Date: November 2, 2009
Title: Cassini’s Deepest Plume Passage: The “E7” Flyby of Enceladus
Podcaster: Anne Verbiscer
Organization: University of Virginia Astronomy
Description: Today the Cassini Spacecraft makes its 7th flyby of Saturn’s enigmatic moon Enceladus. The trajectory takes the spacecraft deeper into the moon’s icy plumes than ever before, skimming a mere 100 kilometers (62.5 miles) above the jet sources found at the small moon’s south pole. (Enceladus itself is only 500 kilometers (or 300 miles) in diameter.) Such a close flyby through the plumes presents the ideal opportunity for Cassini’s fields and particles instruments to sample the plume particles directly and determine their composition, mass, and relative quantities. Cassini’s remote sensing instruments, including its cameras, will take a back seat on this journey, as the spacecraft is moving much too quickly to acquire unblurred pictures, but later this month, on November 21, the roles will reverse when the closest approach distance is about 1600 kilometers (or 1000 miles).
Bio: Anne Verbiscer is a research associate professor at the University of Virginia where she studies the icy bodies of the outer Solar System using spacecraft and ground-based telescopes. Her interest in space was sparked long ago when “moonwalk” was something Apollo astronauts did rather than popular singers. Having forgone the pursuit of placing the first female steps on the Moon, she received her Ph.D. in Planetary Science from Cornell University in 1991. She has been an associate member of both the Voyager and Cassini Imaging teams and currently assists Cassini’s Composite Infrared Spectrometer (CIRS) team with their observations of Enceladus.
Today’s sponsor: This episode of “365 Days of Astronomy” is sponsored by Jim Stratigos: “To my lovely wife Janie who has followed my hobby for years and followed me in search of dark skies.”
Transcript:
Hello, this is Anne Verbiscer from the University of Virginia. I’m a planetary scientist who studies the icy bodies of the outer Solar System, and today I’d like to tell you about an exciting event taking place with the Cassini spacecraft, currently in its sixth year in orbit around Saturn.
One of the most remarkable discoveries that Cassini has made so far in its exploration of the Saturn system is that Enceladus, one of Saturn’s relatively small moons, is geologically active. Geyser-like sprays of water ice particles and water vapor emanate from several source regions near and inside of four prominent fractures located at the moon’s south pole.
These are Enceladus’ “tiger stripes,” known formally as “sulci.” Astronomers had long suspected that Enceladus was unique because its orbit coincides with the densest portion of one of Saturn’s tenuous rings, known as the E ring. Was this placement merely a cosmic coincidence? or were the ring and moon intimately involved? Dynamicists knew that such a tenuous ring should have a relatively short lifetime… unless it had a way to be replenished. We now know, thanks to Cassini and its discovery of the plumes, that Enceladus is in fact the source of the E ring.
Back in 1981, images of Enceladus returned by the Voyager spacecraft revealed that the moon’s surface was dazzlingly bright and quite young, with some regions appearing nearly crater-free and smooth. But back then, it was summer in the northern hemisphere and the south pole was hidden in winter’s darkness. The discovery of the sulci and Enceladus’ plumes would have to wait.. nearly a quarter of a century… for Cassini’s arrival.
Enceladus has long captured my interest. A significant portion of my own Ph.D. thesis was devoted to the photometric analysis of Enceladus’ icy surface as seen in the Voyager images – including those nearly crater-free and smooth terrains. For years those were the best and only images we had of this icy world. As a graduate student twenty years ago I would pore over what now amount to a mere handful of images compared to the bounty supplied by Cassini… carefully examining the manner in which each facet scattered the Sun’s feeble light from a billion miles away… wondering what physical properties made Enceladus the most reflective body in the Solar System.
