Occultations offer huge advantages for learning about an object. When a source of light passes behind Saturn's rings, for example, observing the light's behavior means learning about many characteristics of the rings. If the light is from a star, its pinpoint can reveal minute details in the rings' structure as the star seemingly blinks off and on, grows dimmer and brighter. In a similar situation, watching the Sun light up Saturn's atmosphere from behind, or the atmosphere of its largest moon Titan, provides rich data, including spectral evidence of the atmospheres’ chemical constituents, and measurements of other properties. Cassini has observed stellar occultations and solar occultations at various wavelengths -- ultraviolet, visible, infrared, probing the planet's upper atmosphere, its moons, its rings, and even the icy plume emanating from Enceladus. Cassini itself often takes an active role in occultation experiments, by using its own radio transmitters as sources of "light." This was the case in the typical Radio Science Occultation experiment described here: /resources/15816 .
The orbits that Cassini is flying now, and the 22 Grand Finale orbits beginning next week, provide opportunities for more and more occultations. These science experiments are populating a library's worth of exquisite needlepoint probes of the Saturn system.
Wednesday, April 12 (DOY 102)
Today's occultation experiment occurred when the bright red star Gamma Crucis went behind Saturn's rings, thanks to Cassini's motion. The Visible and Infrared Mapping Spectrometer (VIMS) observed it for three hours, with the Composite Infrared Spectrometer (CIRS) riding along.
Next, the Imaging Science Subsystem (ISS) targeted the region between Saturn's C and B rings. ISS spent two hours observing, using multiple color filters. CIRS, VIMS, and the Ultraviolet Imaging Spectrograph (UVIS), the rest of Cassini's Optical Remote Sensing (ORS) instruments, participated. The results should help constrain models of how Saturn's rings evolve over time.
Following this, UVIS scanned across Saturn's northern polar region to image the aurorae there; CIRS rode along. Then, just 20 minutes before speeding through periapsis, Cassini flew within 12,000 kilometers of Saturn's diminutive moon Atlas -- it was less than one Earth-diameter away. ISS led all the ORS instruments observing Atlas for 2.6 hours. In all its years at Saturn, this approach was by far the closest to the little moon, which orbits Saturn in the space between the broad A ring and the bright, narrow F ring. The Magnetospheric and Plasma Science (MAPS) instruments were making in-situ measurements this whole time, which included the ring-plane crossing. The Radio and Plasma Wave Science (RPWS) instrument increased its sampling frequency to record the signature of dust impacts on the spacecraft, and to study plasma waves near Saturn's magnetic equator. With high-quality Atlas images stored aboard, ISS then turned back to view the receding A ring from its dark side, and led the ORS instruments for 1.3 hours of close-up observations of structures that scientists have dubbed "propellers" (http://go.nasa.gov/17oqTWF).
Studies of Saturn's aurora continued for another 3.7 hours, as VIMS stared at the planet's dark southern polar regions. The lack of sunlight there made it easier to image aurorae; CIRS rode along. Scientific scrutiny then turned back towards some more propellers, with ISS, CIRS, and UVIS observing. Finally, CIRS began a six-hour study of the unlit side of the A ring, with UVIS riding along. Ring temperatures drop abruptly when the particles pass through Saturn's shadow. In perpetual sunlight now that Saturn is near solstice, particles in the A ring are no longer passing through the shadow, so CIRS scanned along the A ring looking for more subtle variations in thermal emission.
Thursday, April 13 (DOY 103)
The next occultation experiment was jointly observed by VIMS and UVIS for 1.2 hours today, when the Sun set behind the A ring. Shortly after passing behind the Cassini Division, the Sun disappeared b