After a year of touring the Saturn system nearly in the equatorial plane, Cassini bid goodbye to its final close encounters with Saturn’s smaller satellites in 2015, including three spectacular flybys of Enceladus. In 2016, 11 Titan flybys punctuated by a handful of large main engine burns will raise Cassini’s inclination to almost 64 degrees. This will set up the 20 F-ring orbits and prepare for the spacecraft's Grand Finale mission phase beginning in April 2017.
Along the way, the Optical Remote Sensing, RADAR, Radio Science, and Ion and Neutral Mass Spectrometer (INMS) will make full use of the Titan flybys to further explore this mysterious world. The smaller satellites are not completely ignored but the encounters will be much more distant. After T125, Cassini’s emphasis will begin to shift to Saturn and its rings as the spacecraft gets some of the closest views since Saturn Orbit Insertion in 2004.
Titan flyby (2,372 miles, or 3,817 kilometers)—T-115: The Composite Infrared Spectrometer (CIRS) will perform limb mapping on both the north and south limbs of Titan on the same flyby which will provide a comparison and contrast between the spring (north) and fall (south) hemispheres where rapid changes in atmospheric circulation are occurring. This is one of the two most scientifically important Titan flybys in the Solstice mission for CIRS. Limb mapping provides a measure of how the gas and aerosol abundances vary with altitude; CIRS will also measure temperature profiles and potentially infer the locations of cloud layers.
Titan flyby (870 miles, or 1400 kilometers) —T-116: UVIS stellar occultations of Epsilon Orionis and Zeta Orionis sample Titan’s atmosphere from roughly 15 – 40 º N. UVIS solar occultations probe both northern and southern latitudes. Occultations are of especially high value because they provide detailed vertical profiles of composition and temperature in the high atmosphere, not accessible by any other instrument and also not by UVIS from reflected sunlight. The solar occultation is also observed by VIMS and provides detailed vertical profiles of composition and aerosols in the atmosphere below 200 km.
Titan flyby (633 miles, or 1018 kilometers) —T-117: An RSS occultation in Titan’s northern hemisphere (latitudes of ~7S and ~30N degrees,) will profile the thermal structure of the atmosphere. The occultation is followed by a short-duration high northern-latitude egress-only bistatic scattering with a ground track likely crossing small lakes, covering the region from about (80N, 190W) to about (70N, 240W) degrees, and capturing near-grazing scattering angle decreasing from about 80 to 75 degrees. . Data from this observation may yield information about surface reflectivity, dielectric constant, and roughness.
Enceladus will pass in front of a star while Cassini’s Ultraviolet Imaging Spectrometer will be carefully measuring how the light from the star decreases as it passes behind the moon’s plume. Scientists will measure the density, velocity, and composition of the plume from these measurements, and try to map out its shape and understand how it is changing with time.
Titan flyby (615 miles, or 990 kilometers)—T-118: This is the only flyby in the mission where UVIS and INMS will observe Titan's atmosphere simultaneously at the same latitude. UVIS will sample remotely by observing the solar occultation signal and INMS will sample the upper atmospheric density directly. Early comparisons of atmospheric density made at different latitudes and times were difficult to reconcile. The UVIS occultations are near North & South polar vortex boundaries; unusual patterns of gas abundances and temperature are seen in these regions in the south. Both hemispheres are interesting because they sample a dynamical regime that is not typical of the rest of Titan's atmosphere.
Titan flyby (603 miles, or 971 kilometers)—T-119: RSS observes a mid-northern latitude atmospheric occultation to profile the thermal structure of the atmosphere and its possible seasonal variation. RSS continues with a short (~ 1hr) high northern latitude bistatic scattering observation with a ground track possibly crossing small lakes. Data from this observation may yield information about surface reflectivity, dielectric constant, and roughness. INMS gathers data at closest approach. This is the only time in the mission that INMS will have the chance to measure Titan’s atmosphere in this geometry: when Titan’s nightside atmosphere is receiving the smallest possible infusion of external energy. MIMI, standing in for the CAPS instrument, will measure the energetic ion and electron environment to estimate the energetic particle input to Titan’s atmosphere and ionosphere, which will help interpret the ionization layers and scale heights observed by RSS.
Titan flyby (606 miles, or 975 kilometers)—T-120: Near closest approach, RADAR will conduct SAR imaging (and global shape via SAR topo) of Titan’s poorly-covered southeastern quadrant, producing a map of Titan’s cloud-covered surface. This will be the last high southern latitude SAR coverage of the mission. The ridealong SAR on INMS covers Shangri-La.
Titan flyby (607 miles, or 976 kilometers) T-121: RADAR carries out prime SAR of Tui Regio and Hotei Arcus to better characterize putative cryovolcanic features and perform change detection, comparing with the T43 and T48 flybys. RADAR will play “switch hitter”, switching from right-look to left-look near closest approach to hit both targets.
Titan flyby (994 miles, or 1,599 kilometers) T-122: The last RSS Titan Gravity flyby of the mission. The main science objectives of gravity measurements at Titan are to 1) assess the presence of a global subsurface ocean by measuring the short-period changes of the gravity field induced by Saturn’s tidal field (eccentricity tides); 2) determine the geoid of the satellite and the presence of large scale gravity anomalies; and 3) determine the rheology of the icy crust by correlative analysis with altimetric data.
Dione will pass in front of a star while Cassini’s Ultraviolet Imaging Spectrometer will carefully measure if the light from the star is dimmed by an atmosphere or plume, indicating possible activity on the icy moon.
Titan flyby (1,102 miles, or 1,774 kilometers) T-123: During this ORS flyby, VIMS will observe the Arcturus occultation that will provide information on the atmospheric composition and its evolution. After the occultation, VIMS will take a high-resolution image of an area on Xanadu. CIRS will make detailed scans of Titan's atmospheric limb near 50N revealing the vertical structure of temperature and trace gas abundances, such as hydrocarbons and nitriles. These data will be used for comparison with observations of equivalent southern latitudes, which are currently experiencing late Fall (south) instead of spring (north). They will also be compared with views of the north earlier in the mission.
Rhea will pass in front of a star while Cassini’s Ultraviolet Imaging Spectrometer will carefully measure if the light from the star is dimmed by the moon’s very thin atmosphere. This atmosphere is composed of oxygen and carbon dioxide that is formed either by surface chemical reactions or by outgassing, and Cassini scientists hope to more fully characterize it.
Titan flyby (984 miles, or 1,584 kilometers) T-124: RSS bistatic scattering observation, one of two opportunities in the entire mission ideal for capturing potential mirror-like surface echoes from Titan's high northern seas. The ground track covers the surface region close to Titan's North pole, crosses Punga Mare(the first bistatic observation of the feature) and other likely liquid-filled close by regions, and ends over the western part of Kraken Mare, a region not explored before by RSS. These measurements will offer a unique opportunity to compare physical properties of Titan’s three major northern seas, and potential differences among different regions of the vast Kraken Mare.
Titan flyby (1,692 miles, or 3,158 kilometers) T-125 VIMS— Gateway to F-Ring orbits and Cassini's Grand Finale mission phase.
The small moon Pandora, which is only about 50 miles (82 kilometers) in size and orbits right outside Saturn’s main ring system, will make its closest passage to Cassini at a distance of 8600 miles (13,800 kilometers). This heavily battered moon appears to be covered with a fine layer of dust and criss-crossed by enigmatic striations.