Welcome back to our five-part examination of Webb’s Cycle 4 General Observations program. In the first and second installments, we examined how some of Webb’s 8,500 hours of prime observing time this cycle will be dedicated to exoplanet characterization, the study of galaxies at “Cosmic Dawn,” the period known as “Cosmic Noon,” and the study of star formation and evolution.
In our final installment, we’ll examine programs that leverage Webb’s unique abilities to study objects in our cosmic backyard—the Solar System!
On March 11th, the Space Telescope Science Institute (STScI) announced the science objectives for the fourth cycle of the James Webb Space Telescope‘s (JWST) General Observations program. Cycle 4 GO. This latest cycle includes 274 programs broken down into eight categories encompassing Webb‘s capabilities. These range from exoplanet study and characterization and observations of the earliest galaxies in the Universe to stellar science and Solar System Astronomy.
One of these categories is dedicated entirely to the study of objects within our Solar System. These programs will use Webb‘s unique capabilities to observe and characterize planets, moons, and asteroids to address several key questions. These include how the Solar System formed 4.5 billion years ago and has evolved since, how water and other volatiles were delivered to Earth, and what is driving various processes observed in previous cycles and by other instruments.
Asteroids
Asteroids are essentially material left over from the formation of the Solar System that has since settled into various orbits. In addition to the populations in the Main Asteroid Belt and the Kuiper Belt, countless bodies have fallen into orbit around planets in the inner and outer Solar System. These include Near-Earth Asteroids (NEAs), Jupiter’s Trojan and Greek populations, and many other “families.”
Based on their composition, asteroids are classified into three broad categories: C-type (carbonaceous), M-type (metallic), and S-type (silicaceous). C-types are the most common, accounting for 75% of known asteroids. They are largely composed of carbonaceous chondrites and are rich in volatiles (like water). M-types have higher concentrations of metals, such as iron and nickel, while S-types are largely composed of silicate minerals.
These bodies have become the focal point of considerable research lately, though many unanswered questions remain. For example, scientists hope to get a full account of asteroid populations. This is the purpose of the GO 8214 program, “Mining JWST data for hidden asteroid gems.” Dr. Artem Burdanov, a research scientist from the Massachusetts Institute of Technology (MIT), is the program’s Principal Investigator (PI).
Using Webb’s Mid-Infrared Instrument (MIRI) in multiple filters, he and his team will search for smaller NEAs, those that measure tens of meters across (decameter asteroids):
“While the smallest bodies in Earth’s vicinity are abundant, they are challenging to observe due to their faintness. Yet, they hold the keys to major endeavors ranging from studying meteorite sources (and thus planetary-defense efforts) to understanding the collisional and dynamical evolution of asteroidal bodies through Solar System history. Fortunately, JWST’s unique infrared capabilities covering the emission peaks of asteroids located up to 10 AU can be combined with synthetic-tracking techniques to detect main-belt asteroids (MBAs) as small as 10 m.”
Their work builds on previous observations where Webb detected 138 decameter asteroids in the Main Belt while observing the TRAPPIST-1 system. These observations allowed astronomers to conduct the first-ever sample study of meteorite parent bodies that travel from the Main Belt to Earth. Following up on this, Burdanov and his team will conduct a Legacy Archival Research program expected to yield information on 600 decameter asteroids in the Main Belt – ten times more data than previous observations.
There’s also the GO 8812 program, “Distribution of water in the Solar System from crossover region spectroscopy of asteroids.” Anicia Arredondo, a graduate student at the Southwest Research Institute (SwRI), is the program’s PI. Using archival MIRI Medium Resolution Spectroscopy (MIRI MRS) data, she and her team will obtain spectra from 32 small bodies in the “crossover region.” As they stated in their proposal:
“For small bodies, spectral flux is dominated by reflected sunlight in the near-infrared and by thermal emission in the mid-infrared. The crossover region (4-8 m) has contributions from both, and their relative contribution to total flux varies based on the asteroid’s distance from the Sun. The crossover region is often excluded from the analysis of asteroid spectra, likely because the signal-to-noise at short wavelengths is lower than the rest of the spectrum.”
