List of Past Planetary Astronomy Lunches (PALS) : 01-Sep-2021 to 31-Dec-2021

Date:   Monday 01-Jan-2021
Speaker:   Dr. Most Valuable Planet (University of the Cosmos)
Title:  Water and Life - there is so much of it here

In this talk I will present my latest research on the endless supply of water on planet XYZ123 and all of the life forms that are hypothesized to exist there. Some models suggest that the life forms are camera shy and do not like the paparazzi as they have never been imaged by spacecraft.

For further information or if you are interested in sharing your reasearch at an upcoming PALS, contact PALS coordinator Dr. Stuck W. Pals at or (301)-405-xxxx.

Date:   Monday 13-Sep-2021
Speaker:   Qicheng Zhang (Caltech)
Title:  Comet C/2021 A1 (Leonard)'s Close Encounter with Venus

Comet C/2021 A1 (Leonard) will pass 0.029 au from Venus this December---closer than any other known comet to Venus in modern history---and in the process, may dust the planet with millimeter-sized meteoroids ejected from the comet many years earlier while it was still far out in the outer solar system. In this presentation, I will introduce the relevant dynamics of the comet's dust tail and the necessary conditions for its meteoroids to become meteors, compare the event at hand with the much closer encounter of comet C/2013 A1 (Siding Spring) to Mars in 2014 and its associated meteor shower, and briefly discuss the plausibility of observing meteors on Venus and/or their aftermath. I will additionally present imagery and spectroscopy from earlier this year that provide a direct look into the current state of the comet as it approaches the Sun, and subsequently speculate on the physical properties of the nucleus and its potential future activity.

For further information contact PALS coordinators Drs. Quanzhi Ye ( and Matej Malik (

Date:   Monday 20-Sep-2021
Speaker:   Robert Melikyan (University of Arizona)
Title:  Estimating the Flux of Bennuid Meteors at Earth

NASA's OSIRIS-REx mission has observed activity at asteroid (101955) Bennu. Hyperbolic ejecta are estimated to be produced at regular intervals about Bennu's orbit. We investigate Bennu's resulting meteoroid stream through the simulation of nearly 2 million Bennuids. Particle production matches the rates, sizes, flux, and ejection mechanisms as constrained by the OSIRIS-REx science team. We characterize the evolution of this meteorid stream along with predicted flux rates through the Earth-Moon system.

For further information contact PALS coordinators Drs. Quanzhi Ye ( and Matej Malik (

Date:   Monday 27-Sep-2021
Speaker:   Michael Zhang (Caltech)
Title:  The Atmospheres of Sub-Neptunes: Photoevaporation, JWST, and more

The atmospheres of planets smaller than Neptune remain largely mysterious. A radius gap divides the larger mini Neptunes from the smaller super Earths; under the prevailing theory, photoevaporation in the planet's youth strips some mini Neptunes of their envelopes, turning them into super Earths. We report the first evidence for escaping primordial atmospheres from two young mini Neptunes, and run extensive 1D and 3D hydrodynamic models to explore the complex effects of stellar wind and metallicity upon the observed signals. The absorption is generally weaker than models predict, pointing to suppression mechanisms such as high atmospheric metallicity or magnetic fields.

In the second part of my talk, I focus on rocky planets. These include 55 Cnc e, where our non-detection of escaping helium to deep limits, and our 3D models, both suggest the planet cannot have a primordial atmosphere. They include GJ 367b, an ultra short period sub-Earth for which a JWST phase curve will reveal atmospheric or surface properties. I will describe the JWST programs targeting these planets, what we will learn from them, and how I plan to improve atmospheric modelling with JWST data.

For further information contact PALS coordinators Drs. Quanzhi Ye ( and Matej Malik (

Date:   Monday 18-Oct-2021
Speaker:   Yun Zhang (Observatoire de la Côte d’Azur)
Title:  Internal properties and structural evolution of top-shaped asteroids as revealed by Bennu and Didymos

Most small bodies with sizes larger than tens of meters are made of numerous pieces of rocks and sometimes ices that have coalesced under the influence of gravity (so called “rubble piles”). As their own gravity is not enough to squeeze them into spheres, these small bodies exhibit a variety of shapes and surface morphologies, recording their life stories. Radar observations have shown that a large fraction of near-Earth asteroids have top-like shapes, which could be the outcome of the rotational reshaping of the so-called YORP effect. Most of top-shaped bodies are accompanied by a small satellite, such as the asteroid Didymos, while a few are alone, such as the asteroid Bennu and Ryugu. Here I will show how top-shaped bodies evolve in general under YORP based on our recent studies. By modeling rubble-pile structural dynamics using the soft-sphere discrete element method, we found that their structural evolution are very sensitive to their internal structure and material properties. The conditions allowing the formation of satellites by YORP-induced mass shedding and implications for Bennu and Didymos will be given.

