Telescopic observations of Kuiper Belt objects have enabled bulk density determinations for 18 objects. These densities vary systematically with size, perhaps suggesting systematic variations in bulk ...composition. We find this trend can be explained instead by variations in porosity arising from the higher pressures and warmer temperatures in larger objects. We are able to match the density of 15 of 18 KBOs within their 2σ errors with a constant rock mass fraction of 70%, suggesting a compositionally homogeneous, rock-rich reservoir. Because early 26Al would have removed too much porosity in small (∼ 100 km) KBOs we find the minimum formation time to be 4 Myr after solar system formation. This suggests that coagulation, and not gravitational collapse, was the dominant mechanism for KBO formation, or the gas disk lingered in the outer solar system. We also use this model to make predictions for the density of Makemake, 2007 OR10, and MU69
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
•Pluto and Charon are rock rich while the small satellites are mostly water ice.•Charon is about 10% icier than Pluto.•A giant impact origin involving partially differentiated precursors ...supported.•Formation of entire PC system in a collapsing, rotating pebble cloud not supported.•Slow, late accretion of impact precursors indicated.
New Horizon's accurate determination of the sizes and densities of Pluto and Charon now permit precise internal models of both bodies to be constructed. Assuming differentiated rock-ice structures, we find that Pluto is close to 2/3 solar-composition anhydrous rock by mass and Charon 3/5 solar-composition anhydrous rock by mass. Pluto and Charon are closer to each other in density than to other large (≳1000-km diameter) Kuiper belt bodies. Despite this, we show that neither the possible presence of an ocean under Pluto's water ice shell (and no ocean within Charon), nor enhanced porosity at depth in Charon's icy crust compared with that of Pluto, are sufficient to make Pluto and Charon's rock mass fractions match. All four small satellites (Styx, Nix, Kerberos, Hydra) appear much icier in comparison with either Pluto or Charon. In terms of a giant impact origin, both these inferences are most consistent with the relatively slow collision of partly differentiated precursor bodies (Canup, Astrophys. J. 141, 35, 2011). This is in turn consistent with dynamical conditions in the ancestral Kuiper belt, but implies that the impact precursors themselves accreted relatively late and slowly (to limit 26Al and accretional heating). The iciness of the small satellites is not consistent with direct formation of the Pluto–Charon system from a streaming instability in the solar nebula followed by prompt collapse of gravitationally bound “pebble piles,” a proposed formation mechanism for Kuiper belt binaries (Nesvorný et al., Astron. J. 140, 785–793, 2010). Growth of Pluto-scale bodies by accretion of pebbles in the ancestral Kuiper belt is not ruled out, however, and may be needed to prevent the precursor bodies from fully differentiating, due to buried accretional heat, prior to the Charon-forming impact.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
Venus is an exceptional natural experiment to test our understanding of atmospheric sulfur chemistry. Previous modeling efforts have focused on understanding either the middle or lower atmosphere. In ...this work, we performed the first full atmosphere analysis of the chemical transport processes on Venus from the surface to 110 km using a 1‐D diffusion model with photochemistry. We focused on the cycling of chemical species between the upper and lower atmospheres and interactions between distinct species groups including SO
x, CO
x + OCS, chlorides, NO
x, O
x, and S
x. We tested different eddy diffusivity profiles and investigated their influences on the vertical profiles of important species. We find that the assumed boundary conditions in previous models strongly impacted their simulation results. This has a particularly large effect for SO
2. We find the high SO
2 abundance in the lower atmosphere is readily transported into the middle atmosphere, far exceeding observed values. This implies some yet unknown chemistry or process limiting SO
2 mixing. We summarize outstanding questions raised by this work and note chemical reactions that should be the highest priority for future laboratory studies and ab initio calculations.
Plain Language Summary
Venus's atmosphere can be broadly separated into lower and middle regions, separated by a thick cloud deck. Chemistry in the lower atmosphere is controlled by the high temperatures below the clouds. In the middle atmosphere, photochemistry stimulated by solar UV radiation is dominant. Previous works have modeled either the lower or middle atmosphere to understand these chemical processes. In this work, we create a single model that encompasses both regions to understand how chemical species are cycled. We find that the large abundance of SO
2 in the lower atmosphere is transported into the middle atmosphere, far exceeding what is observed. We argue that this suggests some as of yet unknown chemistry or process that is limiting the SO
2 flux from the lower atmosphere.
