Jupiter’s second Galilean moon, Europa, is one of the most fascinating planetary objects in our Solar System with its massive subsurface ocean that’s hypothesized to contain almost three times the volume of water as the entire Earth, which opens the possibility for life to potentially exist on this small moon. But while Europa’s interior ocean could potentially be habitable for life, its unique surface features equally draw intrigue from scientists, specifically the large red streaks that crisscross its cracked surface.
While these red streaks are one of the most striking features on Europa, scientists have been unable to identify its chemical signature since no substance on Earth possesses a complementary signature itself. They have previously made their own hypotheses to their origins, with a 2015 study suggesting the red streaks could come from Europa’s interior ocean sea salt that has been blasted with radiation on the surface.
It is these red streaks, and more specifically their chemical origin, that an international team of researchers led by the University of Washington (UW) have addressed in a recent study with the discovery of a new kind of solid crystal that could help explain the scientific processes responsible for the red streaks’ existence on Europa. While this new crystal was created in a laboratory setting, scientists hypothesize it could also form at the bottom of the deep oceans on worlds like Europa, as well. The newly discovered solid crystal is formed from water and table salt (sodium chloride), which are two of the most common substances found on Earth.
Learn more about this remarkable study here!
“It’s rare nowadays to have fundamental discoveries in science,” said Dr. Baptiste Journaux, who is an acting assistant professor in UW’s Department of Earth and Space Sciences, and lead author of the study. “Salt and water are very well known at Earth conditions. But beyond that, we’re totally in the dark. And now we have these planetary objects that probably have compounds that are very familiar to us, but at very exotic conditions. We have to redo all the fundamental mineralogical science that people did in the 1800s, but at high pressure and low temperature. It is an exciting time.”
For the study, the researchers investigated what is known as a hydrate, which is an icy lattice formed in cold water temperatures from the combination of salts and water. Until now, only one sodium chloride hydrate was known to exist, known as hydrohalite, which consists of one salt molecule for every two water molecules.
Using transparent diamonds and cold temperatures, the team compressed a miniscule amount of salty water near 25,000 times Earth’s atmospheric pressure, where they observed two new sodium chloride hydrate crystal structures. The first structure contains two sodium chloride molecules for every 17 water molecules, and the other contains one sodium chloride molecule for every 13 water molecules. It was also discovered that the structure containing 17 water molecules remained stable even near vacuum pressure, which is equivalent to the Moon’s surface, while the structure containing 13 water molecules only maintained its stability at high pressure. It is hypothesized that these unique crystal structures could help explain the “watery” signatures from Jupiter’s moons.
This microscopic image displays the study’s newly discovered hydrate containing two sodium chloride molecules for every 17 water molecules. This crystal structure was produced at high pressure but maintains its stability under cold and low-pressure environmental conditions. Scale bar: 50 micrometers = 0.000050 meters. (Credit: Journaux et al./PNAS)
Previously known sodium chloride hydrate containing one salt molecule for every two water molecules (left); two newly discovered crystal structures with two sodium chloride molecules for every 17 water molecules (center) and one sodium chloride molecule for every 13 water molecules (right). (Credit: Baptiste Journaux/University of Washington)
“We were trying to measure how adding salt would change the amount of ice we could get, since salt acts as an antifreeze,” said Dr. Journaux. “Surprisingly, when we put the pressure on, what we saw is that these crystals that we were not expecting started growing. It was a very serendipitous discovery.”
These same cold, high-pressure environments likely exist on Europa, as scientists postulate its interior ocean could be hundreds of kilometers deep underneath approximately 5 to 10 kilometers of ice, with denser ice structures possibly existing at the bottom of the ocean where the temperatures and pressures would be even colder and more extreme.
For next steps, the researchers wish to create or collect a bigger sample to conduct a more in-depth investigation regarding whether icy moons signatures such as the red streaks found on Europa complement the two recently discovered hydrates.
Both NASA and the European Space Agency (ESA) currently have a few planetary missions scheduled to visit Europa and Titan to explore their potential habitability. These include the ESA’s Jupiter Icy Moons Explorer, also known as JUICE, which is slated to launch in April of this year and arrive at the Jupiter system in July 2031; NASA’s Europa Clipper mission, which is slated to launch in October 2024 and arrive at the Jupiter system in 2030; and NASA’s Dragonfly mission to Titan, which is slated to launch in 2027 and arrive at Titan in 2034. All these missions will attempt to learn more about the chemical compositions of these mysterious and intriguing worlds, which will help scientists determine the best ways to search for life.
Get a recent update on NASA’s Europa Clipper mission here!
“These are the only planetary bodies, other than Earth, where liquid water is stable at geological timescales, which is crucial for the emergence and development of life,” said Dr. Journaux. “They are, in my opinion, the best place in our solar system to discover extraterrestrial life, so we need to study their exotic oceans and interiors to better understand how they formed, evolved and can retain liquid water in cold regions of the solar system, so far away from the sun.”