Astronomy - Saturn and its Moons

Saturn

Diameter 74,000 miles

Distance from Sun 0.89 billion miles 9.54 AU

Its polar diameter is 90% of its equatorial diameter, this is due to its low density and fast rotation.

Saturn turns on its axis once every 10 hours and 34 minutes giving it the second-shortest day of any of the solar system’s planets.

Saturn orbits the Sun once every 29.4 Earth years.

At least 62 moons are known to orbit Saturn, of which 53 are officially named.

Saturn is the sixth planet from the Sun and the most distant that can be seen with the naked eye. the other four being Mercury, Venus, Mars and Jupiter.

Saturn is the second largest planet in the Solar System, after Jupiter.

Like Jupiter, Saturn is a gas giant and is composed of similar gasses including hydrogen, helium and methane.

Saturn Earth Size Comparison

 

Saturn was known to the ancients, including the Babylonians and Far Eastern observers.

It is named after the Roman god Saturnus who was the god of agriculture. And was known to the Greeks as Cronus.

Ancient Chinese and Japanese culture designated the planet Saturn as the "earth star" (土星).

Saturn ring system was first observed in 1610 by the astronomer Galileo Galilei. He thought of them as two moons on Saturn's sides.

It was not until Christiaan Huygens used greater telescopic magnification that this notion was refuted. Huygens discovered Saturn's moon Titan

Cassini later discovered four other moons: Iapetus, Rhea, Tethys and Dione. In 1675, Cassini discovered the gap between A and B ring now known as the Cassini Division.

Four spacecraft have visited Saturn.

Pioneer 11, Voyager 1 and 2, and the Cassini-Huygens mission have all studied the planet.

Pioneer 11 made the first flyby of Saturn in September 1979, when it passed within 20,000 km of the planet's cloud tops.

Images were taken of the planet and a few of its moons, although their resolution was too low to discern surface detail. It also measured the temperature of Titan.

In November 1980, the Voyager 1 probe visited the Saturn system. It sent back the first high-resolution images of the planet, its rings and satellites. Surface features of various moons were seen for the first time.

Voyager 1 performed a close flyby of Titan, increasing knowledge of the atmosphere of the moon.

In August 1981, Voyager 2 continued the study of the Saturn system. More close-up images of Saturn's moons were acquired, as well as evidence of changes in the atmosphere and the rings.

On 1 July 2004, the Cassini–Huygens space probe performed the SOI (Saturn Orbit Insertion) manoeuvre and entered orbit around Saturn.

The orbiter completed two Titan flybys before releasing the Huygens probe on 25 December 2004.

Huygens descended onto the surface of Titan on 14 January 2005, sending a flood of data during the atmospheric descent and after the landing.

Starting in early 2005, scientists used Cassini to track lightning on Saturn. The power of the lightning is approximately 1,000 times that of lightning on Earth.

Saturn’s upper atmosphere is divided into bands of clouds.

The top layers are mostly ammonia ice. Below them, the clouds are largely water ice.

Below that are layers of cold hydrogen and sulphur ice mixtures.

Saturn has oval-shaped storms similar to Jupiter’s.

The region around its north pole has a hexagonal-shaped pattern of clouds.

Scientists think this may be a wave pattern in the upper clouds.

The planet also has a vortex over its south pole that resembles a hurricane-like storm.

Eventually, deep inside, the hydrogen becomes metallic. At the core lies a hot interior.

Saturn has the most extensive rings in the solar system.

The rings are made mostly of chunks of ice and small amounts of dust.

The rings stretch out more than 120,700 km from the planet, but are amazingly thin: only about 20 meters thick.

 

 

Saturns Moons

 

 

 

 

Titan

Largest Moon

Diameter 3,200 miles

Distance from Saturn 759,228 miles

Orbital Period 15.5 days

Titan was discovered on March 25, 1655 by the Dutch astronomer Christiaan Huygens. Huygens was inspired by Galileo's discovery of Jupiter's four largest moons in 1610 and his improvements in telescope technology.

Titan is the largest moon of Saturn. It is the only moon known to have a dense atmosphere, and the only object in space other than Earth where clear evidence of stable bodies of surface liquid has been found.

Titan is the sixth gravitationally rounded moon from Saturn. Frequently described as a planet-like moon, Titan is 50% larger than Earth's Moon, and it is 80% more massive.

It is the second-largest moon in the Solar System, after Jupiter's moon Ganymede, and is larger than the smallest planet, Mercury.

11.4 times larger in the sky of Saturn than the Moon from Earth.

Titan Earth Moon Size Comparison

Titan is primarily composed of water ice and rocky material.

Information from the Cassini–Huygens mission in 2004, including the discovery of liquid hydrocarbon lakes in Titan's polar regions.

The geologically young surface is generally smooth, with few impact craters, although mountains and several possible cryovolcanoes have been found.

Surface of Titan From Huygens Probe

Two new studies from Cornell University in  New York show that the liquid lakes and seas on Titan follow a constant elevation relative to Titan’s gravitational pull.

