question: How do we know, that there is no solid ground on Saturn?
comment: There is a solid core in there somewhere, but it's not very big compared to the volume of the planet. We know Saturn is largely gaseous because it's less dense than water, a situation that's only possible if almost the entire volume of the planet is gas. You can measure the gravitational force of the planet from the orbits of its moons, and that gives you the mass. Divide by the volume and you get a density number that's not possible for a planet with much solid or liquid mass. Earth is more than eight times the density of Saturn.
passages: BULLET::::- Dual Technique Magnetometer (MAG):The MAG was an in situ instrument that measured the strength and direction of the magnetic field around Saturn. The magnetic fields are generated partly by the molten core at Saturn's center. Measuring the magnetic field is one of the ways to probe the core. MAG aimed to develop a three-dimensional model of Saturn's magnetosphere, and determine the magnetic state of Titan and its atmosphere, and the icy satellites and their role in the magnetosphere of Saturn.
   BULLET::::- Subsurface Radar Sounder (SRS) would image faults, stratigraphy and weathering in the upper ~100 m of the areas mapped by VenSAR, to identify structural relationships and geological history. The Principal Investigator of the Subsurface Radar Sounder is Lorenzo Bruzzone, Università di Trento, Italy.
  
  Section::::Gravity Science.
   The 2013-2022 Planetary Science Decadal Survey identified a Saturn probe mission as a high priority mission target for the NASA New Frontiers program due to the need for "in-situ" measurements to depths of 10 bars or more. The SPRITE team explains that "to develop an improved understanding of the formation, evolution, and structure of the Solar System, it is essential that the role played by the giant planets be well understood, and this cannot be accomplished without "in-situ" measurements of the composition, structure, dynamics, and processes of Saturn's atmosphere." In order to accomplish this, the mission scientists have set two main goals:
   As of 2014 the magnetosphere of Saturn has been directly explored by four spacecraft. The first mission to study the magnetosphere was Pioneer 11 in September 1979. Pioneer 11 discovered the magnetic field and made some measurements of the plasma parameters. In November 1980 and August 1981, Voyager 1–2 probes investigated the magnetosphere using an improved set of instruments. From the fly-by trajectories they measured the planetary magnetic field, plasma composition and density, high energy particle energy and spatial distribution, plasma waves and radio emissions. Cassini spacecraft was launched in 1997, and arrived in 2004, making the first measurements in more than two decades. The spacecraft continued to provide information about the magnetic field and plasma parameters of the saturnian magnetosphere until its intentional destruction on September 15, 2017.
   Earth-based radar surveys made it possible to identify some topographic patterns related to craters, and the "Venera 15" and "Venera 16" probes identified almost 150 such features of probable impact origin. Global coverage from "Magellan" subsequently made it possible to identify nearly 900 impact craters. 
   Section::::Results From Viking I Mission.:Analysis of soil.
   In May 2005, "Cassini" began a series of radio occultation experiments, to measure the size-distribution of particles in Saturn's rings, and measure the atmosphere of Saturn itself. For over four months, the craft completed orbits designed for this purpose. During these experiments, it flew behind the ring plane of Saturn, as seen from Earth, and transmitted radio waves through the particles. The radio signals received on Earth were analyzed, for frequency, phase, and power shift of the signal to determine the structure of the rings.
  
  Section::::Selected events and discoveries.:Spokes in rings verified.
   The Mariner 10 mission (1974) mapped about half the surface of Mercury. On the basis of that data, scientists have a first-order understanding of the geology and history of the planet. Mercury's surface shows intercrater plains, basins, smooth plains, craters, and tectonic features.
   Measuring the changes in acceleration experienced by spacecraft as they orbit has allowed fine details of the gravity fields of the planets to be mapped. For example, in the 1970s, the gravity field disturbances above lunar maria were measured through lunar orbiters, which led to the discovery of concentrations of mass, mascons, beneath the Imbrium, Serenitatis, Crisium, Nectaris and Humorum basins.
  
