THE EFFECTS OF IMPACTS ON THE REFLECTANCE SPECTRA OF ALIPHATIC ORGANICS: IMPLICATIONS FOR CERES

In 2017, the imaging spectrometer on the Dawn mission detected a 3.4 mm absorption consistent with organic molecules on Ceres. Some studies have concluded that the organics formed on Ceres as a consequence of aqueous alteration. Other studies raised the possibility that organics were delivered to Ceres by one or more organic-rich impactors. Determining the origin of these organics is a critical clue to decoding Ceres’ evolution and astrobiological potential. If organics are endogenous, Ceres may be a prime and accessible target for investigating potentially habitable worlds.

Impact craters are pervasive on Ceres’ surface, so regardless of origin, organics have been affected by impacts. To date, however, little work has thoroughly examined how impacts affect the spectral properties of the type of organics detected on Ceres under the impact conditions expected at Ceres. Based on first-order calculations and prior work, the distribution of impact speeds at Ceres is such that the outcomes may run the gamut: some impacts might leave the 3.4 mm absorption unscathed whereas other impacts may obliterate this band. Unraveling the nuanced links between impact conditions and reflectance spectra of aliphatic organics will provide valuable constraints for interpreting reflectance spectra of Ceres, discerning the origin of Ceres’ organics, and assessing the relative importance of impacts compared to other processes that might affect the spectra of organics on Ceres (e.g., ion irradiation, solar wind).

To determine the effects of impacts on the reflectance spectra of aliphatic organics similar to those on Ceres, we conducted hypervelocity impact experiments at the NASA Ames Vertical Gun Range (AVGR). Gilsonite, which has the same aliphatic spectral absorptions as the organics detected on Ceres, served as the organic compound of interest. Impact experiments spanned a range of relevant impact velocities and impact angles and included shots with gilsonite in both the target and the projectile. Control experiments with no organics were done to track any possible organic contamination, but no measurable contamination has been found to date. We will report reflectance spectra collected on samples from these experiments and frame the results in terms of implications for Ceres.

This work was supported by DDAP grant 80NSSC19K1237.