Saturn's moon Titan: streams and lakes of liquid hydrocarbons

30 July 2008

DLR scientists publish Nature paper

Until now, the Earth was considered to be the only place in the solar system where it rains and where precipitation feeds streams which eventually drain into standing bodies of water. But using the Cassini space probe, a group of researchers which includes scientists of the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; DLR) have now discovered a lake on Saturn's moon Titan.

They have also discovered that it rains on Titan as well, and that the precipitation feeds streams which flow across the surface and which, through erosion, cut valleys deep into the icy landscape. "We are quite certain that there is at least one large liquid-filled lake on Titan", stated Professor Ralf Jaumann of the DLR Institute of Planetary Research (DLR-Institut für Planetenforschung) in Berlin. "The measurements carried out with the VIMS spectrometer on board the Cassini space probe are all point in the same direction: Close to Titan's south polar region, we have discovered a lake filled with liquid ethane: The lake contains natural gas in a more or less liquid state."

It is likely that the ethane is mixed with other liquids, such as methane or other light hydrocarbons from the alkane series. The newly-discovered lake is called Ontario Lacus, in reference to the 300-kilometre long Lake Ontario near Niagara Falls on the US-Canadian border, because of the similarity in shape between the two.

In a second research project, led by Professor Jaumann, researchers investigated widely branched valleys in other areas of Titan. "These can only have been created under the influence of stream erosion", according to the Berlin-based planetary researcher, who is also prominently involved in the research into Ontario Lacus. This "river network" set in a hilly landscape on Titan was first discovered on images recorded by the Huygens atmospheric entry probe, which was carried to Saturn by Cassini and which on 14 January descended through Titan's atmosphere before securely landing on its surface.

Scientists had already suspected for quite some time that liquid methane, ethane, or other light hydrocarbon compounds exist on Titan, whose dense atmosphere makes it impossible to get a direct view of the surface. Titan's atmosphere is in fact so dense that only a blurred image of the moon's icy surface can be obtained using a regular camera. Only through so-called "atmospheric windows", in very specific, narrowly limited near-infrared wavelengths, can one see details of Titan's landscape.

Valleys carrying almost as much liquid as the Rhine

"In the meantime, we have discovered such valley systems in many other places on Titan by using the spectrometer and the radar instrument", Mr Jaumann explains. "Our analyses show that at least from time to time, considerable amounts of liquids must have flowed through these valleys". Adjusted for the conditions on Titan, up to 1 600 cubic metres of liquid per second could have flowed through the rifts which amounts to about two thirds of the amount of water flowing from the Rhine into the North Sea.

There are only two possible sources for the liquid hydrocarbons: Either it rains methane and ethane from Titan's atmosphere, or volcanic heat from the moon's interior pushes the hydrocarbons, which are liquid at 180 degrees below zero, from the inside out onto the surface. "We are inclined to consider precipitation as the most likely source of the liquids, as the branched valley systems extend over wide areas and do not originate from single sources, but the question can only be answered conclusively by further measurements, which we have planned during the next two years".

First indications for the presence of lakes and streams on Titan.

Measurements carried out by the VIMS spectrometer (Visual and Infrared Mapping Spectrometer), an imaging spectrometer for visible light and wavelengths of the near-infrared spectrum which is carried on board Cassini, provided the scientists with evidence that Ontario Lacus does indeed contain liquid. The VIMS team, of which Ralf Jaumann is also a member, will publish its findings in the current issue of the scientific journal Nature, as well as in the technical journal Icarus.

This marks a significant milestone for research into Titan and the Saturn system: Even before the Cassini mission, there were speculations about the existence of a whole ocean of methane or ethane on Titan, but neither the measurements obtained so far by Cassini from its orbit around Saturn, nor the experiments carried out by the Huygens atmospheric probe, which on 14 January 2005 landed on Titan's icy surface after separating from Cassini, provided conclusive evidence of the existence of such an ocean.

The assumption that there may nevertheless be at least isolated standing bodies of water on Titan dates from mid-2005, when the Cassini camera system recorded parts of the southern hemisphere using a near-infrared filter. The resulting images showed Ontario Lacus as a flat surface hardly reflecting any light. In the high northern latitudes, Cassini's radar later discovered many large, remarkably smooth surfaces resembling lakes.

Just as in the case of Ontario Lacus, the researchers established that this northern 'lake district' has a quite low reflectivity. At the time, the area was also investigated using VIMS. However, as the atmosphere is denser in those parts, the measurements were very 'noisy' and their resolution was too low, so they did not lead to more detailed insights. Late next year, when spring arrives in Titan’s northern hemisphere, Cassini's orbit and the conditions for more precise observations using VIMS will be much more favourable. This will perhaps allow scientists to obtain evidence for the assumption that these surfaces are actually lakes.

