolution of the protolunar disk has yielded great insight into the origin of the Moon's chemistry, and its early thermal history. Here, I show that the community has constructed the elements of an end-to-end theory for lunar origin that matches the overwhelming majority of observational constraints. In spite of the great progress made in recent years, new samples of the Moon, clarification of processes in the impact-generated disk, and a broader exploration of impact parameter space could yield even more insights into this fundamental and uniquely challenging geophysical problem.

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arXiv2017

Formation of Massive Rocky Exomoons by Giant Impact

The formation of satellites is thought to be a natural by-product of planet formation in our Solar System, and thus, moons of extrasolar planets (exomoons) may be abundant in extrasolar planetary systems, as well. Exomoons have yet to be discovered. However, moons larger than 0.1 Earth masses can be detected and characterized using current transit techniques. Here, we show that collisions between rocky planets with masses between a quarter to ten Earth masses can create impact-generated debris disks that could accrete into moons. Collisions between like-sized objects, at oblique impact angles, and velocities near escape speed create disks massive enough to form satellites that are dynamically stable against planetary tides. Impacts of this type onto a superearth between 2 to 7 Earth masses can launch into orbit enough mass to create a satellite large enough to be detected in Kepler transit data. Impact velocity is a crucial controlling factor on disk mass, which has been overlooked in all prior studies of moon formation via planetary collision.

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arXiv2017

Formation of Exomoons: A Solar System Perspective

Satellite formation is a natural by-product of planet formation. With the discovery of nu- merous extrasolar planets, it is likely that moons of extrasolar planets (exomoons) will soon be discovered. Some of the most promising techniques can yield both the mass and radius of the moon. Here, I review recent ideas about the formation of moons in our Solar System, and discuss the prospects of extrapolating these theories to predict the sizes of moons that may be discovered around extrasolar planets. It seems likely that planet-planet collisions could create satellites around rocky or icy planets which are large enough to be detected by currently available techniques. Detectable exomoons around gas giants may be able to form by co-accretion or capture, but determining the upper limit on likely moon masses at gas giant planets requires more detailed, modern simulations of these processes.

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arXiv2016

Recent Tectonic Activity on Pluto Driven by Phase Changes in the Ice Shell

The New Horizons spacecraft has found evidence for geologic activity on the surface of Pluto, including extensional tectonic deformation of its water ice bedrock (see Moore et al., 2016). One mechanism that could drive extensional tectonic activity is global surface expansion due to the partial freezing of an ocean. We use updated physical properties for Pluto and simulate its thermal evolution to understand the survival of a possible subsurface ocean. For thermal conductivities of rock less than 3 W m$^{-1}$ K$^{-1}$, an ocean forms and at least partially freezes, leading to recent extensional stresses in the ice shell. In scenarios where the ocean freezes and the ice shell is thicker than $260$ km, ice II forms and causes global volume contraction. Since there is no evidence for recent compressional tectonic features, we argue that ice II has not formed and that Pluto's ocean has likely survived to present day.

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arXiv2016

Interpreting the Densities of the Kuiper Belt's Dwarf Planets

Kuiper Belt objects with absolute magnitude less than 3 (radius $\gtrsim$500 km), the dwarf planets, have a range of different ice/rock ratios, and are more rock-rich than their smaller counterparts. Many of these objects have moons, which suggests that collisions may have played a role in modifying their compositions. We show that the dwarf planets fall into two categories when analysed by their mean densities and satellite-to-primary size ratio. Systems with large moons, such as Pluto/Charon and Orcus/Vanth, can form in low-velocity grazing collisions in which both bodies retain their compositions. We propose that these systems retain a primordial composition, with a density of about 1.8 g/cm$^3$. Triton, thought to be a captured KBO, could have lost enough ice during its early orbital evolution to explain its rock-enrichment relative to the primordial material. Systems with small moons, Eris, Haumea, and Quaoar, formed from a different type of collision in which icy material, perhaps a few tens of percent of the total colliding mass, is lost. The fragments would not remain in physical or dynamical proximity to the parent body. The ice loss process has not yet been demonstrated numerically, which could be due to the paucity of KBO origin simulations, or missing physical processes in the impact models. If our hypothesis is correct, we predict that large KBOs with small moons should be denser than the primordial material, and that the mean density of Orcus should be close to the primordial value.