Fast forward to the twenty-first century, where we are treated on nearly a daily basis to new raw images just arrived on Earth from the Cassini spacecraft, and every once in a while to a grand feast: new data from close flybys of Enceladus. Cassini’s first four years in orbit around Saturn had been planned for years prior to orbit insertion in July 2004 and included three close flybys of this moon. After the plumes were discovered in 2005, the tour chosen for Cassini’s 2-year-long extended mission (now known as the “Equinox Mission” since the Sun would cross Saturn’s ring plane in 2009) would include no fewer than seven close Enceladus flybys. Now that we knew that Enceladus was a member of a very exclusive club, that of geologically active moons, joining Jupiter’s Io and Neptune’s Triton, we wanted to learn as much as possible about the moon and investigate the very real possibility that reservoirs of liquid water lie just below its surface.
Last year, we marveled at the results from four Enceladus flybys which included the “Skeet shoot” high-resolution images of the jet source regions on and near the four sulci. This year, there are two Enceladus flybys, and they both take place this month. Today, November 2nd, Cassini makes its “E7” or seventh (overall) flyby of Enceladus with closest approach at 7:42 UTC, or coordinated universal time, when the spacecraft will be 100 kilometers (or 62.5 miles) directly above the south pole, and deeper within the plumes than it has ever flown before. Although Cassini came closer to the surface during one of its Enceladus flybys last October, the trajectory was at such an angle that closest approach took place just south of the equator. By the time the spacecraft flew through the plumes then, the spacecraft was nearly 350 kilometers (or ~220 miles) above the south pole.
Today’s low-altitude, deep plume passage allows Cassini’s “fields and particles” instruments such as its Ion and Neutral Mass Spectrometer (INMS) and its Cosmic Dust Analyzer (CDA) to sample plume particles directly and measure their composition. So far, we have learned that the plumes are mostly water vapor with smaller amounts of nitrogen and carbon dioxide, and trace amounts of ammonia, and organics such as methane, hydrogen cyanide, acetylene, and propane. During this flyby, INMS will be able to refine these measurements as well as determine the extent to which any of the jets vary in their production or activity levels when compared with previous flybys.
Just after closest approach, Cassini’s Composite Infrared Spectrometer (CIRS) will make thermal measurements near Enceladus’ south pole to look for anomalously warm areas on the surface. Following that observation, the Ultraviolet Imaging Spectrgraph (UVIS) will observe Enceladus’ plume with Saturn in the background.
For both of these observations, as well as the in situ plume samples, Cassini will turn on its thrusters in order to maintain attitude control of the spacecraft. Normally, Cassini uses its reaction wheels to keep itself properly oriented, but the plumes do exert a non-negligible torque, or force, on the spacecraft when it flies through them. Cassini will use some of its precious hydrazine fuel to keep the plume-sampling instruments and the entire spacecraft pointed correctly while flying through the icy spray.
Next April, Cassini will fly on a similar trajectory through Enceladus’ plumes, but the primary science goal on that flyby will be to obtain gravity measurements of the moon, to get a better idea of its interior structure and answer questions such as “Does Enceladus have a sub-surface ocean?” Such measurements, however, cannot be made while the spacecraft is under thruster control. The data collected during today’s flyby will be used to determine whether or not this “gravity pass” in April can be safely performed using the reaction wheels rather than thrusters to maintain attitude control. So, aside from the scientific goals of today’s flyby, performing this test for the one in April is also a high priority. If the result is that the thrusters must be “on” during the April flyby as well, then the fields and particles instruments will once again perform direct measurements of plume composition.
While today’s trajectory is perfect for the fields and particles instruments to “taste and sniff” the plumes, it’s not ideal for Cassini’s cameras. The spacecraft is moving much too quickly to track features on the surface without smearing the images. The next high resolution views of the south pole will come later this month on November 21st when the closest approach distance will be around 1600 kilometers (or 1000 miles). Now that equinox has passed, the Sun is hanging even lower in the sky above the south pole, so the shadows will be even longer this time than they were a little over a year ago when we last saw the south pole up close.
Nevertheless, I’m really looking forward to learning more about this intriguing little moon from today’s deep plume passage and later this month when Cassini’s cameras will get a closer look. You can always check out the latest images on the mission’s raw image web site at http://saturn.jpl.nasa.gov/photos/raw
And thanks for listening!
End of podcast:
365 Days of Astronomy
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