Planetary Science SEMINARS - Spring 2021
Online over Zoom
Wednesdays 1:10-2:00 PM
For a link to the meeting please contact: swahl @ berkeley . edu
19-JanFirst day of instruction
13-JanRaymond PierrehumberOxfordHabitability at the End of the UniverseLow mass stars such as our nearest neighbour Proxima Centauri, will still be shining a trillion years from now, when the Universe is dilute and dark. Does that mean that planets with habitable atmospheres about such stars have all that time for complex life to evolve? The conventional definition of the outer edge of the habitable zone for planets with a CO2-H2O supported greenhouse assumes that the only constraints on atmospheric CO2 are determined by radiative and thermodynamics limits. In reality, geochemical processes determining the balance between outgassing sources and weathering sinks of CO2 provide important additional constraints. The implications of these constraints for the outer edge of the habitable zone, and for the lifetime of habitability, and for the prospects for emergence of complex multicellular life are explored. The constraints are particularly important of planets orbiting stars somewhat less massive than the Sun, which are correspondingly more long-lived. A key novel feature of the results presented here is the use of the weathering formulation based on work by Maher and Chamberlain, which yields substantially different behaviour from the commonly used WHAK formulation.Dr. William Newman (UCLA)
20-JanKennda Lynch LPISubsurface, Subaqueous, and Salty: Looking for Life in All the Right PlacesOne of the key recommendations from the recent National Academies study on the state of astrobiology science is that “NASA’s programs and missions should reflect a dedicated focus on research and exploration of subsurface habitability…” and in particular the focused study of saline/hypersaline subsurface habitats. On Earth, one such environment that could serve as an excellent model for the study of salty subsurface life on Mars, Ocean or icy worlds, and beyond is the Pilot Valley Basin in Northwest Utah. In particular, recent experimental results show that perchlorate reducing bacteria co-exist with the naturally-occurring perchlorate (NOP) present in the basin. Research results also indicate strong evidence of active microbial reduction of the NOP throughout the basin. This is the first potential finding of in situ perchlorate metabolism in hypersaline environment and could serve as a model for possible extant ecosystems on habitable worlds. In this seminar, I will discuss my groups efforts to understand the progress for understanding the characteristics of microbial (per)chlorate reduction in a planetary analog environment as a guide to elucidating the possible influence of chlorine oxyanions on habitability & life on Mars, Ocean Worlds, and Beyond.Dr. William Newman (UCLA)
27-JanLaurie Barge JPLSearching for Signs of Life and its Origin on Other WorldsIs there life elsewhere in the solar system, and if so how can we find it? Although Earth provides a variety of examples of what biology can look like, examples of the critical steps between abiotic and biotic systems are lacking because the prevalence of life on our planet has erased its record of prebiotic conditions. The distinction between biotic and abiotic is still often unclear, since we are still learning about the limits of life, and also because abiotic chemistry can become more complex when devoid of biological influence. However, prebiotic chemistry may still be a current or formerly active process on other worlds with detected chemical gradients and organics, such as Enceladus, Ceres, or Mars. In this talk I will discuss how astrobiologists approach the search for life on other planets, and describe some of the difficulties in distinguishing living and non-living systems. In particular, I will share some of our group’s lab work on simulating prebiotic chemical systems that aim to bridge the gap between geochemistry and biochemistry, and will discuss some of the challenges in characterizing such systems using mission-relevant instruments.Dr. William Newman (UCLA)
3-FebYoshinori MiyazakiCaltechThe mode of geodynamics in the Hadean and its implication for early life The early Earth experienced large-scale melting owing to giant impacts during the last phase of formation. This indicates the possibility of substantial differentiation during the solidification of a magma ocean, and its aftermath has likely been crucial for characterizing the surface environment of the Hadean. To explore possible mantle structures after the solidification, we solved for the thermochemical evolution of magma ocean, using constraints from recent high-pressure melting experiments. Results suggest that, depending on the mechanism that drives the differentiation, the length scale of chemical heterogeneity differs significantly, which would further affect the plate velocity and crustal thickness of the Hadean mantle. Considering that Earth already had a present-day like moderate climate by the end of the Hadean, we suggest that a wet, depleted mantle with small-scale chemical heterogeneity most naturally explains the swift transition from a fiendish to a habitable surface environment. The resemblance between the Hadean seafloor and the Lost City hydrothermal field will also be discussed, which may imply the emergence of early life.Sean Wahl (UC Berkeley)
10-FebBagheri AmirhosseinETH ZurichDynamical evidence for Phobos and Deimos as remnants of a disrupted common progenitor