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arXiv2015

A Common Origin for Ridge-and-Trough Terrain on Icy Satellites by Sluggish Lid Convection

Ridge-and-trough terrain is a common landform on outer Solar System icy satellites. Examples include Ganymede's grooved terrain, Europa's gray bands, Miranda's coronae, and several terrains on Enceladus. The conditions associated with the formation of each of these terrains are similar: heat flows of order tens to a hundred milliwatts per meter squared, and deformation rates of order $10^{-16}$ to $10^{-12}$ s$^{-1}$. Our prior work shows that the conditions associated with the formation of these terrains on Ganymede and the south pole of Enceladus are consistent with vigorous solid-state ice convection in a shell with a weak surface. We show that sluggish lid convection, an intermediate regime between the isoviscous and stagnant lid regimes, can create the heat flow and deformation rates appropriate for ridge and trough formation on a number of satellites, regardless of the ice shell thickness. For convection to deform their surfaces, the ice shells must have yield stresses similar in magnitude to the daily tidal stresses. Tidal and convective stresses deform the surface, and the spatial pattern of tidal cracking controls the locations of ridge-and-trough terrain.

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arXiv2014

Constraints on planetesimal disk mass from the cratering record and equatorial ridge on Iapetus

Iapetus, the outermost regular satellite of Saturn, has a drastic albedo dichotomy and an equatorial circumferential ridge that reaches heights of 20 km and widths of 70 km. This moon is thought to have formed concurrently with Saturn, and so would have experienced an intense bombardment after its formation. The ridge, which has been inferred to be one of the most ancient features on Iapetus' surface, could reasonably be expected to have been eroded by impacts; however, it has retained long continuous sections and a nearly pristine triangular shape with ridge slopes reaching $\sim$ 40$^{\circ}$. We use these observations, along with crater counts on Iapetus' surface, to constrain the total bombardment mass experienced by the satellite since its formation. The ridge morphology and the crater population recorded on Iapetus indicate it received less than 20% of the bombardment predicted by the classic Nice model for early Solar System evolution. Under the recently proposed scenarios of planetsimal-driven migration of the young outer planets including more realistic disk conditions, our results would imply a planetesimal disk mass of $M_{D}\sim12-34M_{\oplus}$.

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arXiv2014

Topographic Constraints on the Origin of the Equatorial Ridge on Iapetus

Saturn's moon Iapetus has an equatorial ridge system, which may be as high as 20 km, that may have formed by endogenic forces, such as tectonic and convective forces, or exogenic processes such as debris infall. We use high-resolution topographic data to conduct a topographic analysis of the ridge, which suggests a predominantly triangular morphology, with some ridge face slopes reaching 40 degrees, allowing for an exogenic formation mechanism.

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arXiv2014

Tectonic Activity on Pluto After the Charon-Forming Impact

The Pluto-Charon system, likely formed from an impact, has reached the endpoint of its tidal evolution. During its evolution into the dual-synchronous state, the equilibrium tidal figures of Pluto and Charon would have also evolved as angular momentum was transferred from Pluto's spin to Charon's orbit. The rate of tidal evolution is controlled by Pluto's interior physical and thermal state. We examine three interior models for Pluto: an undifferentiated rock/ice mixture, differentiated with ice above rock, and differentiated with an ocean. For the undifferentiated case without an ocean, the Pluto-Charon binary does not evolve to its current state unless its internal temperature $T_i>200$ K, which would likely lead to strong tidal heating, melting, and differentiation. Without an ocean, Pluto's interior temperature must be higher than 240 K for Charon to evolve on a time scale less than the age of the solar system. Further tidal heating would likely create an ocean. If New Horizons finds evidence of ancient tidally-driven tectonic activity on either body, the most likely explanation is that Pluto had an internal ocean during Charon's orbital evolution.

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