Key Points
We perform the first thorough analysis of a Venus atmospheric chemistry model that extends from the surface to 110 km
Some previously proposed chemical pathways break down when more complete chemistry is used
To match observed abundances SO
2 must be inhibited from diffusing through the clouds by some unknown process
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Observations by the Shallow Radar instrument on Mars Reconnaissance Orbiter reveal several deposits of buried CO2 ice within the south polar layered deposits. Here we present mapping that ...demonstrates this unit is 18% larger than previously estimated, containing enough mass to double the atmospheric pressure on Mars if sublimated. We find three distinct subunits of CO2 ice, each capped by a thin (10–60 m) bounding layer (BL). Multiple lines of evidence suggest that each BL is dominated by water ice. We model the history of CO2 accumulation at the poles based on obliquity and insolation variability during the last 1 Myr assuming a total mass budget consisting of the current atmosphere and the sequestered ice. Our model predicts that CO2 ice has accumulated over large areas several times during that period, in agreement with the radar findings of multiple periods of accumulation.
Key Points
Mars' south polar cap contains a deposit with enough CO2 to double the atmospheric pressure if it sublimated
Surface and radar observations show this deposit to be three CO2 subunits capped by water ice
Modeling allows us to constrain the minimum age of this deposit to be ∼300 kyr old
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The circulation in Europa's ocean determines the degree of thermal, mechanical and chemical coupling between the ice shell and the silicate mantle. Using global direct numerical simulations, we ...investigate the effect of heterogeneous tidal heating in the silicate mantle on rotating thermal convection in the ocean and its consequences on ice shell thickness. Under the assumption of no salinity or ocean‐ice shell feedbacks, we show that convection largely transposes the latitudinal variations of tidal heating from the seafloor to the ice, leading to a higher oceanic heat flux in polar regions. Longitudinal variations are efficiently transferred when boundary‐driven thermal winds develop, but are reduced in the presence of strong zonal flows and may vanish in planetary regimes. If spatially homogeneous radiogenic heating is dominant in the silicate mantle, the ocean's contribution to ice shell thickness variations is negligible compared to tidal heating within the ice. If tidal heating is instead dominant in the mantle, the situation is reversed and the ocean controls the pole‐to‐equator thickness contrast, as well as possible longitudinal variations.
Plain Language Summary
Europa, an icy moon of Jupiter, is believed to have a deep salty ocean beneath its ice crust. One of the drivers of ocean circulation is heating from the rocky mantle located under the ocean. This heating is due to (a) the decay of radioactive elements in the mantle (“radiogenic heating”), and (b) the periodic deformation of the mantle as Europa revolves around Jupiter, due to the gravitational force exerted by the gas giant (“tidal heating”). Tidal heating is strongly heterogeneous: higher at the poles, and lower at the points facing and opposite to Jupiter. We investigate the effect of large‐scale heating variations using simulations of the ocean dynamics, although at less extreme parameters than the real Europa ocean, and neglecting the effects of salinity and phase change. We show that if tidal heating is dominant, the ocean circulation does not erase the variations of bottom heating and transposes them particularly well in latitude up to the ice‐ocean boundary. This has consequences on the ice shell equilibrium: if mantle heating is heterogeneous, thickness variations could be controlled by the oceanic heat flux, resulting in thinner ice at the poles. These results now await comparison with measurements from Europa Clipper.
Key Points
We use an idealized model of thermally driven flows in Europa's ocean, neglecting salinity and feedback effects of the ice
Heterogeneous tidal heating in the mantle modifies the mean circulation in Europa's ocean and could drive large‐scale thermal winds
The tidal heating anomaly in latitude is efficiently translated upwards, leading to a higher heat flux into the ice shell at the poles
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Observations by the New Horizons spacecraft have determined that Pluto has a larger bulk density than Charon by 153 ± 44 kg m−3 (2σ uncertainty). We use a thermal model of Pluto and Charon to ...determine if this density contrast could be due to porosity variations alone, with Pluto and Charon having the same bulk composition. We find that Charon can preserve a larger porous ice layer than Pluto due to its lower gravity and lower heat flux but that the density contrast can only be explained if the initial ice porosity is ≳ 30%, extends to ≳100 km depth and Pluto retains a subsurface ocean today. We also find that other processes such as a modern ocean on Pluto, self-compression, water-rock interactions, and volatile (e.g., CO) loss cannot, even in combination, explain this difference in density. Although an initially high porosity cannot be completely ruled out, we conclude that it is more probable that Pluto and Charon have different bulk compositions. This difference could arise either from forming Charon via a giant impact, or via preferential loss of H2O on Pluto due to heating during rapid accretion.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
We model the formation of lunar complex craters and investigate the effect of preimpact porosity on their gravity signatures. We find that while preimpact target porosities less than ~7% produce ...negative residual Bouguer anomalies (BAs), porosities greater than ~7% produce positive anomalies whose magnitude is greater for impacted surfaces with higher initial porosity. Negative anomalies result from pore space creation due to fracturing and dilatant bulking, and positive anomalies result from destruction of pore space due to shock wave compression. The central BA of craters larger than ~215 km in diameter, however, are invariably positive because of an underlying central mantle uplift. We conclude that the striking differences between the gravity signatures of craters on the Earth and Moon are the result of the higher average porosity and variable porosity of the lunar crust.
