Image of Europa from the Hubble Telescope. Note the plumes clearly visible.
The wispy, towering (200 km high) plumes of water vapor, rising above the icy surface of Jupiter's moon Europa, are readily visible in the lower and left periphery of the image. For some time there's been a question of whether these really exist, or are more a mirage like the once popular Martian canals. Now, new images from the Hubble Space Telescope at least strengthens prior evidence for existence, ongoing since the Galileo mission discovered the moon's subsurface ocean in 1996.
One excited researcher is Louise Prockter, director of the Lunar and Planetary Institute in Houston. Quoted in EOS Earth & Space Center News (15 November, p. 11):
"It's potentially great if the images do sow plumes from Europa ...because that means Europa's subsurface is coming at us. We could sample the material without digging through ice."
In fact, as the most recent issue of ASTRONOMY notes (Janaury, 2017, p. 11):
"If Europa has geysers, it opens new possibilities to test if the moon has the right conditions for life. As with Enceladus, space agencies may be able to fly a spacecraft through the plumes and test test the chemical signatures without having to drill into the surface. Additionally, such an encounter could identity areas where the ice shell is weaker and thinner."
In the case of the latest Hubble study, it was led by William Sparks, a researcher at the Space Telescope Science Institute in Baltimore who used a novel method to independently corroborate the plumes which is statistically significant. The plumes, or "geysers", were originally discovered by Lorenz Roth et al in 2012 on surveying a silhouette of Europa against the background of space for spectral lines of hydrogen and oxygen (indirect indicators of water). To read some more on this work, go to:
http://bit.ly/2012-plumes
By contrast, Sparks and co-workers made their observations as Europa passed in front of Jupiter. More of their work, which appeared in the Sept. 29 issue of The Astrophysical Journal, e.g.
http://bit.ly/ApJ-29Sept
can also be found here:
http://www.nature.com/news/europa-s-peek-a-boo-plumes-confirmed-1.20685
Interestingly, exoplanet researchers have commonly used this same technique - called transit photometry- to find their object against the background of the star it orbits. I had actually looked at the basic method in a previous post, e.g.
http://brane-space.blogspot.com/2016/07/carrying-out-yur-own-exoplanet-and.html
Once again, the basic principle is simple: when an object moves in front of a light source it blocks out part of that light. The amount of occlusion and the time duration gives an indication of the size of the exoplanet. Sparks and his team actually went beyond this template, in that Jupiter's glow provided a sufficiently smooth background against which potential plumes could be viewed.
The actual images were made in 2014 during Europa's transit of Jupiter, but processing them to achieve adequate resolution took many months. By using a method distinct from the one (by Roth et al) for the original plume discovery, the Sparks team supplied credibility to the hypothesis that the plumes exist. Still, Sparks urged caution, advising the result "was not one hundred percent verifiable" given that Hubble had operated at its technological limits in making the images.
One optimistic note is provided by recent history in respect of the Saturnian moon Enceladus, which had its own plumes confirmed a decade ago. This suggests similar confirmation may follow with Europa's plumes, but require actual spacecraft observations, since they seem to occur only under certain conditions.
Alas, we won't really know until years after 2022 when the European Space Agency (ESA) launches its Jupiter Icy Moons Explorer Mission. NASA is also building a Europa-focused mission (Europa Multiple Flyby Mission) scheduled to launch between 2020 and 2024. That is, unless Trump and the Republicans cut the NASA budget.
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