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Planetary Exploration Missions

James D. Burke, in Encyclopedia of the Solar System (Third Edition), 2014

2.4 Operations

In even the earliest lunar and planetary missions, it was necessary to keep track of the spacecraft's trajectory and issue commands for onboard functions both engineering and scientific. Gradually a humans-and-machines art developed, represented today by large rooms full of people and displays backed by buildings full of computers and data systems. Initially centered in main theaters, as missions have become more complex, these facilities have become dispersed, providing work spaces for the many specialized flight management and scientific teams working during a mission. With the Internet and other modern communications available, scientists can now reside at their home institutions and participate in missions in real time.

The latest trend is toward increasing onboard autonomy, holding the promise of reducing the large staffing needed round the clock to control missions. Some degree of autonomy, for example, stabilization with Sun and star references, is needed anyway in deep space, simply because of the round-trip signal times to distant spacecraft, tens of minutes for Mars and Venus and many hours in the outer solar system.

Operations have become more and more dependent on software whose design and verification now constitute one of the main cost items in each new mission's budget. With the maturing of the operations art have come numerous stories of remarkable rescues when a distant robot (or, as in Apollo 13, a human crew) got into trouble, but there are also instances where a mistake on the Earth sent a mission to oblivion.

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Volcanism on Mercury

James W. Head, Lionel Wilson, in The Encyclopedia of Volcanoes (Second Edition), 2015

2 Mariner 10 Visits Mercury and the Postmission Debate

The surface morphology of Mercury was initially revealed by images from the Mariner 10 flybys in 1974–1975. Using a clever set of spacecraft trajectories, Mariner 10 flew by Mercury three times and imaged about 45% of the surface at an average resolution of ∼1 km. These flybys showed that, in contrast to the Moon, where there are distinctive composition-related albedo variations between the cratered uplands (relatively high albedo) and the smooth volcanic mare lowlands (relatively low albedo), the albedo of Mercury is relatively uniform across the surface.

Prior to the Apollo 16 mission to the Moon in 1972, a widely distributed smooth, relatively high-albedo plains unit (the Cayley Formation) was mapped in the lunar uplands, lying stratigraphically between the younger, low-albedo maria, and the older, high-albedo impact basins and cratered terrain. One of the purposes of the Apollo 16 mission was to determine the petrology and absolute age of this unit, thought prior to the mission to represent a distinctive premare, highland phase of volcanism. During Apollo 16 surface operations it became rapidly clear that the Cayley Formation consisted of impact breccias, not lava flows, and later assessments suggested that the deposits were a combination of local and regional, basin-related impact ejecta. On the basis of the Apollo 16 results, lunar light plains were subsequently considered by most workers to have been emplaced by impact crater and basin ejecta processes, rather than by extrusive volcanism. Arriving at Mercury shortly after Apollo 16, Mariner 10 revealed the presence of two smooth Cayley-plains-like units alternatively interpreted as representing effusive volcanic deposits or impact basin ejecta. These widespread plains deposits, occurring as relatively smooth surfaces between craters (intercrater plains), and as apparently ponded material (smooth plains), were proposed by some to be volcanic in origin. Others argued that the plains deposits might represent basin ejecta, similar to those found at the lunar Apollo 16 landing site.

Part of the problem concerning interpretation of smooth plains on Mercury as volcanic or impact in origin was the relatively low resolution of the Mariner 10 data. Early on, it was pointed out that the Mariner 10 image data did not have the resolution required to resolve lunar-like volcanic features such as flow fronts, vents, and small domes. Detailed examination of lunar images at resolutions and viewing geometries comparable to those of Mariner 10 readily showed that small shields and cones, elongate craters, sinuous rilles, and flow fronts, all hallmarks of the identification of volcanism on the Moon, would not be resolvable in most of the Mariner 10 images. Furthermore, larger features typical of volcanism on Mars (such as huge volcanic edifices and calderas), and not seen on the Moon, were not observed by Mariner 10 on the ∼45% of Mercury that it observed. Also not observed in the Mariner 10 data were examples of the large (10–30 km diameter) steep-sided domes suggestive of crustal magmatic differentiation processes seen on the Moon and Venus. Lobate fronts exposed at the edge of smooth plains occurrences on Mercury suggested that these might have been volcanic flow margins, but comparisons with marginal basin ejecta deposits on the Moon indicated that such features could also be products of impact ejecta emplacement. Thus, although the surface features observed by Mariner 10 were most similar to lunar plains, there were also fundamental differences between volcanism occurring on the two bodies.

