Irradiation-induced color changes in sodium chloride could reveal whether it came from ocean water mixing with surface water, a key component of the moon’s potential to support life.

A vast, salty ocean likely lurks beneath the smooth water ice crust of Europa, the smallest of Jupiter’s four largest moons. Such an ocean could potentially harbor alien life, and mixing between ocean and surface materials would increase these chances. New research by Poston et al. shows that the color of sodium chloride could reveal whether it may have recently risen to the surface, a key sign of mixing.

The new work builds on a previous finding that Europa’s yellow-brown streaks could get their hue from sodium chloride that was discolored by exposure to radiation from Jupiter’s powerful magnetic field. Such exposure causes defects known as color centers in the salt’s crystal structure, altering its tint.

In the new study, the researchers performed a more detailed investigation of sodium chloride’s response to Europa-like conditions. They placed sodium chloride crystals in a vacuum chamber and exposed them to various temperatures and levels of irradiation. Then, they observed changes in the visible wavelengths absorbed by the crystals over a period of several weeks.

The analysis showed that the ratio between two different color center defects, which result in different wavelength absorption patterns, changed over time as a function of irradiation exposure, largely independent of temperature and irradiation rate. Similar changes could occur for sodium chloride delivered to Europa’s surface by rising ocean waters.

These findings suggest a strategy for determining when potential ocean materials arrived at Europa’s surface. Visible light observations could reveal the color center ratios of any sodium chloride the materials may contain, indicating how recently they may have risen to the surface in a potential mixing event. This method could inform selection of landing sites for probes seeking signs of habitability and life.

Given Europa’s surface conditions and the speed of color center change, the new strategy could work for materials that surfaced up to a few decades before observation. A similar strategy with different parameters could potentially be used to explore other ocean worlds, such as Saturn’s moon Enceladus. (Journal of Geophysical Research: Planets, https://doi.org/10.1002/2017JE005429, 2017)

—Sarah Stanley, Freelance Writer