In other words, just as Earth’s oceans lie at an average elevation that we call sea level, so do Titan’s seas.

Its lakes and seas are filled with hydrocarbons rather than liquid water, and water ice overlain by a layer of solid organic material serves as the bedrock surrounding these lakes and seas.

The new study suggests that elevation is important because Titan’s liquid bodies appear to be connected under the surface in something akin to an aquifer system at Earth.

The atmosphere of Titan is largely nitrogen; minor components lead to the formation of methane and ethane clouds and nitrogen-rich organic smog. The climate including wind and rain creates surface features similar to those of Earth.

Such as dunes, rivers, lakes, seas and deltas, and is dominated by seasonal weather patterns as on Earth.

Rhea

Diameter 949 miles

Distance from Saturn 327,387 miles

Orbital Period 4.5 days

The second-largest moon of Saturn and the ninth-largest moon in the Solar System.

It was discovered in 1672 by Giovanni Domenico Cassini.

Rhea has a rather typical heavily cratered surface, with the exceptions of a few large fractures (wispy terrain) on the trailing hemisphere (the side facing away from the direction of motion along Rhea's orbit)

 

Rhea Earth Moon Size Comparison

Rhea is an icy body with a density of about 1.236 g/cm3. This low density indicates that it is made of ~25% rock (density ~3.25 g/cm3) and ~75% water ice (density ~0.93 g/cm3).

Although Rhea is the ninth-largest moon in The Solar System, it is only the tenth-most-massive moon.

Its surface can be divided into two geologically different areas based on crater density; the first area contains craters which are larger than 40 km in diameter, whereas the second area, in parts of the polar and equatorial regions, has only craters under that size.

This suggests that a major resurfacing event occurred some time during its formation.

Earlier it was assumed that Rhea had a rocky core in the center. However, measurements taken during a close flyby by the Cassini orbiter in 2005 cast this into doubt.

Now considered that Rhea has an almost homogeneous interior (with some compression of ice in the center).

 

Iapetus

Diameter 892 miles

Distance from Saturn 2,212,889 miles

Orbital Period 79.3 days

Iapetus is the third-largest natural satellite of Saturn, eleventh-largest in the Solar System.

Iapetus was discovered by Giovanni Domenico Cassini, in October 1671.

He had discovered it on the western side of Saturn and tried viewing it on the eastern side some months later, but was unsuccessful.

Cassini finally observed Iapetus on the eastern side in 1705 with the help of an improved telescope, finding it two magnitudes dimmer on that side.

 

Iapetus Earth Monn Size Comparison

The low density of Iapetus indicates that it is mostly composed of ice, with only a small (~20%) amount of rocky materials.

The orbit of Iapetus is somewhat unusual. Although it is Saturn's third-largest moon, it orbits much farther from Saturn than the next closest major moon, Titan. It has also the most inclined orbital plane of the regular satellites.

Unlike most of the large moons, its overall shape is neither spherical nor ellipsoid, but has a bulging waistline and squashed poles; also, its unique equatorial ridge is so high that it visibly distorts Iapetus's shape even when viewed from a distance.

These features often lead it to be characterized as walnut-shaped.

Equatorial Ridge

Dione

Diameter 697 miles

Distance from Saturn 2,212,889 miles

Orbital Period 2.7 days

It was discovered by Giovanni Domenico Cassini in 1684. It is named after the Titaness Dione of Greek mythology.

About two thirds of Dione's mass is water ice, and the remaining is a dense core, probably silicate rock.

Data gathered by Cassini Orbiter indicates that Dione has an internal liquid water ocean.

Dione Earth Moon Size Comparison

Gravity and shape data points to a 99 ± 23 km thick ice shell crust on top of a 65 ± 30 km internal liquid water global ocean.

Dione's ice shell is thought to vary in thickness by less than 5%, with the thinnest areas at the poles, where tidal heating of the crust is greatest.

Dione is very similar to Rhea. They both have similar features and varied terrain, and both have dissimilar leading and trailing hemispheres.

Dione's leading hemisphere is heavily cratered and is uniformly bright. Its trailing hemisphere, however, contains an unusual and distinctive surface feature: a network of bright ice cliffs.

The Cassini probe flyby of December 13, 2004, produced close-up images. These revealed bright ice cliffs created by tectonic fractures showing that some of them are several hundred metres high. Dione has been revealed as a world riven by enormous fractures on its trailing hemisphere.

On April 7, 2010, instruments on board the Cassini probe, which flew by Dione, detected a thin layer of molecular oxygen ions  around Dione, so thin that scientists prefer to call it an exosphere rather than a tenuous atmosphere.

Tethys

Diameter 650 miles

Distance from Saturn 183,100 miles

Orbital Period 1.89 days

Discovered by Cassini in 1684 observed using a large aerial telescope he set up on the grounds of the Paris Observatory. And is named after the titan Tethys of Greek mythology.