For further information contact PALS coordinators Drs. Quanzhi Ye ( and Matej Malik (

Date:   Monday 25-Oct-2021
Speaker:   Ariel Graykowski (UCLA)
Title:  Investigation of the Fragments of Split Comet 73P

I will talk about cometary fragmentation in general as well as a case study of split comet 73P/Schwassmann-Wachmann 3. Fragmentation mechanisms remain somewhat mysterious with potential mechanisms including tidal forces, rotational instability, thermal pressure, outgassing pressure, and impacts. Comet 73P has been observed to fragment on several occasions, however the cause of its fragmentation is poorly understood. We analyzed unpublished archival Hubble Space Telescope data in order to understand its fragmentation mechanism. For example, the literature presents a wide range of measured rotational periods for the nucleus of the comet, 73P-C, some that suggest the nucleus might have split due to rotational instability. However, we find the most likely value of the rotation period to be at least ~10.4 hours, much longer than the critical period for rotational instability for any reasonable nucleus density. I will also present our current work on fragments 73P-B and G, which show 100s of mini fragments within their comae. We analyze various properties of these fragments such as their speed on the plane of the sky and their size and frequency distributions in order to further constrain potential fragmentation mechanisms.

For further information contact PALS coordinators Drs. Quanzhi Ye ( and Matej Malik (

Date:   Monday 01-Nov-2021
Speaker:   Teal (University of Maryland)
Title:  Impacts of host star UV uncertainty on exoplanet atmospheres

The upcoming launch of JWST will advance our ability to characterize exoplanet atmospheres. Disequilibrium chemical processes dominate the cooler atmospheres, requiring accurate photochemical modeling of such environments. The host star's UV spectrum is an important input to these models, but most exoplanet hosts lack UV observations. For cases in which the host UV spectrum is unavailable, a reconstructed or model spectrum must be used in its place. We use the MUSCLES catalog and UV line scaling relations to understand how well reconstructed host star spectra reproduce photochemically modeled atmospheres in lieu of real UV observations. We focus on two cases; a modern Earth-like atmosphere and an Archean Earth-like atmosphere that forms significant amounds of hydrocarbon haze. We find that modern Earth-like environments are well-reproduced with UV reconstructions, whereas haze-bearing (Archean Earth) atmospheres suffer from changes at the observable level. Both stellar UV emission lines and continuum significantly influence the chemical state and haze production in our modeled Archean atmospheres, resulting in observable differences in their transmission spectra. Our modeling results indicate that UV observations of individual exoplanet host stars are needed to accurately characterize hazy terrestrial atmospheres. In the absence of UV data, reconstructed spectra accounting for both UV emission lines and continuum flux are the next best option, though model accuracy will suffer.

For further information contact PALS coordinators Drs. Quanzhi Ye ( and Matej Malik (

Date:   Monday 08-Nov-2021
Speaker:   Yu-Dai Tsai (Fermilab/U Chicago)
Title:  Exploring Fifth Forces and Ultralight Dark Matter with Planetary/Asteroidal Dynamics and Space Quantum Technology

We use planetary astrometric data and space Quantum technology to study ultralight dark matter. Using the orbital data collected for asteroids and other minor planets, one can set leading bounds for ultralight dark matter, which mediate long-range forces and induce precessions of the celestial objects. One can also extend this method to study modified gravity theories. With NASA's new Deep Space Atomic Clocks, one can study the ultralight dark matter in unexplored parameter space. I am looking to provide new connections between astrophysics, planetary observations, quantum technology, and fundamental physics.

For further information contact PALS coordinators Drs. Quanzhi Ye ( and Matej Malik (

Date:   Monday 15-Nov-2021
Speaker:   Andrea Guzman-Mesa (University of Bern)
Title:  Exploring the chemical diversity of the atmosphere and interior of sub-Neptune planets

The atmospheres of sub-Neptunes are expected to exhibit considerable chemical diversity, beyond what is anticipated for gas-giant exoplanets. In my talk, we explore self-consistent atmosphere-interior models of sub-Neptunes to explore this chemical diversity and apply this knowledge to the available atmospheric data of GJ 436b and link it with the corresponding plausible internal structures.

