Month: March 2008

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

Well, now it can be told!  It’s frustrating for me to sit on exciting
new results – when we’ve found something cool in our data I want to
tell everyone all about it right away.  But it’s important to present
mission results in a coherent and coordinated way, and of course we
have to take the time to be sure we have our facts straight.  So
we’ve waited till today, Wednesday 26th March, 2 weeks after the
close Enceladus flyby, to spill the beans.

The days after the Wednesday March 12th Enceladus flyby were a blur
of frenzied activity for me as I worked to find the goodies in the
tens of megabytes of data that Cassini’s Composite Infrared
Spectrometer (CIRS) instrument had gathered during the flyby.  My
first peek at the uncalibrated data the next day, on Thursday
afternoon, was already thrilling – the glow of the tiger stripes was
visible not just at the usual 9 – 16 micron wavelength range where
we’d seen them before, but at wavelengths as short as 7 microns.  
Shorter wavelengths mean hotter temperatures (in the same way that
white-hot is hotter than red-hot), so it looked like the fractures
might be warmer than we had thought.  By the time I got all the files
I needed for the full analysis, from CIRS’s home at the Goddard
Spaceflight Center, it was time for dinner.  Precious ones and zeros
that had been flying through the Saturn system onboard Cassini 24
hours earlier, and squirted overnight across the solar system at the
speed of light, made the final leg of their journey to analysis by
bicycle, as I cycled home with my laptop.

After dinner I got comfortable on the sofa and dived in.  By bedtime
I had a preliminary map of the heat radiation from the south pole, at
four times the resolution of our previous best map.  Each of the four
tiger stripe fractures, clearly resolved in the thermal infrared for
the first time, lit up with the bright glow of internal heat.  A
couple of other fractures, not previously suspected to be active,
were warm too.  By Friday afternoon I also had preliminary
temperature estimates for the fractures- yes, they were pretty warm,
at least 180 Kelvin!  That’s -135 Fahrenheit, which isn’t exactly
toasty until you consider that the surrounding terrain is at around
65 Kelvin or -342 Fahrenheit.  The highest temperatures we’d seen on
the previous flyby in July 2005 were much lower, around 145 Kelvin
(-198 Fahrenheit)- not because Enceladus’s fractures were cooler
then, but because the older, more distant, scans were less
sensitive.  If it’s 180 K on the surface it must be even warmer down
below, perhaps approaching the magic number of 273 Kelvin or +32
Fahrenheit, where Enceladus’ ubiquitous ice can melt to form the holy
grail of astrobiology, liquid water.

Since that initial frenzy I’ve been checking calibration and refining
details, preparing graphics for today’s press briefing, and hearing
by e-mail and telecons about all the great data that was gathered by
the rest of Cassini’s arsenal of science instruments during the
flyby, some of which will be revealed by Hunter Waite at today’s
telecon.  We’ve also been modifying plans for upcoming flybys based
on the new data- we now know the hottest and most interesting parts
of the south polar region and can zero in on them next time.  Already
the theoretically-inclined members of the Cassini science team are
honing their models of the south polar plumes based on the new
findings, and the rest of the planetary science community will soon
be following suit.  And this is just the beginning- there are seven
more Enceladus flybys to come in the next two years.

I’ll end this blog with a big thank you to the rest of the CIRS
team.  I’ve been doing the fun job of analyzing these data, but there
would be no CIRS data to analyze without the tireless efforts of the
many folks at Goddard Spaceflight Center who designed and built CIRS
back in the 1990s, keep it running, design the observations, and
calibrate the data.  Special thanks are due to my CIRS colleagues
John Pearl and Marcia Segura, and CIRS Principal Investigator Mike
Flasar.

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

Yesterday was a long day of waiting for our Composite Infrared
Spectrometer (CIRS) data from Wednesday’s Enceladus flyby.  CIRS’s
most important job on this encounter was to map the temperatures in
the active south polar region, to learn more about what warms the
“tiger stripe” fractures and measure the total amount of heat that
Enceladus is generating (the current estimate, based on earlier CIRS
data, is a whopping 6 GigaWatts).  Before I and others could do much
with the new data, they had to be calibrated, and calibration of CIRS
data, which is done by the CIRS team at Goddard Spaceflight Center in
Maryland, is pretty complicated.  It turned out there were some
hiccups in the calibration process, and though the team was working
as fast as they could, everything took longer than expected.

So all through the morning I was reading excited e-mails from members
of other instrument teams, enthusiastically describing their first
look at their data, without being able to contribute any news from
CIRS.  I *did* copy over some files early in the day, but when I
learned that they were only partially calibrated, I decided to wait
for the good stuff – I concentrated on getting my software ready instead.