Tethys has a low density of 0.98 g/cm3, the lowest of all the major moons in the Solar System, indicating that it is made of water ice with just a small fraction of rock. This is confirmed by the spectroscopy of its surface, which identified water ice as the dominant surface material.

The surface of Tethys is very bright, being the second-brightest of the moons of Saturn after Enceladus, and neutral in colour.

Tethys is heavily cratered and cut by a number of large faults/graben. The largest impact crater,  is about 400 km in diameter, whereas the largest graben, is about 100 km wide and more than 2000 km long.

The surface of Tethys is one of the most reflective (at visual wavelengths) in the Solar System. The high albedo indicates that the surface of Tethys is composed of almost pure water ice with only a small amount of a dark material.

This very high albedo is the result of the sandblasting of particles from Saturn's E-ring, a faint ring composed of small, water-ice particles generated by Enceladus's south polar geysers.

The surface of Tethys has a number of large-scale features distinguished by their colour and sometimes brightness. The trailing hemisphere gets increasingly red and dark as the anti-apex of motion is approached.

The leading hemisphere also reddens slightly as the apex of the motion is approached, although without any noticeable darkening.

On the leading hemisphere of Tethys spacecraft observations have found a dark bluish band spanning 20° to the south and north from the equator.

The band has an elliptical shape getting narrower as it approaches the trailing hemisphere.

Enceladus

Diameter 310 miles

Distance from Saturn 148,000 miles

Orbital Period 1.4 days

Enceladus is the sixth-largest moon of Saturn. It is about 500 kilometres (310 mile) in diameter, about a tenth of that of Saturn's largest moon, Titan.

Enceladus is mostly covered by fresh, clean ice, making it one of the most reflective bodies of the Solar System.

 

Enceladus Earth Moon Size Comparison

Consequently, its surface temperature at noon only reaches −198 °C (−324 °F), far colder than a light-absorbing body would be.

Despite its small size, Enceladus has a wide range of surface features, ranging from old, heavily cratered regions to young, tectonically deformed terrains that formed as recently as 100 million years ago.

Enceladus was discovered on August 28, 1789, by William Herschel but little was known about it until the two Voyager spacecraft, Voyager 1 and Voyager 2, passed nearby in the early 1980s.

In 2005, the Cassini spacecraft started multiple close flybys of Enceladus, revealing its surface and environment in greater detail.

Cassini discovered water-rich plumes venting from the south polar region. Cryovolcanoes near the south pole shoot geyser-like jets of water vapor, molecular hydrogen, other volatiles, and solid material, including sodium chloride crystals and ice particles, into space.

Over 100 geysers have been identified. Some of the water vapor falls back as "snow"; the rest escapes, and supplies most of the material making up Saturn's E ring.

In 2014, NASA reported that Cassini found evidence for a large south polar subsurface ocean of liquid water with a thickness of around 10 km (6 mile).

These geyser observations, along with the finding of escaping internal heat and very few (if any) impact craters in the south polar region, show that Enceladus is currently geologically active.

Mimas

Diameter 246 miles

Distance from Saturn 115,289 miles

Orbital Period 0.94 days

Mimas was discovered by the astronomer William Herschel on 17 September 1789.

He recorded his discovery as follows: "The great light of my forty-foot [12 m] telescope was so useful that on the 17th of September, 1789, I remarked the seventh satellite, then situated at its greatest western elongation.“

It is named after Mimas, a son of Gaia in Greek mythology.

A number of features in Saturn's rings are related to resonances with Mimas. Mimas is responsible for clearing the material from the Cassini Division, the gap between Saturn's two widest rings, the A Ring and B Ring.

They orbit twice for each orbit of Mimas. The repeated pulls by Mimas on the Cassini division particles, always in the same direction in space, force them into new orbits outside the gap.

The surface area of Mimas is slightly less than the land area of Spain. The low density of Mimas, 1.15 g/cm3, indicates that it is composed mostly of water ice with only a small amount of rock.

Due to the tidal forces acting on it, Mimas is noticeably prolate; its longest axis is about 10% longer than the shortest.

Mimas's most distinctive feature is a giant impact crater 130 km (81 miles) across, named Herschel after the discoverer of Mimas. Herschel's diameter is almost a third of Mimas's own diameter; its walls are approximately 5 km (3 miles) high, parts of its floor measure 10 km (6 miles) deep, and its central peak rises 6 km (4 miles) above the crater floor.

If there were a crater of an equivalent scale on Earth (in relative size) it would be over 4,000 km (2,500 miles) in diameter, wider than Australia.

The impact that made this crater must have nearly shattered Mimas: fractures can be seen on the opposite side of Mimas that may have been created by shock waves from the impact travelling through Mimas's body.

The surface is saturated with smaller impact craters, but no others are anywhere near the size of Herschel.

Although Mimas is heavily cratered, the cratering is not uniform.

Most of the surface is covered with craters larger than 40 km (25 miles) in diameter, but in the south polar region, there are generally no craters larger than 20 km (12 miles) in diameter.