Planetary Science Seminar - spring-2023

April 6, 2023
noon - 1 p.m.
3853 Slichter Hall

Presented By:

• Dr. Steve Sholes - Jet Propulsion Laboratory/Caltech

Early Mars is often portrayed as having vast oceans covering its northern hemisphere, yet the evidence used to make claims of such ancient oceans is the subject of great controversy with implications on Mars’ past climate, hydrology, and habitability. Primary evidence for past oceans stem from putative ancient shorelines which face criticism over their formational mechanisms and genetic interpretation. After developing a quantitative toolkit for detecting erosional terraces and shorelines we directly test these features and find substantial evidence that they are unlikely to be marine in origin and a wave of other problems in their mapping and post-formational modification. Additionally, we can apply these same tools to better understand the evolution of the different lake levels at Jezero Crater and integrate it with in situ rover observations.

April 13, 2023
noon - 1 p.m.
3853 Slichter Hall

Presented By:

• Dr. Ashwin Vasavada - Jet Propulsion Laboratory/Caltech

Now in its eleventh year on Mars, the Curiosity rover continues to reveal Mars as a once-habitable planet. Early in Mars' history, water persisted in rivers and lakes for millions of years, and even longer in the subsurface. A diversity of organic molecules remains detectable, although degraded, after billions of years. Curiosity has observed the Martian atmosphere over an unprecedented five annual cycles, witnessed a planetary-scale dust storm, and revealed the activity of atmospheric methane. This talk will summarize the scientific findings from the mission, the challenges of exploration, and what lies ahead.

April 20, 2023
noon - 1 p.m.
3853 Slichter Hall

Presented By:

• Prof. Konstantin Batygin - Caltech

Over the course of the last two decades, few astronomical discoveries have generated broader interest than the detection and characterization planetary systems that encircle other stars. The basic properties of the majority of these planets are different from those of our own solar system in a number of ways: extrasolar worlds have orbital periods that are measured in days rather than years; have masses that exceed the Earth by a factor of a few; appear to be silicate-rich though routinely possessing substantial Hydrogen-Helium atmospheres; and frequently occur in multiples. Beyond these basic attributes, recent work has revealed that short-period extrasolar planets exhibit an intriguing pattern of intra-system uniformity, which stands in sharp contrast with the staggering overall diversity of the Galactic Planetary Census. In this talk, I will discuss a new theoretical picture for rocky planet formation that satisfies the aforementioned constraints. Building upon recent work — which demonstrates that planetesimals can form rapidly at discrete locations in the disk — we propose that super-Earths originate inside rings of silicate-rich planetesimals at approximately ~1 AU. Within the context of this picture, we show that planets grow primarily through pairwise collisions, until they achieve terminal masses that are regulated by isolation and orbital migration. Numerical simulations carried out within this theoretical framework produce synthetic planetary systems that bear a close resemblance to compact, multi-resonant progenitors of the observed population of short-period extrasolar planets. I will further discuss how this model fits into the broader context of the formation of the terrestrial planets of the solar system as well as the Galilean satellites. If time allows, I will also give a brief update on all things Planet 9.

April 27, 2023
noon - 1 p.m.
3853 Slichter Hall

Presented By:

• Dr. Bonnie J. Buratti - Jet Propulsion Laboratory/Caltech

Europa Clipper is the first mission to take a detailed, dedicated look at an Ocean World. The mission will derive the properties of Europa's crust and subsurface ocean, including its potential to serve as a potentially habitable environment, understand its geologic evolution, and map its composition. This talk is an overview of the scientific goals of the mission, its instruments, what we know about Europa so far, and how the speaker's research fits into the mission's objectives.