Marcia Segura at Goddard, who did the detailed designs for these
observations, kept me posted on the progress of the work at Goddard,
and by mid-afternoon it looked like the final calibration was still a
couple of hours away.   So I broke down and took a quick-and-dirty
look at the preliminary files I’d downloaded in the morning.  And
they showed …  well, I can’t tell you, because we still need to
properly calibrate and check everything, and make sure we don’t
announce any results that turn out to be wrong on closer inspection.
But I’m pretty happy with what I saw, and we (and the other
instrument teams) hope to have some cool findings to release in the
next few weeks.

Amanda Hendrix,  Cassini Scientist on the Ultraviolet Imaging Spectrograph (bio)

This is a neat movie (click here to download or click here to watch it) put together by Brent Buffington, on the Cassini Navigation team. I love these kinds of movies, because you can really get a good idea of what the spacecraft is doing during a flyby. Everything goes pretty fast, so you may want to watch this a few times to really absorb what’s going on!

In the big left-hand window (you can see a still from the first frame of the movie below), you can see Cassini and a projection of the active (or “prime”) instrument’s field-of-view (FOV) (see key below). The right-hand windows show a view of what the boresights are seeing. Sometimes they’re staring, sometimes they’re scanning. Generally, since the optical remote sensing (ORS) instruments are pretty much bore-sighted (as shown in the upper right window), that means that when one of them is “prime,” the others are “riding along” and also getting data, resulting in a wonderful multi-wavelength suite of measurements.

So we start off with a long, distant stare, with the Visual and Infrared Mapping Spectrometer (VIMS) prime. Even though Enceladus is still far away and small, staring for such a long time gives all the optical remote sensing instruments a nice chance to build up signal and get a good spectrum, useful especially for investigating surface composition.

After VIMS, ISS (i.e., the camera) takes over for a stare at Enceladus (notice that Enceladus is still smaller than a Narrow Angle Camera–NAC, here). Also notice that the phase angle is pretty high, meaning that the body is only partially illuminated and will appear as a crescent (not unlike our own Moon last night!).

After the cameras, radar does a scan of Enceladus. To do this, we have to turn the spacecraft 90 degrees, since the radar is mounted on the spacecraft at a different orientation from the cameras.

After radar, the Composite Infrared Spectrometer (CIRS) is prime and does a scan and a stare to get temperature information, of the north polar region.

Then Ultraviolet Imaging Spectrograph (UVIS) does a slow scan of Enceladus to do surface composition (and potentially gas) measurements.

The Composite Infrared Spectrometer then does another scan (notice that we are getting pretty close to Enceladus and it’s looking bigger!).

For the final remote sensing observation before closest approach, the camera system does a three-panel mosaic of the cratered northern hemisphere. This will be our best look yet at this terrain.

After the cameras, the spacecraft does a big turn to put the in situ instruments’ sensors into the proper orientation for the closest approach (facing into the direction of motion, so they can “scoop up” particles around Enceladus). Remember that the closest approach (about 50 kilometers, or 31 miles) occurs near 20 degrees south latitude – not in the plume. About a minute after the closest-approach, we’ll be over the south pole and in the plume (at about 641 kilometers, or 398 miles). So we’re diving in the plume a little more than we’ve done before! The in situ instruments will get great measurements of particle size and composition, and gas composition, both in the plume and close to the surface of Enceladus.

Now, it isn’t obvious from this movie, but Enceladus goes into eclipse (i.e., it passes into the shadow behind Saturn), very close to the time that Cassini is making its closest approach, and remains in eclipse for a couple of hours. This is a prime opportunity for the Composite Infrared Spectrometer to do temperature mapping of the hot, active south pole region (which is now in view, after Cassini has swung through its closest approach), with no contribution or heating from solar illumination. The other remote sensing instruments will ride along, but without light from the sun, it isn’t clear exactly what they’ll measure – so that will be a surprise! While Enceladus is in eclipse is also a nice time to do radar measurements (radar doesn’t need the sun to illuminate the surface), which we do next, followed by another Composite Infrared Spectrometer scan, post-eclipse.

We complete the flyby with an Ultraviolet Imaging Spectrograph-prime stare at Enceladus (note that by this time, Enceladus is distant and small again), followed by a turn to Earth to downlink the data!

Key to instrument Field of Views:

Optical Remote Sensing, or ORS, instruments:
UVIS (Ultraviolet Imaging Spectrograph) – long narrow magenta field of view
VIMS (Visible-Infrared Mapping Spectrometer) – big red square
ISS (Imaging Subsystem, i.e., the camera) – white square (small=Narrow Angle Camera; large=Wide Angle Camera)
CIRS (Composite Infrared Spectrometer) – red circular field of view and two small red parallel narrow fields of views

HGA – high gain antenna, used for communication with Earth

Radar – green circle (centered on the high gain antenna)

Cheers,
Amanda, still in Houston