Facts About Callisto - The Second Largest Moon Of Jupiter

Callisto - Imaged by Galileo spacecraft in 2001
Callisto - Imaged by Galileo spacecraft in 2001
© wikimedia

In the Jovian system, Callisto is the second-largest moon after Ganymede. It is also known as "Jupiter IV" as it is the fourth Galilean moon of Jupiter by distance. The surface of Callisto is one of the highly cratered and oldest in the solar system. Multi-ring structures, variously shaped impact craters, chains of craters (catenae) and associated scarps, ridges and deposits are the prominent features of the Callistoan world. After its discovery in 1610, the Pioneer 10 and Pioneer 11 spacecraft in the early 1970s gave little but new information about Callisto on their way to Jupiter. Last observed by the New Horizons probe in 2007, some future missions are planned, including human-crewed missions, to explore more about Callisto and its neighbour moons.

Facts about Callisto

Callisto Profile

Name Callisto
Satellite of Jupiter
Distance from Jupiter (avg) 1,880,000 km
Radius 2,410.3 km
Volume 5.9 × 1010 km3
Mass 1.07 × 1023 kg
Surface area 7.30 million km2
Gravity 1.235 m/s2
Max Temperature -108 ℃ or -162 °F
Min Temperature -193 ℃ or -315 °F
Length of day 16.7 Earth days
Length of year 16.7 Earth days
Discovered By Galileo Galilei
Discovered On January 7, 1610

Quick Facts About Callisto

  1. Callistoan surface reflects much more sunlight than our moon, which means it is brighter than our Moon.
  2. The cratered plains are as old as our solar system, about 4.5 billion years old.
  3. Because it is very far from the Jupiter and other three Galilean moons, it is neither involved in an orbital resonance nor tidal heating. Thus, other moons contributed not much to its formation.
  4. Callisto has a little eccentricity of 0.0074. That is, its orbit is nearly circular.
  5. In 1979, the Voyager 1 and Voyager 2 flybys have given the first detailed image of Callisto, picturing more than half the surface.
  6. Later, from 1994 to 2003, Galileo spacecraft imaged the entire surface, approaching as close as 138 km to the surface.
  7. ESA's JUICE (Jupiter Icy Moon Explorer), scheduled to launch in 2022, is the next planned mission to explore the Jovian moons, including Callisto.
  8. Since it is outside Jupiter's main radiation belt and geological stable, Callisto has been chosen as the best place to establish a surface base that would produce rocket fuel to explore the Solar System further.

Detailed Facts About Callisto

  • Callisto was discovered along with the other three Jovian moons

Io, Europa, Ganymede and Callisto (left to right) - The Galilean Moons
Io, Europa, Ganymede and Callisto (left to right) - The Galilean Moons

In January 1610, Galileo Galilei, an Italian astronomer, discovered Callisto and the other three Galilean moons using a 20× magnification telescope. This discovery surprised everybody because the Moon was the only known celestial body revolving around another celestial body (Earth).

Galileo's finding also impacted religious faith because, at the time, it was believed that all the universe revolved around the earth. Galileo's observation of the Jovian moons and other solar system bodies revealed that everything in the universe did not revolve around the Earth.
  • Callisto was named after one of the lovers of Jupiter

The name "Callisto" suggested by Simon Marius, a German astronomer who was one of the first observers of the moons of Jupiter. Callisto, in Greek mythology, was a nymph or the daughter of King Lycaon. She was one of the many women who had secret love with Zeus (the Greek equivalent of Jupiter). However, till the mid 20th Century, the name was not adopted. Instead, Callisto was called "Jupiter IV", a Roman numeral designation system used by Galileo for Jupiter's moon.
  • Like our moon, Callisto is tidally locked with Jupiter

Callisto (bottom left), Jupiter (top right) and Europa (below and left of Great Red Spot) imaged by Cassini–Huygens
Callisto (bottom left), Jupiter (top right) and Europa (below and left of Great Red Spot) imaged by Cassini–Huygens

Callisto is the farthest Galilean moon from Jupiter's surface. It revolves around the giant planet at a distance of nearly 1,880,000 km or 1,168,000 miles which is 26.3 times the radius of Jupiter (71,492 km or 43,441 miles).

Callisto is tidally locked with Jupiter's gravity, meaning that it experiences a similar revolution to our moon. A person on Jupiter can only see one side of Callisto like we see the moon's same face on earth. Moreover, It completes its orbit simultaneously it completes its rotation in about 16.7 earth days.
  • Although it is the same size as Mercury but has comparatively less mass

Callisto compared to terrestrial planets and some prominent satellites of the solar system
Callisto compared to terrestrial planets and some prominent satellites of the solar system
© Flicker

With a radius of roughly 2,410 km, it is about the same size as Mercury. In terms of size, it gets its position in some of the dominant moons of the solar system. This giant moon is the third-largest moon in the solar system, second in Galilean moons and the twelfth largest celestial body in the whole solar system.

