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Spectral Fingerprints of Europa's Ocean

Europa's young surface age, unique geology, and potentially salty surface composition suggest an active past, during which material originating in the subsurface ocean may have been emplaced onto the surface. Using spectroscopic observations of geologically interesting terrains and comparison with laboratory data, I am working to unravel the composition of oceanic material on Europa's surface.

Tasting Europa's Ocean

What does Europa's ocean taste like? This may seem like a strange question, but it's actually a really important one! The potential habitability of Europa's subsurface ocean depends on its composition, and one key unknown is the nature of the oceanic salts. Currently, Europa's geologically young, disrupted surface provides our best window into the salt chemistry, as it likely features fossil ocean material emplaced from below. Investigations using Galileo Near-Infrared Mapping Spectrometer data led to the prevailing view that Europa’s endogenous salts are sulfate-rich, suggesting an ocean composition and chemical history very different from those of Earth's oceans. However, ground-based infrared observations questioned this view, instead suggested that Europa’s endogenic material may actually be rich in chlorides, like we find in our own oceans. The problem was that chlorides have no identifying spectral features at infrared wavelengths, making them impossible to identify in either the ground-based or the Galileo data. Excitingly, when irradiated under Europa-like conditions, sodium chloride (NaCl; basic table salt) becomes discolored due to the growth of radiation-induced defects known as "color centers" and turns a yellow-brown color very similar to some geologically young regions on Europa. Using spectra of Europa obtained with the Hubble Space Telescope, I detected a diagnostic absorption feature of color centers in NaCl within such geologically disrupted terrain (see right). This finding challenged the long-held sulfate hypothesis, instead supporting the idea of a choride-rich ocean more similar in composition to Earth's oceans than previously believed. So, it appears that Europa's ocean may taste pretty familiar after all! For some great write-ups of this finding, check out the Caltech press release here or the following articles in Scientific American and Gizmodo!

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Figure adapted from Trumbo et al. (2019). The yellowish patch in the Galileo image of Europa (left) is the geologically disrupted region Tara Regio, in which we find the strongest NaCl absorptions (right).

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Map of the spectral slope from 650 to 750 nm compared to an approximate true-color mosaic of Europa’s surface (image credit: NASA/JPL/Björn Jónsson) from Trumbo et al. (2020). This slope reflects a broad absorption feature visible across the red wavelengths and corresponds well to the reddish material visible in the imagery. As the broad absorption across the red wavelengths appears common to all of the large-scale geology experiencing sulfur radiolysis, it likely reflects species formed via the bombardment of a mixture of endogenic material and implanted sulfur from Io.

A Salty Source for Europa's Surface Color?

Spacecraft images of Europa show striking color variations across the surface that exhibit marked hemispherical differences and correlations with surface geology. These patterns likely reflect the combined influences of endogenic and exogenic sources on the underlying surface composition. A unique association of color with geologic features pervades the entire surface and hints that compositional fingerprints of the internal ocean may persist within recent geology. Indeed, as discussed above, I have shown that the yellow-brown color of the leading-hemisphere geology partially reflects the presence of discolored NaCl ultimately sourced from the interior. But, a distinct color contrast between the leading and trailing hemispheres, in which the geologic features of the trailing hemisphere are significantly darker and redder than their leading-hemisphere counterparts, appears to also reflect the constant exogenic alteration of the trailing-hemisphere surface chemistry via sulfur radiolysis. Sulfur plasma ions from the volcanos of Io co-rotate with Jupiter’s magnetic field and continuously deposit onto the trailing hemisphere, where bombardment by energetic magnetospheric electrons, protons, and ions drives a chemically active radiolytic sulfur cycle that affects the underlying composition. Indeed, continuous lineae that traverse from the leading to the trailing hemisphere appear to change color, becoming more red as they are exposed to the impinging sulfur plasma.
 

Using spectra from the Hubble Space Telescope, I have examined spectral features unique to the trailing hemisphere and compared their distributions with surface color, geology, and radiation bombardment patterns. This allowed me to isolate features likely reflecting species radiolytically produced from a combination of endogenic material and Iogenic sulfur. In particular, I hypothesized that a broad absorption found to correlate specifically with the red material within geologic features could reflect radiation-induced discoloration of sulfate salts that are produced via the sulfur bombardment and subsequent radiolysis of pre-existing chlorides. If true, then characterizing this radiolytically altered oceanic material will be key to understanding the chemistry of the source material, as well as that of potential exogenic sources to the subsurface ocean in the case of mutual exchange through the ice shell.

Endogenic vs. Exogenic Species

As Europa orbits within Jupiter's magnetosphere, its surface is continuously bombarded with energetic particles trapped within Jupiter's rapidly rotating magnetic field. These include energetic electrons and protons, as well as sulfur and oxygen ions originating from eruptions on Jupiter's volcanic moon Io. This particle irradiation drives much of Europa's surface chemistry by radiolytically processing the surface and creating several new species. Disentangling potential endogenous species from radiolytic products is thus critical to understanding the surface composition of Europa and thereby constraining the chemistry of the ocean below. Though such products do not originate in the subsurface ocean, they may still play an important role in the ocean chemistry and hold clues to the dominant cations and anions in the native material. I am using large ground-based telescopes to spectroscopically identify, map, and study these radiolytic substances, and to disentangle them from species characteristic of more pristine material. Ongoing projects include using adaptive optics to map, and comparison with laboratory experiments to identify, a mystery spectral feature near 3.78 µm in the infrared.

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Figure from our Trumbo et al. (2017) reporting the detection of a previously unseen absorption feature on the trailing hemisphere. Adaptic optics mapping of this feature to constrain its origin is ongoing.