Activity in the outer solar system

For a long time, our picture of the outer solar system was that apart from the four gas giants, it only consists of geologically dead bodies, moons, planets (=Pluto) and asteroids (like the Centaurs), cold clumps of ice and/or rock. Even if they differentiated in the past (like you could expect the largest of them, Ganymede), they are not expected to have interesting phenomena currently going on.

Only exception was Saturn's moon Titan, which in 1944 was discovered to have an atmosphere. Then in 1979, the Voyagers flew past Jupiter, and photographs of the moon Io showed not only a very young surface, and many volcanoes, but even actual plumes from volcanic eruptions!



Since then, slowly but steadily, many other objects turned out to be more interesting than ever imagined, with ongoing geologic processes, or some form of meteorology at least. Let's list them all!

Io

Io's Tvashtar volcano erupting, as seen by the New Horizons spacecraft in early 2007. The volcano itself is behind the limb, and also on the night side, sending material 330 km high and into our view, and also into the path of sunlight. The part in the shadow is only visible because of the reflected light from Jupiter.

Image sequence showing the eruption of Tvashtar. Io's dark side is illuminated by reflected sunlight from Jupiter.

Io also has a varying atmosphere, made up mostly of sulfur dioxide. It's very much dependant on the volcanoes, that supply the otherwise slowly escaping gases. Jupiter's magnetosphere sweeps up gas and dust from Io's atmosphere and they up in the radiation belts around the planet. See an article about Io's atmosphere (the "Ioan" atmosphere?).

Auroral glow on Io, the result of an energetic interaction between Io's atmosphere and Jupiter's powerful magnetic field.

Europa

Color compoiste of Europa. (c) NASA/Ted Stryk

Europa is thought to have a 10-30 km thick crust of water ice, which hides a (probably) 100 km deep ocean. Tidal heating is keeping it from freezing, and drives the geological processes. Crater counts lead to an estimate that the surface is less than 30 million years old. Some think that its long ridges are the ice based analogues of Earth's oceanic ridges, ice/water volcanoes spreading the crust.

Fractured surface. A sign of tectonism?

The closest view of Europa's surface, from Galileo

Ganymede

Global view of Ganymede

Ganymede, the largest satellite in the solar system, which is actually larger than Mercury or Pluto, shows extensive signs of tectonic processes. Its surface is a mix of two types of terrain: very old, highly cratered dark regions and somewhat younger (but still ancient) lighter regions marked with an extensive array of grooves and ridges. Ganymede is not thought to be active any more. However, it is the only moon that has magnetic field of its own. It is not fully understood yet, but it may be generated in a similar fashion to the Earth's, by movements in a molten metallic core.

Tiamat Sulcus region on Ganymede, imaged just after local sunrise.

Auroral glow on Ganymede

Callisto

Callisto from Voyager 2

Callisto has the oldest, most cratered surface of any body yet observed in the solar system; having undergone little change other than the occasional impact for 4 billion years. Callisto experiences the least tidal heating from Jupiter. However, high resolution images from Galileo show deformed craters, and a lack of small craters. Examination of the surface shows that there has been gradual slumping, or "relaxation" of the craters, and what is termed "sublimation-erosion" of the surface.

Close-up on Callisto by Galileo

See JPL news article.


Titan

To a human eye, Titan would only appear as a smooth, orange sphere, because of its thick, opaque atmosphere. Cassini is able to see through the atmosphere using a near-infrared filter.

Titan is the only moon with a real dense atmosphere. It was long theorized that methane and ethane may form clouds, fall to the surface as rain, and create rivers, lakes or oceans. The Cassini spacecraft did indeed discover what appear to be lakes, near the southern and northern polar regions. The moons surface is formed by the atmospheric processes, probably wind, rain, and maybe streams. Ice-volcanic activity is also suspected, but hasn't been confirmed yet. It looks like the basic processes are analogous to Earth's, with hydrocarbons taking the role of water, and ice/water taking the role of rock.

Titan's atmosphere is like a thick, orange smog. Due to the lower gravity, it extends much higher than Earth's. Hundreds of kilometers above the surface, a complicated structure of thin layers of haze are observed. They are composed of complex organic molecules.

These long ridges look like dunes created by wind that curve around obstructions.

Panorama of the boundary between the dark and bright areas. The mosaic was created from images taken by the Huygens probe during its decent onto Titan.

Lakes, based on radar maps (fake colors).


