Astrochemistry

VIS spectroscopy of NaCl-water ice mixtures irradiated with 1 and 5 keV electrons under Europa_s conditions: Formation of colour centres and Na colloids

By Keith Cowing
astro-ph.EP
August 2, 2022
Filed under ,
VIS spectroscopy of NaCl-water ice mixtures irradiated with 1 and 5 keV electrons under Europa_s conditions: Formation of colour centres and Na colloids
CAD of the MoHIS cameras positioned on the side flange of the MEFISTO vacuum chamber. B) closeup view of the samples (here irradiated slabs) in position on the cooled-down table, after re-opening the chamber. C) CAD of the MEFISTO vacuum chamber with the position of the electron gun and the MoHIS cameras. D) Picture of the inside of the MEFISTO chamber during sample insertion. The mass-spectrometer – Time of flight (MS-TOF) is not described in this paper (will be the subject of another publication).

Recent laboratory efforts and telescopic observations of Europa have shown the relevance of a yellow colouration of sodium chloride (NaCl) caused by crystal defects generated by irradiation.

We further investigate this process by irradiating (with energetic electrons) different types of analogues where NaCl is associated in different ways to water ice. We produce two types of icy analogues: compact slabs and granular particles where we investigate two particle sizes (5 and 70 μm). We perform electron irradiation at cryogenic temperatures (100 K) and under high vacuum (10-7 mbar) conditions, with energies of 1 and 5 keV. We observe the formation of two different types of colour centres.

The so-called F-centres (460 nm) were formed in every sample, but the intensity of the absorption band within the compact slabs surpassed any other icy analogues and was comparable to the intensity of the absorption band within pure NaCl grains. M-centres (720 nm) have not been detected at the surface of Europa so far, and were close to the detection limit during our irradiation of compact slabs. The slabs could be good analogues for Europa_s surface as they produce mainly F-centres. Other notable differences have been observed between compact slabs and granular samples, such as the presence of an absorption band at 580 nm attributed to colloids of Na, exclusively within granular samples. Such absorptions have not been reported in previous studies.

Romain Cerubini, Antoine Pommerol, André Galli, Bernhard Jost, Peter Wurz, Nicolas Thomas

Comments: 57 pages, 13 figures
Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Soft Condensed Matter (cond-mat.soft); Chemical Physics (physics.chem-ph)
Cite as: arXiv:2208.01314 [astro-ph.EP] (or arXiv:2208.01314v1 [astro-ph.EP] for this version)
Journal reference: Icarus, June 2022
Related DOI:
https://doi.org/10.1016/j.icarus.2022.114977
Focus to learn more
Submission history
From: Romain Cerubini
[v1] Tue, 2 Aug 2022 08:47:30 UTC (3,864 KB)
https://arxiv.org/abs/2208.01314
Astrobiology

SpaceRef co-founder, Explorers Club Fellow, ex-NASA, Away Teams, Journalist, Space & Astrobiology, Lapsed climber.