  Section::::History.
   Saturn's interior is probably composed of a core of iron–nickel and rock (silicon and oxygen compounds). This core is surrounded by a deep layer of metallic hydrogen, an intermediate layer of liquid hydrogen and liquid helium, and finally a gaseous outer layer. Saturn has a pale yellow hue due to ammonia crystals in its upper atmosphere. Electrical current within the metallic hydrogen layer is thought to give rise to Saturn's planetary magnetic field, which is weaker than Earth's, but has a magnetic moment 580 times that of Earth due to Saturn's larger size. Saturn's magnetic field strength is around one-twentieth of Jupiter's. The outer atmosphere is generally bland and lacking in contrast, although long-lived features can appear. Wind speeds on Saturn can reach , higher than on Jupiter, but not as high as those on Neptune. In January 2019, astronomers reported that a day on the planet Saturn has been determined to be  , based on studies of the planet's C Ring.
   Measuring the changes in acceleration experienced by spacecraft as they orbit has allowed fine details of the gravity fields of the planets to be mapped. For example, in the 1970s, the gravity field disturbances above lunar maria were measured through lunar orbiters, which led to the discovery of concentrations of mass, mascons, beneath the Imbrium, Serenitatis, Crisium, Nectaris and Humorum basins.
   In 1980–1981 the magnetosphere of Saturn was studied by the "Voyager" spacecraft. Up until September 2017 it was a subject of ongoing investigation by Cassini mission, which arrived in 2004 and spent over 13 years observing the planet.
  
  Section::::Discovery.
  
  Immediately after the discovery of Jupiter's decametric radio emissions in 1955, attempts were made to detect a similar emission from Saturn, but with inconclusive results. The first evidence that Saturn might have an internally generated magnetic field came in 1974, with the detection of weak radio emissions from the planet at the frequency of about 1 MHz.
   The first close-up images were received on June 12, 2004, and mission scientists immediately realized that the surface of Phoebe looks different from asteroids visited by spacecraft. Parts of the heavily cratered surface look very bright in those pictures, and it is currently believed that a large amount of water ice exists under its immediate surface.
  
  Section::::Selected events and discoveries.:Saturn rotation.
   Spacecraft magnetometer
  
  Spacecraft magnetometers are magnetometers used aboard spacecraft and satellites, mostly for scientific investigations, plus attitude sensing. Magnetometers are among the most widely used scientific instruments in exploratory and observation satellites. These instruments were instrumental in mapping the Van Allen radiation belts around Earth after its discovery by Explorer 1, and have detailed the magnetic fields of the Earth, Moon, Sun, Mars, Venus and other planets and moons. There are ongoing missions using magnetometers, including attempts to define the shape and activity of Saturn's core.
   Section::::Radar Investigations.
  
  Radar studies provide a wealth of data on elevations, slopes, textures, and material properties of the Martian surface. Mars is an inviting target for Earth-based radar investigations because of its relative proximity to Earth and its favorable orbital and rotational characteristics that allow good coverage over wide areas of the planet’s surface. Radar echoes from Mars were first obtained in the early 1960s, and the technique has been vital in finding safe terrain for Mars landers.
   Some of the satellites of Jupiter have features that may be related to plate-tectonic style deformation, although the materials and specific mechanisms may be different from plate-tectonic activity on Earth. On 8 September 2014, NASA reported finding evidence of plate tectonics on Europa, a satellite of Jupiter—the first sign of subduction activity on another world other than Earth.
  
  Titan, the largest moon of Saturn, was reported to show tectonic activity in images taken by the "Huygens" probe, which landed on Titan on January 14, 2005.
  
  Section::::Other celestial bodies (planets, moons).:Exoplanets.
   For the Jovian moons Ganymede and Europa, the existence of a sub-ice ocean is inferred from the measurements of the magnetic field of Jupiter. Since conductors moving through a magnetic field produce a counter-electromotive field, the presence of the water below the surface was deduced from the change in magnetic field as the moon passed from the northern to southern magnetic hemisphere of Jupiter.
  
