Europa is an ice-encrusted moon of Jupiter with a global ocean flowing underneath its surface. NASA is planning a mission soon that will look for signs of possible life there.
Now, a new finding from old data makes that mission even more tantalizing.
In recent years, the Hubble Space Telescope has spotted what looks like plumes, likely of water vapor, reaching more than 100 miles above the surface.
The plumes, if they exist, could contain molecules that hint at whether Europa possesses the building blocks of life.
In a study published Monday in the journal Nature Astronomy, scientists are reporting a belated discovery that Galileo, an earlier NASA spacecraft that studied Jupiter, appears to have flown through one of the Europa plumes more than 20 years ago.
And that occurred close to one of four regions where Hubble has observed plumes.
“That’s too many coincidences just to dismiss as ‘There’s nothing there’ or ‘We don’t understand the data,’” said Robert T. Pappalardo, the project scientist for NASA’s upcoming Europa Clipper mission, which may launch as soon as 2022.
“It sure seems like there’s some phenomenon, and plumes seem consistent.”
Galileo, which launched in 1989, arrived at Jupiter in 1995 and spent almost eight years examining the planet and its moons until its mission ended with a swan dive into Jupiter in 2003.
During a flyby of Europa on Dec. 16, 1997, instruments on Galileo measured a swing in the magnetic field and a jump in the density of electrons. At the time, scientists noted the unusual readings, but they did not have an explanation.
Then, in 2005, another spacecraft passing by another moon around another planet made a startling observation.
NASA’s Cassini spacecraft — which completed its mission last September — found geysers of ice crystals erupting out of Enceladus, a small moon of Saturn. Enceladus, it turns out, also has an ocean of liquid water under its ice.
That spurred renewed curiosity about Europa and whether it too might burp bits of its ocean into space. The Hubble first recorded signs of possible plumes in 2012, then again in 2014 and 2016.
But at other times, Hubble has looked and seen nothing. That suggests the plumes are sporadic.
Last year, Melissa A. McGrath, a senior scientist at the SETI Institute in Mountain View, Calif. who was not involved in the new study, took a look at some radio experiments conducted by Galileo which examined how signals bent as Europa passed between Earth and the spacecraft.
The experiments showed Europa possesses an atmosphere.
Astronomers will certainly be taking more looks at Europa with the Hubble, trying to better understand how often the plumes erupt.
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