Callisto

Callisto

Callisto is a large, icy, dark-colored, low-density outer moon of Jupiter that is scarred with impact craters and ejecta. It has a diameter of about 3,000 miles (4800 km), the second-largest moon of Jupiter; it is roughly the size of Mercury . The moon is probably composed of a large rocky core surrounded by water and ice, giving it a dark colour. Meteorites have punctured holes in the crust, causing water to spread over the surface and forming bright rays and rings around the crater.

Callisto was first discovered by Galileo in 1610, making it one of the Galilean Satellites . Of the 60 moons it is the 8th closest to Jupiter, with a stand-off distance of 1,070,000 km. It is the 2nd largest moon and is larger than the Earth's moon, with a diameter that is about the distance across the United States, of 4800 km (2983 miles).

 

 

Callisto has the largest-known impact crater in the Solar System, Valhalla, which has a bright patch 600 km across and rings that go out to almost 3000 km. Callisto orbits Jupiter at a mean distance of 1,170,000 miles (1,883,000 km). Its mass is 1.08x10 23 kg. It takes Callisto 400.8 hours = 16.7 days to orbit Jupiter (in a synchronous orbit). It has no atmosphere and is about the same size as Mercury . It is composed of approximately equal proportions of ice-water and rock. Its crust is very old and heavily cratered with remnant rings of enormous impact craters. The largest craters have been erased by the flow of the icy crust over geologic time. Craters and the associated concentric rings are about the only features to be found on Callisto.

Callisto is named after one of Jupiter's many lovers from Greek mythology. Read the myth by using the link below. Callisto's main characteristic is its completely cratered and ancient surface . It is considered one of the Icy Moons because it is mostly made of ice. The Galileo mission discovered that Callisto had a very thin atmosphere.

Surface of Callisto

The surface of Callisto is deeply pockmarked with craters. It looks to be perhaps the most severely cratered body in the solar system. There are also very large craters to be found there. The severity of the cratering indicates that the surface has not been changed by activities from within Callisto, or " resurfaced ". This means that Callisto, alone perhaps among the Galilean satellites, has had no evolution and is preserved, pristine, from the beginning of the solar system. This fact renders Callisto very interesting to scientists, because it represents the "starting point" that other icy bodies may have evolved from.

Very Large Impact Crater

Many examples of the differing types of surface are shown in this image. In the foreground is a huge impact crater, which extends for almost an entire hemisphere on the surface. This crater may be compared with that of Mimas . They both show evidence of large impacts which were almost enough to break the moon apart.

 

 

No signs of crustal movements have been seen on the surface of Callisto! The surface of Callisto was carefully examined for signs of faulting and fracture. These would have provided evidence of pushing and shoving of the crust of Callisto from processes inside.

Examination of the surface of Callisto shows only that there has been gradual slumping, or "relaxation" of the craters, and what is termed "sublimation-erosion" of the surface.

This type of surface is perhaps unique in the solar system. It is certainly a different type of surface that either that of Ganymede or Europa . (The other major moon of Jupiter, Io , exhibits volcanism over the entire surface.) The difference may have to do with the lack of heat in the interior of Callisto.


What causes the surface to change its appearance?

 

Over the course of time there are many things which can cause the surface of a planet to change its appearance. winds , as shown in this picture from the Martian surface. Monument Valley on Earth is an example of weather & water , which cause erosion. Another is volcanism , which pours out a new surface such as on the Moon. C ontinental drift cause slow forces of deformation like those which cause mountains to form, the crust of Ganymede is an example. Another cause is the slumping of craters, mountains and volcanoes such as on the surface of Callisto is an example.

In their earliest histories, every planet & moon was bombarded with the remains of the material which formed them. If a planet's surface does not show many craters, it means that the surface is new, and the planet has been resurfaced, perhaps by one of the processes above. If the planet's surface still shows the many craters left over from it's formation, then that surface is very old, and has not been changed by any activity.

 

Evolution of Callisto

The insides of most of the moons and planets separated while they were forming out of the primitive solar nebula.

Measurements by the Galileo spacecraft have been shown that Callisto is the same inside from the center to the surface, and did not form a core. This means that, unlike Ganymede , Callisto probably never warmed inside enough to allow for the materials to separate. Planets gain heat by many means. The fact that Callisto never warmed could mean one of many things:

  • insufficient warming from forces leading to tides
  • insufficient abundance of radioactive elements

The lack of warmth prevented any movements of the crust which would have changed the appearance of the surface over time.