However, the density of Callisto is 1.83 g/cm³, about one-third of Mercury's density. The low density suggests that its composition is equally divided into rocky material and water ice.
  • There is an ocean lurking beneath Callisto's icy surface

Internal structure of Callisto
Internal structure of Callisto

Data from Galileo spacecraft suggests that probably Callisto is hiding a salty ocean beneath its surface. This theory was first proposed in 1988. Readings from Galileo showed that Callisto has a magnetosphere that interacts with the magnetic fields of Jupiter when it rotates.

The most plausible theory behind the origin of its magnetic fields is that there is possibly a highly conductive fluid with a thickness of at least 10 km underneath its surface. Since the icy surface is not a good conductor and the atmosphere is negligible. The chances are high that the fluid would be a salty ocean since water with salt conducts enough electricity to generate a magnetic field around Callisto.
  • There is nothing but impact craters

Gomul Catena - a chain of craters
Gomul Catena - a chain of craters

When counting craters, Callisto is on the top of the list. Calisto has the most heavily cratered surface in the entire solar system. There are no geological features like large mountains, volcanoes or other endogenic tectonic features. The surface features only contain impact craters and multi-ring structures together with associated fractures, scarps and deposits. The number of craters is very intense that any new crater will tend to erase an older one.

The diameter of craters ranges from 100 m to enormous as over 100 km, while multi-ring structures can extend over 1000 km. Minor craters between 5 - 40 km in diameters have a central peak, while peaks and dome structures are seen in craters over 60 km across. Craters have lesser depth as compared to those of our Moon.
  • Here lies the largest multi-ring impact crater of the Solar System

Valhalla - Largest multi-ring impact crater in the solar system
Valhalla - largest multi-ring impact crater in the solar system

Valhalla, a 3,800 km across multi-ringed crater, is the largest impact feature on Callisto and the solar system. It has a bright centre of 360 km in diameter, with its ringed structure extending up to about 1,900 km from the centre. The crater has been divided into three zones: the central zone, the inner ridge-and-trough zone, and the outer trough zone. The middle part has a relatively smoother surface. The name is derived from Norse mythology. In mythology, Valhalla is a hall where dead warriors were taken after the war.

Its actual age is not determined, but it is the youngest such feature among the five known multi-ring structures on Callisto. Researches suggest that 2 to 4 billion years ago, a giant impact resulted in its formation. The collision probably penetrated its surface and reached the much softer material (warm ice or liquid ocean) underlying the lithosphere. After that, the underlying soft material drifts toward the impact site to replenish the carved surface. The concentric fracturing of the surface after impact gave rise to the ringed structure.
  • Callisto has a very unstable atmosphere

When Galileo was observing Callisto, it detected a very tenuous atmosphere using its Near-Infrared Mapping Spectrometer (NIMS). The atmosphere is composed of Carbon Dioxide and has a surface pressure of approximately 7.5 picobar.

The atmosphere is highly variable that it only lasts for only four days because the ultraviolet radiation from the sun breaks the molecules into ions which subsequently swept away by Jupiter’s magnetic field. However, the slow sublimation of carbon dioxide, frozen on the icy surface, replenishes the atmosphere again.
  • Callisto might be originated from Jupiter's left

Callisto's undifferentiated body suggests that it experienced not enough heat to melt its ice at the early stage of its formation. A completely differentiated planetary body is like earth, whose denser materials sink to the centre while lighter materials come to the surface. Therefore, researchers theorised that its constitute particles slowly accumulated in a low-density Jovian subnebula (a cloud of dust left after Jupiter's formation).

The estimated timescale of slow accretion of Callisto lies then in the range of 0.1 million–10 million years, preventing melting. After that, slow convection currents helped in slow and partial separation and differentiation of rocks and ices inside Callisto.
  • Life can be found there

The essential ingredients to sustain life have been found in many solar system objects such as comets, asteroids and icy moons. It is believed that energy and liquid water are essential ingredients to support life. Therefore, it has been speculated that halophiles, organisms that grow in salt water, can be found in the salty ocean of Callisto.

However, among Galilean moons, the most favourable candidate for habitability is Europa. It also has salty water but has more energy because of the tidal force generated by Jupiter's gravity due to its proximity. Whereas on Callisto, the only sources of energy are impacts, radioactive decay and contraction.

More Callisto Facts

Those are the interesting facts about Callisto. Check out these links to explore more about Callisto:

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