DPS 56: Day 4

Editor’s Note: This week we’ll be writing updates on selected events at the 56th Division for Planetary Sciences (DPS) meeting happening in Boise, Idaho, and online. The usual posting schedule for AAS Nova will resume on October 14th.

Table of Contents:


The Open-Source Science Initiative at NASA (J.L. Galache)

J.L. Galache (Agile Decision Sciences, NASA HQ) opened the session with a brief primer on open-science initiatives at NASA. Essentially, open science is the act of making your data, software, and publications freely accessible to all. Open science benefits the planetary science community by lowering barriers to entry and enhancing reproducibility of research. (Of course, open science also benefits you because your freely accessible data and publications will be cited more often!) NASA approaches open-science implementation through funding (sunpy and astropy have received funding from NASA, among many other recognizable names in science software), training, and policy. Current policy states that data and software should be made available at the time the research is published, and publications should be made available through a pre-print server (e.g., arXiv) if not being published in an open-access journal. Among the existing training options is Open Science 101, an online training module through which 1,900 people have already been accredited.

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Digging Deep: Unveiling Vertical Mixing and the Core Mass of Giant Exoplanets with JWST (David Sing)

David Sing (Johns Hopkins University), winner of the 2024 Alexander Prize for outstanding contributions that have significantly advanced our knowledge of planetary systems, described the journey to understanding exoplanet atmospheres and interiors from Hubble to JWST. Sing focused on the transit method, which has enabled the discovery of thousands of planets beyond our solar system and increasingly allows scientists to probe the atmospheres of those worlds.

With Hubble, scientists were able to test their models of photochemical equilibrium in exoplanet atmospheres, which predicted broad compositional trends with planet temperature. Hubble observations confirmed the basic picture of the chemical composition of hot Jupiters while also revealing complexities like clouds and hazes, which make it difficult to extract chemical abundances from spectra, and the presence of disequilibrium chemistry, which required extra attention from modelers.

plot of mass–metallicity relationship for planets and exoplanets

The mass–metallicity relationship for solar system planets (black points) and exoplanets (colored points). Click to enlarge. [From slide by David Sing]

Hubble observations raised further questions about giant planets, such as the state of their chemical disequilibrium, vertical mixing, horizontal mixing, and even their formation mechanism. Giant planet formation can be probed by measurements of planetary core mass fraction, with the core accretion model predicting that giant planets have roughly 10-Earth-mass rocky cores. In addition, the core accretion model predicts that as a planet’s mass increases, its overall metallicity decreases due to the accumulation of a massive envelope of hydrogen and helium. The giant planets in our solar system follow the predicted relationship, but the evidence for gas giant exoplanets is less clear. What can JWST tell us about the lingering issues in giant planet formation, chemistry, and mixing?

JWST spectrum of WASP-39b

JWST spectrum of WASP-39b’s atmosphere and best-fitting model. [From slide by David Sing]

Enter the JWST Transiting Exoplanet Community Early Release Science Program, which aimed to provide science results from phase curve, primary transit, and secondary eclipse observations of exoplanets while testing and leading to a greater understanding of the capabilities of JWST instruments. Sing highlighted the results of JWST transit observations of WASP-39b, which showed a prominent spectral feature from carbon dioxide (not seen by Hubble) as well as a feature at 4 microns that wasn’t predicted by models. This feature belonged to SO2, showing the first sign of photochemistry in an exoplanet. (SO2 is produced through the reaction of H2S with water molecules that have been split apart by starlight.)

Sing also described the important results from JWST observations of two other exoplanets, HAT-P-18b and WASP-107b. These are both cool/warm gas giants that are expected to have methane in their atmospheres. HAT-P-18b showed no sign of methane in its atmosphere — since the molecule is expected to exist, does this mean it’s being destroyed by photochemistry or by mixing into the deep, hot atmosphere of the planet?