These various observations raised the interesting possibility that there might be no identifiable volcanic units on Mercury. However, crater counts of ejecta facies of the Caloris basin (the largest basin detected on Mercury by Mariner 10) and also on interior/exterior smooth plains deposits indicated that the smooth plains were emplaced after the Caloris basin. On this basis they were interpreted as the product of volcanic eruptions, not contemporaneous ejecta emplacement. Reprocessed Mariner 10 color data, showing some distinctive color boundaries at unit margins, provided additional evidence for the possible volcanic origin of the smooth plains.

Nonetheless, the question of the volcanic origin of the smooth and intercrater plains was hotly debated in the subsequent decades. Could extrusive volcanism not have occurred on Mercury, and if such advective cooling processes did not occur, how did the planet dissipate its accretional heat and that from subsequent decay of radioactive isotopes? One possibility is that partial melting of the mantle may have occurred, but that extrusive volcanism did not. Investigating these possibilities, several workers analyzed the ascent and eruption of magma under Mercury conditions for a wide range of scenarios and found that a thick low-density crust could, as with the Moon, inhibit and potentially preclude intruding dikes from reaching the surface and forming effusive eruptions. This, combined with an apparent global compressional net state of stress in the lithosphere observed by Mariner 10, could produce a scenario in which rising magma intruded the crust but did not reach the surface to produce the level of resurfacing or the array of landforms seen on the Moon, Mars, and Venus. Indeed, these workers showed how easy it was, given the range of conditions known to occur in the history of terrestrial planetary bodies, to create a planet with little to no extrusive volcanic activity. Superficially, Mercury looks like the Moon, but Mariner 10 and terrestrial remote sensing data told us that it must be very different in many fundamental respects. Could Mercury be a Moon that did not undergo surface evolution by endogenic processes (e.g., mare volcanism) subsequent to the period of heavy impact bombardment?

The dominant endogenic geologic process on the Earth, Moon, Mars, and Venus is volcanism, characterized by massive extrusions of basaltic lavas, significant resurfacing of their surfaces, and emplacement of large volumes of intrusive magmas. Such fundamental outstanding questions concerning Mercury's volcanism and thermal evolution highlighted the importance of obtaining new spacecraft image, mineralogical, and elemental data. The MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) mission was designed to provide a complete picture of Mercury in the context of the early evolution of the terrestrial planets. One of the major goals of the MESSENGER mission was to assess the presence, age, and distribution of volcanic deposits.

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Physics of Terrestrial Planets and Moons

J.E.P. Connerney, in Treatise on Geophysics (Second Edition), 2015

10.06.4.2.1 Observations

The discovery of Saturn's magnetic field was thus left to Pioneer 11, arriving in September 1979, following its Jupiter swingby. The particle and field investigations on board Pioneer 11 (see previous section) obtained observations along a spacecraft trajectory that remained within ~ 6° of the equator (Figure 16). The most remarkable feature of Saturn's magnetic field proved to be the close alignment of the magnetic dipole and rotation axes (Acuña and Ness, 1980; Acuña et al., 1980; Smith et al., 1980a), also noted by the charged particle investigations (Simpson et al., 1980; Van Allen et al., 1980). Pioneer 11 approached to within 1.35Rs, measuring a maximum field of ~ 8200 nT at − 6° latitude (Smith et al., 1980a,b).

Figure 16. Spacecraft flybys (Pioneer 11, Voyager 1 and 2, and Cassini) of Saturn in a cylindrical coordinate system in which the z-axis is aligned with Saturn's magnetic dipole axis and rotation axis. The spacecraft distance from the equator plane (z) as a function of distance from the z-axis (ρ) is given in units of Saturn radius. The positions of satellites Mimas, Enceladus, Tethys, Dione, and Rhea are indicated. The shaded region approximates the innermost portion of a washer-shaped region of azimuthal ring currents.

The Voyager 1 encounter in November 1980 and the Voyager 2 encounter in August 1981 provided the first observations of Saturn's magnetic field at high northern and southern latitudes (Ness et al., 1981, 1982). Voyager 1 sampled relatively high latitudes (− 40°) at close radial distance (3.07Rs) but remained in the southern hemisphere while inside of 6Rs radial distance. Voyager 1 measured a maximum field of 1093 nT at − 40° latitude and 184° SLS longitude, just prior to closest approach (Ness et al., 1981). Voyager 2's closest approach of 2.69Rs occurred at 323° longitude, diametrically opposite to those of Pioneer 11 and Voyager 1. Voyager 2 sampled relatively high latitudes in both hemispheres and measured a maximum field of 1187 nT just prior to closest approach (Ness et al., 1982).

