Tuesday, April 12, 2005

Titan-5 Flyby Info

View of Titan from Cassini 30 minutes into ISS' high resolution observation (sub-spacecraft point=7.16N, 26.75W). The plot was generated in GISS' Titan 24 software. The map is derived from ISS map released last month (ask for full version used here, just adds blank space north of 34N). The full version of above plot is available. The data for plot was obtained from Mission description document.

JPL has released the mission description for this week's 1025-km altitude, Titan flyby over the sub-Saturnian hemisphere. The usual goodies are include like instrument goals for the flyby, timeline of events, and the playback table to you (and I) can have a reasonable idea of when the data will be played back. Like T4, most of the data is played back at a reasonable hour, the early to mid morning period on Sunday. Some earlier encounters had data coming back in the late evening.

T5 is largely a magnetospheric flyby with "MAPS" instruments like MAG, MIMI, RPWS, and CAPS taken up much of the observing time after closest approach.

A few facts about this encounter:
  • Occurs on April 16 at 1:28 pm PDT
  • Closest Approach Distance = 1025 km (bumped up from 950 km)
  • Relative speed WRT Titan = 6.1 km/sec
  • Closest Approach Lat and lon = 74N, 272.5W
  • First near-polar pass (74N at C/A)
  • Outbound flyby (sunlit inbound)
  • Phase angle at -3hours = 57 deg.
Here are some highlights in the flyby goals:
  • ISS will once again look at the sub-Saturnian hemisphere. Unlike the last flyby where ISS imaged much of the visible hemisphere at global-scale (~1.5 km/pixel) resolution, on this encounter, ISS will get 12, higher resolution (725-250 m/pixel) snap-shots across the disk, including some along the T03 SAR swath). This is to designed to understand features seen upclose on the last encounter as well as to give VIMS optimum spatial coverage.
  • This is the most important Titan flyby for MAG and is also very important for CAPS. The flyby geometry (both in terms of its low altitude and approach geometry) is close to optimum for electromagnetic studies of Titan's interior via induction effects. Both CAPS and MAG will examine Titan's Alfven currents that couple Titan to Saturn's magnetosphere. This will allow better understanding of the origin of Alfven wings and slow mode wings. MAG will use the low altitude to search for an internal magnetic field with a dipole near the rotation axis. CAPS will be prime for 16 hours after closest approach to observe signatures of Titan's interaction with Saturn's magnetosphere.
  • INMS (Ion and Neutral Mass Spectrometer) will use its open source channel at closest approach to measure minor ion and neutral densities and to help calibrate data taken during the Titan flyby last October.
  • CIRS (Composite Infrared Spectrometer) will search for new species at high northern latitudes (~55 deg) and will map CH4, CO, and HCN, etc. distribution by examining lon-wavelength rotational lines.
  • VIMS will map parts of the northern sub-Saturnian hemisphere as well as map the sub-Saturnian hemisphere during the ride-along with ISS mentioned earlier.
  • UVIS will map Titan's atomic emissions, acetylene distribution (a chemical that has taken on new prominence), and haze properties.
  • RPWS (Radio and Plasma Wave Science instrument) will measure large-scale and distant aspects of Titan s interaction with Saturn s magnetosphere by observing during entire period around closest approach and from 10 to 25 Saturn radii.
Images start being played back at 6:56 am on Sunday morning. Playback continues (for imaging) until 11:30 am. How does this translate into being placed on the JPL Raw images page? On T4, images didn't show up until almost a day after images had been returned to earth, to the disappointment of the public and to the consternation of the imaging team who couldn't even release slightly processed images until after JPL had the images up on their raw images page. Hopefully, there won't be the same delay on this flyby but I can't make any guarantees. Depending on how JPL has the system setup, given that it is a Sunday when these images come down, it could be Monday before the images start showing up.


Blogger Brian said...

Stupid question, but anyone know whether Cassini can detect an induced magnetic field like Galileo did, providing signficant evidence of subsurface oceans? I assume it can, although maybe Saturn's weaker magnetic field might make it harder to detect.

Almost as stupid question, but would a very cold water/ammonia ocean generate a different or weaker induced field than a "regular" ocean?

I suppose these questions would be relevant to Enceladus too.

Thanks for the link to your blog! I'll poke around.

-Brian (pwokenyaw at HZ))

4/12/2005 03:08:00 PM  
Blogger Bruce Moomaw said...

