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Register a new userA new perspective on interiors of ice-rich planets: Ice-rock mixture rather than a layered structure
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Ice-rich planets formed exterior to the iceline and thus are expected to contain substantial amount of ice (volatiles). The high ice content leads to unique conditions in the interior, under which the structure of a planet may be affected by ice interaction with other metals. We use experimental data of ice-rock interaction at high pressure, and calculate detailed thermal evolution for possible interior configurations of ice-rich planets. We model the effect of migration inward on the ice-rich interior by including the influences of stellar flux and envelope mass loss. We find that rock and ice are expected to remain mixed, due to miscibility at high pressure, in most of the planet interior (>99% in mass) for a wide range of planetary masses. We also find that the deep interior of planetary twins that have migrated to different distances from the star are usually similar, if no mass loss occurs. Significant mass loss results in an interior structure of a mixed ice and rock ball, surrounded by a volatile atmosphere of less than 1% of the planets mass. In this case, the mass of the atmosphere of water / steam is limited by the ice-rock interaction. We conclude that when ice is abundant in planetary interiors the ice and rock tend to stay mixed for giga-years, and the interior structure differs from the simple layered structure that is usually assumed. This finding could have significant consequences on planets observed properties, and it should be considered in exoplanets characterisation.
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We present a phase-space model that simulates Pleistocene ice volume changes based on Earths orbital parameters. Terminations in the model are triggered by a combination of ice volume and orbital forcing and agree well with age estimates for Late Pleistocene terminations. The average phase at which model terminations begin is approximately 90 +/- 90 degrees before the maxima in all three orbital cycles. The large variability in phase is likely caused by interactions between the three cycles and ice volume. Unlike previous ice volume models, this model produces an orbitally driven increase in 100-kyr power during the mid-Pleistocene transition without any change in model parameters. This supports the hypothesis that Pleistocene variations in the 100-kyr power of glacial cycles could be caused, at least in part, by changes in Earths orbital parameters, such as amplitude modulation of the 100-kyr eccentricity cycle, rather than changes within the climate system.
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Exoplanets number in their thousands, and the number is ever increasing with the advent of new surveys and improved instrumentation. One of the most surprising things we have learnt from these discoveries is not that small-rocky planets in their stars habitable zones are likely common, but that the most typical size of exoplanet is that not seen in our solar system - radii between that of Neptune and the Earth dubbed mini-Neptunes and super-Earths. In fact, a transiting exoplanet is four times as likely to be in this size regime than that of any giant planet in our solar system. Investigations into the atmospheres of giant hydrogen/helium dominated exoplanets has pushed down to Neptune and mini-Neptune sized worlds revealing molecular absorption from water, scattering and opacity from clouds, and measurements of atmospheric abundances. However, unlike measurements of Jupiter, or even Saturn sized worlds, the smaller giants lack a ground truth on what to expect or interpret from their measurements. How did these sized worlds form and evolve and was it different from their larger counterparts? What is their internal composition and how does that impact their atmosphere? What informs the energy budget of these distant worlds? In this we discuss what characteristics we can measure for exoplanets, and why a mission to the ice giants in our solar system is the logical next step for understanding exoplanets.
The discovery of a large putative impact crater buried beneath Hiawatha Glacier along the margin of the northwestern Greenland Ice Sheet has reinvigorated interest into the nature of large impacts into thick ice masses. This circular structure is relatively shallow and exhibits a small central uplift, whereas a peak-ring morphology is expected. This discrepancy may be due to long-term and ongoing subglacial erosion but may also be explained by a relatively recent impact through the Greenland Ice Sheet, which is expected to alter the final crater morphology. Here we model crater formation using hydrocode simulations, varying pre-impact ice thickness and impactor composition over crystalline target rock. We find that an ice-sheet thickness of 1.5 or 2 km results in a crater morphology that is consistent with the present morphology of this structure. Further, an ice sheet that thick substantially inhibits ejection of rocky material, which might explain the absence of rocky ejecta in most existing Greenland deep ice cores if the impact occurred during the late Pleistocene. From the present morphology of the putative Hiawatha impact crater alone, we cannot distinguish between an older crater formed by a pre-Pleistocene impact into ice-free bedrock or a younger, Pleistocene impact into locally thick ice, but based on our modeling we conclude that latter scenario is possible.
Using the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM), we map the temporal variability of water ice absorption bands over the near-polar ice mound in Louth crater, Mars. The absorption band depth of water ice at 1.5 microns can be used as a proxy for ice grain size and so sudden reductions can time any switches from ablation to condensation. A short period of deposition on the outer edge of the ice mound during late spring coincides with the disappearance of seasonal water frost from the surrounding regolith suggesting that this deposition is locally sourced. The outer unit at Louth ice mound differs from its central regions by being rough, finely layered, and lacking wind-blown sastrugi. This suggests we are observing a new stabilizing effect wherein the outer unit is being seasonally replenished with water ice from the surrounding regolith during spring. We observe the transport distance for water migration at Louth crater to be ~4km, and we use this new finding to address why no water ice mounds are observed in craters <9km in diameter.
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