More than two decades passed before the arrival of Cassini at Saturn. The Cassini spacecraft has been in orbit about Saturn since its orbit insertion (SOI) on 30 June 2004 and has remained relatively distant subsequent to the orbit insertion maneuver (Dougherty et al., 2004), targeting multiple satellite flybys from the vantage point of a near-equatorial orbit. During SOI, the spacecraft periapsis was just 1.33Rs; this is the closest Cassini would come to Saturn throughout its prime mission. Cassini is instrumented with two magnetometers, a vector fluxgate and a helium magnetometer, the latter operated in a mode to measure field magnitudes near the planet (Dougherty et al., 2004). However, a large number of orbits have been executed thus far, many with periapsides < 4Rs. The spacecraft is just now executing a series of maneuvers designed to increase the inclination of the orbit, which will provide a more favorable distribution of observations for internal field analysis, particularly near the end of mission, with many orbits planned with periapsides < 3Rs.

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OBSERVATIONS PLATFORMS | Rockets

M.F. Larsen, in Encyclopedia of Atmospheric Sciences (Second Edition), 2015

Introduction

The influence of the lower atmosphere on our daily lives is immediately clear because of the weather processes that occur there. The processes that occur in the upper atmosphere are also important, although that did not become evident until the space age began at the end of World War II. As we become more dependent on space and satellite communications, as well as traditional radio communications, understanding those processes becomes critical. The ionosphere, which covers the region from approximately 80 km to altitudes above 1000 km, is a mixture of neutral gas constituents and charged particles. The plasma, as it is known, and the electrodynamic forces that drive it can produce small-scale fluctuations in the charged particle densities that can affect radio and satellite communications significantly. The temperatures, composition, and density of the neutral gas component can also have large variations in response to both local processes and changing activity on the Sun. Such effects can alter satellite and other spacecraft trajectories enough to be a concern. Measuring the properties of the atmosphere at high altitudes where the dynamical properties are both interesting and important but where the densities are low enough to make measurements difficult has been a continuing challenge, especially in the period since World War II. The measurement techniques used extensively include ground-based remote sensing with radars and optical techniques, measurements from satellites, and measurements from suborbital rockets, also known as sounding rockets. This article will focus on the techniques used to obtain measurements from the rocket platform.

The layers of the atmosphere closest to the surface of the Earth are the troposphere and stratosphere. Above 45 to 50 km altitude, where the stratosphere ends, the temperature again decreases with altitude in the layer known as the mesosphere, followed by a region of steadily increasing temperature above 90 km, known as the thermosphere, where the absorption of extreme ultraviolet radiation from the sun is primarily responsible for the temperature increase. Radiation from the Sun also leads to the ionization of neutral particles in that region which, because of the importance of the charged constituents, is also known as the ionosphere.

Between the surface of the Earth and the region near 100 km altitude, the density of the atmospheric gas decreases by a factor of approximately 1 000 000, and traditional techniques for making measurements in situ from platforms such as balloons or aircraft do not work, since the platforms cannot be supported by the atmosphere. New possibilities for probing the upper-altitude regions became available at the end of World War II when rockets developed by the Germans during the war and captured by the Allies became available for scientific investigation.

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Ganymede and Callisto

Geoffrey Collins, Torrence V. Johnson, in Encyclopedia of the Solar System (Third Edition), 2014

3.1 Interior Structures

The ice/rock bulk composition inferred for Ganymede and Callisto from their densities naturally led to the suggestion that even modest heating from accretion and the decay of radioactive elements in the rocks would melt the ice in the interior and allow the rocks to sink, leading to differentiated interiors—that is, a layered structure with the denser rock and metal constituents concentrated closer to the center of the satellite and the ice in the outer layers. Most analyses following the Voyager mission operated on the assumption that Ganymede and Callisto had similar differentiated interior structures, but the data to test this assumption would not come until the Galileo mission.

Determining the interior structure of a planetary object is intrinsically difficult, particularly from remote observations alone. Most of the information about the interior of our Earth, for instance, comes from over a century of study of seismic data, where waves created by earthquakes travel deep through the Earth and provide clues to the density and composition throughout the interior [see Earth Surface and Interior]. So far the only other world for which we have seismic data is the Moon, acquired with seismometers left by the Apollo astronauts (see The Moon).

An extremely important quantity that can be used to assess the distribution of mass inside an object is a dimensionless number known as the moment of inertia; a sphere with uniform density throughout has a moment of inertia of 0.4, with lower values indicating increasing degrees of mass concentration near the center. The moments of inertia for Ganymede and Callisto were measured indirectly by the radio experiment on Galileo, which measured the perturbations of the spacecraft's trajectory as it flew by the satellites on multiple occasions at low altitudes and different latitudes. Although perfect spheres with different moments of inertia have identical external gravity fields, the key to this experiment is that the distribution of mass in the interior of a satellite does affect the way its shape is perturbed from a perfect sphere by rotation and tides. The rotation rates of Ganymede and Callisto, although slow by terrestrial standards (a little over a week for Ganymede, and over 2 weeks for Callisto), are still sufficient to cause a slight equatorial bulge and polar flattening, while Jupiter's strong gravity raises tidal bulges on the sub- and anti-Jupiter hemispheres. The combination of these two effects leads to distinctly nonspherical components to the external gravity field (in mathematical terms, the description of the satellites' gravity in a spherical harmonic expansion contains significant J2 and C22 terms). The magnitude of these nonspherical terms depends on the degree of internal mass differentiation, and they are related directly to the moment of inertia as long as the object responds as a fluid to spin and tidal distortion (i.e. hydrostatically).