The crossover region contains spectral features indicative of the rock-forming minerals olivine, pyroxene, and water. This will provide a representative sample that will reveal vital data on how water (a key ingredient in the emergence of life) was distributed throughout the Solar System. “The study of the distribution and evolution of hydrated small bodies has direct implications for how water was delivered to Earth and how water could be delivered to other extrasolar planets,” they summarize.
Near-Earth
Similarly, but closer to home, you have the GO 8782 program, “Nominally anhydrous asteroids as reservoirs of water in the inner solar system.” Led by PI Matthew Hedman, an Associate Professor from the University of Idaho, this program will use data obtained by Webb’s NIRSpec instrument to assess the abundance and distribution of water in non-carbonaceous asteroids. Current models for the origins of Earth’s water have focused on C-type asteroids from the outer Solar System.
However, isotopic studies suggest that non-carbonaceous materials may have also played a significant role in delivering water to Earth. As they indicate, this highlights the need for more research to improve our understanding of water distribution in these bodies:
“By studying these features, we aim to determine whether nominally anhydrous asteroids exhibit evidence of hydration and if OH/H2O abundance correlates with heliocentric distance. These observations will provide new insights into the diversity of water sources in the solar system, ultimately advancing our understanding of whether the early inner solar system contained sufficient water to contribute significantly to Earth’s water supply.”
In recent years, astronomers have discovered two quasi-satellite asteroids orbiting Earth, Kamo’oalewa, discovered in 2016, and 2023 FW13, discovered in 2023. Characterizing the former is the goal of the GO 8663 program, “Is the Moon an Active Source of Near-Earth Asteroids? Testing the Origins of Earth Quasi-Satellite Kamo’oalewa.”
Led by PI Benjamin Sharkey, a Visiting Senior Faculty Specialist from the University of Maryland, this program will conduct NIRSpec observations of Kamo’oalewa, which was discovered in 2016. At the time, astronomers declared it Earth’s “second Moon,” while a third (2023 FW13) was discovered in 2023. Previous observations led to the hypothesis that it may represent the first known asteroid that formed from a lunar impact. Its faintness, as they explain, has prevented more detailed characterization until now:
“By observing Kamo‘oalewa with NIRSpec, we will comprehensively characterize the presence of these materials and provide the first measurements of its size and albedo. This program will enable strict tests of the possible lunar origin for this object, testing whether the moon acts as an entirely unpredicted source of near-Earth asteroids in conflict with fundamental population models.”
Outer Satellites
Of course, no observation cycle would be complete without dedicating some time to studying the many curious satellites in the outer Solar System. These programs will have implications for astrobiology, providing data on “Ocean Worlds” so scientists can learn more about their potential to support life, and the physical processes that led to the formation and evolution of others. For example, there’s the GO 7847 program, “Saturn’s E ring in 2025: A rare opportunity to observe material from inside Enceladus.”
Matthew Hedman, an Associate Professor of Engineering at the University of Idaho, is the program’s PI. He and his colleagues will use Webb’s NIRSpec and NIRCam to study Saturn’s diffuse E-ring, which is composed primarily of particles erupting from Saturn’s moon, Enceladus. Since the Cassini mission explored the system (2004-2017), scientists have been eager to get a closer look at the plume activity emanating from its southern polar region.
During Cycle 4, the JWST will have a rare opportunity to observe this ring almost edge-on, which will not occur again for the next 15 years. This, they propose, will help astrobiologists constrain the presence of potential biosignatures coming from Enceladus’ interior. “NIRSpec IFU spectra of the brightest part of the E ring will clarify the carbon content of the particles erupted by Enceladus. Meanwhile, deep NIRCam images of the entire E ring at multiple wavelengths will constrain how efficiently material from Enceladus is transported throughout the Saturn system.”
There’s also considerable interest in Io, Jupiter’s volcanic moon. Recent missions like the Juno probe have led to pictures of the moon’s over 300 active volcanoes. This is the goal of the GO 8857 program, “Io’s Auroral Emissions as a Tool to Investigate Atmosphere-Plasma Torus Interactions.” Zachariah Milby, a graduate student in the Division of Geological and Planetary Sciences at the California Institute of Technology (Caltech), is the PI for this program.
