In 2014, scientists with Southwest Research Institute and collaborators published a paper describing what they thought were giant plumes of water vapor erupting from the surface of Europa, an icy moon orbiting Jupiter.
Now, they’re not so sure.
In a follow-up study published in May, researchers concluded that the signatures they identified over a decade ago could be explained equally well by a statistical quirk.
Whether such plumes exist is important given that Europa is one of the most promising candidates for potential life beyond Earth within our solar system.
A hidden ocean
Jupiter, the fifth planet from the sun and by far the largest in our solar system, is orbited by four large moons. Europa is the smallest of those moons — slightly smaller than Earth’s moon — and covered in a relatively smooth icy surface.
There’s evidence that a giant liquid saltwater ocean sits below Europa’s thick ice sheet, potentially containing twice as much water as Earth’s oceans, and maybe even life.
Researchers believe that all the necessary ingredients for life, at least as we understand it — namely liquid water, certain chemical elements and energy — could be present, making it one of the best places to look for lifeforms beyond our own planet, according to NASA.
That’s part of the reason why the water plume discovery from 2014 was exciting, explained Kurt Retherford, a planetary astronomer at Southwest Research Institute and co-author of the paper.
Using special imaging from the Hubble Space Telescope, Retherford and his collaborators detected ultraviolet emissions from Europa’s south pole, consistent with plumes of water vapor erupting 124 miles above the surface.
If these plumes are real and originate from the moon’s subsurface ocean, it would make it a lot easier for scientists to get a clearer picture of the ocean’s chemical soup, and whether it could support life.
As Retherford explained, flying an unmanned spacecraft through a plume to collect that data is a much easier engineering project than having to drill through the moon’s thick ice sheets.
“We could taste and sense what’s in that plume in terms of those minerals or other kind of chemicals that we’re interested in learning more about,” Retherford said.
Statistical quirk
Retherford said that the research team did their best to poke holes in their own findings back in 2014, but that these kinds of measurements are vulnerable “to some other kind of little nuance that you only realize years later,” he said.
“We provided the paper to Science Magazine, and we originally included a sentence saying, ‘These results need to be verified with additional future measurements,'” Retherford said. “And they didn’t like that sentence so much, so we took it out.”
After re-analyzing their findings and the method they used to identify the signatures, Retherford and his collaborators concluded that the readings could just as easily be explained by the positioning of the Hubble Space Telescope in relation to Europa.
It doesn’t rule out water vapor plumes, but Retherford no longer sees the signatures as solid evidence of their existence. “(We have) hints of data, but no real smoking gun for water vapor plumes,” he said.
In 2030, scientists will get some clarity. A basketball-court-sized spacecraft known as Europa Clipper launched by NASA in 2024 is on its way to the moon to collect a trove of data from its atmosphere. The mission’s main objective is to determine if Europa has conditions suitable for life.
It will do that by flying instruments as low as 16 miles above the moon’s surface, conducting nearly 50 flybys at different locations. “It has a full suite of different instruments that are all well suited to go search for plumes and characterize them, if they exist,” Retherford said.
Two of these instruments were developed by SwRI engineers and scientists in San Antonio, including Retherford, and are designed to work in tandem in search for certain chemical compounds in Europa’s atmosphere.
fter transmitting its data back to Earth, Europa Clipper is planned to crash into Ganymede, Jupiter’s largest moon, in 2034.
Finding life or the conditions that make it possible on other worlds is an important mission, as it can inform our understanding of how life began on Earth, Retherford explained.
“Europa is pretty attractive in terms of chances to find more of an ecosystem there, or at least signs of habitability,” Retherford said. “Understanding where we all came from … is a big question that we get to ask and help the community understand better. So I’m excited about that.”
