[For trouble viewing the images/movies on this page, go here]
On September 10, 2007, the Cassini spacecraft will perform an encounter with Saturn's outermost large, icy satellite: Iapetus. The encounter will occur 2:15 pm UTC and will be at a distance of 1,644 km (1,022 mi) above the surface of Iapetus. The slow speed encounter (~2.9 km/sec, 5368 mi/hr with respect to Iapetus) allows for a much larger data return than encounters of similar distance with Saturn's other icy satellites. The slow speed provides a much longer period during which high-resolution observations can be acquired, allowing more instrument modes to be used than possible when the encounter schedule is tighter, and allowing for more intricate mosaic designs. The detailed schedule of events for the encounter is described below... The encounter on September 10 is Cassini's first and only close encounter with Iapetus during its four-year primary mission (and no such encounter is being considered for the possible two-year extended mission). On New Year's Day 2005, Cassini approached Iapetus at a distance of 123,000 km (76,000 mi), providing images (with pixel scales approaching 700 meters/pixel) and other data over Cassini Regio, a hemispheric-scale area of dark terrain on the satellite's leading hemisphere. The yin-yang-like distribution of bright and dark material on Iapetus' surface was noticed as far back as the seventeenth century. Shortly after he discovered Iapetus, Giovanni Cassini (for whom the Cassini spacecraft is named) noticed that Iapetus was brighter (and thus easier to see) on one side of its orbit around Saturn than on the other. The two encounters with Iapetus by the Cassini spacecraft are designed to examine the origin of Iapetus' dark material. Results from the encounter in 2005 suggest that the dark material was recently deposited from an outside source (such as one of the numerous small, outer satellites of Saturn). Additionally, the encounter revealed a few surprises. A large ridge lies along the equator within the Cassini Regio, and in places reaches 20 km high. The images also revealed a very ancient surface with at least 5 impact basins larger than 350 km across. Flyby encounter observations of Iapetus begin on September 9 shortly before 5:00 am UTC, 33 hours before closest approach. Iapetus will almost fill the narrow angle field of view with a resolution of 1.7 km/pixel (1.05 mile/pixel). Immediately following this ISS observation, the RADAR instrument will perform the first of three observations inbound to Iapetus. This observation will use the scatterometry mode on the RADAR instrument, which measures the roughness of a planetary surface. CIRS is up next, 28 hours before closest approach, and will perform a mid-infrared scan of Iapetus' visible crescent in order to measure surface temperatures. The final observation before a 9-hour downlink to Earth is a two-frame ISS mosaic of the visible crescent, at a resolution of 1.3 km/pixel (0.8 mi/pixel).
Following the downlink, ISS will observe the sunlit crescent of Iapetus with a two-frame mosaic at 0.8 km/pixel (0.5 mi/pixel). An additional frame will be acquired over the south polar region on the night-side in an attempt to observe this region illuminated by Saturn. This area was poorly observed during the New Year's 2005 encounter due to the far-north trajectory of that encounter. This frame will include the southern transition zone between the bright and dark terrain as well as a large impact crater that lies right along the boundary. Following the ISS observation, the CIRS instrument will perform a polarization measurement with its FP3 filter over the southwestern portion of a 550-km (340-mi) wide impact basin in northeastern Cassini Regio on Iapetus' night-side. This observation requires the spacecraft to rotate 180 degrees while keeping CIRS pointed at Iapetus. Combined with scatterometry data from the RADAR instrument, this type of observation can also be used to determine surface roughness. Following CIRS' polarization observation, the RADAR instrument will perform its second scatterometry scan across Iapetus' night side. The RADAR instrument will also stare at Iapetus with its radiometry mode, which is used to determine the dielectric constant, which can place constraints on the composition of the surface. The RADAR team is particularly interested in the depth of the dark material on Iapetus leading hemisphere as well as determining the amount of ammonia that might be present in the upper few decimeters of the surface. At 11.5 hours before closest approach, VIMS has its first prime observation of Iapetus, observing the moon's night-side and crescent. ISS' next prime observation occurs 10 hours before closest approach, at 4:20 am UTC on September 10. This observation has four parts. First, ISS will acquire a 4-frame mosaic of the entire sunlit crescent. Next, ISS will acquire two, 3-frame mosaics over the north and south polar regions, both in Saturn-shine (night-side areas illuminated only by reflected light from Saturn). These two mosaics are designed to image the northern and southern bright-dark transition zones. Then, a single frame will be acquired over the sunlit portion of the equatorial ridge at around 135° West Longitude. This image will later be combined with images acquired seven hours later to create a digital terrain model (DTM) of the ridge in the area. A lower resolution DTM created from images taken during the New Year's 2005 encounter suggests that this area is the highest point along the ridge, reaching 13 km above the reference spheroid. Finally, a single frame will be acquired of the large impact basin in northeastern