Twenty-five years after the Voyagers' brief surveys, SwRI instruments on board Cassini will spend four years examining Saturn and its satellites
by J. Hunter Waite Jr., Ph.D., and David T. Young, Ph.D.
When the Cassini spacecraft leaves Earth in October 1997 to begin its seven-year, 1.4 billion-kilometer journey to Saturn, it will carry the most sophisticated set of experimental equipment yet flown to any planet. There are a dozen instruments on the Cassini orbiter and a further half dozen aboard the Huygens probe, destined to travel to the surface of Saturn's gigantic moon, Titan. These instruments have been designed and built by leading space scientists at institutions around the world for the coordinated investigation of the surface, atmosphere, and magnetosphere of Saturn, its rings, and its satellites.
The data returned by Cassini will contribute not only to our knowledge of the fascinating Saturn system but also to our understanding of the formation and evolution of the solar system and, possibly, of the origin of life within it as well.
Southwest Research Institute (SwRI) serves as the principal investigator organization for the Cassini Plasma Spectrometer (CAPS), which will collect data on the composition and dynamics of the plasma (a dilute ionized gas) in Saturn's magnetosphere. In addition, SwRI leads a Cassini science team that will use the Ion and Neutral Mass Spectrometer (INMS) to investigate the important neutral component of Titan's upper atmosphere and its interaction with the magnetosphere.
Together, the two instruments will provide complementary data on the sources of plasma in the Saturn system and on the processes that couple the plasma and neutral gas regimes. These data not only will tell us about the present state of Saturn's magnetosphere but also will provide valuable clues to processes that played an important role in the evolution of the Saturn system.
This mission represents a high point for SwRI's space research program, which since 1977 has been playing an increasingly important role in the National Aeronautics and Space Administration (NASA) space physics and planetary missions, through instrument design and fabrication, theoretical research, and data systems development.
Centuries of ground-based observations established the fundamental properties of the Saturnian system: size, mass, density and probable composition, rotation period (10 hours), and length of solar orbit (29-and-a-half-years). Astronomers learned that the rings of the planet were composed of a thin, flat layer of independently orbiting particles and discovered the approximate size and motion of its numerous moons, including the giant Titan - the second largest moon in the solar system and the only one known to be shrouded by a dense atmosphere.
This basic knowledge was multiplied manyfold by the first generation of spacecraft observations: Pioneer 1 (1979), Voyager 1 (1980), and Voyager 2 (1981). These flights to the distant planet provided preliminary measurements of the composition, pressure, and temperature of Saturn's upper atmosphere. Spectacular images showed the ring systems to be far more complex than originally envisaged. A Saturnian magnetosphere was detected that varied enormously in size in response to changes in pressure of the solar wind (sometimes including Titan within it, sometimes not). A number of new satellites were discovered and a variety of strange satellite surface features noted. Titan's dense atmosphere was confirmed, and tantalizing hints were found about the possibility of lakes of ethane as well as conditions conducive to the formation of organic chemicals.
Unlike previous flyby missions, when the Cassini/Huygens spacecraft reaches Saturn in July 2004, it will be placed into orbit around Saturn to conduct four years of intensive investigation of the Saturnian system, nominally ending in June 2008. The Huygens probe will be released for its descent to the surface of Titan in November 2004. There will be about 60 orbits of Saturn, to include at least 40 further flybys of Titan at altitudes as low as 950 km, as well as 1,000-km flybys of Enceladus, Dione, Rhea, and Iapetus, and more distant flybys of these and other moons. Numerous opportunities for observing the occultation of the spacecraft by Saturn, Titan, and the rings will be available, and observations will be made as Cassini goes into the shadow of all of these objects.
Cassini will be the best-equipped spacecraft yet sent to another planet. Instruments on the orbiter include imaging devices that operate from the far ultraviolet to the far infrared, a synthetic aperture radar mapper to produce high-resolution radar images, instruments to measure dust particles and radio waves, and a complete set of electromagnetic fields and atomic particle instruments, including CAPS and INMS, to characterize the neutral and plasma environments through which the spacecraft passes during its four-year exploration of the Saturn system. In addition, the Huygens probe carries six instruments that will provide the first detailed in situ measurements of Titan's atmosphere and surface.
With its diverse set of sophisticated instruments, the Cassini mission will achieve a number of important space physics and planetary science objectives. The orbiter will provide new information on the composition, dynamics, and vertical structure of Saturn's atmosphere. It will probe Saturn's rings, gathering information on the composition, size, and radial distribution of the ring particles and on the interaction of the rings with the magnetospheric plasma.
Data from the Huygens probe, together with complementary data from the orbiter, will tell researchers about the structure and composition of Titan's atmosphere and about the multilayered clouds made up of complex organic compounds that mask Titan's surface. The probe also will provide images of Titan's surface, along with data on its physical properties.
Finally, Cassini will explore Saturn's magnetosphere, the cavity formed in the interplanetary medium by the planet's intrinsic magnetic field, and radio back to Earth data on the composition and dynamics of the magnetospheric charged particle populations and on their interactions with the rings, satellites, and Saturn's upper atmosphere.
Puzzles at Titan
Titan is by far the largest of Saturn's many moons and is the second largest moon (after Jupiter's Ganymede) in the solar system. It is far larger and more massive than our own Moon or the planet Pluto, and even larger in diameter, though less massive, than the planet Mercury. Titan's significant atmosphere led to its selection as a primary target for investigation by Voyager 1 in 1980. The few minutes of this flyby at a distance of 4,000 km provide most of the exciting information that we now have - although much remains a mystery, including the surface, because Titan's atmosphere is rendered opaque to visible light by a persistent haze layer about 300 km above the moon's surface.
