Why don’t Europa’s plumes have heat signatures?
Does Jupiter’s moon Europa have water vapor plumes? A growing body of evidence in the last few years suggests the answer is yes, but a final confirmation has remained elusive. If the plumes do exist on Europa, they seem to be less active than those on Saturn’s moon Enceladus, where huge geyser-like plumes erupt from the moon’s south pole on a regular basis. Data from the Hubble Space Telescope has strengthened the case for plumes on Europa. But a new finding – presented on October 22, 2018, at the Division of Planetary Sciences meeting in Knoxville, Tennessee – has thrown a wrench into the possibility.
Researchers led by Julie Rathbun of the Planetary Science Institute said they found a lack of the expected heat signatures at locations where Europa’s plumes should originate. In other words … no hot spots.
Rathbun explained in a statement:
We searched through the available Galileo thermal data at the locations proposed as the sites of potential plumes. Reanalysis of temperature data from the Galileo mission does not show anything special in the locations where plumes have possibly been observed. There are no hotspot signatures at either of the sites.
This is surprising because the Enceladus plumes have a clear thermal signature at their site of origin, so this suggests that either the Europa plumes are very different, or the plumes are only occasional, or that they don’t exist, or that their thermal signature is too small to have been detected by current data.
Previous observations had suggested a plume originating from an area north of Pwyll Crater on Europa, and reanalysis of Galileo magnetometer and plasma data also supported the existence of a plume source about 600 miles (1,000 km) northeast of the first site.
But if the plume locations on Europa don’t show any heat signatures – as they do on Enceladus – that result suggests that either Europa’s are different from those on Enceladus, or that Europa’s plumes don’t even exist. Rathbun put it simply when she said:
Europa was expected to be active.
Rathbun said there are four possible explanations for the lack of heat signatures on Europa. The plumes may be intermittent. They may be fundamentally different from those on Enceladus and not be associated with anything hot. They may be smaller than expected. Or, they don’t exist; however, other data from Hubble provide strong evidence that they do exist.
On Enceladus, the plumes originate from a salty global subsurface ocean, where the water makes its way to the surface through large cracks called “Tiger Stripes” in the outer ice shell near the south pole. Data from the Cassini spacecraft, which orbited Saturn until September of 2017, also showed that there is likely geothermal activity on the ocean floor. That’s intriguing because, on Earth, geothermal vents on the sea floor provide conditions suitable for life.
It’s thought that conditions similar to those on Enceladus probably exist in Europa’s global subsurface ocean as well. But finding out for sure will require a return mission to the vicinity of Jupiter. Europa’s subsurface ocean has given it a powerful allure for space scientists, and a future planetary mission is already bound there. NASA’s upcoming Europa Clipper mission, to be launched in the early-mid 2020s, will be able to study the marine environment in Europa’s ocean better than ever before. It will be able to examine deposits left on the moon’s surface by evaporating ocean water. Some water may simply seep to the surface, but if there are plumes, they could deposit a large amount of minerals on the surface, which Europa Clipper could analyze.
It’s even possible that Europa Clipper could fly directly through the plumes and analyze their composition, much as Cassini did at Enceladus, when that spacecraft discovered complex organic molecules in Enceladus’ plumes.
Whatever the explanation for Europa’s plumes turns out to be, it will provide scientists with valuable insight into how plumes occur on other worlds, including the nitrogen gas ones on Neptune’s moon Triton. Those plumes have nothing to do with water, but are active geysers of extremely cold nitrogen gas – something not seen anywhere else in the solar system.
Bottom line: New research shows that Europa’s plumes may be significantly different from those on Saturn’s moon Enceladus, since they don’t exhibit the same heat signatures. The upcoming Europa Clipper mission should be able to help finally determine what is happening (or not) on Europa.