The surprising results of the Galileo tracking experiment showed that Ganymede and Callisto have distinctly different interiors. The derived moments of inertia for both satellites were lower than they would be for bodies of uniform density, as expected. Ganymede's measured value of 0.31 is so low that it implies essentially complete separation of its water ice from the heavier rock and metal. However, Callisto has a significantly larger moment of inertia, 0.35. This is small enough to imply some differentiation, but too large to be compatible with full separation of light and heavy components. Callisto probably has some significant portion of its interior composed of a rock–ice mixture.

The measured moments of inertia can be combined with the values for the total mass, size, and properties of ice and rock under pressure to construct models of the satellite's interiors that match all the known quantities. As there are several unknown parameters regarding interior chemistry, the exact compositions and thicknesses of layers in such models are inherently uncertain, but reasonable assumptions can be made to construct a general model. Figure 37.3 shows the best current estimates of their internal structures. Ganymede is shown with a three-layer structure: a metallic core, a rock mantle, and a deep water-rich upper layer; Callisto is shown with a two-layer structure: a large rock-ice core, with the fraction of dense material increasing toward the center, and an upper water-rich layer. Note that the water-rich layers on both satellites are shown with liquid water oceans trapped below thick layers of surface ice, as discussed in the next section.

FIGURE 37.3. Cutaway diagrams showing current models for the interior structures of Ganymede and Callisto based on Galileo gravity data. Ganymede (left) is highly differentiated, with a molten iron core surrounded by a rocky mantle, in turn surrounded by a thick outer layer of ice. An interior ocean of liquid water may exist sandwiched between the surface layer of Ice-I and the higher pressure phases of ice below. Callisto (right) has an interior composed of a mixture or rock and ice, slowly increasing in density toward the center. The outermost layer is relatively clean water ice, with a liquid water ocean at its base.

Zareh Gorjian and Eric De Jong, NASA/Jet Propulsion Laboratory.

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Exploration and Analysis of Planetary Shape and Topography Using Stereophotogrammetry

Jürgen Oberst, ... Frank Preusker, in Encyclopedia of the Solar System (Third Edition), 2014

2 Missions and Cameras

The coordinates of a 3-D point are determined by computing viewing vectors from two or more different observation positions and by computing the intersection point of the rays defined by these viewing vectors. The exterior orientation of the camera, i.e. the positions of the viewing posts at the time of exposure and the camera pointing (more specifically, the rotation matrix relating the target coordinate system to image coordinates) must be precisely known or inferred to determine the viewing vectors. For convenience, positions and viewing vectors are computed in the coordinate system fixed to the target body. In the case that the target-fixed coordinate system (e.g. a planet's rotational model) is not sufficiently well known, this must be modeled in the process.

Viewing vectors are computed from the exterior orientation parameters of the camera in combination with the measured image coordinates of features and of calibration data describing the imaging system itself. The latter include focal length, image distortion parameters, and mounting offsets of the sensor system typically determined from calibration campaigns carried out either before flight on ground or in-flight, e.g. by using observations of star fields.

As navigation and calibration data are typically not as well known as one might wish, sophisticated techniques for reconstructing orbit and/or pointing data in combination with parameters of the imaging system have been developed. Depending on the properties of the available data and the desired characteristics of the surface model, these steps play an important role in establishing a reliable geometric framework for the reconstruction of topography.

2.1 Framing Cameras

Framing cameras are equipped with frame Charge-Coupled Device (CCD) or Active Pixel Sensor (APS) array sensors or even (in the early mapping of the Earth and Moon) photographic film. Here, mathematically simple geometric conversions exist that relate positions of a feature on the sensor surface to viewing vectors for each sensor pixel. The image geometry is very stable, i.e. the relative positions of the pixels are precisely known. Stereo images are acquired by pointing the camera from different body-fixed positions to the surface feature of interest. Alternatively, for small objects as asteroids, the camera may remain at some inertially fixed position, while the asteroid rotates beneath the camera position.

