By Shannon Stirone
Jupiter’s icy moon Europa is believed to be one of the most promising places to search for alien microbes, thanks to the presence of a subsurface ocean. Plumes of water thought to emerge from the surface would make it even easier to hunt for life – but it now seems these plumes could just be warm rocks.
Key to the mystery is a property called thermal inertia, a measure of how quickly a material absorbs and releases heat, which is different for all rocky material. For example, sand has a low thermal inertia, heating up quickly during the day and cooling fast at night. Larger pebbles or boulders take a while to heat up, but retain that warmth well into the night.
This slower kind of retention could be taking place at the Pwyll crater on Europa. NASA’s Galileo probe, which orbited Jupiter between 1995 and 2003, observed a “hotspot” in this crater – normally indicative of an underground heat source that could fuel a spout of water. But although researchers suspected the heat source may be a plume, the results were never confirmed.
Now Samantha Trumbo and Mike Brown at the California Institute of Technology have used the ALMA telescope array in Chile to watch the anomalous spot during Europa’s daytime and compared it to the night-time observations previously made by Galileo.
“All we can say is that our data are not consistent with a current subsurface heat source,” says Trumbo. “That doesn’t necessarily mean there’s no plume there.” It just means that they’re not seeing a heat signature with that plume like they have on Enceladus, Saturn’s icy moon.
Sampling from space
Testing the liquid that researchers strongly suspect lies beneath Europa’s icy crust would be easier if material was being shot out into space, where a spacecraft could collect samples. The alternative would involve the tricky feat of finding a way to drill through the thick ice.
There is still hope, even if there are no plumes in that spot: previous observations with the Hubble Space Telescope hint at possible plumes elsewhere on Europa, but the ALMA results may have crossed the Pwyll crater off the list.
Two spacecraft missions are planned to visit Europa in the 2020s, so finding out whether the plumes exist in advance is a top priority. Trumbo and Brown have been approved for more telescope time on ALMA, where they will dig deeper into the mysteries of the Pwyll hotspot in the hope of finding an answer.
“It’s really interesting, because one of the big problems we have with Europa is the lack of data,” says Cynthia Phillips at NASA’s Jet Propulsion Laboratory. “If there was a confirmed plume, that would be really exciting, but if there’s no plume, it doesn’t mean the ocean is less habitable.”