April 28, 2023
1 p.m. - 2 p.m.
3853 Slichter Hall

Presented By:

• Dr. Anthea Coster - MIT Haystack Observatory

Lise Meitner was one of the pioneers of nuclear physics and co-discoverer, with Otto Hahn and Fritz Strassmann, of nuclear fission. Albert Einstein once called her the most significant woman scientist of the 20th century.´ Yet by the 1970's, her name was nearly forgotten. With the publication of the book by Ruth Lewin Sime, Lise Meitner, A life in physics,´ to some extent her name has resurfaced. The chronology of the discovery of fission is considerably more complex than the facts, and clouded by events beyond the world of science. The facts are that on January 6, 1939, Hahn and Strassmann reported in Naturwissenschaften their chemical findings for fission. On February 11, 1939, Meitner and Frisch published in Nature the physical interpretation of the process they named fission. In 1944, Otto Hahn alone received the Nobel Prize in Chemistry "for his discovery of the fission of heavy nuclei." I became familiar with Lise Meitner and her story when, in 1972, Dr. Sime started writing my father for details about Lise Meitner's escape from Germany. This is because in July 1938, my grandfather, Dirk Coster, was the person who escorted her out of Germany. In Sime's book, Meitner's escape from Germany reads like a spy novel, except that it is completely based in fact. At age 59, Meitner left Germany forever with 10 marks in her purse, one small suitcase, and a diamond ring given to her by Otto Hahn that he had inherited from his mother. This talk will be a combination of facts, excerpts from the film, Path to Nuclear Fission: The Story of Lise Meitner and Otto Hahn´ (a film by Rosemarie Reed), and personal stories heard from my father, aunts, and uncles. Lise Meitner's early years, her role in the discovery of nuclear fission, her escape from Germany, and the consequences that followed will be covered.

May 4, 2023
noon - 1 p.m.
3853 Slichter Hall

Presented By:

• Dr. Renyu Hu - Jet Propulsion Laboratory/Caltech

JWST provides revolutionary capabilities to measure the atmospheric compositions of small exoplanets and the possibility of near-term observational studies of exoplanet habitability. While the number of temperate and rocky exoplanets suitable for JWST characterization is limited, profound progress in understanding the diversity, occurrence rate, and formational conditions of habitable worlds will be made by observing temperate sub-Neptunes (planets with radii between 1.5 and 2.6 times Earth radius) as well as hotter rocky planets. A subset of temperate sub-Neptunes may support liquid-water oceans if they do not have massive H2 atmospheres and are thus not too hot at the bottom of the atmospheres. I will present a novel method to combine transmission spectroscopy and planetary chemistry modeling to determine if temperate sub-Neptunes are capped by thick H2 atmospheres. With a growing sample of temperate sub-Neptunes that straddle the runaway greenhouse limit, these observations may also provide a cornerstone for this foundational theory in planetary science. Hotter rocky planets, on the other hand, are suitable for observations in the thermal infrared and thus provide a testing ground for models of atmospheric evolution and retention. I will discuss using thermal emission spectra to determine the surface or atmospheric compositions in the context of a JWST survey of ~10 likely rocky planets. The deep characterization of temperate or rocky exoplanets will bring us new questions and usher in an era for understanding the workings of small exoplanets with combined observations and models.

May 11, 2023
noon - 1 p.m.
3853 Slichter Hall

Presented By:

• Prof. Robin Wordsworth - Harvard University

Modeling the climate of Earth and other planets presents many challenges, not least of which is the huge variation in scales over which key physical processes operate. Here I discuss a series of case studies across this range of scales, drawn from recent work by our group. I begin by summarizing some of our recent advances in global 3D climate modeling. I then transition to the mesoscale, where I show how generalized convection modeling can provide new insights into clouds, hydrology and atmospheric chemistry on past Earth and on exoplanets. At even smaller scales, fundamental work on cloud and haze microphysics can help elucidate the nature of precipitation on other worlds, as well as contemporary problems in warm rain initiation on Earth. In the final part of the talk, I show how a first-principles analysis of the quantum mechanics of the CO2 molecule can yield new insight into anthropogenic global warming. I conclude by discussing the utility of theoretical approaches to climate problems that bridge Earth and planetary science.

May 18, 2023
noon - 1 p.m.
3853 Slichter Hall

Presented By:

• Prof. Eddie Schwieterman - UC RIverside

The search for habitable and inhabited worlds beyond our own is a civilizational goal and a prime motivator behind the proposed Habitable Worlds Observatory resulting from the 2020 Astronomy & Astrophysics Decadal Survey. Earth is our only example of a living world and will always provide the greatest richness of information to guide our search for remotely detectable biosignatures. Earth represents not just one but a palette of possible (bio)geochemical states for rocky exoplanets with secondary atmospheres, as our planet has undergone titanic shifts in atmospheric composition over geologic time. However, a growing literature portends great challenges in biosignature interpretations, illuminating multiple avenues of the abiotic accumulation of molecular oxygen and other potential biosignature gases. This talk will include a general overview of proposed exoplanet biosignatures, their possible false positives, and plausible strategies for disentangling them. I will specifically discuss recent work on methane, nitrous oxide, and halomethane biosignatures, and how we may anticipate differences in biosignature abundance, detectability, and specificity depending on planetary and astrophysical context (such as the spectral characteristics of the host star). Through interdisciplinary collaborations between geoscientists, biologists, and astronomers, we can maximize our chances of succeeding in the search for life beyond our solar system.

