Investigating Detectability of Organics on Europa's Surface

Detection of organics on the surface of Europa, especially amino acids, will bolster the case for potential habitability of the subsurface ocean. This project investigates detection of such species through model spectra of intimate mixtures.

We start with a simple, 1-D geometric optics model. Using the optical constants of water ice, sulfuric acid octahydrate (SAO) and alanine at cryogenic temperatures, as available in literature, we find that a water ice and alanine mixture spectrum is almost indistinguishable from a pure water ice spectrum for low alanine concentration (below 5%). However, the reflectance values of a water, SAO and alanine mixture are higher than a pure water ice spectrum values, and hence distinguishable at similar concentration levels.

We are currently revising the model to incorporate angle dependence of reflectance and fine-tune it to Europa's surface conditions. We also plan to extend this work to beyond 2.5 microns region, which is more favorable for detection of organics. Radiolytic processing of organics and its effect on the spectra also needs to be explored. Other radiolytic products like H₂O₂, CO₂ and SO₂ can also be included in the mixture.

The biggest limitation to our preliminary work has been the lack of quality laboratory data of optical constants of the end members, especially the organics, at the appropriate cryogenic temperatures. There is a similar dearth of reflectance spectra in online databases, surprisingly of water ice. The available spectra are mostly not well characterized in terms of the particle grain size distribution, temperature of the sample, etc.

Our work points out the need for additional laboratory work tuned to Europa conditions, so as to maximize the utility of space and ground based spectra of this intriguing ocean world.