Despite numerous flyby and ground-based observations, the origin of the Martian moons, Phobos and Deimos, remains elusive. While their surface morphology, and cratered surfaces suggest an asteroidal origin, capture has been questioned because of potential dynamical difficulties of achieving the current near-circular, near-equatorial orbits. To circumvent this, in situ formation models of the moons, have been proposed as alternatives. Yet, explaining the current separation of the moons, their small sizes, and any apparent compositional differences with Mars has proved more challenging. Here, we combine geophysical and tidal evolution modeling of the Mars-satellite system, to demonstrate that Phobos and Deimos originated from a common parent body. We show that tidal dissipation within a Mars-satellite system, enhanced by the physical libration of the moons, circularises the highly eccentric orbits in <2.7 Gyr and makes Phobos descend from its point of origin close to or even above Mars’s synchronous radius to its present orbit, explaining the current separation of the moons. While the parent body could have been formed in situ, we demonstrate that Phobos is unlikely to have originated alongside Mars, given a lifetime of <3 Gyr. By exploiting seismic data from the ongoing Mars InSight mission improved knowledge of the frequency-dependence of dissipation and therefore the satellites’ orbital history is obtainable. The novel geophysical and tidal evolution models presented here can be employed to shed further light on the origin and dynamical evolution of the moons in the outer Solar system and exoplanetary objectsBurkhard Militzer (UCB)
17-FebMichael SoriPurdue Ice and fire on Mars and Triton: Do polar caps reveal planetary heat?Internal heat is an essential quantity in understanding a planet’s geophysical evolution, but is challenging to measure beyond Earth. A partial remedy is in surface features that are products of or affected by geothermal heat, and thus may be used to constrain it. Here, I consider extraterrestrial ice sheets. On Mars, I will consider the H2O polar caps and a proposed identification of basal liquid water. I will show that this putative detection requires unusually high geothermal heat that implies either local magmatism or that the liquid water interpretation is incorrect. On Neptune’s moon Triton, I will consider the N2 ice cap and its unusually great lateral extent. I will show that the nitrogen ice’s large extent can plausibly result from viscous spreading or basal melting. Viscous spreading can be sourced from radiogenic heat alone, but basal melting requires an additional heat source that would explain the moon’s youthful surface and endogenic activity. On Mars and Triton, the same physics creates different outcomes because of differences in geothermal heat, surface temperature, and ice properties, leading to a more holistic understanding of the connections between internal heat and surface features throughout the Solar System.Anton Ermakov (UCB)
24-FebFrederica CoppariLLNLLaser-compression and X-ray diffraction experiments to understand material behavior deep inside planetsThe use of lasers to induce extreme compression states has enabled the study of material properties and equations of state at unprecedented pressures and temperature conditions. Combined with ultra-fast x-ray diagnostics these techniques allow us to probe in-situ the transformation happening in matter subjected to extreme conditions, providing key insights into material behavior at pressures and temperatures existing deep inside planets.
In this talk, I will present the results of recent x-ray diffraction experiments on laser-compressed water ice and oxides, the main components of the mantle of ice giant and terrestrial planets. (This work was performed under the auspices of the US Department of Energy by Lawrence Livermore National Laboratory under Contract No. DE-AC52-07NA27344)
Burkhard Militzer (UCB)
3-MarCaroline DornUniversity of ZurichDiversity of Super-Earth interiorsThe increasing number of newly discovered extrasolar planets reveal a remarkable diversity in planet sizes and mean densities. Among the most frequently occurring planets are super-Earths and mini-Neptunes, whose interiors are largely unknown. Detailed interior characterization is challenging since data are few and have significant uncertainties. Given data of mass and radius alone, many interiors can be modelled and it is key to make additional constraints available. With the focus on Super-Earths, I will discuss a range of additional constraints that help to improve interior estimates and I will highlight main achievements in finding exotic and less exotic worlds.Diogo Lourenço (UCB)
10-MarSuzanne SmrekarJPLA once and future Earth? Geodynamic lessons from VenusVenus is often referred to Earth’s twin due to its similar size, density, and distance to the sun. Its intense greenhouse atmosphere creates a parched surface that acts as a cautionary climate tale. It also creates a hot lithosphere, possibly similar to that of early Earth. Venus is the only other body besides Earth that may have analogs of continents, and subduction – but no plate tectonics. There is little data from the period when subduction and continents first formed. Studying these processes on another body provide insights into the conditions needed to produce subduction, the first step in plate tectonics. This talk discusses new research on the evidence for present day volcanism, subduction, and variable lithospheric thickness along with the implications for geodynamic evolution of non-plate tectonic planets.Will Newman (UCLA)
17-MarSarah MillhollandPrincetonTidal Sculpting of Short-Period ExoplanetsMultiple-planet systems composed of close-in super-Earth/sub-Neptune-sized planets are ubiquitous, representing a dominant outcome of planet formation. This population exhibits predictable hallmarks of architectural regularity and uniformity, such as low eccentricities and inclinations, similar orbital spacings, and intra-system correlations in planetary masses and radii. On top of this first-order structure, however, these systems also exhibit surprising anomalies that require explanation. Examples include (1) ultra-short period planets, whose extremely-irradiated orbits have been separated off from the rest of their systems; (2) planets piled up wide of mean-motion resonances; and (3) a subset of Neptune-sized planets that show signs of radius inflation. In this talk, I will propose that tidal dynamics can account for these specific anomalies and more. Specifically, I will discuss the critical role of enhanced tidal dissipation due to non-zero planetary axial tilts (obliquities), which arise by way of prevalent dynamical resonances. I will highlight strategies for testing these tidal theories and observing obliquities directly in the future.Marta Bryan (UCB)
