Todd J. Barber, Cassini lead propulsion engineer
I recently sat down with Dr. Amanda Hendrix, a science team member for the Ultra-Violet Imaging Spectrograph (UVIS) instrument on Cassini. Her specialty is icy satellites of the outer planets, so this November with twin Enceladus flybys must seem like an early holiday present for her and her team. Dr. Hendrix obtained her PhD studying ultraviolet observations of Earth’s moon and the main-belt asteroids Gaspra and Ida, all using data from the Galileo spacecraft en route to Jupiter. Cassini’s UVIS observations continue this line of inquiry with perplexing Enceladus, including assessments of surface composition and evolution, interactions with the magnetospheric plasma around Saturn, and even the icy E-ring particle grains themselves. The eighth flyby of Enceladus (“E8”) is imminent, but first I queried Dr. Hendrix about our recent encounter with E7, the closest-yet pass near Enceladus’ south pole and the deepest traverse within the plume to date.
E7 results are still being analyzed, but the short story is that the flyby was exciting, informative, and 100 percent successful. Data from E7 actually allow us to baseline the E9 flyby on reaction wheels instead of thrusters. E9 is scheduled for April, 2010, nearly a carbon copy of E7, but performing this flyby using the precision of reaction wheels will enable exquisite measurements of Enceladus’ gravity field, which in turn may reveal secrets of its interior structure (for example, possible mass concentrations, or “mascons” near the south pole). I asked Dr. Hendrix about the latest thoughts for Enceladus’ water sources and energy balance, enabling a true crash course in Enceladus geology. Essentially, she mentioned that the Cosmic Dust Analyzer (CDA) measured sodium salts in a flyby prior to E7, strongly suggesting subsurface liquid water interacting with rocks. I had heard some ground-based equipment could not detect sodium in the Enceladus plumes, but Dr. Hendrix enlightened me to the fact that the concentration was low enough that both results are consistent. It’s hard not to get excited about this small, frigid moon whose interior seems more alive the closer we examine it! As for the energy source, Dr. Hendrix told me that tidal heating (due to the 2-to-1 orbital resonance with Dione) was still the favorite theory, but radiogenic heating is needed as well for Enceladus to remain geologically active some 4.6 billion years after formation.
For E7 itself, I asked her about any initial results from CDA or the Ion & Neutral Mass Spectrometer (INMS), two of the “sniffing” instruments prime during this deep plume passage. She couldn’t say much yet, but her one comment was the signal-to-noise ratio for the data from these two instruments was so high that we may anticipate some excellent spectra, not just for the overall plume but for the embedded jets as well. For her instrument, UVIS, a neat experiment was executed as well at E7, basically studying the plume with the disk of Saturn directly behind Enceladus as the “light” source! I’ve heard of stellar occultations, but this was a cool way to probe the Enceladus’ plume in the ultraviolet. Dr. Hendrix informed me that this has been done before with Jupiter as a backdrop for Io, and that E10 in May of 2010 will actually employ a solar occultation, probing sunlight as it streams through Enceladus’ spewing ice geysers! E10 UVIS observations will be sensitive to a shorter wavelength band, thus perhaps allowing a distinction between diatomic nitrogen (N2) and carbon monoxide (CO), two molecules which have the same molecular weight (28 grams per mole) if I remember my high school chemistry.
Lest I get caught up in the excitement of E7, our E8 flyby is only a couple of days away as I write this column, so I pulsed Dr. Hendrix about this encounter to close out our meeting. Cassini will pass within about 1600 kilometers of Enceladus, again nearly over the south pole but this time at much higher altitude. The Composite and Infra-Red Spectrometer (CIRS) will be prime for this flyby, executing a nifty scan of the tiger stripe “Baghdad Sulcus” (source #1) at high resolution. Other instruments “riding along” for this flyby include the Imaging Science Subsystem (ISS), the Visual & Infrared Mapping Spectrometer (VIMS), and, of course, UVIS. ISS will image terrain in Enceldaus’ leading hemisphere at higher resolution than seen previously, while VIMS may find super-hotspots in the tiger stripes, if they exist. UVIS will contribute to surface composition maps, including water ice, grain sizes, and even the presence of ammonia (NH3). UVIS spectra of Enceladus are typically dark, suggesting small but non-zero amounts of NH3 on the surface (and consistent with VIMS data). A water-ice-only surface of Enceladus would appear bright in UVIS spectra. My favorite quote from Dr. Hendrix was about the ubiquitous nature of Enceladus, making its presence felt throughout the Saturnian system (e.g., in generating the E-ring). She said Enceladus was “contaminating the system, but in a good way.” Classic! What a wonderful meeting I had with Pasadena native Dr. Hendrix, a young woman enamored with space science and JPL since childhood, now helping to unlock the vast mysteries of Saturn’s icy moons.