Key Points
The positive gravity signature of craters is due to initial porosity compaction
Porosity is responsible for the observed scatter in the Bouguer anomalies
Mantle uplift dominates the gravity for craters larger than ~215 km in diameter
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Recent results from the Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) instrument have been interpreted as evidence of subsurface brine pooled beneath 1.3 km‐thick South Polar ...Layered Deposit (SPLD). This interpretation is based on the assumption that the regionally high strength of MARSIS radar reflections from the base of the ice cap is due to a strong contrast in dielectric permittivity across the basal interface. Here, we demonstrate that the high‐power reflections could instead be the result of a contrast in electric conductivity. While not explicitly excluding a liquid brine, our results open new potential explanations for the observed strong radar reflections, some of which do not require liquid brine beneath SPLD. Potential basal materials with suitably high conductivity include clays, metal‐bearing minerals, or saline ice.
Plain Language Summary
Previous work reported a regionally strong radar reflection under Mars' south polar ice sheet. Due to its brightness, this radar reflection was interpreted as liquid water (likely with high concentration of dissolved salts). A radar reflection can be bright due to a large contrast in either dielectric permittivity or electric conductivity. Previous work only considered contrasts in dielectric permittivity. We find that contrasts in electric conductivity between materials could also explain the brightness of the reflection. We suggest that this difference could be due to clays, metal‐bearing minerals, or saline ice under the polar ice sheet.
Key Points
Radar reflections can be caused by contrasts in either dielectric permittivity or electric conductivity
Reflections observed by Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) at Mars' south polar region can be explained by contrasts in electric conductivity and do not require liquid water
We propose that a polar‐ice substrate containing clays, other conductive minerals, or saline ice are the most plausible candidate materials
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Magnetic induction measurements and astrometry provide constraints on the internal structure of Io, a volcanically active moon of Jupiter. We model the tidal response of a partially molten Io using ...an Andrade rheology which is supported by silicate deformation experiments. This model uses material properties similar to the Earth's mantle and includes feedbacks between partial melting, tidal heat production, and melt transport. We are able to satisfy constraints provided by the measured imaginary part of the tidal Love number Im(k2), the inferred depth and melt fraction of a near‐surface partially molten layer, and the observed equatorial concentration of volcanic landforms. We predict a value for the real part of the tidal Love number of Re(k2) = 0.09 ± 0.02, much smaller than the value of Re(k2)≈0.5 predicted for an Io with a fluid magma ocean. Future spacecraft observations should be able to measure this value and test which model is correct.
Key Points
We self‐consistently model dissipation and melt production in the interior of Io
If Io has a partially molten magma reservoir, as opposed to a magma ocean, that reservoir must be greater than 100 km thick
We make a prediction of the tidal Love number k2. This value can be measured by spacecraft and used to determine if Io has a magma ocean
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We derive a topography data set from images of Pluto and Charon that contain the body edge (i.e., limb profiles) which will help in understanding the comparative history of the binary system. We use ...the profiles to derive topographic variance spectra and find that while the variance spectrum of Pluto fits a single power law, Charon's spectrum displays a clear breakpoint at ∼150 km wavelength. Assuming the breakpoint is a result of topographic flexure, we find that Charon's elastic thickness must have been 20 ± 10 km during topography formation. A lack of a breakpoint for Pluto sets a minimum elastic thickness for Pluto of 60 km. We use these elastic thickness estimates to calculate a maximum heat flux of ∼13 m Wm−2 on Pluto during and after topography formation. On Charon, however, we find that the heat flux during topography formation was 35−15+44 m Wm−2. This range of values far exceeds the likely radiogenic heat production and is consistent with either heat released following the Charon‐forming impact event or (more likely) tidal heating during Charon's early history.
Plain Language Summary
Studying the topography of planetary bodies provides key insights into the geologic processes of their surfaces and interiors. In this work we develop a topography data set for Pluto and Charon by mapping variations in the height along the worlds’ edges in images from New Horizons. We analyze the data to determine roughness using the mean amplitude of mountains and valleys for a range of widths. Pluto shows the expected result of a single slope decreasing in roughness at shorter widths, but Charon has a change in the slope at ∼150 km. Mountains and valleys on Charon wider than this are respectively shorter and shallower than expected. This gives insight into how the landforms on Charon formed as well as the ability of Charon's crust to support variations in elevation. Charon's landforms must have formed at the observed size or decreased over time to have modern amplitudes. Either case implies that Charon had a thinner ice shell, and was relatively hotter, than Pluto in the ancient past. This extra heat is consistent with a Charon‐forming impact or (more likely) tidal heating during the Charon's initial history.
Key Points
We derived new limb profile topography datasets of Pluto and Charon based on the body edge in images
For Charon, the topographic variance spectrum displays a distinct change in slope at ∼150 km wavelength
Charon's topography records high heat fluxes from either tidal heating or a giant impact
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