May 25, 2023
noon - 1 p.m.
3853 Slichter Hall

Presented By:

• Dr. Jessica Weber - Jet Propulsion Laboratory/Caltech

Prebiotic organic reactions are key for understanding the origins of life on Earth and how life could emerge on other worlds. Exploring chemical reaction networks in laboratory analog systems are also critical for understanding and differentiating between abiotic and biotic reactions as this can provide context for returned mission data and samples, ultimately facilitating life detection across the solar system. In this talk I will discuss work that explored a number of chemical experimental scenarios that address how complex metabolically-relevant reactions can be driven by various minerals in planetary analogs in the absence of complex enzymes. The results of this work can be useful for exploring abiotic/prebiotic chemistry under a variety of conditions, including those analogous to early Earth, Mars, and ocean worlds.

June 1, 2023
noon - 1 p.m.
3853 Slichter Hall

Presented By:

• Dr. Fei Dai - Caltech

Thousands of exoplanets have been discovered in the past ~30 years. However, there are still major gaps of knowledge in our understanding of planet formation. The most extreme exoplanets are often ideal for identifying, isolating, and investigating critical aspects of plant formation. In this talk, I will highlight three extremes of planet formation: 1) the ultra-short-period planets (<1 day, < 2 REarth) are the hottest possible planets. They may help understand the composition, surface mineralogy, and formation pathways of terrestrial planets in general. 2) I will showcase a multi-planet system with all six planets locked deep in a chain of mean-motion resonances. The orbital period ratios deviate from exact integer commensurability by merely a few parts in10-4. This 700 Myr-old system is likely the deepest in resonance. It is likely a fossil record of the orbital architecture produced by convergent disk migration before dynamical processes could disrupt the initial resonant configuration. 3) Could planet formation be starved out in extremely metal-poor environments? With TESS’ full sky coverage and ground-based medium resolution surveys, we examined whether planet formation is suppressed for metal-poor host stars ([Fe/H] < -0.5). Preliminary results suggest that sub-Neptune planet form readily in low-metallicity environments. The result possibly hints at the early inward drift of solids in protoplanetary disks.

June 8, 2023
noon - 1 p.m.
3853 Slichter Hall

Presented By:

• William Misener - UCLA Graduate Student

Sub-Neptunes are among the most common class of exoplanet discovered to date, but there is considerable uncertainty surrounding their interiors. One model consistent with observations is that these planets consist of silicate cores surrounded by hydrogen envelopes. At the conditions of the magma-atmosphere interface of sub-Neptune planets, substantial silicate vapor is expected to be in chemical equilibrium in the atmosphere. These species could greatly alter the atmospheric structure and evolution of these exoplanets, but previous models have neglected this compositional coupling. I present a coupled chemical equilibrium and atmospheric structure model, including silicate gas and its interactions with the background hydrogen. We find that silane, SiH4, and water, H2O, are the main products of atmosphere-interior interactions in sub-Neptune planets. These vapor products act as condensable species, decreasing in abundance with altitude. The resultant mean molecular weight gradient inhibits convection at temperatures above ∼2500 K, inducing a non-convective layer near the magma surface. This layer decreases the planet's radius compared to a planet with the same base temperature and a convective, pure H/He atmosphere. Therefore, we expect silicate vapor to have major effects on the inferred envelope mass fraction and thermal evolution of sub-Neptune planets. The presence of silicon species in the atmosphere may also be observable, presenting a window into the interiors of these planets.

June 8, 2023
noon - 1 p.m.
3853 Slichter Hall

Presented By:

• Jacob Widmer - UCLA Graduate Student

Seminar Description coming soon.