The Gory Details

John SpencerJohn Spencer, Cassini Scientist on the Composite Infrared Spectrometer (bio)

For the nerds out there (you know who you are), here’s a somewhat technical presentation that I gave a couple of weeks ago to the rest of the Satellites Orbiter Science Team (SOST) — the group that plans the details of all the non-Titan satellite encounters.  SOST holds a “preview” telecon before each major satellite encounter, where each team reminds the rest of us what their instrument will be up to, so this presentation summarizes what the Composite Infrared Spectrometer (CIRS) will be doing during tomorrow’s Enceladus flyby.

CIRS_061EN_FP3HOTSPT001 is the observation that I’ll be pouncing on once the data are calibrated and available on Thursday.  Occupying the time between 15 and 63 minutes after closest approach, it will give us by far our most detailed look so far at the heat from the south polar fractures.  As you see on slides 6 and 7, that one observation has a lot of different things crammed into it.

Here’s a secret decoder ring to translate some of the Cassini-speak in the presentation:

CIRS: The Composite Infrared Spectrometer.  CIRS has three different focal planes sensitive to different wavelengths of infrared radiation.  Yes, there is no focal plane 2- it was eliminated early in the design phase as a cost-cutting measure.

FP1:  CIRS focal plane 1, which measures long-wavelength heat radiation (wavelengths longer than 16 microns).  These are the wavelengths where most of Enceladus’ heat is radiated, so we hope to get improved measurements of Cassini’s total internal heat flow from this part of CIRS.  FP1 has rather coarse spatial resolution, represented by the red circles on the observation preview diagrams.

FP3:  CIRS focal plane 3, measuring shorter-wavelength radiation (wavelengths between 10 and 16 microns).  This detector can see much finer details than FP1, and is good for measuring the higher temperatures along the tiger stripes.  The FP3 field of view is  represented by the pink rectangle, which contains a row of ten pixels, each making independent measurements.

FP4: CIRS focal plane 4, measuring radiation at wavelengths shorter than 10 microns.  Enceladus doesn’t put out much radiation at these short wavelengths, unless we find some *really* hot spots along the tiger stripes, but FP4 will be recording data anyway, just in case.

Rev. 11, 32, 61:  Cassini orbit numbers.  Rev. 61 is the encounter coming up tomorrow.

C/A-02:30, etc.:  Times measured relative to closest approach time, in hours and minutes.  Closest approach is at 19:06:12 Universal Time on March 12th.  Other times are in Universal Time.

CIRS_061EN_FP1INMAP001, etc.:  The official names used to identify each discrete Cassini observation.

Bolometric Albedo:  The fraction of the sunlight hitting Enceladus (at all wavelengths), that is reflected back into space (in all directions).  The remaining sunlight is absorbed by the surface and heats it, so understanding Enceladus’ bolometric albedo is important for understanding out how much heat escapes from the interior.  Enceladus’ bolometric albedo is about 0.8, higher than for any other known planetary body, due to its coating of plume fallout, which is almost pure, white, water ice.

Thermal Inertia:  A measure of how well a planetary surface can store heat.  A high thermal inertia surface will not cool down much at night, while low thermal inertias mean rapid nighttime cooling.  Again, it is important to understand Enceladus’ thermal inertia to separate out the warmth of absorbed sunlight from the warmth of its internal heat.

MAPS: Magnetospheric and Plasma Science, the group of instruments on Cassini that directly measure gases, plasmas, dust, and magnetic fields.  Tomorrow’s flyby is a very important one for the MAPS instruments, as they directly sample the material in the plume as we fly through.  MAPS will be in charge of the spacecraft for the critical few minutes around closest approach.

Plume source VI: The source of one of the jets identified from analysis of the Cassini plume images, by Joe Spitale and Carolyn Porco in a recent “Nature” paper.

Hot spot C: A discrete region of high surface temperatures seen by the CIRS instrument during the July 2005 flyby, listed in a 2006 “Science” paper by myself and the rest of the CIRS team.  Plume source VI seems to correspond to hot spot C, so that’s a region we’ll  
be focusing on.

Here’s the full presentation.

Wish us luck!