The Influence of Temperature and Photobleaching on Irradiated Sodium Chloride at Europa-like Conditions

By Keith Cowing
January 5, 2022
Filed under
The Influence of Temperature and Photobleaching on Irradiated Sodium Chloride at Europa-like Conditions
Spectra of irradiated sodium chloride at 120K with our high irradiation dosage and no photobleaching. M-center growth is observed after F-center saturation.

Europa’s leading hemisphere chaos regions have a spectral feature at 450 nm that has been attributed to absorption by crystal defects in irradiated sodium chloride, known as F-centers.

Some discrepancies exist between the laboratory data of irradiated sodium chloride and the observations, including a ∼10 nm shift in central wavelength of the F-center band and the lack of the prominent 720 nm absorption on Europa from M-centers, which result from the coalescence of pairs of F-centers. Here, we perform irradiation experiments on sodium chloride in an attempt to understand these discrepancies. We show that careful control of the temperature of the sample at a temperature of 120 K yields F-centers with an absorption wavelength comparable to that of Europa.

In addition, we measure the effect of photobleaching — the destruction of F-centers by photons — and show that at the energetic particle and photon flux on Europa, an equilibrium will be reached where only a modest F-center absorption develops. The density of F-centers never reaches high enough values for the creation of secondary M-centers. Our experiments predict that F-centers grow during the night on Europa in the absence of photobleaching and then partially decay during the daytime. We show observations from the Hubble Space Telescope consistent with this prediction. All observations of the 450 nm F-center on Europa are now consistent with laboratory measurements of sodium chloride, confirming the presence of this salt on Europa.

William T.P. Denman, Samantha K. Trumbo, Michael E. Brown

Comments: 11 pages, 8 figures, Published in conjunction with two other manuscripts, Planetary Science Journal, in press
Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Instrumentation and Methods for Astrophysics (astro-ph.IM); Chemical Physics (physics.chem-ph)
Cite as: arXiv:2201.01332 [astro-ph.EP] (or arXiv:2201.01332v1 [astro-ph.EP] for this version)
Submission history
From: William Denman
[v1] Tue, 4 Jan 2022 20:00:53 UTC (1,005 KB)
Astrobiology, Astrochemistry

Explorers Club Fellow, ex-NASA Space Station Payload manager/space biologist, Away Teams, Journalist, Lapsed climber, Synaesthete, Na’Vi-Jedi-Freman-Buddhist-mix, ASL, Devon Island and Everest Base Camp veteran, (he/him) 🖖🏻