A Planetary Cold Case: Using JWST to Uncover the Catastrophic History of Neptune’s Moons

Why It Matters

The findings overturn assumptions about ice‑giant moon formation, showing that a single capture event can erase regular satellite systems and alter planetary evolution models.

Key Takeaways

• JWST spectra reveal Neptune’s inner moons lack water‑ice signatures.
• Broad 3‑micron absorption indicates hydrated minerals, not ice.
• Uranian ring moons show typical water‑rich Kuiper‑belt spectra.
• Findings suggest a catastrophic event reshaped Neptune’s satellite system.
• Triton’s capture likely destroyed any original regular moons around Neptune.

Summary

The lecture focused on new James Webb Space Telescope observations of Neptune’s tiny inner moons and rings, aiming to decipher how the ice giant’s satellite system formed and evolved. Dr. Riley Davis presented high‑resolution IFU spectra that capture reflected sunlight from these faint bodies, a feat previously impossible due to their proximity to the bright planet. The data reveal a deep, broad 3‑micron absorption feature on all Neptune inner moons, yet they lack the accompanying water‑ice overtone bands that dominate Kuiper‑belt objects and Uranian ring moons. This suggests the presence of hydrated minerals without detectable ice—a spectral signature never seen elsewhere in the outer solar system. Davis highlighted the surprise of the team, noting that the spectra “look nothing like what we expected” and contrasting them with Uranus’s inner moons, which display classic water‑rich signatures. The comparison underscores a fundamental compositional divergence between the two ice‑giant systems. If Neptune once hosted a regular moon family similar to Uranus’s, the capture of Triton—a retrograde, Pluto‑sized body—likely shattered that system, leaving only the anomalous, debris‑derived inner moons. This scenario forces a revision of moon‑formation models for ice giants and informs how catastrophic capture events may shape exoplanetary satellite architectures.

Original Description

The rings and small inner moons of the giant planets are remnants of past events that shaped their planetary systems. Around Uranus and Neptune, these faint “ring-moons” have remained largely unexplored, and their origins have been uncertain for decades. In this talk, I will show how new observations from the James Webb Space Telescope (JWST) allow us to use their surfaces to reconstruct that history. Neptune’s inner moons do not resemble the primitive icy bodies we might expect. Instead, their compositions point to a more complex origin—one that suggests they are the remnants of much larger ocean worlds that were broken apart and later reassembled. This points to a catastrophic destruction of Neptune’s original satellite system, likely associated with the capture of its largest moon, Triton.

By turning faint points of light into detailed compositional measurements, JWST is opening a new window into the outer solar system. These observations show how even the smallest and most distant moons can be used to reconstruct the events that shaped planetary systems billions of years ago.

About the Speaker:

Ryleigh Davis investigates how the surfaces of icy satellites record their formation conditions, radiation history, thermochemical evolution, and interior composition. During her PhD, she has studied icy moons across the Jovian, Uranian, and Neptunian systems. Her work has examined how radiation alters Europa, how deeper material is exposed on Callisto, and how Europa’s surface chemistry evolves over millions of years. Most recently, she has used JWST to show that Neptune’s small ring-moons are likely the broken remnants of ancient, water-rich worlds that once hosted subsurface oceans.

About the Series:

The Everhart Lecture Series is a forum encouraging interdisciplinary interaction among graduate students and faculty, the sharing of ideas about research developments, as well as a space to discuss controversies. Everhart Lectures allow for the recognition of individual Caltech student's exemplary presentation and research abilities. Each year, three graduate student are selected as Lecturers and receive a $2000 honorarium and recognition at graduation.

This series is cohosted by the Caltech Graduate Office and Graduate Student Council

This lecture was recorded on April 21st, 2026

Produced in association with Caltech Academic Media Technologies.

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