Yes, Brian, those questions are STUPID. STUPID, STUPID, STUPID! How could you possibly BE so stupid?

Anyway, moving on: the short answer is that -- at least in the case of Titan -- Cassini definitely can do so. There was, in fact, a poster presentation on the subject at either a recent DPS meeting or last year's Astrobiology Conference at Ames; I have a copy of it somewhere on my CD-ROMs and will type up the text from it here pretty soon.

As for Enceladus: I don't absolutely know that it can be done, but I take for granted that if they do it for Titan they can do it for Enceladus -- especially with 4 close flybys in the primary mission alone.

4/12/2005 10:23:00 PM  
Blogger Bruce Moomaw said...

The poster in question was by F.M. Neubauer et al at the 2003 DPS meeting ( http://www.aas.org/publications/baas/v35n4/dps2003/58.htm ):

"One important objective of observational studies of Titan's interior is the characterization of an ammonia-water ocean. Whereas the strong deviations from axissymmetry of the Jovian magnetic field can be used to electromagnetically sound the interior of the Galilean satellites of Jupiter, this technique is not available at Titan. In Titan's case an inducing field is applied to Titan's interior by the strong temporal variations of the shielding ionospheric currents as Titan revolves around Saturn within 16 days. We have modelled the magnetic field in Titan's close vicinity at various important locations with altitudes below 1000 km as a function of Saturnian local time for a variety of possible models of Titan's interior. We briefly discuss the potential of the technique applied to the magnetic field data to be obtained during the close flybys of the Cassini mission to resolve ice thickness, ocean conductivity and ocean thickness. We finally note that suitable Titan encounters during an Extended Mission of Cassini are particularly interesting from this point of view."

The poster itself contains a lot of nice graphs (which I can't make head or tail out of), but the only significant additions to this in its text are that (1) Saturn's field is generally shielded by ionospheric currents except during a brief period when Titan is near its direct anti-Sun position during each of its revolutions around Saturn; and (2) that therefore "Titan encounters during solar eclpises by saturn during its exteneded mission (2008-09) have high potential."

I hadn't even thought of the fact that Saturn's magnetic field, unlike Jupiter's, is not significantly tilted relative to its rotational axis, and therefore that all its moons closer than Iapetus orbit almost exactly along the borderline between the northern and southern halves of Saturn's magnetic field -- thus requiring some other means to make the direction and strength of Saturn's magnetic field passing through those moons change direction in a way that would make a search for a moon's induced magnetic field possible. In the case of Titan, this is because Titan's orbit is right near Saturn's daylight magnetopause -- that is, the edge of the planet's magnetosphere -- and so Titan's ducking in and out through the outer fringes of Saturn's magnetosphere provides the necessary fluctuations in the magnetic field near Titan. But, for the same reason, I am no longer sure that Enceladus (well inside Saturn's inner magnetosphere, and also with an orbit precisely along Saturn's magnetic equator) is a suitable target for Cassini to look for an induced field.

4/12/2005 11:18:00 PM  
Anonymous Anonymous said...

Thanks Bruce, and I'd be happy to have you call me stupid as much as you want if you also wrote more articles in SpaceDaily.

Too bad about Enceladus, which also has the problem of virtually no atmosphere, so presumably no ionosphere to generate currents.


4/15/2005 12:36:00 PM  
Blogger Bruce Moomaw said...

Thanks for the compliment, Brian, and I don't REALLY think you're stupid. Just naive...

However, I think you're wrong about Enceladus -- if only Saturn's magnetic field was tilted relative to Enceladus' orbit, it woudn't make much difference whether it had an ionosphere or not. Europa has very little of one -- but Jupiter's magnetic field (and its changes in orientation relative to Europa) generate a very nice induced field in Europa's subsurface ocean. The only difference with Titan, I think, is that the changes in the orientation of Saturn's magnetic field (out near its magnetosphere's periphery, anyway) produce a similar effect INDIRECTLY in Titan's case -- because Titan has a thick ionosphere, any changes in Saturn's field generate changing currents in that ionosphere, which in turn generate a changing maqnetic field of their own, which in turn can generate electrical curents and thus an induced field within any Titanian subsurface ocean. (I keep saying "I think" because I'm not absolutely certain of my reasoning on this. One would almost think I was stupid.)

4/15/2005 05:01:00 PM  

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