UCLA quarter ends March 19
24-MarUCB spring break
31-MarVictor RobinsonICTP TriestePhase transitions in molecules and metals under pressureMolecular mixtures at planetary conditions, such as water+ammonia and water+methane, are expected to be found in many celestial bodies such as Uranus and Neptune. Here we find new ionic phases and mixtures of ammonia and water were predicted up to 500 GPa, and now recently measured [1], using crystal structure searching methods. These solid mixtures were then heated to reveal the expected superionic transition before melting, but also find a plastic regime at lower temperatures associated with various forms of molecular movements. At lower pressures, liquid methane and water have been observed to mix and here we perform long AIMD simulations between 0 and 2.3 GPa combined with neutron scattering data to examine the structure and increased solubility of methane in water. The second half of this talk examines the effect of high pressure on the previously considered simple alkali metals. These elements enter complex phases, such as host-guest and electride structures, following the typical bcc and fcc once compressed. Here we investigated the ideal host-guest atom ratio and found that these phases have moderate electride character, shown by a build of electron localization in non-nuclear sites. These host-guests structures also show an order-disorder transition, known as "chain-melting", and here we trained a machine-learned interatomic potential to reveal how these guest chains disorder with temperature. Making further use of this potential and AIMD for the liquid, we show that alkali metals localize electrons on non-nuclear sites as "pseudoanions" under sufficient pressure during a continuous liquid-liquid transition.
[1] Phys. Rev. Lett. 126, 015702 (2021)
7-AprYifan ZhouUT AustinMeasuring the Accretion onto the Young Gas Giant Planet PDS 70 b with HST UV and H-alpha imagesThe discoveries of protoplanets PDS 70 b and c offer an excellent opportunity to probe the planet formation process directly. Residing within the transition disk gap, these two planets exhibit clear evidence of ongoing accretion through their intense Hα emissions. An accurate mass accretion rate measurement requires constraining hydrogen continuum emissions, which likely carry most of the energy released from the planetary accretion shocks. We observed the PDS 70 system with HST in the U (F336W) and the H-alpha bands. We incorporated a suite of novel image processing and angular differential imaging techniques in our observations and detected the planet PDS 70 b in both bands with high significance. These results led to the first direct imaging detection of an exoplanet in the UV and a new accretion rate measurement for PDS 70 b. The U-band photometry of PDS 70 b confirmed that accretion excess emission significantly elevates the planet's spectrum on the blue side of the Balmer jump. Our observations also placed an upper limit on the planet's Hα variability over a five-month timescale. In this presentation, I will introduce the observational methods that enabled the detections, demonstrate our new accretion rate measurement, and discuss the new insight into the formation process of PDS 70 b. Marta Bryan (UCB)
14-AprSei-ichiro WatanabeNagoya UniversityHayabusa2 at Ryugu: journey to the origin of EarthHayabusa2 explored C-type asteroid Ryugu from June 2018 to November 2019 and returned to Earth with surface samples. In December 2020, more than 5 g of particles in the sample catchers were successfully retrieved and now under analysis. I’ll review the interdisciplinary science of Hayabusa2 mission based on the proximity observations through visible and thermal IR imaging, NIR spectrometer, LIDAR, and the Small Carry-on Impactor (SCI) experiment. The SCI experiment reveals crater-to-impactor size ratio on Ryugu is more than 60 [1], contrast to the lower value of 10 derived from the size-frequency distributions of the main-belt asteroids and craters on asteroids already explored [2]. This will change the chronology of the material transport process from the main asteroid belt to the Earth region. Based on the SCI scaling law, the surface age of Ryugu is estimated to be ~16 Ma, which is much younger than the estimated ages (100 Ma to 1 Ga) of the candidate source collisional families in the inner asteroidal belt. This suggests that Ryugu is a product of a higher generation of the parent body disruption [3] or global resurfacing due to YORP induced past rapid rotation [4]. Visible color variations on Ryugu suggest the reddening of surface material by solar heating and/or space weathering under a temporal excursion near the Sun [5]. Proximity observations using NIRS3 reveals the presence of global OH-bearing minerals on Ryugu [6], whereas the amount is smaller compared with hydrated carbonaceous chondrites and Bennu [7]. Both partial dehydration and incipient aqueous alteration might produce the weak OH absorption [3], which is about to be distinguished by return sample analyses. Size-frequency distribution of surface particles as well as aggregate’s constituent particles are important for identifying what kind of dust grains planetesimals are consist of. I hope to discuss a vision of exploration-based reconstruction of planetesimals.

