To date, scientists have focused their investigations on single channels due to the fact that radar and spectral data have been captured only for some narrow areas of the surface below the thick nitrogen atmosphere of this mysterious moon of Saturn. This data jigsaw puzzle is being filled in through more flybys of Titan by NASA’s Cassini spacecraft. Now, for the first time, the DLR team has developed a global perspective of the deposits of liquid hydrocarbons, such as methane and ethane, and their affected forms of erosion.

In addition to Earth, Titan is the only body in the solar system where liquids directly have been proven to exist. The moon’s exceptionally thick atmosphere, where chemical reactions occur at freezing temperatures of -290° Fahrenheit (-179° Celsius), makes this second largest moon of the solar system of special interest for planetary science.

On this equatorial band, bright continent areas and extensive dune regions can be distinguished. Dark spots on the continent areas are of special interest because they are supposed to be fluvial deposits. Additional radar data show channels precisely linked to them, which are dry, canyon-like, and broadly distributed.

Towards the north pole, the picture is much richer. There is a dense network of branching, active river systems similar to those on Earth. They are visible down to small tributaries on radar images and can be seen flowing into multiple lakes. Contrastingly, hardly any channels are found at the south pole.

“The observations of the extensive river structures at the north pole have led the team to a fascinating conclusion Ᾱ there must be heavy and frequent rain of liquid hydrocarbons. The measured channels provide the first clues about the composition and relative age of different regions of Titan,” said Langhans.

As Ganymede and Io orbit Jupiter, they interact with regions of plasma and generate electromagnetic waves. These waves are projected along Jupiter’s magnetic field lines toward Jupiter’s poles, where they cause auroral bright spots. Scientists from the University of Liege in Belgium have used thousands of images taken by the Hubble Space Telescope in ultraviolet wavelengths to monitor these auroral features in unprecedented detail.

“Each of these auroral structures is telling an ongoing story about vast transfers of energy taking place far away from the planet,” said astrophysicist Denis Grodent. “By analyzing the exact locations of these features and how their shape and brightness changes as Io and Ganymede move in their orbit around Jupiter, we have created the most detailed picture to date of how Jupiter and these moons are electromagnetically interconnected.”

Uniquely among Jupiter’s moons, Ganymede has a strong enough magnetic field to carve a protective magnetic bubble within Jupiter’s powerful magnetosphere. Analysis of the Hubble images by Grodent and his colleagues has allowed them to measure accurately the size of the Ganymede auroral footprint for the first time. They have found that it is too big to be a simple projection of Ganymede’s cross-section. However, using a three-dimensional computer model to map the footprint back along the field lines, the team has found that it corresponds well with the diameter of Ganymede’s mini-magnetosphere.

In addition, the sequences of Hubble images revealed unexpected brightness variations of Ganymede’s auroral footprint at three different timescales — 100 seconds, 10 to 40 minutes, and 5 hours.

“Each of these timescales appears to refer to a specific aspect of the Ganymede-Jupiter interaction and allows us to identify possible actors of this interaction,” said Grodent. “The 5-hour variation appears to be linked to the rotational period of Jupiter’s magnetic field and the movement of Ganymede through the tilted plasma sheet that surrounds the planet. The 10-40 minute variations could be due to sudden changes in energy due to plasma being injected into the system, and the 100-second pulses may be linked to bursts of magnetic energy being suddenly released when Jupiter and Ganymede’s magnetic field lines connect. However, we are not sure at this stage.”

The team has also mapped the positions of all possible locations of the auroral footprint of Jupiter’s volcanically active moon, Io, with unprecedented accuracy. Io’s footprint consists of a series of spots and a long tail that swirls out about 19,000 miles (30,000 kilometers) in the direction of the planet’s rotation. The angle of observation in some of the Hubble images has allowed the team to measure the altitude of the tail for the first time with accuracy.

“We found that the tail is at an altitude of approximately 600 miles (900 kilometers) above Jupiter’s cloud tops,” said Bertrand Bonford. “Interestingly, although the brightness of the tail decreases as it gets further away from the main spot, the altitude remains relatively constant. We also saw spectral absorption indicating that methane is present, which is unexpected at such a high altitude.”

Io’s footprint arises as a result of the moon’s motion through a doughnut-shaped torus of charged particles, which accumulates along Io’s orbit from material ejected by its volcanoes. In this flow of particles, Io acts as a boulder in a stream, generating powerful waves that propagate toward Jupiter’s poles. These waves have the special property to project electrons in both directions along the magnetic field lines. When these electrons finally hit Jupiter’s atmosphere, they create aurora in the form of luminous spots. In addition, Io drags on the plasma, briefly slowing it down. When the plasma is reaccelerated to normal speed, it generates electric currents that form the tail.