For further information contact PALS coordinators Drs. Quanzhi Ye ( and Matej Malik (

Date:   Monday 22-Nov-2021
Speaker:   Tad Komacek (University of Maryland)
Title:  Clouds, storms, and variability in exoplanet atmospheres

Aerosols are ubiquitous in exoplanet atmospheres, and large-scale dynamical instabilities and thermodynamic and cloud-radiative feedbacks are expected to drive weather patterns in planetary atmospheres. This talk will discuss current theory and modeling efforts to study vertical mixing, cloud lofting and cloud-radiative feedback, time-variability, and large-scale storms in exoplanet atmospheres. Specifically, I will describe four main topics. First, I will discuss a theoretical scaling for vertical mixing in the atmospheres of hot Jupiters and implications for their general circulation. Second, I will describe current modeling efforts for hot Jupiters and extensions to study patchy clouds in the ultra-hot Jupiter regime. Then, I will discuss time-variability in the atmospheres of hot Jupiters and tidally locked rocky exoplanets, and the implications of this variability for upcoming JWST observations of these planets. If time permits, I will finish by describing recent efforts to model tropical cyclones in tidally locked exoplanet atmospheres with high-resolution simulations.

For further information contact PALS coordinators Drs. Quanzhi Ye ( and Matej Malik (

Date:   Monday 29-Nov-2021
Speaker:   Tim Stubbs and Ramin Lolachi (NASA/GSFC)
Title:  Light Scattering by Dust in the Lunar Environment: Natural and Exploration-driven Sources

Dust is ubiquitous on the Moon and represents the finest component of the lunar regolith (particles < ~100 µm). It is primarily formed by continual bombardment of the lunar surface by meteoroids, which act to break-up rocks and regolith into finer particles. The size distribution of the irregularly-shaped regolith particles eventually reaches a steady state in which the very smallest particles (dust) can cohere together to form agglomerates. The meteoroid impacts eject dust into the lunar exosphere such that they form a permanent, asymmetric cloud around the Moon. The properties of this dust ejecta cloud are important to our understanding of the meteoroid environment at the Moon, as well as the evolution of the lunar surface. One of many lessons learned from the Apollo program is that lunar dust can be both a nuisance and hazard during operations on the Moon. During future exploration by the Artemis program, monitoring dust activity will be critical to understanding and mitigating problems with dust. Here we report on: (i) the first optical detection of the impact-generated dust ejecta cloud surrounding the Moon; and (ii) a concept for a commercial-off-the-shelf (COTS) camera that would be capable of sensitively monitoring dust activity at an exploration site. We will also discuss aspects of the light scattering simulation suite being developed at NASA Goddard. Our findings demonstrate how the observation and interpretation of sunlight scattered from lunar dust can be a powerful tool for science and exploration at the Moon.

For further information contact PALS coordinators Drs. Quanzhi Ye ( and Matej Malik (

Date:   Monday 06-Dec-2021
Speaker:   Shang-Min Tsai (Oxford)
Title:  The case of the missing molecules -- how atmospheric chemistry modeling can solve some mystery of exoplanets

Exoplanet research has revolutionized how we think about life and planets. The ground- and space-based telescopes, such as HST and Spitzer, have allowed us to progress from the stage of detection to probe fundamental properties of the planets. The upcoming space telescopes, such as JWST and ARIEL, promise a bright outlook for characterizing exoplanet atmospheres. We are in an exciting era to study exoplanet atmospheres, driven by the goal of understanding the principal physical and chemical processes and ultimately identifying habitable worlds. In this talk, I will review the role of chemical modeling in helping us understand the atmosphere. I will summarize the highlights in my recent work Tsai et al. (2021a), Tsai et al. (2021b) on various chemical processes in diverse exoplanet atmospheres, and what we can learn about the surface conditions when certain gases are present or missing. Lastly, I will discuss the modeling challenges and how we can go beyond the limit of 1D models.

For further information contact PALS coordinators Drs. Quanzhi Ye ( and Matej Malik (

Date:   Monday 13-Dec-2021
Speaker:   Prabal Saxena (NASA/GSFC)
Title:  Turning Up the Difficulty Setting on Observations of Directly Imaged Exoplanets

Observations of directly imaged exoplanets are coming! With JWST and Roman coming relatively soon and the Astrophysics Decadal recommending a large optical/UV/IR space telescope capable of directly imaging Earth-sized planets, a range of worlds will be observed in ways they never have been before. With those new worlds will come additional challenges. I'll talk about observing two potentially interesting targets with Roman and also a potential issue that will need to be considered when targeting Earth-sized planets in the future.

For further information contact PALS coordinators Drs. Quanzhi Ye ( and Matej Malik (

This page was automatically generated on: 19-Jan-2022.