Cassini Regio, to correspond with the earlier CIRS polarization observation, in Saturn-shine. Of particular interest in this planned image is a crater located at 6° North, 36° West. Earlier observations revealed a landslide deposit covers most of the floor of this crater (see PIA06171). This observation is followed by a 3-hour downlink with Earth. In addition to downlinking data, the Doppler shift of Cassini's signal will be measured as the spacecraft is accelerated in response to Iapetus' gravitational pull. This information can be used to better understand Iapetus' internal structure, including whether it is a homogenous body or is layered, like Earth. Following this second downlink and first radio science observation, CIRS will perform a 2-hour-long scan, starting at 8:50 am UTC, of the entire visible surface using the FP1 filter. This observation is designed to measure small-scale variations in surface temperature caused by differences in local slopes and thermal inertia (which is predominately a measure of how "fluffy" a planetary surface is). During this observation, ISS will acquire a ride-along image over the low southern latitudes within the sunlit crescent. Three hours before closest approach, the RADAR instrument will once again be pointed at Iapetus. This time, RADAR will perform Synthetic Aperature Radar (SAR) observations of Iapetus' leading hemisphere. Normally, this mode is only used for imaging Titan's surface, since the radio waves Cassini's antenna transmits and receives to build up an SAR image can easily pierce the thick haze layers of that satellite's atmosphere. However, using SAR requires precious fuel to build up an image and takes up time that could be used for higher resolution optical remote sensing instruments like ISS. Therefore, to this point, SAR has not been used for any target other than Titan. The slow speed of this encounter allows more time for the RADAR instrument, which allows that instrument team to experiment with some of their other modes. The antenna will sweep north and south in a zig-zag pattern, producing several long strips with a top resolution of 4 km. In addition to the SAR swaths, RADAR will also use its altimetry mode to measure the height of the equatorial ridge near 74° West Longitude (digital terrain models from the 2005 encounter suggest that this might be a location where the ridge actually becomes a low trough).
ISS returns at 12:30pm UTC (1.75 hours before closest approach) with a single frame (with both narrow-angle and wide-angle images) over the equatorial ridge at a scale of just 85 meters (280 feet) per pixel. The wide-angle image from this sequence will be combined later with an observation at 4:20am UTC to provide stereo imaging over this portion of the equatorial ridge and provide a look at topography along the illuminated limb. Next up is UVIS, which will perform a scan of the northern half of the sunlit crescent, mapping its brightness in the ultraviolet. This observation will be combined with UVIS observations from after closest approach to estimate the fine-scale structure of Iapetus' surface. ISS will be riding along with this observation, imaging the sunlit, northern bright-dark transition zone. UVIS will then perform a stellar occultation, observing the star Sigma Sagittarii (also known as Nunki) as it passes behind Iapetus. This observation is designed to search for an atmosphere, though none is expected. As the star nears occultation, ISS will acquire a single wide-angle frame over the southern part of the sunlit crescent, with the potential for stereo when combined with a prior observation. At 1:20 pm UTC, less than one hour before closest approach, VIMS and ISS begin their highest resolution observations of Iapetus. Many of these observations focus on the equatorial ridge and its western extension known as the "Voyager Mountains" and the western portion of Cassini Regio. At closest approach, ISS will acquire an 8-frame mosaic along the equatorial ridge at 10 to 12 meters (32 to 39 feet) per pixel. With the spacecraft track across the surface moving quickly near closest approach, smear could be a concern with this highest resolution images. Wide-angle images (WACs) acquired at the same time as the narrow-angle images (NACs) should be fairly crisp.
Following closest approach, ISS and the other optical remote sensing instruments will continue to acquire high-resolution observations. At 15 minutes following closest approach, CIRS will acquire a high-resolution scan of the dark terrain near 12° South, 168° West. This will be followed by an ISS and VIMS observation of the same location. ISS NAC resolution during this observation approaches 20 meters (66 feet) per pixel. Next, the Cassini spacecraft rotates 90 degrees while ISS, VIMS, and CIRS perform a scan across a bright-dark transition zone in southwestern Cassini Regio. Forty minutes following closest approach, ISS will acquire a 2-frame mosaic over the bright terrain near 43° South, 209° West at 33.5 meters (110 feet) per pixel. This is followed by a six-frame mosaic over the Voyager Mountains along the equator. As the nickname for these features suggest, these bright features within western Cassini Regio were first seen by the Voyager 2 spacecraft, poking up above the limb. These observations will be combined with those taken earlier near closest approach to better understand the detailed geomorphology of these structures that may reach as high as 20 km above the surrounding cratered plains. The VIMS instrument will then take spectra of representative bright terrain. ISS will be riding along with this observation, and will acquire images in multiple filters.
