The vanishing cryovolcanoes of Ceres Sori, Michael M.; Byrne, Shane; Bland, Michael T. ...
Geophysical research letters,
16 February 2017, Volume:
44, Issue:
3
Journal Article
Peer reviewed
Open access
Ahuna Mons is a 4 km tall mountain on Ceres interpreted as a geologically young cryovolcanic dome. Other possible cryovolcanic features are more ambiguous, implying that cryovolcanism is only a ...recent phenomenon or that other cryovolcanic structures have been modified beyond easy identification. We test the hypothesis that Cerean cryovolcanic domes viscously relax, precluding ancient domes from recognition. We use numerical models to predict flow velocities of Ahuna Mons to be 10–500 m/Myr, depending upon assumptions about ice content, rheology, grain size, and thermal parameters. Slower flow rates in this range are sufficiently fast to induce extensive relaxation of cryovolcanic structures over 108–109 years, but gradual enough for Ahuna Mons to remain identifiable today. Positive topographic features, including a tholus underlying Ahuna Mons, may represent relaxed cryovolcanic structures. A composition for Ahuna Mons of >40% ice explains the observed distribution of cryovolcanic structures because viscous relaxation renders old cryovolcanoes unrecognizable.
Key Points
We hypothesize that viscous flow significantly modifies cryovolcanic structures on Ceres
We find that cryovolcanoes on Ceres are modified on 107–109 year timescales if they are >40% ice by volume
Viscous relaxation as a modification mechanism is consistent with cryovolcanism occurring throughout Cerean history
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FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
Ceres, the largest body in the asteroid belt (940 km diameter), provides a unique opportunity to study the interior structure of a volatile-rich dwarf planet. Variations in a planetary body's ...subsurface rheology and density affect the rate of topographic relaxation. Preferential attenuation of long wavelength topography (≥150 km) on Ceres suggests that the viscosity of its crust decreases with increasing depth. We present finite element (FE) geodynamical simulations of Ceres to identify the internal structures and compositions that best reproduce its topography as observed by the NASA Dawn mission. We infer that Ceres has a mechanically strong crust with maximum effective viscosity ∼1025 Pa s. Combined with density constraints, this rheology suggests a crustal composition of carbonates or phyllosilicates, water ice, and at least 30 volume percent (vol.%) low-density, high-strength phases most consistent with salt and/or clathrate hydrates. The inference of these crustal materials supports the past existence of a global ocean, consistent with the observed surface composition. Meanwhile, we infer that the uppermost ≥60 km of the silicate-rich mantle is mechanically weak with viscosity <1021 Pa s, suggesting the presence of liquid pore fluids in this region and a low temperature history that avoided igneous differentiation due to late accretion or efficient heat loss through hydrothermal processes.
•We use Dawn mission data and FE models to understand the interior of Ceres.•The crust has low density and high strength, suggesting substantial hydrated salts.•Any dehydrated mantle is at least 100 km deep, implying low peak temperatures <600 °C.•The salt and volatile-rich crust is most likely due to freezing of global surface fluids.•The low deep interior temperatures suggest late accretion or convective heat loss.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPUK, ZRSKP
Abstract
The stellar, gaseous and young stellar disks in the LITTLE THINGS sample of nearby dwarf irregular galaxies are fitted with functions to search for correlations between the parameters. We ...find that the H
i
radial profiles are generally flatter in the center and fall faster in the outer regions than the
V
-band profiles, while young stars are more centrally concentrated, especially if the H
i
is more centrally flat. This pattern suggests that the H
i
is turning into molecules in the center, and the molecular clouds are forming stars and FUV. A model that assumes the molecular surface density is proportional to the total gas surface density to a power of 1.5 or 2, in analogy with the Kennicutt–Schmidt relation, reproduces the relationship between the ratio of the visible to the H
i
scale length and the H
i
Sérsic index. The molecular fraction is estimated as a function of radius for each galaxy by converting the FUV to a molecular surface density using conventional calibrations. The average molecular fraction inside 3
R
D
is 23% ± 17%. However, the break in the stellar surface brightness profile has no unified tracer related to star formation.
•We use the shape and gravity field of Vesta determined from the Dawn observations to constrain the asteroid’s internal structure.•Vesta’s shape is not in hydrostatic equilibrium.•Terrains in Vesta’s ...northern hemisphere preserve the pre-impact shape of Vesta and are closer to hydrostatic equilibrium.•The floors of the giant impact basins have the thinnest crust. The basins are surrounded by a belt of a thicker crust.•Inversion for crustal thickness does not reveal an uplift of the crust-mantle boundary to the surface of the giant basins.
We use the shape and gravity field of Vesta determined from observations of the Dawn spacecraft to place constraints on the asteroid’s interior structure. We compute a three-layer interior structure model by minimizing the power of the residual gravity anomaly. The densities of the mantle and crust are based on constraints derived from the Howardite–Eucrite–Diogenite (HED) meteorites.
