Planetary scientists in the US have traced carbon on the surface of Jupiter’s moon Europa to the icy ocean beneath it, revealing new information about the ocean’s nature and origin. The discovery raises astrobiologists’ hopes that the carbon, which exists in the form of carbon dioxide, could stem from biological processes taking place under the ice. However, a search for water plumes bursting out of Europa’s surface came up empty, and scientists involved in the observations say that better measurements will be needed to distinguish between biological and geological sources of carbon.
We know there’s an ocean on Europa thanks to Jupiter’s immense magnetosphere, which induces a magnetic field within the salty liquid water. Astrobiologists have speculated about the habitability of this ocean for years, but it is difficult to study because it is buried beneath the moon’s 23–47-kilometre-thick ice shell.
Instead of digging through the ice to probe the ocean directly, the latest studies used the Near-Infrared Camera (NIRCam) and Near-Infrared Spectrometer (NIRSpec) on the James Webb Space Telescope (JWST) to bring the ocean closer to us. Among the features on Europa’s surface are regions full of irregularly shaped blocks crisscrossed by discoloured ridges. Known as chaos terrain, these regions have been interpreted as sites where material from the ocean wells up and reaches the surface, and it’s here that scientists in two separate teams hunted for evidence of the ocean’s composition.
The data showed four strong spectral signatures of carbon dioxide in Tara Regio, which is an 1,800-kilometre-wide area of chaos terrain on Europa’s leading hemisphere. The scientists also identified a weaker signal of carbon dioxide in another area of chaos terrain called Powys Regio.
Signatures of carbon dioxide at spectral wavelengths of 4.25 and 4.27 microns brought particular attention. While the latter is the expected infrared emission of pure carbon-dioxide ice, the former suggests a mixture of carbon dioxide and other molecules.
One of the teams, led by Geronimo Villanueva of NASA’s Goddard Space Flight Center, identified this mix as water ice laced with carbon dioxide and methanol. Intriguingly, laboratory experiments suggest that the 4.25-micron signature could stem from salts being brought to the surface from the ocean and becoming irradiated. The carbon dioxide-water ice-methanol mixture then either forms a thin film around the salt crystals or is trapped inside them.
A primordial origin
The ratio of carbon-12 to carbon-13 isotopes on Europa is also of profound interest. Villanueva’s team measured this ratio as 83 (+/–19), placing it firmly within the bounds of ratios measured on Saturn’s moons, the near-Earth asteroid Ryugu visited by Japan’s Hayabusa-2 mission, and Earth, which has a carbon-12 to carbon-13 ratio of 89 for inorganic carbon (that is, carbon not bonded to hydrogen). This commonality suggests that, unlike water, which occurs in different isotopic ratios on different bodies, the carbon built into the worlds and moons of our solar system comes from the same source.
“The isotopic values, within the accuracy that we achieved, are indeed consistent with that of other moons and also of some primordial materials,” Villanueva tells Physics World.
As such, measurements of Europa’s carbon provide more information about the composition and distribution of materials in the proto-stellar disc that formed the solar system some 4.5 billion years ago.
An oxidized ocean
The second team, consisting of Samantha Trumbo of Cornell University and Michael Brown of the California Institute of Technology, focused on the origins of Europa’s carbon. Since the JWST detected no complex organic molecules on Europa’s surface, Trumbo and Brown say this eliminates any chance that the carbon dioxide formed via photodissociation of those organics as the radiation environment around Jupiter breaks them apart. Instead, the observations indicate that the carbon was already in the form of carbon dioxide when it reached the surface, suggesting that this carbon dioxide must therefore be dissolved in the ocean.
On this basis, Trumbo and Brown drew some general conclusions about the state of Europa’s ocean. They suggest that the ocean is highly oxidized, which is consistent with models depicting the downwards motion through the ice of oxidants such as molecular oxygen and hydrogen peroxide that formed in the radiation environment on the surface. However, even NIRSpec’s powerful eye could not determine whether the carbon dioxide came from living organisms. “More measurements and higher accuracies will be needed to further establish the formation and evolution processes of the observed carbon on Europa,” Villanueva agrees.
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Something else that will require more measurements is the plumes of water spraying high above Europa’s surface. Although the Hubble Space Telescope detected such plumes on three occasions over the past 10 years, the JWST saw none during its observations in November 2022. While this doesn’t mean the plumes aren’t real, it does place an upper limit of 300 kilograms per second on the mean rate of material spewing out. It also means that the plumes, if they exist, must be intermittent.
Further information is likely to arrive within the next decade, with the European Space Agency’s Jupiter Icy Moons Explorer (JUICE) due to perform two fly-bys of Europa once it arrives in the Jovian system in 2031. NASA’s Europa Clipper mission is also due to set sail for Jupiter in 2024, with a planned arrival date in 2030. The JWST’s observations will play a vital role in determining where, and what, the two missions should study on Europa’s surface.