2.2 Scanners

For spacecraft in low planetary orbits, it is advantageous to use line scanners rather than framing cameras for topographic mapping. Line scanners are equipped with linear CCD or APS sensors, which are operated at high rates. Images are obtained along the spacecraft ground track, while the spacecraft moves ("pushbroom mode"). The stereo processing of scanner images or the combination of several such scans requires that the spacecraft trajectory and attitude of the sensor are known or modeled on a quasi-continuous basis, according to the high sampling rate of the imager. By using multiple line sensors side by side, large image blocks can be obtained. For example, the High-Resolution Imaging Science Experiment (HiRISE) camera on Mars Reconnaissance Orbiter has 14 separate line sensors on its large focal plane. Sensors equipped with filters can provide multispectral images. One of the advantages of scanners over framing cameras is that they can produce quickly very large-format images, limited by available onboard data storage only.

2.3 Cameras on Surface Landers and Rovers

A special but increasingly important case of planetary photogrammetry is the recovery of topography from stereo images obtained by cameras on surface landers or rovers. Such spacecraft are typically equipped with pairs of framing cameras mounted on a mast, on the rover body, or both. Dual lenses, separated horizontally by a fixed and precisely calibrated base length are used to obtain the perspective effects. The stereo reconstruction process is challenged by the fact that ranges to features of interest and corresponding perspective effects vary greatly. Perspective effects are large near the camera, and approach zero at the horizon, depending on the separation and angular field of view of the dual cameras. Also, due to the specific viewing conditions ("frog perspective") and a possibly rough surface, large parts of the image may be hidden from the camera, thus preventing the construction of a contiguous terrain model from only one pair of stereo images.

2.4 Built-in Stereo Capability

Using multiple sensors pointed at different along-track viewing directions, stereo images can be obtained simultaneously during a single orbital pass. For spacecraft in orbits dedicated to surface mapping, it may be worthwhile carrying cameras with such a built-in stereo capability. As the first camera of this kind, the high-resolution stereo camera (HRSC) line scanner on Mars Express is equipped with nine sensors, five of which are pointed at different viewing directions and are dedicated to stereo imaging. While the spacecraft moves, the stereo sensors will scan an area several times under different viewing angles. Similar camera systems featuring dual or triple line arrays for terrain mapping have flown on the Japanese Kaguya, the Indian Chandrayaan-1, and the two Chinese Chang'e missions. Alternatively, dual-frame cameras pointed forward and backward can be used. The built-in stereo capability does not require any dedicated pointing or spacecraft maneuvers. The stereo images are typically obtained within minutes under near-identical illumination conditions, which facilitates the data analysis.

2.5 Acquisition of Ancillary Orbit and Pointing Data

For the stereo processing excellent navigation data are required. Spacecraft orbits are usually obtained from radio range and Doppler measurements. Long tracking arcs of several hours are used to determine the trajectory of the spacecraft at any time. The attitude of the spacecraft and, hence, pointing of the cameras, is obtained by star sensors in combination with gyroscopes mounted on the craft.

2.6 Camera Calibration

For the processing, also good knowledge of the camera geometric parameters is required. This includes the focal length and principle point of the camera as well as alignment parameters with the spacecraft coordinate system. Parameters of image distortion can be relevant as well, usually expressed in polynomial functions relating the image coordinates at which a feature is observed to the "ideal" coordinates for a distortionless camera. Camera geometric properties are normally determined on the ground before flight. However, after launch, updates of camera calibration parameters may be required. Typically, dedicated imaging campaigns of star fields (in which positions of stars are accurately known) are used to verify the camera calibration parameters.

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Ganymede and Callisto

Geoffrey Collins, Torrence Johnson, in Encyclopedia of the Solar System (Second Edition), 2007

1. Exploration

1.1 Discovery

Ganymede and Callisto were discovered by Galileo Galilei in 1610, when he first trained his telescope on Jupiter and shortly thereafter published his results in the Siderius Nuncius. Along with Io and Europa, they became the first natural satellites, other than the Moon, known to science. Galileo immediately recognized the significance of the "new stars" traveling with Jupiter and changing their positions every night. The orbits of what are now known as the Galilean satellites were rapidly calculated and found to be essentially circular and in the same plane as Jupiter's equator. Because Galileo made these observations centuries ago, his records of satellite eclipses provide a long timeline to compare with modern measurements, and they are still used to constrain calculations of the dynamical evolution of Jupiter's satellite system under the influences of tidal dissipation and the satellites' mutual gravitational interactions.

1.2 Astronomical Observations

The Galilean satellites are large enough to exhibit distinct discs (on the order of ∼1 arc second in angular diameter) when viewed through even moderate power telescopes, and it was thus known from simple geometry that they must be bodies comparable in size to the Moon. Precise measurements of their sizes proved difficult with conventional astronomical techniques, with published estimates from different observers disagreeing significantly. Even these relatively uncertain size estimates were sufficient, when combined with the satellites' brightness, to indicate that their surfaces are highly reflective compared with that of the Moon.