Interior of Callisto

When the Galileo spacecraft flew by Callisto, it made measurements which showed that Callisto was made almost entirely of the same thing. This means that Callisto never separated into layers, but is probably formed of a unusual material which is made of rock and ice combined.

At the same time, there were some measurements that suggested that there might be a thin salty-slushy layer of watery-ice right under the solid surface of Callisto that creates an electric current. This watery-ice layer is thought to be on Ganymede and Europa as well. This is a picture that shows the interiors of Io, Europa, Ganymede and Callisto together .

Callisto's Atmosphere

The Galileo mission discovered an amazing thing: Callisto seems to have its own atmosphere, although it is very, very thin, and may come and go with time. The atmosphere is created when molecules from the magnetosphere, moving very fast, hit the surface and knock out a water molecule.

One of the instruments on the spacecraft measured the presence of an Ionosphere , a part of the atmosphere that is filled with ions. But it wasn’t always there.

Even though Ganymede and Callisto have thin atmospheres, there does not seem to be a Ganymede or Callisto torus, as there is at Europa and Io .

Possibly the most highly cratered body in the Solar System, Callisto is similar to Ganymede in its composition of water ice and rock. Unlike Ganymede, however, the satellite reveals no evidence of any internal geologic activity. Although Callisto is only the second largest of the Galilean satellites, it is still larger than the planets Mercury and Pluto. A far encounter scene from Voyager 1 shows that Callisto superficially resembles Ganymede with areas of light and dark terrain. Bright craters have ejected fresh water and ice with little rock material, and are the youngest features on the satellite. The large circular feature, named Valhalla, is the largest impact structure in the Solar System. In some ways it is similar to multi-ringed basins on the Moon and Mercury, but there is no depression in the center of the structure. The icy crust of the moon, much weaker than rock, is not able to support either mountains or depressions over long periods of time (millions of years). Elevation differences gradually adjust to the average level of the surface.

 

The Interior

In a new study, Javier Ruiz of the Universidad Computense de Madrid found that heat generated in the interior of Callisto may be trapped there, warming a subsurface ocean but not detectable on the surface.

The research, published in the July 26 issue of the journal Nature, shows that the heat would be generated radioactively, in the moon's core, and that it warms a subterranean ocean but does not radiate through Callisto's dense, thick crust of ice. The work supports previous suspicions that Callisto might harbor subsurface water.

What Khurana and others have been looking for, since the Galileo spacecraft first observed Callisto's supposed magnetic field in 1998, is an explanation for why the magnetometer noted variations in Jupiter's magnetic field whenever Callisto was observed. It turns out the moon does not have its own magnetic field, but instead it acted like an enormous electromagnet: a magnet made from a conductive material, and interacted with Jupiter's colossal magnetic field.

The only explanation that scientists could come up with was that Callisto was filled with some sort of conductor.

"Any conductor would do," explained Khurana, "However it's hard to imagine very large amounts of conductors inside the moon beside salt water."

The misunderstood moon

Several strikes were against Callisto. For one, unlike Europa, whose underground ocean is kept in a liquid state from Jupiter's gravity tugging on it, an effect called tidal heating, Callisto is only heated from its small radioactive core. Because it was more obvious Europa had an ocean, this difference made Callisto a poor candidate for water.

Also, scientists relied on evaluations of Callisto that used only average temperatures, surface thickness and ice viscosity. The results suggested water in Callisto had to be frozen, despite its ability to behave electromagnetically.

But Ruiz modeled Callisto with a wide range of these properties and found that the moon could hold water if the ice was thick enough and dense enough. Ice viscosity, or how easily the ice grains slide past each other, was one major source of variation in the modeling.

"The viscosity (of the ice) in the past was modeled as a fixed number for the whole crust. Where as in this work, it's found that the top-most layer would be stable against convection of heat, because it is so very cold and rigid. When that is taken into account, the rest of the ice would have a higher viscosity than one imagined before," said Khurana.

A higher viscosity would hold in the small amount of heat from the core and also explains Callisto's unique appearance.

David Stevenson, a professor of planetary science at the California Institute of Technology, cautions that settling on one value of ice viscosity is still just guesswork.

"The parameters of Callisto are still poorly known, and not well enough known to settle on a definitive characterization," Stevenson said.

