Best bet on what this is - Hubble may have spotted the water plume from Europa again. This is what it spotted last time.
Hubble Sees Evidence of Water Vapor at Jupiter Moon
Note that the photograph of Europa was not taken by Hubble. It just made the observation of the water plume - those large blue pixels. If it has taken new photos showing a water plume, they will be similar resolution to this.
There isn't much else Hubble can see at this distance. It’s not going to see details of the surface. Europa is just too far away for that. Some plume of some sort, and water is the most likely bet there as for last time.
This is potentially exciting news because the last time it spotted a plume of water, it was a once off observation and didn’t get repeated. So after the initial excitement many astronomers concluded it was probably just a very rare asteroid impact on Europa sending water up into space. Europa's Elusive Water Plume Paints Grim Picture For Life - Astrobiology Magazine
However if it has spotted it again, this makes asteroid impact a very unlikely explanation. If so this would show that it is probably a huge geyser.
This would be very exciting news for astrobiologists, because such a huge geyser would suggest the water is coming from deep below the surface, maybe even from the subsurface ocean 100 km below the surface.
This in turn is very interesting because this is one of our best chances in the solar system for finding extraterrestrial life and even possibly, complex multicellular life like Earth. And almost no chance of life transferred from Earth to Europa or vice versa over the entire history of the solar system. And if it is sent into space in a geyser, one of our spacecraft could sample it just by flying through it with no need for a lander (which is already a possibility for Enceladus).
This shows an actual photograph of the geysers of Enceladus:
They vent into space (as ice of course, once they hit the vacuum of space) and escape its gravity, feeding one of the minor rings of Saturn.
If the Hubble observations indicate a geyser, it’s a powerful geyser to send material all the way into space far enough away for Hubble to spot it, because the geyser would have to overcome the much greater gravity of Europa, of 1.315 m/s² similar to the Moon’s 1.62519 m/s² (it’s 81% of lunar gravity).
We also have a camera in orbit around Jupiter, Junocam. Juno doesn’t go very close to Europa, but it does occasionally get close enough so that it could image an active geyser as two pixels instead of less than one pixel. So if there is a visual element, such as dust, then maybe it can spot that too.
So then, there's a chance of a follow up observation with Junocam on Juno - to see if there is anything visual such as particles as for Enceladus - but probably not until March 2017 and then September / October 2017 as the best configuration for Juno to observe Europa - still at a distance but far closer than we can from Earth and a plume could span 2 pixels, enough for a chance of information about it if it is more than just water vapour.
This is what Emil Laksawalla says about using JunoCam to image Europa:
"JunoCam or ASC can only detect plumes if they contain fine particles. The Hubble discovery (if real) only shows the presence of water vapor. We can predict by analogy to Enceladus that water vapor plumes will also contain particles. However, it is important to remember that the Hubble discovery was of gas, not particles. If the putative Europa plumes are Enceladus-like and do contain particles, they would not be as tall as Enceladus', because of Europa's higher gravity. Scaling for Europa’s gravity gives a maximum plume height of under 140 kilometers. To detect plumes, we need at least two pixels, so the image spatial scale would need to be better than 70 kilometers, at a relatively high phase angle where the particles would forward-scatter light to JunoCam and ASC.
"To achieve resolutions better than 70 kilometers per pixel, UVS needs to be within 40,000 kilometers of Europa; JunoCam, 100,000 kilometers; and ASC, 170,000 kilometers. For the cameras, given the low expected height of the plumes, there is not much flexibility.
"There are just four orbits that have Europa flybys that are closer than 300,000 km. Juno reaches the best available geometry in September 2017 as the rotation of the line of apsides brings Juno’s orbit close to Europa’s orbit:
"2017-03-08 253,118 km
2017-09-19 264,043 km
2017-10-03 92,267 km
2017-10-17 204,654 km"
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