Vesta’s present-day shape is not in hydrostatic equilibrium. The Rheasilvia and Veneneia impact basins have a large effect on Vesta’s shape and are the main source of deviation from hydrostatic shape. Constraining a pre-giant-impact rotation rate and orientation of the spin axis from an ellipsoidal fit to the parts of Vesta unaffected by the giant impacts, and using the theory of figure, we can constrain the shape of the core.
Our solution for Vesta’s crust–mantle interface reveals a belt of thick crust around Rheasilvia and Veneneia. The thinnest crust is in the floor of the two basins and in the Vestalia Terra region. Our solution does not reveal an uplift of the crust–mantle boundary to the surface in the largest basins. This, together with the lack of olivine detected by the Visible and Infrared Spectrometer (VIR) data in Rheasilvia and Veneneia, indicates that Vesta’s presumed olivine mantle was either not brought to the surface by these large impacts or was covered by ejecta from subsequent impacts.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPUK
•Here we provide an introduction to the Icarus special issue about Occator crater.•We summarize studies of Ceres that occurred before the Dawn mission.•We review the updates Dawn-derived studies have ...made to our understanding of Ceres.•We introduce Occator crater and discuss its regional setting.•We discuss the motivation/goals of our investigation of Occator and its faculae.
Ceres, the largest body in the asteroid belt, was explored from orbit for the first time by the Dawn mission. The ∼92 km-diameter Occator crater contains bright regions, or faculae, which are one of the most remarkable discoveries of Dawn's exploration of Ceres. A pit within the center of the crater contains both the Cerealia Facula and a central dome. The Vinalia Faculae are located in the eastern crater floor. Occator's faculae are composed mostly of sodium carbonate and their single scattering albedo is the highest of any material on Ceres’ surface. The faculae do not occur in other impact craters on Ceres and Occator's extensive interior lobate materials are unusual. Understanding the driving forces behind the formation of Occator crater and its faculae has the potential to lead us to a new understanding of the processes and conditions in Ceres’ past, and to possibly provide constraints about the present-day internal state. In this special issue we present a variety of investigations, which use Dawn data, theoretical modeling and laboratory experiments to investigate Occator crater and its faculae. The results of these investigations are summarized and synthesized in the final paper of this collection. Herein we introduce Ceres and the Dawn mission and summarize the geophysical, geological, mineralogical and geochemical properties of Ceres as derived from Dawn data. We also discuss the regional setting of Occator crater and introduce Occator crater itself.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPUK
The asteroid Vesta is a differentiated planetesimal from the accretion phase of Solar System formation. Although its present-day shape is dominated by a non-hydrostatic fossil equatorial bulge and ...two large, mostly unrelaxed impact basins, Vesta may have been able to approach hydrostatic equilibrium during a brief early period of intense interior heating. We use a finite element viscoplastic flow model coupled to a 1D conductive cooling model to calculate the expected rate of relaxation throughout Vesta’s early history. We find that, given sufficient non-hydrostaticity, the early elastic lithosphere of Vesta experienced extensive brittle failure due to self-gravity, thereby allowing relaxation to a more hydrostatic figure. Soon after its accretion, Vesta reached a closely hydrostatic figure with <2km non-hydrostatic topography at degree-2, which, once scaled, is similar to the maximum disequilibrium of the hydrostatic asteroid Ceres. Vesta was able to support the modern observed amplitude of non-hydrostatic topography only >40–200My after formation, depending on the assumed depth of megaregolith. The Veneneia and Rheasilvia giant impacts, which generated most non-hydrostatic topography, must have therefore occurred >40–200My after formation. Based on crater retention ages, topography, and relation to known impact generated features, we identify a large region in the northern hemisphere that likely represents relic hydrostatic terrain from early Vesta. The long-wavelength figure of this terrain suggests that, before the two late giant impacts, Vesta had a rotation period of 5.02h (6.3% faster than present) while its spin axis was offset by 3.0° from that of the present. The evolution of Vesta’s figure shows that the hydrostaticity of small bodies depends strongly on its age and specific impact history and that a single body may embody both hydrostatic and non-hydrostatic terrains and epochs.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPUK
The Mercury gravity field, spin-pole axis, rotation period, Love number, and ephemeris have been determined using the complete four years of MESSENGER tracking data from March 2011 to April 2015. The ...pole location and obliquity (1.99 ± 0.12 arcmin) is consistent with previous determinations. Since MESSENGER was in a highly elliptical orbit with periapsis in the far northern hemisphere, the gravity resolution over the surface of Mercury varies greatly from harmonic degree n = 12 at the south pole to n = 154 in a small region near the north pole which was covered with exceptionally low periapsis data near the end of mission. The gravity field MESS160A is determined to n = 160 and shows notable improvement in the correlation with topography. Three different constraint methods are used to generate the gravity field. The nominal method is a Kaula power law of 5 × 10−5/n2 to constraint the coefficients. One alternate constraint uses surface acceleration measurements, which only constrains the unobserved portion of the gravity field. The other constraint method sets the gravity uncertainties using the harmonic spectrum of the gravity derived from topography. The Mercury tidal Love number solution k2 = 0.53 ± 0.03 is larger than previous results but within suggested error bounds. In addition, the technique for estimating the Mercury ephemeris is discussed.
