Atmospheric mass loss due to giant impacts: the importance of the thermal component for hydrogen-helium envelopes

Systems of close-in super-Earths display striking diversity in planetary bulk density and composition. Giant impacts are expected to play a role in the formation of many of these worlds. Previous works, focused on the mechanical shock caused by a giant impact, have shown that these impacts can eject large fractions of the planetary envelope, offering a partial explanation for the observed spread in exoplanet compositions. Here, we examine the thermal consequences of giant impacts, and show that the atmospheric loss caused by these effects can significantly exceed that caused by mechanical shocks for hydrogen-helium (H/He) envelopes. When a giant impact occurs, part of the impact energy is converted into thermal energy, heating the rocky core and the envelope. We find that the ensuing thermal expansion of the envelope can lead to a period of sustained, rapid mass loss through a Parker wind, resulting in the partial or complete erosion of the H/He envelope. The fraction of the envelope lost depends on the planet's orbital distance from its host star and its initial thermal state, and hence age. Planets closer to their host stars are more susceptible to thermal atmospheric loss triggered by impacts than ones on wider orbits. Similarly, younger planets, with rocky cores which are still hot and molten from formation, suffer greater atmospheric loss. This is especially interesting because giant impacts are expected to occur $10{-}100~\mathrm{Myr}$ after formation. For planets where the thermal energy of the core is much greater than the envelope energy, the impactor mass required for significant atmospheric removal is $M_\mathrm{imp} / M_p \sim \mu / \mu_c \sim 0.1$, approximately the ratio of the heat capacities of the envelope and core. When the envelope energy dominates the total energy budget, complete loss can occur when the impactor mass is comparable to the envelope mass.Comment: 10 pages, 9 figure

Integrating Machine Learning for Planetary Science: Perspectives for the Next Decade

Machine learning (ML) methods can expand our ability to construct, and draw insight from large datasets. Despite the increasing volume of planetary observations, our field has seen few applications of ML in comparison to other sciences. To support these methods, we propose ten recommendations for bolstering a data-rich future in planetary science.Comment: 10 pages (expanded citations compared to 8 page submitted version for decadal survey), 3 figures, white paper submitted to the Planetary Science and Astrobiology Decadal Survey 2023-203

Constraints on the Distances and Timescales of Solid Migration in the Early Solar System from Meteorite Magnetism

© 2020. The American Astronomical Society. All rights reserved.. The migrations of solid objects throughout the solar system are thought to have played key roles in disk evolution and planet formation. However, our understanding of these migrations is limited by a lack of quantitative constraints on their timings and distances recovered from laboratory measurements of meteorites. The protoplanetary disk supported a magnetic field that decreased in intensity with heliocentric distance. As such, the formation distances of the parent asteroids of ancient meteorites can potentially be constrained by paleointensity measurements of these samples. Here, we find that the WIS 91600 ungrouped C2 chondrite experienced an ancient field intensity of 4.4 ± 2.8 μT. Combined with the thermal history of this meteorite, magnetohydrodynamical models suggest the disk field reached 4.4 μT at ∼9.8 au, indicating that the WIS 91600 parent body formed in the distal solar system. Because WIS 91600 likely came to Earth from the asteroid belt, our recovered formation distance argues that this body previously traveled from ∼10 au to 2-3 au, supporting the migration of asteroid-sized bodies throughout the solar system. WIS 91600 also contains chondrules, calcium-aluminum-rich inclusions and amoeboid olivine aggregates, indicating that some primitive millimeter-sized solids that formed in the innermost solar system migrated outward to ∼10 au within ∼3-4 Myr of solar system formation. Moreover, the oxygen isotopic compositions of proposed distal meteorites (WIS 91600, Tagish Lake and CI chondrites) argue that the CM, CO, and CR chondrites contain micrometer-scale dust and ice that originated in the distal solar system

Implications of Philae Magnetometry Measurements at Comet 67P/Churyumov-Gerasimenko for the Nebular Field of the Outer Solar System