In the two decades leading up to the first spacecraft exploration of the Jupiter system, astronomical techniques advanced rapidly, particularly in the area of sensors in the visible and near-infrared spectral range (∼0.3–2.5 μm). The pioneering planetary astronomer Gerard Kuiper used early infrared detectors to show that Ganymede's reflectance at 2 μm was much lower than in the visible range and suggested that water ice might be responsible. Vassily Moroz, a planetary scientist working at the Crimea Observatory in the Soviet Union, made even more detailed infrared color measurements and concluded that water ice was the best explanation for Ganymede's spectrum.

At the spectral resolution and signal-to-noise ratio of these pioneering measurements, however, a conclusive identification of the surface composition could not be made because several other candidate materials, including ices of carbon dioxide and ammonia, were known to have absorptions in the same part of the infrared spectrum. The issue was settled conclusively for Ganymede and Europa by a team led by Carl Pilcher, then at MIT, who published the first high-resolution infrared reflection spectra for these satellites in 1972 and compared them in detail with laboratory spectra of ices at low temperature. All the significant absorption features in the 1- to 2.5 μm region matched spectra of water ice and ruled out any major contribution from other ices. Callisto's spectrum also displays water ice and hydrated silicate features, although the water signature is subdued compared with Ganymede's strong water ice spectral absorptions, due to the larger amount of dark material mixed with the ice on Callisto's surface.

Figure 2 shows a compilation of the best telescopic spectra of Ganymede and Callisto compared with Io and Europa. The dominant features in all the spectra except Io's are the deep absorptions at wavelengths longer than 1 μm due to the presence of hydrated materials and water ice. Laboratory studies of water ice reflectance and theoretical simulations of spectra from mixtures of material have demonstrated that the observed spectra can be explained by water ice/frost, mixed with varying amounts of a spectrally neutral darker component with a reddish color in the visible portion of the spectrum (i.e., one having absorption at ultraviolet and blue wavelengths). The nature of the non-water-ice component in the satellites' surfaces is still under investigation, but spectra of different regions on both satellites taken by the Near-Infrared Mapping Spectrometer (NIMS) instrument on the Galileo mission are providing clues to the identification of this material (see Section 3).

FIGURE 2. Compilation of the best telescopic spectra of Ganymede (green) and Callisto (blue) compared with Io (red) and Europa (yellow).

1.2.1 MASSESAND DENSITIES

The mass of a distant planetary object is normally impossible to determine from remote astronomical observations alone, unless it happens to have a companion whose orbit can be determined, as is the case for the giant planets with their satellite systems, the Pluto/Charon system, and more recently numerous asteroids and several trans-Neptunian objects. The Galilean satellites represent a more difficult case. They do not themselves have satellites, but the mutual gravitational attraction among these large satellites produces significant and measurable changes in their orbits about Jupiter. The mathematician Pierre Laplace studied these interactions in the late 19th century, and subsequent developments in this new branch of dynamical astronomy permitted reasonable estimates of the satellites' masses to be made in the early 20th century. When combined with the still uncertain size estimates, the best estimates of masses prior to 1970 suggested that the inner satellites, Io and Europa, had rock-like densities, similar to the Moon's, and that Ganymede and Callisto appeared to be less dense, suggesting the possible presence of large amounts of ice in their constituent materials.

In 1972, observations of a stellar occultation by Ganymede from two stations on the Earth provided the first high-precision measurement of its diameter. This was closely followed by the first spacecraft exploration of Jupiter by the Pioneer 10 and 11 missions in 1973 and 1974, which greatly improved the mass estimates of the satellites from tracking the gravitational perturbations in the spacecraft trajectories caused by the satellites. This led to the first accurate determination of Ganymede's density of about 1900 kg/m3, adding more evidence to the hypothesis that its bulk composition is a mixture of rock and ice. The Voyager 1 and 2 Jupiter encounters in 1979 provided even more data on the satellite's masses and accurate determination of their shapes and volumes. These data showed Callisto is very similar to Ganymede in its bulk properties, with a density of about 1800 kg/m3. Interior structure models, taking into account the high-pressure behavior of water ice, show that the average bulk composition for both satellites is a mixture of 50–60% (by mass) anhydrous silicate "rock" with water ice.

1.3 Spacecraft Exploration

Seven spacecraft have visited the Jupiter system to date: Pioneer 10 and 11, Voyager 1 and 2, Galileo, Ulysses, and Cassini. Ulysses, a joint European Space Agency/NASA mission to study the Sun's environment at high latitudes, made measurements of Jupiter's magnetic fields, radiation belts, and dust environment but did not study the satellites directly. Cassini, on its way to its rendezvous with Saturn, flew by Jupiter in 2000 and returned spectacular observations of its atmosphere and magnetosphere, but its trajectory was too far from the Galilean satellite system to provide high-resolution views of the satellites.