As part of the JWST’s Early Release Science (ERS) program, the NIRSpec was able to spatially resolve emissions of sulfur (S) and sulfur oxide (SO). These observations suggested that the same process may excite these two emission sources. These emissions play an important role in Io’s atmosphere (like its auroral activity) and feed the plasma torus surrounding Jupiter. As they propose, a closer examination using Webb’s unique capabilities will yield new information about the interactions between Io and Jupiter’s magnetic field:
“Consequently, we propose to observe Io in eclipse on four occasions, sampling across the range of magnetic latitudes Io experiences to determine the excitation mechanism of the SO emissions. Additionally, we propose to use the spatially-resolved S emissions to map for the first time the precipitation of the torus into Io’s atmosphere and determine how it varies with Io’s position within Jupiter’s magnetosphere.”
The Giants
Speaking of Jupiter’s atmosphere, the JWST is uniquely qualified to reveal more about the processes powering its massive anticyclonic storms, streaks, and auroral activity. Consider the GO 8173 program, “The evolution of comet-impact products in Jupiter’s atmosphere: a benchmark for auroral chemistry in giant (exo)planets.” Led by PI Pablo Rodriguez Ovalle, a Ph.D. student at the Observatoire de Paris, this program will rely on archival MIRI MRS data to monitor chemical abundances in Jupiter’s atmosphere – specifically water (H2O), carbon dioxide (CO), and hydrogen cyanide (HCN).
These chemicals were introduced by the comet Shoemaker-Levy 9 (SL9) impact in 1994 and have since exhibited a puzzling chemical behavior. Observations made during previous Cycles reveraled unexpected trends in their abundances across Jupiter’s southern hemisphere – particularly around the South Pole Region. These findings challenge existing models of polar chemistry and suggest multiple processes are at work, including an influx of oxygen from Io. Per their proposal:
“[W]e propose a comprehensive mapping of HCN, H2O, and CO2 across both hemispheres, targeting regions within and outside auroral zones. By comparing the temporal abundance trends since 1994, and by studying the impact of auroral precipitation, we aim to uncover the key chemical pathways governing the destruction and production of these molecules. These observations will not only refine our understanding of Jupiter’s atmospheric chemistry but also provide insights into similar processes occurring on other giant planets and exoplanets, with potential implications for understanding the chemical evolution of exoplanetary atmospheres under cometary impacts.”
Right now, NASA’s Lucy mission is on its way to Jupiter to study its Trojan asteroid population, which remains a mystery to planetary scientists. In the meantime, studying them with IR spectroscopy is the purpose of the GO 9078 program, “Probing the origin and interiors of Jupiter Trojans through the study of collisional fragments.” According to current models of Solar System evolution, the Trojans would have formed in the Trans-Neptunian region alongside the Kuiper belt objects.
Therefore, a detailed study of Trojans’ surface properties could reveal clues to the origin of objects in the outer Solar System. In addition, observing the debris resulting from collisions would allow the team to probe the composition of these objects. Webb obtained NIRSpec data on the collision fragment Eurybates during Cycle 1, revealing the possible presence of water ice and carbon dioxide – two key biosignatures.
Ian Wong, a staff scientist with the Space Telescope Science Institute, is the program’s PI. They will conduct spectroscopic observations of additional Trojan collision fragments per their proposal to build on the earlier results:
“We plan to observe 3 smaller members of the Eurybates family to assess whether the 4.25 micron feature on Eurybates is representative of the entire parent body. We will also target 2 members of the Ennomos collisional family to probe whether there are systematic differences in bulk composition among the Trojan population. Rotationally resolved spectra of Ennomos will explore the surface variability apparent in previous ground-based observations.”
The study and characterizing of objects in the Solar System is vital to ascertaining how, where, and when the ingredients were distributed throughout the Solar System. Addressing these questions will also assist astrobiology missions searching for possible evidence of life beyond Earth. And, of course, a more complete understanding of how the Solar System evolved could help point the way towards potentially habitable exoplanets.
This concludes our look at the programs that make up the JWST Cycle 4 General Observation program. Thank you for reading!
Further Reading: STScI