JWST Measurements of the Three Largest Low-Albedo Asteroids

Motivation: Ceres, Pallas, and Hygiea are the three largest low-albedo objects in the main asteroid belt, and contain a little more than half of the belt’s mass. Ceres has been visited by the Dawn spacecraft, which has shown it to be a geologically varied relict “Ocean World” and dwarf planet, with cryovolcanism perhaps still occurring today [1,2]. Hygiea appears to be a spectral twin of Ceres, suggesting a similar composition (and perhaps a similar history?). Pallas, by contrast, is thought to be more consistent with the carbonaceous chondrite meteorites, and could plausibly be the largest undifferentiated object in the solar system.  The launch of JWST provided the opportunity to extend the wavelength coverage of these objects into spectral regions unobservable from Earth, and to use Dawn’s visit to Ceres to calibrate quantitative estimates of hydrogen content from these, and future, observations.

Observations: Two instruments were used for the observations: the Near-Infrared Spectrograph (NIRSpec) and the Mid-Infrared Instrument (MIRI).  In both cases an integral field unit (IFU) was used. The observations were made as part of GTO program 1244, part of a larger GTO effort spearheaded by Heidi Hammel. Observations took place on 1 August 2022 (Hygiea MIRI and NIRspec), 1 October 2022 (Pallas NIRSpec), 17 December 2022 (Pallas MIRI), and 18 January 2023 (Ceres MIRI and NIRSpec). 

NIRSpec: The NIRSpec data were downloaded from the Mikulski Archive for Space Telescopes (MAST) and processed using typical methods for IFU observations that are detailed in the literature [3-5, etc.]. Because all three targets are extended sources for the NIRSpec IFU (ranging from 2.3 pixels for Hygiea to 6.4 pixels for Ceres), boxes of 13 pixels for Ceres and 9 pixels for Pallas and Hygiea were used, encompassing ~98% of calibrated flux, though the final analyses were done on normalized spectra so the results are insensitive to the absolute flux calibration.  The spectra of Ceres and Pallas were saturated beyond roughly 3.7 and 3.2 µm, respectively, and those spectral sections were not analyzed. It is possible that some of the saturated wavelength regions can be recovered (see below), but the wavelength regions uniquely observable by JWST (2.5—2.85 µm) were unsaturated.  Following spectral extraction, the thermal flux contribution was calculated for each object and removed, resulting in a reflectance-only spectrum for analysis.  

All three objects have an absorption band near 2.72 µm, with Hygiea and Ceres showing band centers at identical wavelengths that only differ by 0.001 µm from Pallas’s band center. This is interpreted as due to Mg-OH indicating very Mg-rich phyllosilicates. Ceres and Hygiea additionally show features near 3.05 µm and 3.31 µm, with band centers that differ by < 0.01 µm. These are interpreted as due to NH₄⁺ in minerals. In the 3-µm region, the two differ mainly in band depths, with Ceres showing a relatively deeper band due to NH₄⁺than the OH band, suggesting Ceres has a more NH₄⁺ -rich mineralogy than Hygiea. Ceres and Hygiea appear practically identical across all wavelengths that have been studied, not only in the JWST data but in other wavelengths by other workers. This similarity opens the intriguing possibility that Ceres and Hygiea had similar histories and processes, though modeling of Hygiea’s history will be necessary to demonstrate whether that was likely. 

The NIRSpec data were also used to estimate the hydrogen content in the regolith of Ceres, Pallas, and Hygiea, with results suggesting concentrations of roughly 0.5-1 weight percent, consistent with what is seen in CM chondrites.

MIRI: The sensitivity of JWST/MIRI and the brightness of Ceres, Pallas, and Hygiea at mid-infrared wavelengths combine to make it impossible to collect a completely unsaturated spectrum across all wavelengths with any setting. With this in mind, the observations were undertaken with an eye to analyzing those wavelength regions that were unsaturated and understanding the conditions where saturated data could be usefully recovered.

After exploring sophisticated recovery techniques, we determined that many such techniques that estimate the brightness of saturated pixels [6] were foiled by JWST’s very tight PSF. In this situation, the surrounding non-saturated pixels had low enough signal compared to the central pixel that extrapolating to the central pixel flux was not robust. As a result, our initial analyses focus on extracting the SED of the unsaturated pixels and scaling them to match unsaturated spectral segments where there is overlap.

We will discuss the NIRSpec results and the current state of the MIRI analysis. 

References: [1] Scully, Jennifer EC, et al. The Planetary Science Journal 2.3 (2021): 94. [2] Ruesch, O., et al. Science 353.6303 (2016): aaf4286. [3] Emery, J. P., et al. Icarus 414 (2024): 116017. [4] Wong, Ian, et al. The Planetary Science Journal5.4 (2024): 87. [5] Rivkin, Andrew S., et al. The Planetary Science Journal 6.1 (2025): 9. [6] Pope, B. J. S., et al. Monthly Notices of the Royal Astronomical Society: Letters 455.1 (2015): L36-L40.