The team’s analysis shows that the charged particles that generate Io’s auroral features have a wide range of energies, meaning some electrons penetrate deep into the atmosphere while others lose most of their energy in the upper atmosphere.

Stevenson, together with colleagues Jan Kleyna and David Jewitt, began observing comet Holmes in October 2007 soon after it was reported that the small 2.2-mile (3.6-kilometer) wide body had brightened by a million times in less than a day. They continued observing for several weeks after the outburst using the Canada-France-Hawaii Telescope in Hawaii and watched as the dust cloud ejected by the comet grew to be larger than the Sun.

The astronomers examined a sequence of images taken over 9 nights in November 2007 using a digital filter that enhances sharp discontinuities within images. The filter, called a Laplacian filter, can pick out faint small-scale features that would otherwise remain undetected against the bright background of the expanding comet. Numerous small objects moved away from the nucleus radially at speeds up to .08 mile per second (125 meters per second). These objects were too bright to simply be bare rocks, but instead were more like mini-comets creating their own dust clouds as the ice sublimated from their surfaces.

“Initially we thought this comet was unique simply because of the scale of the outburst,” said Stevenson. “But we soon realized that the aftermath of the outburst showed unusual features, such as these fast-moving fragments that have not been detected around other comets.”

While cometary outbursts are common, their causes are unknown. One possibility is that internal pressure built up as the comet moved closer to the Sun and sub-surface ices evaporated. The pressure eventually became too great and part of the surface broke away, releasing a huge cloud of dust and gas, as well as larger fragments.

Surprisingly, the solid nucleus of comet Holmes survived the outburst and continued on its orbit, seemingly unperturbed. Holmes takes approximately 6 years to circle the Sun, and it travels between the inner edge of the asteroid belt to beyond Jupiter. The comet is now moving away from the Sun, but it will return to its closest approach to the Sun in 2014, when astronomers will examine it for signs of further outbursts.

“The maps were created from measurements taken by the Mars Odyssey neutron spectrometer,” said Thomas Prettyman, Planetary Science Institute senior scientist.

The spectroscopy data, gathered during 2 martian years, allowed Prettyman and his colleagues to accurately determine the thickness of the martian ice caps over time.

The amount of carbon dioxide ice at the poles varies in response to seasonal changes in sunlight, and about 25 percent of the atmosphere is cycled through the seasonal caps, Prettyman said.

“We need a detailed understanding of the present atmosphere on Mars in order to answer fundamental questions about the planet’s climate history, including whether conditions on Mars could have been suitable for life in the distant past,” he said.

The local thickness of the polar caps depends on several factors such as the amount of solar energy absorbed by the surface and atmosphere and the flow of warm air from lower latitudes that accompanies carbon dioxide condensation at the poles, Prettyman said.

In the northern polar region, carbon dioxide deposition is skewed toward an area known as Acidalia. Thicker carbon dioxide ice in that region may be caused by frigid winds coming from Chasma Boreale, a large canyon near the martian north pole.

In the southern hemisphere, carbon dioxide ice accumulates more rapidly in a region known as the south polar residual cap, which is offset from the pole and contains perennial carbon dioxide ice deposits.

Prettyman and his colleagues concluded that the asymmetry in the south polar seasonal cap is caused primarily by variations in surface composition.

“The regions outside the residual cap consist of water ice mixed with rocks and soil that are warmed during summer,” he said. “This delays the onset of carbon dioxide ice accumulation in the fall. In addition, heat stored in water-rich regions is gradually released during fall and winter, further limiting ice accumulation.”

Prettyman and his colleagues also used neutron spectroscopy to determine how much non-condensable gas (argon and nitrogen) remains in the atmosphere at high latitudes as carbon dioxide condenses.

“We observed strong enrichment of non-condensable gas at the south pole during fall and winter,” Prettyman said. “The observed time variation of the concentration of nitrogen and argon provides information about local circulation patterns. This includes the timing and strength of the polar winter vortex, a large-scale, cyclonic flow that inhibits mixing of nitrogen and argon with air from lower latitudes.”

Accurate data on the thickness of carbon dioxide ice and its distribution, as well as data on seasonal concentrations of non-condensable gases, will allow scientists to refine martian general circulation models. This will give them a deeper understanding of atmospheric dynamics and the planet’s climate change over time.