The first Jupiter missions, Pioneer 10 and 11, were designed to provide the first reconnaissance of the system and to establish the intensity of the radiation belts. The Pioneer program's major contribution to knowledge of Ganymede and Callisto, as mentioned earlier, was improving the mass estimates of the satellites, leading to the first precision bulk density measurements.

In 1979, Voyager 1 and 2, with powerful remote sensing payloads and close targeted flybys of each Galilean satellite, provided the first in-depth reconnaissance of the satellites and set the stage for the geological and geophysical exploration of these worlds. Voyager's cameras showed that Ganymede and Callisto, alike in many large-scale properties, have divergent geological histories (Fig. 1). Callisto's surface is heavily cratered at all scales, from large impact scars over a 1000 km in diameter down to craters a few kilometers in diameter, the smallest scale resolvable on Callisto by the Voyager cameras. This battered, uniform surface stands in stark contrast to Ganymede's varied landscape. Ganymede's surface can be divided into two distinct types of terrain, based on a sharp albedo contrast. The darker areas (named "dark terrain") are heavily cratered and exhibit palimpsests, much like the surface of Callisto. The brighter parts of Ganymede's surface (named "bright terrain") form wide lanes through the dark terrain and are less heavily cratered, implying a younger surface. Voyager images showed the bright terrain to have some areas that appeared to be smooth, while other areas exhibit sets of parallel ridges and troughs.

One of the major objectives of the Galileo mission was to perform detailed observations of the big satellites. The mission design allowed multiple close flybys at ranges 100–1000 times closer than the Voyager encounters, enabling high-resolution studies of their surfaces and detailed measurements of their gravity fields and interactions with Jupiter's magnetospheric environment. High-resolution images of the different terrains first identified by Voyager have illuminated their origins, described in detail in subsequent sections of this chapter. The close flybys also enabled more detailed spectroscopic observations, which identified some of the non-water-ice components on the satellite surfaces, including carbon dioxide embedded in the surface and evidence for carbon compounds.

Repeated close flybys enabled Galileo to make precision gravity and magnetic measurements, resulting in several major discoveries. First, Ganymede has a strongly layered internal structure, with heavier rock and metal concentrated in the center, whereas Callisto has a more homogenous structure. Second, Ganymede was found to have a relatively strong internal magnetic field, creating its own "mini-magnetosphere" embedded within Jupiter's vast magnetosphere. Finally, the interactions of Ganymede and Callisto with Jupiter's rotating, tilted magnetic field show that both satellites exhibit an induced magnetic field interpreted as evidence for an electrically conducting liquid water ocean beneath their icy crusts.

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Probing the Interiors of Planets with Geophysical Tools

W. Bruce Banerdt, ... Suzanne E. Smrekar, in Encyclopedia of the Solar System (Third Edition), 2014

3 Investigating Planetary Interiors Using Gravity and Dynamics

Historically, the most common method for exploring planetary interiors has been through observed variations in their gravitational fields, orientation, and rotation, which can be collectively described as geodesy. The data can be obtained as a by-product of the communication and navigation systems of planetary probes flying by, in orbit around, or landed on the planets. Gravity and rotation provide information primarily on the distribution of mass within a planet, including density differences due to overall structure (crust, mantle, and core) and from processes such as volcanism and tectonic deformation.

The major disadvantage of these techniques is their inherent ambiguity. It can be shown mathematically (Gauss' theorem) that for any gravity field measured outside a body, there are an infinite number of distinct mass distributions inside the body that could generate that field (a similar result holds for rotational dynamics). While this seems daunting, in practice, there is usually other information that can be used to constrain the possible (or likely) distributions. Thus, geodetic methods are most powerful when used in combination with other techniques.

Spatial variations in the gravity field of planetary bodies are obtained by monitoring the trajectory of passing or orbiting spacecraft. For example, if an orbiter passes over a buried body with higher density than its surroundings, it will experience a higher gravitational attraction than otherwise and will be pulled slightly downward in its orbit. This deviation is determined via Doppler (which gives velocity) and ranging (which gives distance) measurements on radio links between the Earth and the spacecraft. These measurements use the precision transmitters and receivers at large deep space antennas such as the ESTRACK stations (European Space Agency (ESA) TRACKing stations) at Perth and Madrid and the NASA DSN (Deep Space Network) antennas at Goldstone, Madrid, and Canberra (see Figure 55.4). The precision possible with these systems is impressive, with resolution of velocity changes of fractions of a millimeter per second and absolute locations in inertial space of a few centimeters now possible.

FIGURE 55.4. Deep Space Network 70-m station at Goldstone, California.

