What is an inner Moon?
An inner moon, such as the Tethys moon, is also referred to as an inner natural satellite.
When astronomical bodies such as moons and satellites orbit a planet they do so in one of two directions, known as:
- Prograde/direct motion – is an orbit in the same direction as the rotation of the primary object
- Retrograde motion – is an orbit in the opposite direction as the rotation of the primary or central object
An inner moon, or inner natural satellite, follows a prograde low inclination orbit, inwards of the outer and larger satellites orbiting the parent body.
These inner satellites/moons differ from other satellites as they are located closer to their parent astronomical body. They are usually smaller with a low density, and take a shorter time to orbit their Planet than regular satellites.
The Four Giant Planets
There are four giant planets in our solar system: Jupiter, Neptune, Uranus and Saturn.
Each giant Planet has inner moons orbiting them.
There are 30 inner moons/satellites currently identified with these 4 Planets. They were formed at the same time as their Planet.
They are small and difficult to see by naked eye from Earth as they suffer from glare from their Planet.
Each inner moon of Saturn has been observed by the NASA Cassini mission and discovered that some are icy moons constantly having small particles from Saturn’s rings falling onto them, ranging from ice particles to ice grains.
The inner moons are all different differing in size, characteristics and shape. Their organic molecules content determines their color.
Generally the innermost moons contain iron and red organic molecules and material are redder in color than the outer ones.
The outermost of the ‘inner icy moons’, are blue in color because they have an icy crust and icy surface due to the water ice and water vapor from the liquid plumes that spew out of their larger neighboring moon Enceladus.
Saturn’s Moons and Rings:
Saturn is the second largest Planet in our solar system, after Jupiter, and the 6th furthest from the Sun.
It is probably mostly recognized as the large Red Planet with rings – Saturn’s Rings. There are 14 sub-divisions of its rings with the widest ring, the B ring, measuring 25,500 km.
There is not just one moon of Saturn, it has a total of 62 moons in orbit around it and most are fairly smallwith a low density. Of the 62 moons orbiting Saturn and of those 53 are named and of those, 24 are regular satellites with a prograde orbit.
The 24 regular satellites include the 7 major moons, another 4 smaller moons that exist in a Trojan orbit with the bigger moons, 2 co-orbital moons that guard Saturn’s F Ring, and the others are located around the other Rings such as the G Ring between Mimas and Enceladus.
The 7 major moons closest to Saturn are:
- Mimas, Enceladus, Tethys, Dione, Rhea, Iapetus, Hyperion and Titan (the largest moon of Saturn)
FACT: Many Space objects have been named after the Titans but just who were the Titans? In Greek Mythology the Titans were the race of immortals that ruled the universe, before the rise of Olympians, with Cronus as King, and Rhea as Queen. Cronus (or Kronus) was the father of Zeus.
Let’s take a look at Saturn’s Moon Tethys
Saturn’s Moon Tethys is one of the many Planets and Moons named after a Giant from Greek or Roman Mythology.
Tethys is a mid-sized moon and ranks as the 5th largest of the 7 major Saturnian moons, and is around 20% of the size of Titan (the largest of the seven)
It is the 16th largest moon overall in the Solar System and is more massive than the combined size of all the other known moons that are smaller that it.
Who discovered Tethys and the other icy moons of Saturn?
Tethys, also referred to as Saturn III, was one of the 4 moons of Saturn discovered in 1684 by the Italian-French mathematician and astronomer Giovanni Cassini.
Giovanni Cassini also compiled solar tables and data about the moons of Jupiter and used those calculations to more accurately measure the circumference of Planet Earth. He also attempted to measure the size of the Solar System by comparing distances between Mars and Earth.
In 1669, King Louis XIV of France appointed Cassini as Director of The Paris Observatory and to serve as his court astronomer/astrologer.
Cassini continued to balance his interest in mathematics and astronomy. In 1684 he discovered a pair of moons orbiting Saturn – they were named Iapetus and Rhea. He then discovered another 2 moons of Saturn and called them Tethys and Dione in honor of the Titans.