[1] Arakawa+ (2020), Science 368, 67; [2] Bottke+ (2020), Astron. J. 160, 14; [3] Sugita+ Science 364, eaaw0422, [4] Watanabe+, Science 364, 268; [5] Morota+ (2020), Science 368, 654; [6] Kitazato+ (2019), Science 364, 272; [7] Hamilton+ (2019), Nature Astron. 3, 332.
Anton Ermakov (UCB)
21-Aprcancelled - no seminar
28-AprRuth Murray-ClayUC Santa CruzTDBTDBBurkhard Militzer (UCB)
5-MayJuliette BeckerCaltechLifetimes of Multi-Planet Systems Around Evolving StarsRecent observational advances have allowed the discovery of thousands of exoplanets and an initial characterization of their orbital and physical parameters. As this population has grown, it is increasingly apparent that existing models of planet formation are incomplete. In this talk, I will describe what we can learn about the lifetimes of exoplanet systems through studying a large number of exoplanet systems of varying ages with diverse orbital architectures. Resolving the conflicts in existing theories of planet formation requires using the current dynamical state and histories of benchmark systems to infer the range of allowable system states and how those states change with time. I will describe several recent exciting discoveries and the constraints they have provided on the boundary conditions of our understanding of planet formation.Marta Bryan (UCB)
Last day of instruction at UCB
12-MaySarah ArvesonUCBMelting Experiments at High Pressures & Immiscibility in Iron Alloys: Layered Liquids in Earth’s Outer Core?Earth's outer core is composed of a liquid iron alloy with up to 10% of unknown light elements, likely silicon, oxygen, sulfur, carbon, or hydrogen. The release of these light elements upon freezing of the solid iron inner core plays an important role in sustaining Earth’s magnetic field, but the exact chemical makeup of the core is widely debated. In this talk, I will discuss high-pressure, high-temperature melting experiments and first-principles simulations on iron alloys containing silicon and oxygen which reveal that two distinct liquids form at high pressures. The presence of immiscible liquids may explain a seismically observed stratified layer atop the outer core and suggests that an Fe-Si-O composition can explain multiple observations of the outer core.Diogo Lourenço (UCB)
19-MaySonia TikooStanfordVariability of the ancient lunar dynamo fieldThe ancient Moon generated a dynamo magnetic field that started no later than 4.25 billion years ago (Ga) and ceased sometime after 1.8 Ga. During the period between approximately 3.85 and 3.56 Ga, the lunar surface field intensity at times rivaled that of the modern terrestrial field. However, during the same period, Apollo-era lunar paleointensity studies indicate that the field may have fluctuated in intensity by an order of magnitude or more on <100 My timescales. In this presentation, I will explore possible origins of variability in the lunar paleointensity record and discuss how they may be explained by various hypothesized lunar dynamo generation mechanisms. Anton Ermakov (UCB)
26-MayEdwin KiteUniversity of ChicagoHow did Mars' surface become uninhabitable?Mars’ river valleys are dry today. What allowed rivers and lakes on Early Mars, which received just 1/3 of the modern Earth’s insolation? And why did Mars’ surface become uninhabitable? Data from rovers and orbiters have revealed a rich stratigraphic record of climate-sensitive deposits, allowing models to be tested. We have found that the greenhouse effect of high-altitude water ice clouds is a possible explanation for the warm climates – but only if the surface was arid, consistent with the geologic record. A new synthesis of geologic data and models suggests that water loss, CO2 loss, and loss of non-CO2 greenhouse forcing combined in surprising ways to set Mars’ habitability trajectory. While lake-forming climates on Mars occurred over a time span of >1 Gyr, now Mars’s surface is too cold and dry for life. I will discuss ways in which Martian surface habitability could be re-enabled.Diogo Lourenço (UCB)