Between Rhea and Iapetus is the large (5,150 km [3,200 mi]) satellite Titan, which is a "maverick" among the icy group, in that it is quite different in its appearance owing to the presence of an atmosphere with a distinctive color described as a brownish-orange. In a sense it is as odd amongst its peers as Io is within the Galilean satellites.
Some of the variations in the atmosphere can be discerned as Titan rotates. Here are four views obtained through the Hubble Space Telescope, using selected bands in the infrared that penetrate the atmosphere.:
A diagramatic summary of what is known about Titan's atmosphere, prior to the Cassini mission, appears here:
Estimates of the relative amounts of atmospheric constituents, in percentages and parts per million, are given in this table; nitrogen is the principal constituent:
Titan has been studied from Earth. Photos taken through ground telescopes over several years show variations in lighter-toned areas which are assumed to be some type(s) of clouds that come and go; these are mainly just above the surface:
The European Southern Observatory (ESO) Very Large Telescope (VLT) in Chile has obtained sharp images using an instrument called the Simultaneous Differential Imager (SDI) camera, which obtains Near-IR images at 1.575 and 1.600 µm (surface sources) and 1.625 µm (atmosphere). SDI data taken on different nights can be reprocessed by differencing to give this group of images representing changes which may be due to the surface; the actual surface itself is likely to be partially obscured by the organic haze in the atmosphere:
The color version can be enhanced to bring out as many details as possible - rendered in reds and yellows - rather than in the natural color that is a medium brownish-yellow. Titan appears unlike the others because its dense atmosphere (with surface pressures of 1.5 atm, about 50% higher than Earth's) hides its surface. Nitrogen is the main constituent of the atmosphere but the 2% of methane and associated organics cause the colors noted. The continuous clouds are somewhat analogous to the haze or smog found above urban areas such as Los Angeles. We show some of this atmospheric structure here:
What is below the clouds has been revealed somewhat by the Cassini mission, described on this page. At Titan's atmospheric temperatures (typical value: -179° C), the ethane (and probably methane and nitrogen) may have condensed into a liquid "ocean" that is up to a kilometer deep. Below this, Titan probably consists of a layered mix of silicates and water ice. An infrared image, acquired by the Hubble Space Telescope, suggests that there may be several upwellings of (solid?) material, similar to "continents" (see yellow area in the image above). Recent studies now point to some of the variations in the atmosphere imagery are related to topographic irregularities on Titan's surface. In fact, prominences as high as 2000 meters have been postulated to account for some of the patterns in the image. A map has been prepared that purports to indicate the height variations in that surface:
This atmosphere consists of about 90% molecular nitrogen, 6% argon, with the remainder being methane and organic derivatives, such as hydrogen cyanide, ethane, acetylene, and CO2. Solar radiation breaks some of these constituents into colored compounds, analogous to urban smogs on Earth. Ammonia, now a trace constituent, may once have been more abundant, until dissociating into carbon-nitrogen compounds.
This mysterious satellite (larger than Mercury and Pluto) - totally disparate, compared to the other saturnian moons - may harbor still other organic molecules of unusual nature. This surmise just cries out for follow-up study, since no other planet besides Earth or planetary satellite in the Solar System seems so promising for finding and identifying the more complex molecules that may be present.
Just as the Galileo spacecraft has added significant knowledge of Jupiter, a sophisticated mission to Saturn is underway. That probe is part of the Cassini-Huygens mission, named for the 17th Century French-Italian astronomer who pioneered telescopic observations of Saturn. The Cassini spacecraft successfully launched from Cape Canaveral at 4 AM, on October 15, 1997, to begin a 6.7 year journey to the Ringed Planet. This vehicle has taken a round-about trip to gain velocity by gravity assisted "kicks". By passing Venus twice, then Earth again, and Jupiter later, the spacecraft used their gravitational pulls to increase its speed enough to reach Saturn. JPL manages the mission, with experiments from NASA, the European Space Agency, the Italian Space Agency, and other participants. A good overview of the Cassini mission is found on JPL's Cassini website. The Cassini Orbiter's task is to explore Saturn's magnetosphere, its rings, its icy satellites, its atmosphere, the atmosphere of the moon Titan and, with luck, its surface as well.
