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In season 1 episode 1 of Space's Deepest Secrets (IMDB), Dr. Chris McKay, a planetary scientist for NASA, says around 1h:40m into the show, "This is the single most important and interesting place for astrobiology in our Solar System beyond the Earth."

Why is that? I'll admit having a bias towards Europa, which also seems to hold the possibility of harboring life. But - is Enceladus the better place to spend our money? Why?

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    $\begingroup$ The evidence for liquid water on Enceladus seems a little stronger than for Europa, but beyond that it seems like a tossup. $\endgroup$ – Russell Borogove Jun 19 '18 at 2:12
  • $\begingroup$ @RussellBorogove - Good thought. Given that there are water plumes visible in pictures of Enceladus, the evidence seems unassailable. Still, I thought the confidence of water on Europa is high, but I need to seek that out. $\endgroup$ – Don Branson Jun 19 '18 at 2:17
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    $\begingroup$ I think that Europa has a better chance of harboring life. My understanding is that Europa has enough heat that if its ocean was liquid at the time of formation, it would have stayed liquid throughout its history. However, Enceladus's ocean may be more recent. There would be lots more time for life to develop on Europa than Enceladus. $\endgroup$ – Phiteros Jun 19 '18 at 3:04
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According to the recent discoveries Europa can have plume activity too.

Hubble's observations in ultraviolet and re-analysis of Galileo magnetometer data show the possibility of the plumes at Europa.

Cassini took lots of photos of water plumes at Enceladus. But the Galileo probe had a problem with a failed main antenna, that dramatically reduced its downlink capacity. So Galileo could make only scarce number of photos. Some time ago I digged NASA photo archives to answer - whether Galileo made any photo of Europa in conditions favorable for plume observation? Looks like it did not. At least I didn't find it.

To see plumes we need some overexposed photos - plumes are faint compared with the bright surface of the icy moon. Also a high Sun-moon-spacecraft angle is desirable for less illumination of the moon's surface. For example look at this photo of Enceladus:

enter image description here

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in the photo's description:

The view was acquired at a distance of approximately 502,000 miles (808,000 kilometers) from Enceladus and at a sun-Enceladus-spacecraft, or phase, angle of 176 degrees. Image scale is 3 miles (5 kilometers) per pixel...

Enceladus' intriguing south-polar jets are viewed from afar, backlit by sunlight while the moon itself glows softly in reflected Saturn-shine.

Galileo made normal exposure photos of Europa's sunlit surface. The main purpose was mapping.

So, if Galileo could take enough pictures of Europa, including over-exposed ones, probably it could reveal Europa's plumes too.

Currently we have 100% evidence of Enceladus' liquid water. For Europa the conclusions about liquid water are based on indirect evidence at this time.

There is a known human bias to prefer most exciting and promising explanations of observed phenomena, and scientists should remember this. Sometimes reality turns out to be "more boring" that our imagination and theories. There is small chance that indirect evidences at Europa can be explained by other model. But currently the water ocean is the most likely explanation of the phenomenas observed.

To answer the question - most likely Europa is more interesting place for astrobiology than Enceladus.

As comments say, we don't know whether Enceladus' ocean is billions-years-old, or if it's recent. Thermal sources for Enceladus are a mystery, looks like tidal heating is not enough... For Europa there is a greater chance that the ocean existed through history and life could occur in it.

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