  Section::::Indicators, methods of detection and confirmation.:Geological indicators.
  
  Thomas Gold has posited that many Solar System bodies could potentially hold groundwater below the surface.
   However, the way the samples were handled prohibited an exact measurement of the amount of water. But, it was around 1%. Studies with magnets aboard the landers indicated that the soil is between 3 and 7 percent magnetic materials by weight. The magnetic chemicals could be magnetite and maghemite, which could come from the weathering of basalt rock. Subsequent experiments carried out by the Mars "Spirit" rover (landed in 2004) suggest that magnetite could explain the magnetic nature of the dust and soil on Mars.
  
  Section::::Results from the "Viking 2" mission.:Search for life.
   3 May 2005 – "Cassini" begins Radio occultation experiments on Saturn's Rings, to determine ring particle size distribution, on the scale of centimetres.
   Magnetometer readings showed that the planet's magnetic field is not globally generated in the planet's core, but is localized in particular areas of the crust. New temperature data and closeup images of the Martian moon Phobos showed that its surface is composed of powdery material at least 1 metre (3 feet) thick, caused by millions of years of meteoroid impacts. Data from the spacecraft's laser altimeter gave scientists their first 3-D views of Mars' north polar ice cap.
   Mars is the fourth planet from the Sun; it is about 1/2 size of Earth with 1/3 the gravity, and orbits between about 1.3 to 1.7 Earth-Sun distances. It has an almost 25-hour day, and has a 2-year orbit of the Sun for its year. Flybys and landings to gather scientific data about Mars have been conducted since the 1960s, but quality seismological studies – which would provide detailed information about the interior of Mars – have yet to be performed.
   In June 2008, the Visual and Infrared Mapping Spectrometer on "Cassini" confirmed the presence of liquid ethane beyond doubt in Ontario Lacus. On December 21, 2008, "Cassini" passed directly over Ontario Lacus and observed specular reflection in radar. The strength of the reflection saturated the probe's receiver, indicating that the lake level did not vary by more than 3 mm (implying either that surface winds were minimal, or the lake's hydrocarbon fluid is viscous).
   BULLET::::- Determining the three-dimensional structure and dynamic behavior of the rings of Saturn.
  
  BULLET::::- Determining the composition of the satellite surfaces and the geological history of each object.
  
  BULLET::::- Determining the nature and origin of the dark material on Iapetus's leading hemisphere.
  
  BULLET::::- Measuring the three-dimensional structure and dynamic behavior of the magnetosphere.
  
  BULLET::::- Studying the dynamic behavior of Saturn's atmosphere at cloud level.
  
  BULLET::::- Studying the time variability of Titan's clouds and hazes.
  
  BULLET::::- Characterizing Titan's surface on a regional scale.
   During a press conference on March 2, 2004, mission scientists discussed their conclusions about the bedrock and the evidence for the presence of liquid water during their formation. They presented the following reasoning to explain the small, elongated voids in the rock visible on the surface and after grinding into it (see last two images below).
   On September 20, 2006, a "Cassini" probe photograph revealed a previously undiscovered planetary ring, outside the brighter main rings of Saturn and inside the G and E rings.
  
  In July 2006, "Cassini" saw the first proof of hydrocarbon lakes near Titan's north pole, which was confirmed in January 2007. In March 2007, additional images near Titan's north pole discovered hydrocarbon "seas", the largest of which is almost the size of the Caspian Sea.
  
presuppositions: There is no solid ground on Saturn. Saturn has no solid ground. 
corrections: Saturn has a small solid core. Although not big compared to the planet, Saturn does have a solid core. 
raw_presuppositions: There is no solid ground on Saturn. Saturn has no solid ground.  There is no solid ground on Saturn.
raw_corrections: Saturn has a small solid core. Although not big compared to the planet, Saturn does have a solid core.  Saturn has a small solid core.