He collectively named these 4 moons ‘Sidera Lodoicea’, meaning the ‘Stars of Louis’. This was in recognition of his King and employer, King Louis XIV of France.
Neighboring ice moons Mimas and Enceladus were discovered by William Herschel in 1789.
It was NASA’s Cassini spacecraft mission that brought us images in graphic detail of these moons from its close flybys of these ice moons in 2005.
Tethys has a low density, suggests it is composed of water ice like its neighboring moons, Rhea and Dione.
How were the inner moons of Saturn discovered?
Saturn’s moon Tethys, also known as Saturn III, and the other inner moons of Saturn have been observed and photographed in detail over the years by NASA spacecraft flybys missions by ‘Voyager’, (Voyager 1, Voyager 2), and NASA’s Cassini spacecraft.
FACT: E.E. Barnard discovered the very first inner moon/satellite in 1892. It orbits the Planet Jupiter and is called Amalthea.
In 1966 the Saturnian moons – Janus and Epimetheus – were first observed. They are interesting as they share the same orbit, which initially caused come confusion. Could there really be two moons in the same field?
We are grateful to NASA for its variety of inner moon images, which helped clarify the positions and movement of these Saturnian moons.
Future missions by NASA and Planetary Science institutes
Future NASA missions and planetary science research over the next 20 years is likely to target the Ocean Worlds of the Solar System.
This could be through a combination of robotic missions and laboratory simulation attempting to investigate and even sample the complex organics, surface features, and cratering of the ocean world.
Tethys is a heavily cratered moon with a large number of fissures or fault lines over its surface. The largest surface fault on Tethys is called Ithaca Chasma.
Ithaca Chasma is around 1243 miles (2000km) in length and around 249 miles (400 km) wide. Bodies like the Space Science Institute (SSI) and the Planetary Science Institute (PSI) in the meantime will attempt to simulate activity on and beneath the surface of these moons.
Several moons are popular for further investigation, especially Europa, a moon of Jupiter, and Enceladus, a moon of Saturn, especially as it is thought that life could exist in a warm liquid water interior ocean like the Enceladus Ocean.
The success of these NASA space missions requires years of planning and research before launch. A recent study in the Icarus scientific journal addressed the composition of planetary surface features and how through simulation based on remote photo polar metric observations, it can appear real before it happens.
The study’s lead author, Robert M. Nelson is a senior scientist the Planetary Science Institute (PSI) and scientists from JPI and Caltech were a co-author of the study.
The success of these missions may be affected by the very low-density surfaces, which warns that these icy surfaces may be too soft to land on.
Cassini spacecraft research missions
As some of the inner moons of Saturn are not visible by naked eye we rely on observations from various Nasa spacecraft missions for details.
The close proximity of Tethys to Saturn and its rings makes it difficult to spot from Earth. Even though it is the brightest object in the Solar System.
It was Pioneer 11 that carried out the first flyby of Saturn in 1979 and then Voyager completed another flyby. This provided further detailed photos of the Planet Saturn’s orbiters such as Saturn’s Moon Tethys.
NASA has been instrumental in completing several successful missions into Space to observe the Planet Saturn, Saturn’s rings and Saturn’s moons.
Images have been captured and shared by Cassini, JPL and the Space Science Institute (SSI)
FACT: NASA aims to explore, discover and expand our knowledge for the benefit of mankind. As well as facilitating spacecraft missions, via Voyager 1 and Voyager 2 and the Cassini mission Spacecraft, it also has a National Laboratory in Space.
What does this Saturn Moon Tethys look like?
The Surface Features of Tethys
Saturn’s Moon Tethys is an irregular shaped, cold and airless cratered moon.
It looks like an unusual eyeball due to the location of a huge impact crater called Odysseus, and a network of central peaks, on its surface.
It was the impact of a massive collision that shaped the surface appearance of this moon. This impact formed the Odysseus crater and its rebound impact is likely to have caused the central peak shapes in the center of the crater too.
There is no geological activity on Tethys.