Neutron spectroscopy is key to determining carbon dioxide ice thickness during the long polar nights. “Unlike optical techniques, neutron spectroscopy does not require sunlight and can peer into the darkness, revealing details of the seasonal caps that have never been observed before,” he said.

The current thunderstorm on Saturn is the ninth that has been measured since Cassini swung into orbit around Saturn July 2004. Lightning discharges in Saturn’s atmosphere emit very powerful radio waves that are measured by the antennas and receivers of the Cassini Radio and Plasma Wave Science (RPWS) instrument. The radio waves are about 10,000 times stronger than their terrestrial counterparts and originate from huge thunderstorms in Saturn’s atmosphere with diameters around 1,900 miles (3,000 kilometers).

“These lightning storms are not only astonishing for their power and longevity, but also the radio waves they emit are useful for studying Saturn’s ionosphere, the charged layer that surrounds the planet a few thousand miles above the cloud tops,” said Georg Fischer of the Austrian Academy of Sciences. “The radio waves have to cross the ionosphere to get to Cassini and thereby act as a natural tool to probe the structure of the layer and the levels of ionization in different regions.”

Observations of Saturn lightning using the Cassini RPWS instrument are being carried out by an international team of scientists from Austria, the United States, and France. Results have confirmed previous studies of the Voyager spacecraft, indicating that levels of ionization are approximately 100 times higher on the day-side than the night-side of Saturn’s ionosphere.

Lightning storms on Saturn usually occur in a region nicknamed “Storm Alley,” which lies 35° south of Saturn’s equator.

“The reason why we see lightning in this peculiar location is not completely clear,” Fischer said. “It could be that this latitude is one of the few places in Saturn’s atmosphere that allow large-scale vertical convection of water clouds, which is necessary for thunderstorms to develop. However, it may be a seasonal effect. Voyager observed lightning storms near the equator, so now that Saturn has passed its equinox August 11, we may see the storms move back to equatorial latitudes.”

Saturn’s role as the source of lightning was given added confirmation during Cassini’s last close flyby of Titan August 25. During the half hour that Cassini’s view of Saturn was obscured by Titan, no lightning was observed.

“Although we know from Cassini images where Saturn lightning comes from, this unique event was another nice proof for their origin,” Fischer said.

There are only a handful of known comets where this phenomenon of temporary satellite capture has occurred, and the capture duration in the case of Kushida-Muramatsu, which orbited Jupiter between 1949 and 1961, is the third longest.

An international team led by Katsuhito Ohtsuka modeled the trajectories of 18 “quasi-Hilda comets,” objects with the potential to go through a temporary satellite capture by Jupiter that results in them either leaving or joining the “Hilda” group of objects in the asteroid belt. Most of the cases of temporary capture were flybys where the comets did not complete a full orbit. However, Ohtsuka’s team used recent observations tracking Kushida-Muramatsu over 9 years to calculate hundreds of possible orbital paths for the comet over the previous century. In all scenarios, Kushida-Muramatsu completed two full revolutions of Jupiter, making it only the fifth captured orbiter to be identified.

“Our results demonstrate some of the routes taken by cometary bodies through interplanetary space that can allow them to either enter or escape situations where they are in orbit around the planet Jupiter,” said Asher.

Asteroids and comets can sometimes be distorted or fragmented by tidal effects induced by the gravitational field of a capturing planet or may even impact with the planet. The most famous victim of both these effects was comet D/1993 F2 (Shoemaker-Levy 9), which was torn apart on passing close to Jupiter and whose fragments then collided with that planet in 1994. Previous computational studies have shown that Shoemaker-Levy 9 may well have been a quasi-Hilda comet before Jupiter captured it.

“Fortunately for us, Jupiter, as the most massive planet with the greatest gravity, sucks objects towards it more readily than other planets, and we expect to observe large impacts there more often than on Earth. Comet Kushida-Muramatsu has escaped from the giant planet and will avoid the fate of Shoemaker-Levy 9 for the foreseeable future,” said Asher.

The object that impacted Jupiter this July, causing the new dark spot discovered by Australian amateur astronomer Anthony Wesley, may also have been a member of this class, even if it did not suffer tidal disruption like Shoemaker-Levy.

“Our work has become very topical again with the discovery this July of an expanding debris plume, created by the dust from the colliding object, which is the evident signature of an impact. The results of our study suggest that impacts on Jupiter and temporary satellite capture events may happen more frequently than we previously expected,” said Asher.

The team has also confirmed a future moon of Jupiter. Comet 111P/Helin-Roman-Crockett, which has already orbited Jupiter three times between 1967 and 1985, is due to complete six laps of the giant planet between 2068 and 2086.