Observations of WASP-107b provided more clues: in addition to CO2 and water, JWST observations of WASP-107b showed at last a hint of methane. However, there was about a thousand times less methane than expected given the planet’s temperature. By modeling both vertical mixing and photochemistry, Sing’s team showed that the methane depletion was due to mixing. Going further with these data, they used interior structure modeling to estimate the mass of the core at 11.5 Earth masses — consistent with predictions of the core accretion model.

Up next is the exoplanet grand tour spectroscopic survey, which will observe 25 exoplanets during 125 hours of JWST time. Stay tuned for news from this survey — the first data are being collected roughly 24 hours after this talk!

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Tracing the Heating of the Solar System Through Geochemical Signatures (Katherine de Kleer)

Katherine de Kleer (California Institute of Technology), winner of the 2024 Harold C. Urey prize for outstanding achievements in planetary science by an early-career scientist, described recent work on the history of heating in the solar system. De Kleer and collaborators use the Atacama Large Millimeter/submillimeter Array (ALMA) to understand where, when, and how objects in the solar system were heated and how that drives the chemical evolution of and geological activity on those objects today.

Let’s take things all the way back to the early days of the solar system: in the beginning, there was aluminum-26, a once-abundant, radioactive form of aluminum that heated the infant solar system. Heating from aluminum-26 melted early planetesimals and caused them to differentiate, separating into distinct compositional layers. Some of these planetesimals clumped together to form what are today the planets in our solar system, while others collided, leaving their remains scattered through the solar system as asteroids. The fragments of planetesimals in the asteroid belt can tell us about differentiation and, thus, heating in the protoplanetary disk that birthed our solar system.

thermal emission from asteroid Psyche

The thermal glow of asteroid 16 Psyche as seen by ALMA. [From slide by Katherine de Kleer]

Using ALMA, de Kleer observed the thermal emission from large main-belt asteroids — with an incredible resolution of 30 kilometers — and modeled the observations. The data show high emissivity (indicative of large metal content) but low polarization (indicative of scattering due to metal inclusions). This suggests that rather than having metal tied up in minerals, the metal in these asteroids likely exists in the form of numerous metallic chunks.

While heating is still ongoing in the solar system today, radiogenic heating — heating from the decay of radioactive material — is not as important as it once was. Now, small bodies are largely heated through tidal heating. The most dramatic example of the effects of tidal heating is Jupiter’s moon Io — the most volcanically active body in the solar system. Tidal heating melts Io’s rocky interior, which then gushes through the moon’s 400 active volcanoes to its surface.

Many images of Io and its volcanoes

Many views of the infrared glow of Io and its many volcanoes. Wavelength increases toward the bottom. Click to enlarge. [Slide by Katherine de Kleer]

By studying when and where Io’s volcanoes erupt, de Kleer’s team hopes to learn about Io’s interior, including where tidal heating is deposited. Tidal heating models predict specific distributions of Io’s volcanoes, but after 300 nights of observing Io (covering many revolutions around Jupiter), there doesn’t appear to be any cyclical behavior to the eruptions, nor does the spatial distribution appear to match the models well. This could mean that convection in Io’s mantle is obscuring the signature of tidal heating, or there could be stochastic geological processes at play.

Scientists have studied volcanic activity on Io for forty years, but the moon has been active much, much longer. If Io has been trapped in a resonance with fellow moons Europa and Ganymede for billions of years, as research suggests, it must have lost the equivalent of its entire mantle contents many times over, churning through and reprocessing this mantle material repeatedly. Another way of looking at Io’s evolution over time is through its atmosphere. Sulfur — an abundant component of Io — is lost to space from the moon’s upper atmosphere. Because different isotopes of sulfur have different masses, lighter isotopes are held less tightly to the moon and are preferentially removed from the atmosphere. By measuring the ratio of different isotopes of sulfur, de Kleer showed that Io has lost 94–99% of its available sulfur, supporting the picture of a dramatically, dynamically heated and evolving world.

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