Built at a cost of $3.27 billion dollars, the complete Cassini spacecraft, the size of a school bus (5.7 m [22 ft]; 5.83 tons), is shown here in its assembly room before being taken to its launching rocket:
The Orbiter contains a variety of instruments, as listed below. Power for the spacecraft is a RTG (Radioisotope Thermoelectric Generator). The general layout of its major components is portrayed in this schematic:
* Imaging Science Subsystem (ISS) - observes particle properties, vertical distributions (~6 km/px, 0.6 Mbit/frame with 2x2 summing). ISS will also examine wind/cloud motions; (3-12 km/px, 3 images/timestep in CB1 filter to increase SNR), and search for and monitor lightning/aurora (High-resolution imaging, 50-200 m/px, special targets, emission angles < 45� prefer IR-polarizer (phase 45-110)).
* Cassini Plasma Spectrometer (CAPS) - investigates large-scale and distant aspects of the Titan interaction with Saturn's Magnetosphere by observing during the entire period around an encounter from 10 to 25 RS.
* Composite Infrared Spectrometer (CIRS) - obtains information on trace constituents in Titan's stratosphere. Integrate on limb at two positions. Obtain information on CO, HCN, CH4. Integrate on disk at air mass 1.5-2.0. Pointing: -Y to Titan, X away from Sun.
* Ultraviolet Imaging Spectrometer (UVIS) - observes the star Beta Ori as it becomes occulted by Titan's atmosphere.
* Visible and Infrared Mapping Spectrometer (VIMS) - provides new high resolution images that will help understand Titan's geology and the fate of CH4. VIMS will also search for and study the evolution of mid-latitude clouds, and search for lightning and hot spots.
* Magnetometer (MAG) - examines large-scale and distant aspects of the Titan interaction by observing during the entire period around an encounter from 10 to 25 RS. T13 is an equatorial wake flyby under plasma conditions near Saturnian local midnight with 1852 km altitude at closest approach. Thus it is very similar to T13 even according to local time.
* Magnetospheric Imaging Instrument (MIMI) - investigates micro-scale and near aspects of the Titan interaction by observing during about one hour period around an encounter. With -Y pointed toward Titan, when within 30 minutes of the targeted flyby, optimise secondary axis for co-rotation flow as close to the S/C -X, � Z plane as works with the other constraints on Pointing. Also, measure Titan exosphere/magnetosphere interaction by imaging in ENA with INCA (when Sun is not in INCA FOV).
* Ion and Neutral Mass Spectrometer (INMS) - procures data regarding Titan's atmospheric and ionospheric composition and thermal structure. INMS will also observe the magnetospheric/ionospheric interaction.
* Radio and Plasma Wave Spectrometer (RPWS) - performs observations in the immediate vicinity of Titan, including thermal plasma density and temperature measurements with the Langmuir probe, search for lightning and other radio emissions, characterization of plasma wave spectrum, search for evidence of pickup ions. Langmuir probe within 90� of spacecraft ram at closest approach, co-rotational ram outside of � 15 minutes.
* RADAR - conducts low and high resolution SAR (Synthetic Aperture RADAR) imaging of Titan's surface. Additionally, RADAR will collect Altimetry, Radiometry, and Scatterometry Data. SAR swath cuts right across Xanadu, as well as some areas where there exists good ISS/VIMS hires coverage, enabling useful comparative studies.
The nearly 7 years needed for Cassini-Huygens to travel the 3.5 billion kilometers (about 2.1 billion miles) involved a series of 4 gravity "kicks" (velocity boosts) by flying the spacecraft relativelly close to planets, as shown in this diagram (from the Cassini-Huygens Home Page):
After arrival on late June 30, 2004 Cassini underwent a series of fuel burns to progressively insert it into orbits of different radii and ellipticity around Saturn. This diagram gives the shifts during the first 7 months:
Cassini, which cost in excess of 3 billion dollars (and raised protests from some environmentalists fearful of its 77 grams of plutonium that will serve as a power source), is the most sophisticated and ambitious probe yet sent to explore the planets.