Tethys is one of Saturn´s larger icy moons, sometimes presenting with a bright and colored cratered surface. The trailing hemisphere of Tethys becomes darker and redder as the anti-apex of motion is approached.
The moon surface streaked areas are sometimes described as red arcs.
FACT: Anti-apex is the point opposite the apex of the Sun’s motion in Space
Brightness and Color – the Albedo Scale
FACT: Albedo is the measure of the diffuse reflection of solar radiation, out of the total solar radiation. The Albedo scale in visible light ranges from:
- 0 – a low albedo representing a black colored object like charcoal
- 1 – a high albedo representing a white colored object like snow or ice particles
Ocean surfaces and forests have low albedo, whereas an area of desert has a high albedo.
The Tethys Moon is highly reflective with a visual albedo of 1.229
Cratering and Fissures
The crater Odysseus is the largest on Saturn’s moon Tethys and covers around 40% of this moon’s diameter. Odysseus is one of the largest known impact craters in the Solar System and measures 277 miles across (445 kilometers).
This massive crater earns it the name ‘the Death Star Moon’. (Made famous after being used as the name of the Space Station in the Star Wars Movies).
Another fascinating surface feature is the major fissure that wraps around Tethys’ circumference. Tethys’ circumference is a massive 3,349 km.
The surface of Tethys has two different types of terrain:
- The ancient region is densely packed with craters
- The less ancient areas are darker with less cratering, but with fissures and fault lines.
How does a Moon get its name?
Giovanni Domenico Cassini discovered and named the icy moon Tethys. This moon like many other inner moons of Saturn was named after a Titan from Greek Mythology.
Tethys, from Greek Mythology, was a Titan and daughter of Uranus and Gaia. Her other siblings, who now share their names with other moons are:
- Titan, Crius, Cronus, Coeus, Hyperion, Lapetus, Mnemosyne, Oceanus, Phoebe, Rhea, Theia and Themis
It was Giovanni Domenico Cassini, in 1675, who discovered that ‘the Ring of Saturn’ was not one single ring, but rather it was several smaller rings.
The rings are named from A to G, in the order in which they were discovered. They are not solid rings but are composed of ice particles and water ice, with gaps in between them.
The distance between the gaps is not equal in each ring they vary considerably.
The closest ring to Planet Saturn is the D Ring, then C Ring,B Ring, followed by A Ring and then F Ring, and finally the two fainter rings , G Ring and E Ring.
Saturn’s E Ring is the outermost ring of the major rings.
The Moon Tethys orbits within the densest area of Saturn’s E Ring, located between the Moons Enceladus and Dione.
The Cassini Division
The largest gap between the rings is the gap between the ‘B ring’ and the ‘A ring’, with a distance of approximately 4,800 kilometers in between them.
This gap is known as the ‘Cassini Division’.
Physical Characteristics of the Tethys Moon
Saturn III ‘s orbital distance from Saturn is 183093 miles (294,660 km)
A cross section of the moon Tethys would show a rocky core in the center, then a global ocean and an ice crust on the surface
The icy surface of Tethys has a temperature of around -187 degrees Centigrade (-307 degrees Fahrenheit).
This image of the highly reflective surface of this icy moon Tethys has been captured many times by NASAs Cassini Spacecraft. Then reported back to their scientists at the Southwest research Institute (SwRI).
Like many of its neighboring moons, of Saturn, Tethys is also bombarded by ice particles spewing forcefully and travelling in their direction from the geysers on the icy active moon Enceladus.
The Tethys Moon is 5th largest of the moons orbiting Saturn and the 16th largest in the whole Solar System with a radius of 330 miles (531km)
Tethys is a very bright and reflective moon as its surface is primarily covered in water ice and this becomes reflective when sunlight makes contact.
Most of the larger moons (also known as satellites) of Saturn keep one side facing Saturn as they rotate within their orbital period.
The average orbit distance from Tethys to the center of Saturn is around 295,000km. This is the equivalent of around 4.4 Saturn’s Radii. The orbital period takes the equivalent of 1.89 Earth days (45.4 hours approximately to orbit round Saturn)
The orbital inclination is about 1%.