For the analysis of these radio science data and for theoretical simulations of planetary interiors with which to compare them, sophisticated numerical codes are used to compute accurate orbits of spacecraft from radio tracking data. Spatial and temporal variations in the gravity field can be used to determine physical properties of the interior and atmosphere of the planet. Since the beginning of the space age, the large-scale structure of the gravity field of planets and moons has been successfully used to determine the moment of inertia, which is a measure of the global density distribution and an important constraint on the interior structure.

In addition to global mass distribution, regional or local properties above target surface features can also be measured from the deviation of spacecraft trajectories. As an example, the Mars radio science experiment (MaRS) aboard Mars Express acquired gravity data during a number orbits above the Tharsis volcanoes, which form the largest volcanic region in the solar system.

The data analysis shows that the overall density of the volcanoes is higher than the average density of the Martian crust, in agreement with the basaltic composition of many Martian meteorites probably originating in the Tharsis area. One volcano, Ascraeus Mons, differs from the others in being of lower density in its upper part, although its overall density remains high. If the Tharsis Montes were built in succession by a single moving mantle plume, this suggests that Ascraeus Mons formed as the last of the Tharsis Montes. These data also show that Olympus Mons, the highest mountain in the solar system, lacks a low-density root, which indicates that it was built on a lithosphere of high rigidity, whereas the other volcanoes partly sank within a less rigid lithosphere.

Solid-body tides, which can be observed through their time-variable effect on the gravity field, can also provide information on the deep interior. These are particularly sensitive to global fluid layers such as a liquid iron core in terrestrial planets or an internal subsurface ocean in icy satellites. From the latest available data on the moment of inertia and the tidal amplitude of Mars, the best estimates yet obtained have been determined for the core size and composition of Mars. These show that the core size is expected to be between 1715 and 1850 km and that the weight fraction of sulfur in the core is between 13% and 18%. The addition of sulfur decreases the melting temperature of iron, and for the current estimates of the internal temperature of Mars, this high sulfur estimate implies that the core of Mars is entirely liquid and contains no solid inner part, in contrast to the Earth.

Constraints on planetary interiors can also be obtained from rotation variations. Three broad classes of rotation variations are usually considered: rotation rate (also called length-of-day) variations, orientation changes of the rotational axis with respect to inertial space (precession and nutation), and orientation changes of the planet's surface with respect to the rotation axis (polar motion and polar wander). These are due to both internal (angular momentum exchanges between solid and liquid layers) and external (gravitational torques) causes. As the rotational response depends on the planet's structure and composition, insight into the planetary interior can also be obtained. This is particularly so for the rotational variations due to well-determined external gravitational causes, such as for the nutations of Mars and the librations of Mercury and natural satellites.

Observation of the rotation may be performed using orbital measurements or, more precisely, by direct tracking of landers on the surface of a planet. The latter will be the case with Rotation and Interior Structure Experiment (RISE) onboard the InSight Lander. Precise Doppler tracking from the Earth of a location on the Martian surface over extended periods (months to years) can be used to obtain Mars' rotation behavior. More specifically, measuring the relative position of the lander on the surface of Mars with respect to the terrestrial ground stations allows the reconstruction of Mars' time-varying orientation and rotation in space. Precession (long-term changes in the rotational orientation occurring over many tens of thousands of years) and nutations (periodic changes in the rotational orientation occurring on subannual timescales) as well as polar motion (motion of the planet's surface with respect to its rotation axis) are determined from this experiment and are used to obtain information about Mars' interior. Precession measurements improve the determination of the moment of inertia of the whole planet, which is particularly sensitive to the radius of the core. Using geochemical constraints to specify a plausible range of possible compositions, the core radius is expected to be determined with a precision of a few tens of kilometers (compared to ∼150 km currently). A precise measurement of variations in the orientation of Mars' spin axis also enables an independent (and more precise) determination of the size of the core via the core resonance in the nutation amplitudes. The amplification of this resonance depends on the size, moment of inertia, and flattening of the core. For a large core, the amplification can be very large, ensuring the detection of the free core nutation and determination of the core moment of inertia.

At the same time, measurement of variations in Mars' rotation rate reveals variations of the angular momentum due to seasonal mass transfer between the atmosphere and polar caps and zonal winds.

Investigation of the dependence of rotation variations, gravity field, and tidal variations on interior and atmosphere properties and orbital motion characteristics is essential to understand the interior and evolution of terrestrial planets. These studies include the development of advanced models of rotation, the construction of detailed models for the structure and dynamics of solid and fluid layers of the planets, the investigation of the dynamical response of these models to both internal and external forcing, the modeling of the orbital motion of large bodies of our solar system, and the inclusion of general relativistic effects into the data analysis.

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