Rounded pebbles of about 10 cm, lying near the descended probe. They're probably made of ice


Enceladus

In 2005, the Cassini probe observed a localized atmosphere at the south pole of Enceladus, through a stellar occultation, and also from particle measurements. It also imaged a hot area, in that region, with its infrared spectrometer. Later it was able to directly observe the water vapour plumes emanating from the southern parts. (see here)

The geysers can be seen by Cassini camera (false colour image)

These stripes appear to be 10 degrees warmer than the surrounding area, as measured by Cassini

A model of the Enceladus geysers

Geysers that spew ice/water would mean that there's a source of liquid water. Is that possible at -200 °C that is the observed temperature of the surface? Well, the depths of Enceladus may be heated by remnant radioactivity or tidal heating.

However, another hypothesis was also put forward, which doesn't need a liquid layer, rather, simply the (violent) decomposition of clathrates (see article, and the original in Science).

The plumes of Enceladus are also replenishing Saturn's E Ring with tiny particles.

Dione

Color image of Dione from Cassini

Researchers are now looking for possible geysers on Saturn's smooth icy moon Dione - which appears to be a less active version of Enceladus (see article and
mission news).

Iapetus





Iapetus has an old equatorial ridge about 1,300 km long, 20 km wide and 13 km high, making the moon look somewhat walnut-shaped. Its origin is not explained yet.

Iapetus one of the weirder bodies in the Solar System. It's not just its weird equatorial ridge. It's not just that it's two-faced, with its leading hemisphere as dark as soot, and its trailing hemisphere among the brightest. It's even more intriguing that there's no grey part at all - no transition between the two sort of covering, as far as the highest resolution images show.

Is it white on black?

Is it black on white?

NASA scientists think this is a result of thermal segregation - a process in which the slightly darker ice absorbs more sunlight, thus it sublimates, only to become trapped on the colder surface of slightly brighter ice.


Dione

Color image of Dione from Cassini

Researchers are now looking for possible geysers on Saturn's smooth icy moon Dione too - which appears to be a less active version of Enceladus (see article and
mission news).


Ariel



Ariel, a satellite of Uranus, shows a young surface, with few ancient craters, and the existing ones distorted. It also has large faults, canyons, with smooth floors. It certainly underwent considerable activity in the past. A theory is that the driving force behind this was tidal heating, due to a different, more eccentric orbit in the past. Ariel has high water content, and the highest reflectivity of the Uranian moons. Can it be a result of fresh ice crystals on the surface, from cryovolcanism still going on? We don't know yet.

Other Uranian moons, Miranda and Titania also show a relatively young surface, compared with the other large moons in their neighborhood.


Triton

Neptune's largest moon, Triton has a surprisingly complex surface

Triton is Neptune's largest moon. It's the 7th largest satellite in the Solar System, with a diameter only 20% smaller than our Moon's. But it is much more alive than the Moon! It has a very young surface, it has polar caps, it has a very tenous atmosphere (estimated to have surface pressure of 0.01 mbar). A tenous atmosphere which nevertheless is enough to hold an observable cloud - which is created by cryovolcanism, as directly observed by Voyager-2!

Patches

A tenous cloud above Triton

A plume on Triton, with a 10 km high column that has a tail extending horizontally about 100 km.


Movie showing geysers on Triton


Pluto

Pluto's surface (determined from brightness variations during Pluto-Charon occultations)

Pluto's atmosphere was detected through stellar occultations. It varies considerably with time, as most of this atmosphere is thought to freeze out onto the surface as Pluto reaches its farther parts of its orbit around the Sun. However, recently, some spikes were observed in the density of the atmosphere, even though Pluto is right now receding (see article).

These polygonal structures near in the northern polar regions of Mars, are thought to be created by the movement (expansion&contraction) of an ice layer. Of course, Pluto's ice layer would be extremely thin, but given enough time, couldn't it also wear the surface?

Charon

Our best "map" of Charon

Unlike Pluto, its moon Charon has no atmosphere. However, it appears to be host to cryovolcanism, or ice geysers (see New Scientist's article, and original article). The source is probably a liquid water-ammonia layer.

Other KBO's

The presence fresh water ice crystals on the surface of Charon are inferred from its spectra. However, some of the Kuiper Belt Objects, recently discovered, appear to share this spectral feature with Charon. Its not impossible, that some of these objects also have cryovolcanic activity.