One hour and forty minutes after closest approach, ISS and VIMS will begin a large, 21-frame mosaic over the equatorial transition zone between bright and dark terrain. ISS narrow-angle images in this mosaic will have a resolution of 82 to 131 meters (269 to 429 feet) per pixel. This mosaic will also allow VIMS to acquire high-resolution spectra over both bright and dark terrain. While bright terrain appears consist predominately of water ice frost, dark terrain is characterized by dark organic material (like tholins), cyanides, and CO2. The final observations of the high-resolution observation sequence are of the terminator, the boundary between day and night) along 295° West Longitude and of the large "Moat" craters near 10° North, 315° West in Saturn-shine. At 5:15 pm UTC, four hours after closest approach, CIRS begins two hours of temperature mapping across Iapetus' visible surface. This includes two north-south scans using CIRS' FP3 filter across the bright-dark transition zone and over the southern bright terrain and a full-disk temperature map using the instrument's FP1 filter. Like the temperature maps produced of the night-side before closest approach, these measurements are designed to examine the thermal inertia of different regions across Iapetus. ISS will acquire ride-along images during this period, examining the southern bright terrain including the eastern rim of a large impact basin centered near 45° South, 255° West.
VIMS is prime next, starting at 7:15 pm UTC on September 10. The spectrometer will acquire two image "cubes" (mapping spectra acquired over many wavelengths): the first over the center of the large impact basin in the southern portion of Roncevaux Terra (the official name of Iapetus' bright terrain) and the second near the bright-dark transition zone at 10° South, 230° West. ISS will be riding along during this observation, acquiring multi-color, narrow-angle observations of these two areas. Following this VIMS observation, Cassini's high-gain antenna will be pointed to Earth for another gravity pass and short downlink of three hours. Combined with the observation prior to closest approach, the Radio Science team hopes to pin down the internal structure of Iapetus, which is expected to an undifferentiated, homogenous world like its twin, Rhea. At 10:40 pm UTC (8.5 hours after closest approach), ISS will begin its full-disk, global mosaic of Iapetus. The first frame over Hamon crater will use multiple, full-resolution color filters. Next, ISS will observe the visible part of the night-side of Iapetus in Saturn-shine. Finally, ISS will image a north-south swath of bright terrain in multiple color filters, looking for possible latitudinal differences in color.
CIRS will next perform another polarization observation using its FP1 filter by rotating the spacecraft as it observes a single spot on the surface, this time centered on the Voyager Mountains. Twelve hours after closest approach, at 2:20 am UTC on September 11, ISS will once again be prime. ISS will once again attempt to observe the "Moat" craters in Saturn-shine as well as the equatorial transition zone using ISS' full compliment of color filters in a 2-frame mosaic. Finally, the RADAR instrument will observe Iapetus' day-side in scatterometry and radiometry modes in its first of two passes. With all this important data acquired, Cassini will then point its high-gain antenna at Earth for data playback. To ensure no data is lost, two playback passes are planned.
Following the long playback period, Cassini will point its instruments back at Iapetus for its final sequence of observations during this encounter. VIMS will observe Iapetus starting at 8:45 pm UTC on September 11, creating an image cube of the entire visible surface. ISS will be riding along, providing global-scale imaging of Iapetus' trailing hemisphere in multiple color filters at 1.6 km (1 mi) per pixel. CIRS will then perform a scan of Iapetus' trailing hemisphere at mid-infrared wavelengths and then at longer infrared wavelengths. Following the RADAR instrument's final scatterometry and radiometry scan of Iapetus and an ISS observation of Saturn, ISS will again observe Iapetus at 2.25 km (1.4 mi) per pixel using multiple color filters.
While only one targeted flyby of Iapetus is planned for the entire Cassini mission, this flyby will provide an immense dataset that will hopefully answer many of the remaining questions about Iapetus. Is the equatorial ridge a global feature and how did it form? What is the origin of the dark terrain and what is it made of? To answer these questions, Cassini will unleash its full arsenal of instruments over a 3-day period to gain a better understanding of this bizarre, ancient world.
Image products created in Celestia. Iapetus map by Steve Albers.
Images created with Celestia. Iapetus map by Steve Albers.