Cassini to Swing Low Into Titan’s Atmosphere

César Bertucci
César Bertucci

This weekend, Cassini will embark on an exciting mission: trying to establish if Titan, Saturn’s largest moon, possesses a magnetic field of its own. This is important for understanding the moon’s interior and geochemical evolution.

For Titan scientists, this is one of the most anticipated flybys of the whole mission. We want to get as close to the surface with our magnetometer as possible for a one-of-a-kind scan of the moon. Magnetometer team scientists (including me) have a reputation for pushing the lower limits. In a world of infinite possibilities, we would have liked many flybys at 800 kilometers. But we went back and forth a lot with the engineers, who have to ensure the safety of the spacecraft and fuel reserves. We agreed on one flyby at 880 kilometers (547 miles) and both sides were happy.

Artist's concept of Cassini's Titan flyby
Cassini flies to within 880 kilometers (547 miles) of Titan’s surface during its 71st flyby of Titan, known as “T70,” the lowest in the entire mission. Image credit: NASA/JPL/Space Science Institute
› Full image and caption

Flying at this low altitude will mark the first time Cassini will be below the moon’s ionosphere, a shell of electrons and other charged particles that make up the upper part of the atmosphere. As a result, the spacecraft will find itself in a region almost entirely shielded from Saturn’s magnetic field and will be able to detect any magnetic signature originating from within Titan.

Titan orbits within the confines of the magnetic bubble around Saturn and is permanently exposed to the planet’s magnetic disturbances. Previous measurements by NASA’s Voyager spacecraft and Cassini at altitudes above 950 kilometers (590 miles) have shown that Titan does not possess an appreciable magnetic field capable of counterbalancing Saturn’s. However, this does not imply that Titan’s field is zero. We’d like to know what the internal field might be, no matter how small.

The internal structure of Titan can be probed remotely from its gravitational field or its magnetic properties. Planets with a magnetic field — like Titan’s parent Saturn or our Earth — are believed to generate their global-scale magnetic fields from a mechanism called a dynamo. Dynamo magnetic fields are generated from currents in a molten core where charge-conducting materials such as metals are flowing around each other and also undergoing other stresses because of the planet’s rotation.

We might not find a magnetic field at all. A positive detection of an internal magnetic field from Titan could imply one of the following:

a) Titan’s interior still bears enough energy to sustain a dynamo.
b) Titan’s interior is “cold” (and therefore has no dynamo), but its crust is magnetized in a similar way as Mars’ crust. If this is the case, we should find out how this magnetization took place.
c) Something under the surface of Titan got charged temporarily by Saturn’s magnetic field before this Cassini flyby. While I said earlier that the ionosphere shields the Titan atmosphere from Saturn’s magnetic bubble, the ionosphere is only an active shield when the moon is exposed to sunlight. During part of its orbit around the planet, Titan is in the dark and magnetic field lines from Saturn can reach the Titan surface. A temporary magnetic field can be created if there is a conducting layer, like an ocean, on or below the moon’s crust.

Once Cassini leaves Titan, the spacecraft will perform a series of rolls to fine-calibrate its magnetometer in order to assess T70 measurements with the highest precision. We’re looking forward to poring through the data coming down, especially after all the negotiations we had to make for them!

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    10 Responses to “Cassini to Swing Low Into Titan’s Atmosphere”

  1. ralph isaacs Says:
    June 18th, 2010 at 8:43 am

    Why is the the ionosphere active as a shield only when exposed to sunlight? Is it that the ionsphere recombines (”neutralizes”) without the sunlight replenishing it?

    Cesar Bertucci responds:

    Exactly, in the absence of sunlight, the charged particles in the ionosphere recombine, reducing its ability to carry currents that would prevent Saturn’s field from penetrating the atmosphere. However, the nightside ionosphere is not completely neutral. There are other mechanisms of ionization that are not related to the sun (e.g., impact of electrons from Saturn’s magnetosphere) and it is believed there is transport of charged particles from the dayside into the nightside. However, this amount is not enough to provide efficient shielding.

  2. snoopyloopy Says:
    June 19th, 2010 at 10:54 am

    good hunting, any new findings, data gathered on this flyby continues a legacy of discovery ongoing over a three hundred year span,of Saturn system, kudos to the navigation team…..

  3. Dan Gibbons Says:
    June 20th, 2010 at 6:40 pm

    Good luck with the MF scan.

    I am most excited about this one. Only Hueguins was bigger then this job, hope the bird makes a clean pass for you.

    I also hope you get to negotiate one more time….When Cassini mission is almost finished. Lets put her hard through the soup as many times as she ‘ll take it.

    Saturn offers little promise for terrestrials, but this little rock (titan) is special….!

    Dan Gibbons
    St. George Utah

  4. Kimberly Says:
    June 20th, 2010 at 10:22 pm

    This is exciting! As I began reading, I wondered if it might be possible for Titan to become magnetized due to the influence of Saturn’s magnetic field on a component found on/in/under the moon. You answered that for me. Thank you so much!

  5. Michael Suede Says:
    June 22nd, 2010 at 4:46 pm

    Dynamo theory pushes the bounds of reason.