Unlike Europa, whose ocean continually cracks and smoothes its patina, Callisto's icy terrain appears to have gone unchanged since its creation roughly four billion years ago, except for the countless dimples of meteorite impacts. This undynamic appearance enforced Callisto's reputation as an unlikely candidate for having a liquid middle.

But Ruiz's evaluation states that the surface on Callisto must be so thick, as much as 93 miles (150 km), that the minimal heat from within would never smooth out the surface.

Still, cautioned Stevenson, "This is not a problem that you can calculate and find an answer of 'yes' or 'no' to."

Stevenson said some properties that are being estimated, and which therefore leave the results lacking a definitive answer, are the grain sizes of the ice and the heat flow of the planet. Nonetheless, Stevenson believes the results are promising.

Of course, where there is water there is a possibility for life as we know it to flourish. But Khurana and Stevenson agreed that it's very unlikely that a fertile incubator lies underneath Callisto's ancient surface.

Khurana believes that because Callisto's insides are in a form of equilibrium, there will be no environmental niches like temperature gradients or reactions between water and minerals for life to colonize. Nonetheless, Ruiz's calculations allow science to expand its strict parameters to include Callisto as a fascinating satellite of Jupiter.

 

The Galilean Moons of Jupiter

Group

Name

Diameter (km)

Mass (kg)

Mean orbital
radius (km)

Orbital period

2

Io

3632

8.92×10 22

421,600

1.76 days

Europa

3138

4.8×10 22

670,900

3.55 days

Ganymede

5262

1.49×10 23

1,070,000

7.16 days

Callisto

4820

1.08×10 23

1,883,000

16.69 days

 

The largest of the four Galilean Moon’s is Ganymede. Ganymede is similar to Callisto in that it too has many markings on the surface. Many of these are craters caused by meteorites, as on Callisto. The fact that both moons are fairly large probably plays a part in this, attracting meteorites by virtue of their gravity, which otherwise would head straight for their parent planet of Jupiter. Callisto is very different in appearance and nature from its sister moons of Europa, the ice moon and Io the fiery volcanic moon.

Our Experiments

We were particularly interested in the surface of Callisto and how the markings had occurred. We researched that these craters would have been formed by meteorite collisions and set up an experiment to investigate this further using small marbles to simulate meteorites and sand to represent the surface of Callisto.

 

 

Some of our results are included below, taken from those marbles dropped directly overhead.

Height of drop Width of crater Depth of crater

10cm

2.5cm

0.5cm

20cm

3cm

0.5cm

30cm

3.5cm

0.75cm

40cm

4.5cm

0.75cm

50cm

6cm

1cm

60cm

7cm

0.25cm

70cm

7cm

1.5cm

80cm

7cm

1cm

90cm

6cm

1.75cm

100cm

5.5cm

1.5cm

We tried the experiments first of all using a range of drops – using a chute (steep and shallow) and from directly overhead. We found that only the marbles dropped from directly overhead produced the scatter patterns as observed on Callisto, converting almost all the kinetic energy to impact damage and heat. Those released with a steep angle produced ‘lop-sided’ craters, with one high-banked side at the end of a grooved run. The very shallow chute marbles left very little impact, retaining some of their kinetic energy to bounce off.

The graph shows some of the results taken from the ‘overhead’ marbles. We found that at heights above 80cm, the scatter of the crater stopped increasing. We think that, at this point, the crater was too deep for more debris material to be ejected from the crater. Instead, the displaced material remained loose inside the crater.

 

 

 

 

 

 

 

Our conclusions

We think that the amount of loose debris remaining inside the craters could give information on the presence of atmosphere and weather patterns on Callisto. The amount of expeted displaced material inside craters of a certain depth could be assessed in the future by probes. It would give information not only of composition but of possible erosion or shifting of the moon’s crust. The loose debris could also absorb substances in or around the surface of Callisto, even though not much atmosphere would be expected due to the relative small size of the moon and its subsequent low gravity. The debris inside the crater would also give important information regarding the impact material from the meteorites. We look forward to the results of the further space probes on their missions to Callisto.

 

Research information taken from:

http/galileo.jpl.nasa.gov/moons/callisto

www.solarviews.com

www.the-planet-jupiter.com

www.nssdc.gsfc.nasa.gov/photo_gallery/photogallery-callisto.html