The remanent magnetization of solar system bodies reflects their accretion mechanism, the space environment in which they formed, and their subsequent geological evolution. In particular, it has been suggested that some primitive bodies may have formed large regions of coherent remanent magnetization as a consequence of their accretion in a background magnetic field. Measurements acquired by the Rosetta Magnetometer and Plasma Monitor have shown that comet 67P/Churyumov-Gerasimenko (67P) has a surface magnetic field of less than 0.9 nT. To constrain the spatial scale and intensity of remanent magnetization in 67P, we modeled its magnetic field assuming various characteristic spatial scales of uniform magnetization. We find that for regions of coherent magnetization with ≥10 cm radius, the specific magnetic moment is ≲5 × 10-6 . If 67P formed during the lifetime of the solar nebula and has not undergone significant subsequent collisional or aqueous alteration, this very low specific magnetization is inconsistent with its formation from the gentle gravitational collapse of a cloud of millimeter-sized pebbles in a background magnetic field 3 μT. Given the evidence from other Rosetta instruments that 67P formed by pebble-pile processes, this would indicate that the nebular magnetic field was ≲3 μT at 15-45 au from the young Sun. This constraint is consistent with theories of magnetically driven evolution of protoplanetary disks. ©2019NASA Emerging Worlds Program (grant no. NNX15AH72G)U.S. Rosetta Program (grant no. CREI 1576768

Searching for Subsurface Oceans on the Moons of Uranus Using Magnetic Induction

The icy moons of Uranus may contain subsurface oceans. Such oceans could be detected and characterized using measurements of magnetic fields induced by Uranus' time-varying magnetospheric field. Here we explore this possibility for Uranus's five major moons, with a focus on Ariel. We find that the magnetic field at each moon is dominated by the synodic frequency with amplitudes ranging from ∼4 nT at Oberon up to ∼300 nT at Miranda. If these bodies contain oceans with sufficient thicknesses (>∼3–40 km) and conductivities (>2 S m−1) even underlying relatively thick (∼50 km) ice shells, the induced surface fields should have amplitudes exceeding the typical ∼1 nT sensitivity of spacecraft magnetometry investigations. Furthermore, the magnetic field variations at the moons span periods ranging from 1 to 103 h. These could enable long-term measurements to separately constrain ocean and ice thicknesses and ocean salinity

Magnetic Field Modeling and Visualization of the Europa Clipper Spacecraft

Abstract The goal of NASA’s Europa Clipper Mission is to investigate the habitability of the subsurface ocean within the Jovian moon Europa using a suite of ten investigations. The Europa Clipper Magnetometer (ECM) and Plasma Instrument for Magnetic Sounding (PIMS) investigations will be used in unison to characterize the thickness and electrical conductivity of Europa’s subsurface ocean and the thickness of the ice shell by sensing the induced magnetic field, driven by the strong time-varying magnetic field of the Jovian environment. However, these measurements will be obscured by the magnetic field originating from the Europa Clipper spacecraft. In this work, a magnetic field model of the Europa Clipper spacecraft is presented, characterized with over 260 individual magnetic sources comprising various ferromagnetic and soft-magnetic materials, compensation magnets, solenoids, and dynamic electrical currents flowing within the spacecraft. This model is used to evaluate the magnetic field at arbitrary points around the spacecraft, notably at the locations of the three fluxgate magnetometer sensors and four Faraday cups which make up ECM and PIMS, respectively. The model is also used to evaluate the magnetic field uncertainty at these locations via a Monte Carlo approach. Furthermore, both linear and non-linear gradiometry fitting methods are presented to demonstrate the ability to reliably disentangle the spacecraft field from the ambient using an array of three fluxgate magnetometer sensors mounted along an 8.5-meter (m) long boom. The method is also shown to be useful for optimizing the locations of the magnetometer sensors along the boom. Finally, we illustrate how the model can be used to visualize the magnetic field lines of the spacecraft, thus providing very insightful information for each investigation