Tethys orbit is deep within the magnetosphere of Saturn, and this means the plasma co-rotating with Saturn strikes the trailing hemisphere of Tethys.
Electrons and ions, in the magnetosphere, constantly bombard the Moon Tethys. It is also affected by tidal and rotational forces, which affect its eccentricity.
Tethys has two co-orbital moons, they are called Calypso and Telesto and orbit close to the Lagrange Points (or lagrangian Points) of Tethys.
The Lagrange points for Calypso and Telesto’s orbit are 60 degrees ahead and 60 degrees behind Tethys in the same direction.
FACT: The Langrangian Points are special places in the Universe where the forces of gravity are balanced. They are sometimes also referred to as the liberation points or L-Points.
FACT: Eccentricity is the measure of the deviation of the Earth’s orbit from a circular object, using the scale where 0 is a circular orbit, anything from 0 to1, is elliptical to highly elliptical orbit (HEO), and 1 and over is parabolic.
For Tethys its eccentricity of orbit is almost negligible, meaning it is closer to an elliptical orbit.
This is more of a distorted triaxial ellipsoid shape than the round shape we usually associate with a Moon.
Tethys has a circumference of 3349 km.
Tethys is one of the better known inner moons of Saturn and with a diameter of around 659 miles (1060 kilometers).
Tethys Moon is only around 0.083 of the size of Earth.
Many of our images showing the detail of the surface, shape and possible density of Tethys is thanks to the efforts of the NASA Cassini spacecraft, travelling quite close to Tethys on its 2005 flyby mission
This is an icy moon, that’s shiny and patterned with craters and unusual streaks on the surface that looks like red arcs. These streaks are only a few miles wide, which is narrow in Space terms and they are several hundred miles long.
The surface of Tethys looks like it has been cracked and pitted. The surface is mainly covered with fresh ice particles, which makes it highly reflective due to light bouncing off it.
Tethys is one of the most reflective of all of the Saturnian moons as it reflects the Sun’s light. The appearance of this moon when captured by NASA looks like it has red arcs on the surface and a huge crater. It also has the largest valleys in the solar system created from water ice.
Tethys has an orbital inclination of 1% which means that it is locked in an inclination resonance with another of Saturn’s Moons Mimas, although it does not cause any notable orbital eccentricity or even any tidal heating. For this reason we now know that surface of Tethys is not geologically active.
FACT: Geological activity is the movement of tectonic plates beneath the surface of a Planet or Moon. For geological activity to occur an energy source, namely heat is required. It is the heat that causes the tectonic plates to shift.
The driving force behind the geological activity on a moon is still somewhat unclear, other than it certainly involves a heat source.
NASA’s Cassini-Huygens spacecraft during its flybys in 2005 has captured detailed images of many different moons during the same mission. It captured Tethys in a flyby approach this to within an altitude distance of 1500 km, above the surface of this moon.
The surface of Tethys and other Saturnian moons is a mix of ice and impurities, although some display a hint of color like Tethys. The main feature on the western hemisphere of Tethys is the large shallow crater Odysseus, measuring 400 km.
This moon has a lowest density of all the major moons in the Solar System measuring a density of 0.98 g/centimeters cubed (less than water which is 1gm/cm3) meaning it is almost completely composed of water ice. If it were located closer to the Sun it would melt.
Tethys remains as a triaxial ellipsoid shape due to tidal and rotational forces. It is likely that it has a homogenous interior but no subsurface ocean.
Notable Features of Tethys Moon
Tethys is an icy moon and has one of the brightest and most reflective surfaces of all bodies in the Solar System. It is the third furthest large moon from the Planet Saturn.
Perhaps its most notable feature is its large impact crater and red arcs on the surface.
Tethys orbits Saturn within the densest part of the E Ring, the outermost of Saturn’s rings in a prograde/direct motion direction following in thedirection of the rotation of Saturn.
It orbits at a distance of 147,886miles (238,000 kilometers) from the center of Saturn, at an average speed of 25km/s.