Cassini began its process of insertion into orbit around Jupiter at 6:30 PDT on June 30, 2004 by traveling through a gap in its rings. One of its antenna was deployed to act as a shield against microparticles of ice in the ring complex. A burn sequence of nearly 90 minutes was required to achieve a proper orbit for the first phase of exploration. Total success during this phase was achieved. A series of hundreds of spectacular images of the rings was obtained in this timeframe. Since they show imagery similar (but at better resolution) to the Voyagers, we will present only a few of these here:
In this next image of Ring A, the closeup detail shows the individual ice block nature of the larger particles making up the ring itself.
Cassini has determined that there is a gradual increase in size of the ice particles going from the inner main ring to the outer main ring:
This variation in particle size and in possible non-ice "dirt" are factors in these striking color renditions of the A and the B-C ring characteristics, as determined from UV images interpreted by a group at the University of Colorado-Boulder:
Another discovery, using the Visual and IR Imaging Spectrometer, is that the Cassini Gap, and by inference the Encke Gap, is not a region of minimal particles but instead contains "dirt", particles of non-ice composition. These may be remnants of the rocky portion of a satellitic fragment composed of low reflectance basaltic material.
The F ring is narrow and seemingly isolated, as seen in the first image below. The F ring has always merited special attention because of its twisted appearance. Cassini shows that in this mosaic image. The contortions are caused by gravitational interactions with the small satellites Prometheus (145 x 85 x 62 km) and Pandora (90 x 53 x 39 km), spoken of as "shepherd" moons because they help to guide and maintain the ring material:
Here are Cassini views of the shepherd moons Prometheus and Pandora:
Athough some data relating to the mean temperatures in the individual rings had been obtained prior to Cassini's visit, those data were refined to give more exact values. A temperature profile was obtained during passage and used to assign colors to the rings by extrapolating the strip profile to the entire rings. In the image below, blue represents a temperature range clustered around 70° C (-333 ° F), green 90° C (-298° F), and red 110° C) (- 261 ° F):
This next Cassini image shows a novelty - yet not unexpected. Under certain solar lighting conditions, sunlight is variably absorbed by Saturn's rings. The result is a series of thin light to dark bands on the saturnian surface, representing shadows. B and C ring shadows appear in this image; the black dot at the bottom is the shadow image of the moon Mimas.
Cassini has also produced near-true color images of Saturn's surface. This next image shows shades of blue in the northern hemisphere. This color is due to scattering by gases, much like that observed in the Earth's sky during a clear day. The variable shading is caused by ring shadows.
Temperature and wind speed variations in the atmosphere above the saturnian clouds are shown in this next diagram. Highest temperatures are towards the upper atmosphere; strongest winds at this time in the saturnian year are near the equator.
As Cassini approached Saturn, its Ion and Neutron Camera was able to take data that lead to a determination of the Magnetosphere and Magnetopause. In the image below, excitation of hydrogen ions gives rise to the orange glow that indicates the extent of the magnetosheath.
During the approach to Saturn Cassini's magnetometer encountered several magnetic spikes that are equivalent to magnetopause bow shocks within Saturn's magnetosphere.
The magnetometer detected a very small secondary magnetic field around Titan, induced by interaction of its interior with Saturn's field.
From a far distance as it approached during its first major swing around Saturn, Cassini's Magnetosphere Imager has measured the distribution of trapped radiation in the Main Magnetic Belt around the planet, as shown below
This belt begins (from the surface) out at 70000 km (48000 miles) and reaches to about 783000 km (489 miles). Cassini also detected a previously unknown inner belt near the surface that is about 6000 km (4000 miles) thick.
One of the discoveries, made by the UV spectrometer, is that there is a small but detectable concentration of oxygen over much of Saturn's environment beyond the rings. One tentative hypothesis: Ionic bombardment of the E ring caused a sequence of reactions in the ice particles that led to free molecules of oxygen that dispersed in the pattern shown here that covered much of December 2003. Readings taken in 2004 showed an increase in oxygen for several months, but the amounts may now be abating.
There are now so many informative images from the Cassini mission that it is helpful to add another page. So proceed to page 19-19a by clicking on Next below.