    Thornhill comments on an alternative:

    “It seems obvious to propose that a stellar or a planetary magnetic field is a combination of the field due to a rotating charged body and the field due to moving electric currents impinging on that body. The interplay between the two, together with the effects of uneven and moving distribution of charge within the rotating body, gives rise to the complex and changing fields that we observe. This obvious suggestion never gained acceptance because to provide the Earth’s magnetic field, for example, a current of one billion amperes is required. That would imply a tremendously strong electric field at the Earth’s surface, which does not exist. But the fallacy in that argument lies in the use of an idealized electrostatic model with the Earth moving in a perfect vacuum and a zero potential at infinity. The Earth moves in plasma. The clear air electric field of the Earth shows that the Earth is charged. The Earth’s electric field is confined within the Earth’s plasma sheath (double layer) at the magnetospheric boundary. The potential difference between the solar plasma and the Earth is largely confined to the plasma sheath. And certainly the solar plasma is not at zero potential (however that is measured). “

  6. Craig Carmichael Says:
    June 22nd, 2010 at 6:52 pm

    I gather there’s nothing like Ganymede’s field on Titan. Could surface liquid (CH4) create an induced field like Callisto’s, which seems to result from an H2O ‘magma’ layer deep in?

    Would Saturn’s deadly ionizing radiation penetrate to the surface if its field isn’t stopped by the ionosphere and there’s no local field, or are the ions absorbed in the upper atmosphere? (and could H+’s supply hydrogen to Titan to make up for what escapes to space?)

    The nature of the magnetic field (or lack of one) would seem to have important implications for Titan life.

    Cesar Bertucci responds:

    Could surface liquid (CH4) create an induced field like Callisto’s, which seems to result from an H2O ‘magma’ layer deep in?
    It could be possible for Titan to generate an induced field like Callisto or Europa if it possesses a conducting element within its interior (an ocean, for example). Titan presents two main complications when compared to the two Galilean satellites: 1) Saturn’s field at Titan’s location does not vary as regularly as Jupiter’s, which is why that hypothetical induced field (and, in particular, its temporal variability) is hard to predict. 2) Compared with Europa and Callisto, Titan possesses a substantial dayside ionosphere which produced induced fields, too. As a result, the detection of an induced field should be carried out when Saturn’s magnetic field impinges on Titan’s nightside.

    For T70, we are working to measure whether there is a permanent intrinsic field in addition to any externally induced field.

    Would Saturn’s deadly ionizing radiation penetrate to the surface if its field isn’t stopped by the ionosphere and there’s no local field, or are the ions absorbed in the upper atmosphere? (and could H+’s supply hydrogen to Titan to make up for what escapes to space?)
    Titan orbits at the distance of about 20 Saturn radii, whereas Saturn’s radiation belts extend up to a few planetary radii. As a result, the moon is in a less energetic environment. However, during the reconfiguration of Saturn’s magnetosphere, high-energy ions and electrons impact on the upper atmosphere of Titan, accelerating local neutral particles, as observed by Cassini’s MIMI (magnetospheric imaging instrument). This is one of the mechanisms by which the Titan atmosphere is eroded in the absence of a significant internal magnetic field.

    The nature of the magnetic field (or lack of one) would seem to have important implications for Titan life.
    Yes, indeed, the presence of a dynamo type of magnetic field would imply that sources of energy might be residing below Titan’s surface. Given the strong limitations for Earth-type life on Titan’s surface due to the low temperatures, scientists wonder if Titan’s interior would provide enough energy to sustain any kind of life underground. Cassini’s magnetometer and radio science experiments are trying to provide relevant evidence to address this question.

  7. Torbjörn Larsson, OM Says:
    June 24th, 2010 at 3:36 pm

    Thornhill comments

    That is presumably “The Electric Universe” Wallace Thornhill, a supporter of Velikovsky and a doubter of relativity (I gather). Pity such anti-science should pollute NASA blogs.

    Anyway, it doesn’t matter whether a sheath is placed between a planet and the universe - the immense room for Debye lengths screens out charge. As for the EM effect, I recently heard a seminar where an astrophysicist modeled the resulting induction charge. The result? ~ 40 C (distributed over a sphere of ~ Earth dimensions of course)! ROTFL on the suggestion that “the use of an idealized electrostatic model” is a “fallacy”; it is the best approximation I heard in a long time.

  8. Torbjörn Larsson, OM Says:
    June 24th, 2010 at 3:37 pm

    Oh, I forgot: yay for Cassini!

  9. Craig Carmichael Says:
    June 25th, 2010 at 9:39 am

    Why did I bother to comment? 3 days later the conversation has moved along, but mine is still “awaiting moderation”. Seems to me by the time it’s shown, it’ll be way up in the middle somewhere and no one will see it, and I won’t be taking the trouble to return to this blog again to check for feedback.

    JPL responds:
    Thank you for your feedback, and we apologize for the delay in posting your comment. We’ve passed your earlier question along to a scientist for a response, but due to the busy schedules of many of the scientists, we often cannot provide an official response right away. You do, however, make an excellent point about opening these questions/comments up for public discussion and commentary. So we’ve decided to post your questions and others pending in the spirit of inviting responses from our blog community. (We will continue to provide official responses from our scientists and experts when they become available.) Thanks again for bringing this to our attention, and we hope that you’ll return to read and comment on future blogs about the latest science findings from JPL.

  10. terry james fraser Says:
    October 29th, 2010 at 12:45 pm

    nasa gets 40% of the aniti matter engine that runs on water where the hydrogen is seperated from the plant (organic) “andonitanoatherdrome” angelene jolene gets,10%-yes i am pattening the quantuim computer- works like this, todays silicon chips (sand)will process a max of 10 billion bits, same age as the earth, in which it takes .01 seconds for light to pass (electrons)but a white dwarf was a super jiant the size of andromeda galaxy which takes light 400,000 light years to pass, so you use those meterites for the second drive after the standerd usbs are feeding the white dwarf (black widow R)processors, terry fraser #18 falmere wy ne. calgary alberta canada.

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