This orbit of Saturn takes less than 24 hours and during this time it is facing the direction of the Sun the whole time.
This orbit position is described as being tidally locked to the Planet.
The escape velocity
The escape velocity of a space object is the speed it needs to be travelling at in order to break away from the gravity of its main object (for example Saturn).
Basically the larger the object the faster it has to travel to break free. For Tethys, the escape velocity is 0.394 km/s.
Orbital resonance occurs when orbiting astronomical bodies (usually a pair) bring regular, periodic gravitational influence to each other.
The mass of Tethys is estimated at 6.22e20 kg. (6.17 x 1020kg) which is the equivalent of 0.000103 of the mass of Planet Earth
It has a mass of less than 1% of Earth
It is believed that Tethys is almost entirely composed of water ice and not certain if it has a rocky core and ice mantel.
Tethys is estimated to be a few billion years old, up to 4.5 billion years of age, possible as old as the moon of Planet Earth.
The Geography of this Moon
This moon is unusual as it is almost entirely composed of ice, its covered in craters and fractured terrains, of all shapes and sized and not uniform at all. It is bright reflecting sunlight on its icy surface. It thought to have five different types of terrain:
- Densely cratered terrain, lightly cratered, young smooth terrain, linear cracks and trenches, and bright ice patches.
It is the 3rd closest innermost moon to Saturn, and Saturn’s 5th largest moon.
Tethys has a huge trench on the surface, which is estimated to be around 40 miles wide (65km) and runs to a depth of several kilometers. This is called Ithaca Chasma.
It also has a dark bluish belt that runs across the surface. Although the reason for this is still unknown, it could be something connected to ice in its small craters.
The darkening or coloration on this moon’s surface is typical for Saturnian mid-sized satellites.
Observations – what’s close, what’s passing by, what’s changing?
The Moons of Saturn are all quite different, in size and shape depending on their own gravity.
Tethys is tidally locked with its parent Planet Saturn and therefore the same side always faces Saturn during orbit.
- Saturn is sometimes referred to as ‘The Jewel of the Solar System’.
- The low temperature at the point where Saturn is in the Solar nebula suggests that water ice was the main solid component from which all these moon were formed
- It is believed that Saturn and Jupiter are relatives, as they are very similar in their atmospheric composition
- Planetary Science is fascinated in the idea of finding alien life on another planet or moon, so where there is a confirmed water source and atmosphere we are particularly interested in any investigations
- Saturn has a number of moons not yet officially confirmed as moons
- Saturn is the least dense and flattest planet in the Solar System
- Earth passes through the Saturn ring plane approximately every 13 to 15 years
- The Tethys moon is only around 1% of the size of Planet Earth
- Tethys is said to resemble a giant eyeball
- The Odysseus crater was named after the Greek King in the novels by Homer “The Iliad” and “The Odyssey”.
- Saturn has 56 confirmed Moons and the other 3 Moons, often associated with Saturn, are not yet confirmed as Moons. For this reason Saturn is sometimes referred to as the ‘Moon King’
- The Planet Saturn is tilted
- NASA’s Cassini spacecraft was in the Saturnian system from 2004 to 2017
- Four Giants: https://www.worldatlas.com/r/w960-q80/upload/30/a0/3b/shutterstock-1029243295.jpg
- tethys Moon : By NASA/JPL-Caltech/Space Science Institute – https://commons.wikimedia.org/w/index.php?curid=40829129
- Tethys Surface: https://www.nasa.gov/sites/default/files/styles/full_width_feature/public/thumbnails/image/pia19637_main.jpg
- giovanni domenico cassini: Public Domain, https://commons.wikimedia.org/w/index.php?curid=44215
- saturns ring: By NASA / JPL-Caltech / Space Science Institute – https://commons.wikimedia.org/w/index.php?curid=29592312
- cassini division: https://starchild.gsfc.nasa.gov/docs/StarChild/solar_system_level2/cassini_division.html
- cassini space craft: By NASA/JPL – https://commons.wikimedia.org/w/index.php?curid=626636