Place around Jupiter

Io is the 3rd largest of the four Galilean moons in the Jovian system. It completes one orbit around Jupiter in 42.5 hours, fast enough to be observed in a single night. The moon is in 2:1 and 4:1 orbital resonance with Europa and Ganymede respectively, meaning it revolves twice per orbit of Europa and four times per orbit of Ganymede. Io is tidally locked with Jupiter and hence only one side of IO is facing Jupiter. 

The Orbit of Io is not completely circular due to its orbital resonance, and hence Io experiences tremendous tidal forces from its host gas giant. This bulges Io's solid surface up to 100 meters(significantly huge, compared to tidal forces on liquid water here on earth: 18 meters).

Jupiter’s magnetosphere and Io

The gas giant has an extremely powerful magnetic field around it and Io’s orbit crosses these fields in some of the strongest sections. Io acts as an electric generator releasing ions that enlarge Jupiter's magnetic field to more than twice its original size. 

The volcanic eruptions on Io create a thin atmosphere and a ‘neutral cloud’ made of sulfur, oxygen, sodium, and potassium atoms. This material floats in orbit around Jupiter and interacts with ions in the Torus shaped region of ionized particles. The particles in this plasma torus travel at speeds way faster than Io’s orbital velocity. The interaction between neutral cloud and plasma torus excites the particles to higher energy states and eventually feeding them into Jupiter’s magnetosphere. 

Surface of Io

Io has a geologically young surface constantly renewed by active volcanism depositing materials like sulfur, sulfur dioxide, and silicates. Which is a major reason why Io lacks impact craters as seen on many of the moons and planets in the solar system. The volcanic activity has resulted in numerous lava lakes and mountains on the surface. 

Sulfur and Sulfur dioxide are abundant on the surface of Io. The majority of the equatorial region of the moon is filled with these chemical compounds making it look yellowish with large patches of white and grey. Intense radiation causes stable sulfur molecules to break at polar regions, giving it reddish-brown colour. 

Key Facts

• Io is the most volcanically active object in the solar system. 

• Some of the Volcanoes on Io are so powerful that they can be seen from powerful telescopes from Earth. 

• Io has the least amount of water out of any known body in the solar system.

Place around Jupiter 

Europa is the 6th closest moon of planet Jupiter at an average distance of 5.2 astronomical units from the Sun. It orbits Jupiter every 3.5 Earth days and only one side of Europa faces the planet due to gravitational lock.

The innermost 3 satellites of the Jovian system are in an orbital resonance. For every one orbit of Ganymede, Europa Orbits twice and Io orbits four times. This also means, sometimes all these moons align, with Io and Europa aligning most frequently.  

Europa’s orbit is not exactly circular and hence, as Europa approaches closer to Jupiter, the gravitational attraction elongates the moon in the direction towards the planet and away from it. And as it reaches the farthest point from Jupiter, Europa relaxes back into a more spherical shape.  

The Surface of Europa

The icy surface of Europa is filled with long and dark ridges and cracks known as ‘Lineae’. This is believed to occur due to the active glaciers sliding on the surface. These patterns and features of shifting Icy tectonic plates are a result of Jupiter’s Gravity tugging and pulling the Europa’s Surface causing friction and melting in the ice plates. Not all surface patterns are explained due to this phenomenon, some sections of the surface have rotated at certain angles as if the surface is floating above a liquid beneath. 

A subsurface ocean? How?

Instruments onboard Galileo spacecraft during its mission, detected a magnetic field generated by Europa. This Magnetic field appeared to be shifted during subsequent flybys. Scientists explain this anomaly by understanding that the Europa behaves like a conductor with a special induced magnetic field around it. Which is believed to be caused by an electrically conductive fluid present between the crust and mantle (Salt Water). 

Galileo spacecraft was also able to fly through the plumes of water-rich material ejected from Europa’s surface. Much like Saturn’s moons: Enceladus and Triton, further strengthening the hypothesis of the subsurface ocean.

Furthermore, scientists have also been pointing Hubble Space Telescope towards the moon. It has also detected the presence of water-rich plumes at the south pole of the Europa by taking ultraviolet images of Europa as it was passing in front of Jupiter. 

Fun Facts

• Europa is 5 times more reflective than our moon due to its surface of water ice.

• Europa’s mass exceeds the combined mass of all the smaller moons in the Solar System. 

• Europa has the smoothest surface compared to any other known body in the solar system. 

• Radiation levels per day on the surface of Europa are 1800 times the average annual dose of a human here on Earth.

A bit of History 

Discovered by William Herschel on August 28, 1789, its name comes from Greek mythology(Giant Enceladus). Not a lot was known until Voyager 1 and Voyager 2 spacecraft passed nearby the planet in 1980 and 1981. Later in 2005, the Cassini spacecraft discovered the presence of water and captured some incredible pictures of the coldest satellite of Saturn.

Place around Saturn

From the total of 82 moons of Saturn, Enceladus is the 6th largest moon of the planet. 2nd closest major moon(There are some objects in Saturn’s rings which may or may not be considered as moons) to Saturn after Mimas. It orbits Saturn every 32.9 hours, fast enough to be observed in a single night. 

It is in a 2:1 orbital resonance with the moon: Dione(fourth closest major moon of Saturn). This orbital resonance means Enceladus completes 2 revolutions around Saturn for every 1 Revolution of Dione around the planet. This plays a major role in the orbit of the Enceladus, forcing it to be non-circular (eccentricity = 0.0047). This results in Tidal forces which heat up the interior of the moon.

Cryovolcanism on Enceladus

The moon is located at 9.5 Astronomical Units from our star. A major reason why water cannot exist in the liquid state on its surface. But the Tidal forces acting on the moon heats up the core, melting the innermost layer of ice and forms a subsurface ocean (Process known as ‘Tidal Heating’). This water then erupts from the weakest parts of the crust with a speed of about 1200 km/hr. This exceeds the escape velocity on Enceladus. With no gravitational influence of the moon on this material, it adds the water crystals to the E-Ring of Saturn. Some water crystals gravitate back towards the surface of Enceladus, renewing it continuously.  

During the Cassini Mission, the spacecraft was able to pass through the plumes of the volcanoes. And with the help of Cosmic Dust Analyzer on-board, it was able to detect the presence of amino acids, carbon dioxide, methane, nitrogen, and water in the plumes, contributing to the possibility of life beneath the surface of Enceladus.

Fun Facts

• The icy surface on the active south pole is constantly stretching and buckling due to gravitational forces. 

• Enceladus has the whitest, most reflective surface in the solar system.

• Enceladus sprays geysers of water ice, creating a ring of its own around Saturn(E Ring).

• Enceladus is small enough to fit into the length of the United Kingdom.

Can you imagine looking up at the sky towards a huge, most beautiful, spinning ball of gas decorated by rings of water ice and dust? This is what you will see from the surface of Saturn’s moon, Titan.