Long-term MIR photometry and spectroscopy of (1) Ceres from SOFIA

The FORCAST instrument on the Stratospheric Observatory for Infrared Astronomy (SOFIA) regularly obtained mid-infrared data of large asteroids as calibrators for other observations. This previously unused dataset represents a long-baseline series of photometric and spectroscopic observations that can be used to investigate how thermal and compositional properties may change over time. We present multi-filter photometry and spectroscopy in the wavelength range 5-38 μm of (1) Ceres which was observed on seventeen nights between 2015-2022. These nights correlate to different portions of Ceres’ orbit (Figure 1). While there are no data near perihelion, the full data set covers a wide spread of Ceres’ orbit.  

Figure 1. Location of Ceres along its orbital path (grey line) around the Sun (yellow star) for each of our observations (colored markers). Perihelion is marked with a black cross.

We performed aperture photometry on the imaging data, which consisted of between six and twelve different filters per night. We fit the Near-Earth Asteroid Thermal Model (NEATM) to the extracted photometric points to derive the beaming parameter (η), which is a proxy for thermal properties. We find that η does not change with Ceres’ point in its orbit or with the sub-observer longitude of the observation; however, we find a general increase in η over the time scale of our observations (Figure 2). The cause of this increase is likely instrumental, as it is also seen in a similar study of (4) Vesta. 

Figure 2. We do not find a correlation between η and point in orbit or sub-observer longitude, indicating that there are no significant rotational or seasonal thermal effects on Ceres. We do see a correlation between η and julian date, which is likely an instrumental artifact.  

Spectra between 17.6-37.1 μm were obtained on all nights, and additional spectra between 4.9-13.7 μm were obtained on three nights. The average spectrum (Figure 3) shows a broad feature near 20 μm due to phyllosilicates. We did not find any significant differences in the size or shape of this feature on any of the nights, indicating that it does not vary rotationally or seasonally. Ceres' lack of rotational and seasonal changes enables it to be a good calibrator for other astronomical observations, as expected. 

Figure 3. Average spectrum of  Ceres from all seventeen nights of data.