I've been having a hard time understanding what flying through Enceladus's plume accomplishes. Cassini is an orbiter. It seems odd that it would have instruments that could even analyze the plume at all. I know dust counters are pretty common for similar missions, but it seems like Cassini would avoid flying through anything that would have enough material to sample. What can Cassini accomplish by flying through the Plume, and why was the capability designed in the first place?
Cassini's INMS, the Ion Neutral Mass Spectrometer, is an in situ instrument that measures the neutral and plasma gas composition of what it ingests. It was intended for the measurement of Titan's atmosphere, Saturn's magnetosphere plasma, ring composition, and in fact the composition of icy satellite effluents.
And yes, the Cosmic Dust Analyzer, CDA, also is an in situ instrument that detects ice particles as it flies through the plume. The CDA was intended mainly to measure ring particles. The CDA is sensitive enough to detect particle fluxes that would not harm the spacecraft. Otherwise there would be little sense in having such an instrument.
Concise version from the pre-flyby media teleconference announcement:
Cassini scientists are hopeful the flyby will provide insights into how much hydrothermal activity is occurring within Enceladus, and how this hot-water chemistry might impact the ocean’s potential habitability for simple forms of life. If the spacecraft’s ion and neutral mass spectrometer instrument (INMS) detects molecular hydrogen as it travels through the plume, scientists may get the measurements they need to answers these questions.
"Confirmation of molecular hydrogen in the plume would be an independent line of evidence that hydrothermal activity is taking place in the Enceladus ocean, on the seafloor," said Hunter Waite, INMS team lead at Southwest Research Institute in San Antonio. "The amount of hydrogen would reveal how much hydrothermal activity is going on."
Using Cassini's cosmic dust analyzer (CDA) instrument, scientists expect the flyby will lead to a better understanding of the chemistry of the plume. The low altitude of the encounter is, in part, intended to increase the spacecraft’s access to heavier, more massive molecules -- including organics -- than the spacecraft has observed during previous, higher altitude passes through the plume. The CDA instrument, which is capable of detecting up to 10,000 particles per second from the plume, also is expected to reveal how much material the plume is spraying from the moon's ocean into the space around Saturn.
And more detailed explanation of scientific expectations can be found in this Enceladus Flyby 'E-21' page:
Key scientific expectations for this flyby
Scientists are looking forward to several important scientific results from the Oct. 28 flyby. These results will not be available immediately -- they will take several months of careful analysis, and would be published in a peer-reviewed journal.
1. Confirm presence of molecular hydrogen (H2)
- This measurement will be accomplished using Cassini's sensor that sniffs the gases in the plume (called INMS)
- Confirmation of H2 would be an independent line of evidence that hydrothermal activity is taking place in the Enceladus ocean, on the seafloor
Amount of H2 Cassini measures would reveal how much hydrothermal activity is going on in the ocean.
• This has implications for the amount of energy available for creating a habitable environment in the ocean
2. Better understand the chemistry of material in the plume
Cassini's dust detector (called CDA) will obtain spectra of the heavier particles only found at low altitudes nearer to the plume's source
• Among these heavier particles, Cassini may detect new, more complex organic molecules (albeit with not enough resolution to confirm if they are biological in nature)
• Scientists think these heavier particles carry material from the sub-surface ocean
• Scientists are doing laboratory experiments to create a catalog they can refer to of chemical fingerprints (or spectra) for fragments of complex organic molecules Cassini might detect
3. Determine the nature of the plume sources
- Is the plume made up of tight, column-like jets or curtain-like eruptions that run along the length of the tiger stripe fractures (or both)?
- How much icy material are the plumes actually spraying out? Scientists are still not sure, and the amount has major implications for how long the moon might have been active.
- This measurement will be accomplished by part of Cassini's CDA instrument called the high-rate detector, which can count the impacting ice particles from the plume (over 10,000 per second) in real-time.
I know, pretty dry reporting* from me this time, but the rest is already in Mark's answer. I just thought to share this since I watched October 26th flyby teleconference. Recording of it is here, accompanying visuals here.
*OK, OK, here's a contrast-enhanced crop of a nice image of Enceladus' fairly recent South polar plume activity, taken by Cassini on October 14, 2015 (N00249604), two weeks before its flyby in the thick of it:
Cassini Solstice Mission scientists might have their own expectations, but I'll be also hoping for some awesome closeup images. :) It has been mentioned during the teleconference that they might suffer from substantial motion smear due to such high relative velocity (8.5 km/s) and bad lighting conditions since the Enceladus' South polar region will only be illuminated by Saturn shine during the flyby, but that it should be possible to digitally enhance them in post-processing and get some nice vistas of its surface anyway.
An update on the measurement's "accomplishments". The data gleaned from the analysis of the plume chemistry suggests not only a renewable source of energy for potential microorganisms in Enceladus' ocean, but helps identify characteristics of Earth organisms that might also live there if (accidentally) transported there. While the Methanogenic Archaea used in the example may not be common contaminants, it is a proof-of-principle.
Thus the plume analysis makes it even clearer the level of caution that would need to be exercised to avoid accidental contamination of Enceladus' "brothy" ocean.
A recent and open access paper in Nature Biological methane production under putative Enceladus-like conditions describes an in-depth analysis of sources of energy in Enceladus' ocean and suggests that molecular hydrogen as H2 may be present and "eatable" by some organisms currently doing similar things here on Earth, e.g. Methanogenic forms of the Archaea domain single-celled microorganisms. From the introduction:
The most prominent potential source of H2 in Enceladus’ interior may be oxidation of native and ferrous iron in the course of serpentinization of olivine in the chondritic core. Olivine hydrolysis at low temperatures is a key process for sustaining chemolithoautotrophic life on Earth9 and if H2 is produced in significant amounts on Enceladus, then it could also serve as a substrate for biological CH4 production.
You can read more about serpenization in the answers to the question What is serpentinization, in the context of disappearance of surface water on Mars?
The detection of silica-rich dust particles, as an indication for ongoing hydrothermal activity, and the presence of water and organic molecules in the plume of Enceladus, have made Saturn’s icy moon a hot spot in the search for potential extraterrestrial life. Methanogenic archaea are among the organisms that could potentially thrive under the predicted conditions on Enceladus, considering that both molecular hydrogen (H2) and methane (CH4) have been detected in the plume. Here we show that a methanogenic archaeon, Methanothermococcus okinawensis, can produce CH4 under physicochemical conditions extrapolated for Enceladus. Up to 72% carbon dioxide to CH4 conversion is reached at 50 bar in the presence of potential inhibitors. Furthermore, kinetic and thermodynamic computations of low-temperature serpentinization indicate that there may be sufficient H2 gas production to serve as a substrate for CH4 production on Enceladus. We conclude that some of the CH4 detected in the plume of Enceladus might, in principle, be produced by methanogens.