Titan's surface atmospheric pressure is about 1.6 bar (1.6 times sea level pressure on Earth). Remote sensing data and modeling studies suggest that the atmosphere is composed primarily of molecular nitrogen (like Earth's), with a small percentage of argon and methane, plus trace elements such as ethane and carbon dioxide. Given the surface temperature (-178 degrees Celsius) and pressure, it seems possible that there may be liquid ethane lakes on Titan, and that chemical conditions may be propitious for the production of organic molecules.
Substituting direct observation for inference, the Cassini INMS will make in-situ measurements of the composition of Titan's neutral upper atmosphere during more than 40 close flybys between 2004 and 2008. Within three months of the entry of the Huygens probe, the spacecraft will pass through the high-altitude region (900-1,000 km) above the moon. In that region, photochemical reactions produce complex hydrocarbon/nitrogen compounds that condense as aerosols near the tropopause and may eventually precipitate onto the surface of Titan, possibly forming hydrocarbon-nitrile lakes. The orbiter findings will be correlated with measurements made by the Huygens probe during its descent to the surface of the moon. The probe will provide data on composition, temperature, and pressure, as well as images and any indication of liquid on Titan's surface.
While speculations about liquid hydrocarbon lakes may not be confirmed, let alone those about the presence of relatively elaborate organic molecules, repeated sampling by INMS during its many Titan flybys will provide the information needed to characterize the composition, thermal structure, energetics, dynamics, and variability of Titan's upper neutral atmosphere.
Saturn's magnetosphere is a vast plasma-filled region extending roughly 1.5 million kilometers sunward of the planet and, because of the drag of the solar wind, perhaps 10 to 100 times that distance away from the sun. Its overall size varies with time in response to variations in the intensity of the solar wind. Within its sunward boundary the magnetosphere contains several icy inner moons, the entire ring system, and - except at times of increased solar wind pressure - Titan. Even more distant satellites plunge in and out of Saturn's magnetosphere, interacting sometimes with the magnetosphere and sometimes with the solar wind. These objects, buried in Saturn's rapidly co-rotating magnetic field, are thought to be sources of the planet's magnetospheric plasma, which is accelerated by electric fields induced by the motion of Saturn's rapidly rotating magnetosphere and by the solar wind. Its high ratio of neutrals to plasmas, due to the sputtering of water from the rings and icy satellites, makes Saturn's magnetosphere unique among the planets.
The science goals of the CAPS investigation are to understand the sources as well as the losses of Saturn's plasma, its acceleration and transport, and the contributions that plasma makes to satellite resurfacing and to Titan's atmospheric processes. These processes all significantly affected the evolution of the Saturn system over its 4.5 billion years of existence. By understanding the changes brought about by these processes and by sampling the isotopic and elemental composition within Saturn's vast magnetosphere, we will add yet another piece to the puzzle of how the solar system formed and evolved.
CAPS was designed to provide continuous high-resolution data on plasma composition, density, flow velocity, and temperature throughout the Saturnian system. For example, CAPS' sensitivity allows it, in a single two-second energy scan, to measure the composition, density, and velocity distribution of a 10-million Kelvin plasma (typical of Saturn) with a density of as little as 0.001 particles per cubic centimeter, to an accuracy of 10 percent or better. Its measurement capabilities are at least 10 times better than those from comparable Voyager instruments.
More important, though, is the capability of the various Cassini instruments to work together to elucidate problems. For example, comparison of CAPS plasma data with INMS neutral and low-energy ion data obtained during the Titan flybys will provide the first in-situ description of the satellite's ionosphere, the existence of which has been inferred from the Voyager 1 plasma and magnetic field measurements of Titan's magnetospheric wake. Furthermore, a clearer picture will be obtained of the interaction of Titan's ionosphere and neutral upper atmosphere with Saturn's magneto-spheric plasma.
This interaction is important both because it supplies plasma to the magnetosphere and because it results in the loss of light gases such as hydrogen from the atmosphere. Over geological time the loss of such constituents may have significantly altered the composition of Titan's atmosphere. Moreover, the interaction of Titan's upper atmosphere with Saturn's magnetosphere is a source of free energy that drives the production of the complex carbon/nitrogen compounds that have been observed in Titan's atmosphere.
Another example of cooperative functioning of CAPS and INMS is the case of the 10 or so close passes by the icy satellites Enceladus, Tethys, Dione, and Rhea. These satellites are exposed to bombardment by high-energy magneto-spheric particles (characterized by CAPS), and their surfaces may react by "sputtering" neutral and low-energy ionized particles, thus returning material to the magnetosphere. The mechanics of this process can be illuminated by the cooperative efforts of these two instruments.
The sophistication and diversity of the 12 Cassini orbiter and six Huygens probe instruments provide scientific breadth but also bring operational complexity. During the mission's long cruise phase (the trip to Saturn, including the complex "slingshot" passes by Earth and Venus, takes nearly seven years), the CAPS and INMS teams will be completing flight software, developing software for operations and data analysis, and validating and training for every aspect of the complex set of orbital operations that Cassini will carry out during the "tour" phase of the mission, between 2004 and 2008.
Expecting the unexpected is the hallmark of planetary research. Cassini thus will no doubt have a strong exploratory component. The breadth and adaptability of Cassini's instrumentation will be a key to taking full advantage of new discoveries, while at the same time providing the level of measurement sophistication and diversity to turn exploration into new understanding.
Published in the Fall 1997 issue of Technology Today®, published by Southwest Research Institute. For more information, contact Joe Fohn.