•Mercury gravity field has a maximum resolution of n = 154 or 50 km.•Mercury tidal response Love number estimate is k2 = 0.53.•Localized studies show larger admittance and smoother GTR results.•The orbit of Mercury is converged with an iterative process with the gravity field.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPUK
Heavily cratered terrains dominate the surfaces of asteroid 4 Vesta and dwarf planet 1 Ceres. The data from the Dawn spacecraft allowed reconstruction of high‐resolution shape models of these bodies. ...We used the stereophotoclinometric shape models to compute gravitational slopes and topographic roughness of Vesta and Ceres. We compute the slope distributions of Vesta and Ceres and compare them to those of the other bodies with heavily cratered terrains. The distribution of slopes of Vesta and Ceres has a kink at ≈34°. The slope distribution is steeper for slopes higher than ≈34°. The Moon and Mars have a similar kink but at a lower (shallower) slope (≈29°–30°). We hypothesize that this kink corresponds to the angle of repose, which is higher on the two minor bodies compared to that of Mars and the Moon. We have also analyzed the topographic roughness of Vesta and Ceres. Vesta's topography is characterized by lower roughness in the southern hemisphere that was resurfaced by giant impacts. The roughness of Ceres is mostly controlled by regional‐scale geology with no significant large‐scale variations. Large, fresh craters have smooth ejecta blankets on both Vesta and Ceres unlike previously observed rough ejecta on the Moon, Mars, and Mercury. On Ceres, the smooth ejecta craters also have smooth floors explained by impact slurry. Lineaments of elevated roughness are abundant on Ceres, whereas are nearly absent on Vesta. The roughness maps we produced provide a synoptic view of the surface texture and could be used to aid geologic mapping.
Plain Language Summary
We have computed and analyzed surface slopes of Vesta and Ceres. We find a deficiency of slopes steeper than a certain critical angle. We hypothesize that this angle is the angle of repose. Angle of repose is the natural maximum angle at which a pile of granular material can stay at rest. In addition, we find that this critical angle is approximately 4°–5° higher on Ceres and Vesta compared to Moon and Mars. This reason of this difference is unknown. We have also looked at the surface of roughness of Vesta and Ceres. We found that large, fresh craters on Vesta and Ceres have smooth surroundings, unlike craters on the Moon, Mars, and Mercury that have rough surroundings. On Ceres, craters also have smooth floors explained by impact slurry. Rays of high roughness are often seen on Ceres, whereas are nearly absent on Vesta. The roughness maps we produced provide a global view of the surface texture and could be used to aid geologic mapping.
Key Points
We have computed the gravitational slopes of Vesta and Ceres using the stereophotoclinometric shape models
The Vesta and Ceres slope distributions indicate that the angle of repose could be different on these bodies compared to Mars and the Moon
The produced roughness maps provide a synoptic view of the surface texture and can help geologic mapping of Vesta and Ceres
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
The 27 satellites of Uranus are enigmatic, with dark surfaces coated by material that could be rich in organics. Voyager 2 imaged the southern hemispheres of Uranus’s five largest “classical” ...moons—Miranda, Ariel, Umbriel, Titania, and Oberon, as well as the largest ring moon, Puck—but their northern hemispheres were largely unobservable at the time of the flyby and were not imaged. Additionally, no spatially resolved data sets exist for the other 21 known moons, and their surface properties are essentially unknown. Because Voyager 2 was not equipped with a near-infrared mapping spectrometer, our knowledge of the Uranian moons’ surface compositions, and the processes that modify them, is limited to disk-integrated data sets collected by ground- and space-based telescopes. Nevertheless, images collected by the Imaging Science System on Voyager 2 and reflectance spectra collected by telescope facilities indicate that the five classical moons are candidate ocean worlds that might currently have, or had, liquid subsurface layers beneath their icy surfaces. To determine whether these moons are ocean worlds, and to investigate Uranus’s ring moons and irregular satellites, close-up observations and measurements made by instruments on board a Uranus orbiter are needed.
Planetary gravity fields are measured by various means. The accuracy of the recovered gravitational potential model depends, of course, on the number and accuracy of the measurements, and in ...non-trivial ways upon the measurement configuration and orbit geometry. We derive and present simple analytic expressions which yield estimates of the resulting error spectra, with reasonable accuracy. Actual recovery of gravity models from the relevant data requires much more rigorous analysis, but these models are expected to be helpful in early mission planning activities. We present results of an application to Vesta, used as a validation exercise, and show possible future results for Mars and Venus.
•Simple analytic expressions are presented for error in gravity field estimates.•These are useful in early phase of mission planning.•3 different orbital configurations are examined.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPUK, ZAGLJ, ZRSKP
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