Unveiling Comet Nuclei Surface Spectra: Validating a Coma Subtraction Technique for IFU Comet Observations

Cometary nuclei preserve clues to our early solar system and spectroscopic studies of both their surfaces and surrounding comae—clouds of gas and dust that activate during solar approach—offer critical insight into their composition and evolution. Specifically, cometary surface spectra reveal the materials present on the object’s outermost layers, which record evidence of past and ongoing surface processing. Characterization of these surfaces also enables comparisons—and potential connections—to other small body populations (e.g., asteroids, centaurs, and trans-Neptunian objects) providing perspective into the myriads of gravitational rearrangements experienced by these objects since our solar system’s formation. However, obtaining spectroscopic data for a large number of cometary nuclei remains challenging. Their small, dark surfaces often make them difficult to observe during inactive periods, which typically occur at larger heliocentric distances. Conversely, during observations at smaller heliocentric distances, their proximity to the Sun increases the likelihood of activity, where the enshrouding presence of comae leads to an entanglement of the signals received and complicates efforts to isolate nucleus-only information.

To address this observational challenge, we investigate whether an established coma modeling and removal technique, originally developed for broadband imaging [e.g., 1, 2, 3], can be adapted to spectroscopic integral field unit (IFU) observations of comets. IFU systems capture both spatial and spectral information, where each spectral wavelength element corresponds to a reconstructed 2D image of the instrument’s field of view (FOV), such as that produced by JWST’s NIRSpec IFU (see Figure 1). This data structure presents a potential opportunity to isolate high-resolution, nucleus-only spectra by modeling and removing the coma’s contribution from each of the spectrum’s wavelength resolution elements. A successful coma subtraction requires sufficient details of the coma’s 2D surface-brightness distribution to generate an accurate coma model, a task enabled by imaging’s larger FOVs. However, IFU instruments typically have smaller FOVs, raising uncertainties about their ability to provide enough coma information for accurate modeling. Our work aims to evaluate the efficacy and limits of the nucleus extraction technique’s application to IFU data.

Rather than relying on real comet data for the method’s validation, where coma and nucleus flux contributions are inherently uncertain a priori, our approach utilizes synthetic comet “observations” created to mimic real data, but with known input signals for each component. These simulated datasets create a controlled environment that enables rigorous validation of coma modeling accuracy and the potential for recovering clean nucleus spectra under realistic observing circumstances. Here, we present our progress towards this goal and preliminary simulation results focused on comet data acquired with the JWST NIRSpec IFU in Prism mode.

Figure 1. A JWST NIRSpec spectrum of the active comet/centaur 29P/Schwassmann-Wachmann 1 [4] is shown where individual datacube wavelength slices are identified. The coma’s surface brightness is clearly visible in the wavelength slices. Our project seeks to identify if the coma modeling and removal procedure proven successful in broadband imaging studies can be applied to smaller FOV IFU data. For reference, the JWST NIRSpec IFU FOV is 3” x 3”.

References:

[1] P. Lamy and I. Toth, "Direct detection of a cometary nucleus with the Hubble Space Telescope," Astronomy and Astrophysics, vol. 293, pp. L43-L45, 1995.

[2] C. M. Lisse, "The Nucleus of Comet Hyakutake (C/1996 B2)," Icarus, vol. 140, no. 1, pp. 189-204, 1999.

[3] Y. R. Fernandez, "Physical properties of cometary nuclei," PhDT, 1999.

[4] Faggi, S., “Heterogeneous outgassing regions identified on active centaur 29P/Schwassmann–Wachmann 1”, Nature Astronomy, vol. 8, no. 10, pp. 1237–1245, 2024.