Titan is one of the most exciting "test labs for prehistory" in the solar system

With its diameter of 5 150 kilometres, Titan is the second largest moon in the solar system – and the only one with an atmosphere of any significance. Titan’s atmosphere consists of about 95 percent nitrogen, with the remainder consisting mainly of trace amounts of methane and ethane. In this it resembles the Earth’s atmosphere as it was before it started to change under the influence of the evolution of life, three to four billion years ago. The existence of organic molecules, i.e. hydrocarbon compounds, in Titan’s atmosphere makes it an exciting laboratory for studying the chemical processes which occurred on Earth three to four billion years ago. Some researchers even think that there may be a layer of meltwater under Titan’s icy crust, an ocean several kilometres below the surface. All this makes this satellite of Saturn one of the most interesting subjects for research into our solar system.

The Cassini-Huygens mission is a joint project of NASA, the European Space Agency ESA and the Italian Space Agency ASI. The Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena (California), manages the mission for NASA. Several DLR scientists are involved in the mission. DLR Space Agency (DLR Raumfahrt-Agentur) supports the mission through ESA, and it supports scientists from German universities and from the Max Planck Society (Max-Planck-Gesellschaft) who are involved in the mission. Very recently, on 1 July, the Cassini mission was extended by another two years. Until mid-2010, the probe will circle around Saturn 60 more times, and it will perform 26 close flybys of Titan during this period.

Last modified: 09/05/2011 13:51:00

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Branched system of drainage channels at the Huygens landing site

Verzweigtes System von Abflusskanälen an der Huygens%2dLandestelle

During the slow descent through the atmosphere of Saturn, the Huygens landing probe from the European Space Agency (ESA) that had been in the Saturn system with NASA's Cassini orbiter since 14 January 2005, sent back pictures of a complex network of valleys that form a larger flat, low-lying area. The pictures, which show a mosaic of peaks of between seven and thirteen kilometres in height, caused a sensation in the science world, although they revealed that the satellite Titan did not have liquid hydrocarbon oceans, though at certain times fluid did appear to flow on the surface, which have eroded the valleys out of the icy landscape. This may well be what can be seen in the lower third of the picture. The colourful mosaic is probably the result of differences in light intensity on the surface of Titan under its atmosphere of nitrogen and methane.

Ontario Lacus, a lake near to Titan's South Pole

Ontario Lacus, ein See nahe dem Südpol des Saturnmondes Titan

This view of Titan's south pole reveals the intriguing dark feature named Ontario Lacus and a host of smaller features dotting the south polar region. The true nature of this feature, seen here at left of centre, is not yet known with absolute certainty. However, the feature's darkness, the shore-like smoothness of its perimeter, and its presence in an area where frequent convective storm clouds have been observed by Cassini and Earth-based astronomers made it the best candidate for an open body of liquid on Titan when this image was taken in June 2005.

Ontario Lacus, a hydrocarbon sea with a 'beach' on Titan

Ontario Lacus, ein mit flüssigen Kohlenwasserstoffen gefüllter See mit "Strand" auf dem Titan

Ontario Lacus is a sea close to the South Pole of Titan, one of Saturn's moons, filled with liquid ethane, probably mixed in with liquid methane, nitrogen or other light alkene compounds. The picture shows the sea in a cartographical projection, firstly in an infra-red image taken by Cassini's ISS (Imaging Science Subsystem, top left) camera and also in three negative images obtained from combinations of images taken through various infra-red filters of VIMS (the Visual and Infrared Imaging Mapping Spectrometer on board Cassini). The negatives show the south-east section of the sea as a dark blue area. A bright, narrow band can clearly be seen around the sea: it is obviously a transitional zone separating the sea from the surrounding area. VIMS scientists call this 'the beach'. The Cassini camera first photographed the sea in mid-2005, when it showed up as a dark area which hardly reflected any light (ISS-NA, top left). Due to its shape and size, which recalled Lake Ontario, to the east of Niagara Falls on the border between the USA and Canada, it was given the Latin name Ontario Lacus. However, the picture gave no further clues as to the composition and condition of the fluid (or otherwise) in Ontario Lacus. During one of the later fly-pasts of Titan, on 4 December 2007, VIMS was able to get a picture of the sea from a distance of 1 100 kilometres at the relatively high resolution (for a spectrometer) of 500 metres per pixel. Because of Titan's thick nitrogen and methane atmosphere, actual detail of the surface of the icy satellite can only be registered in specific, narrow wavelengths.

Ontario Lacus

Ontario Lacus

VIMS (the Visual and Infrared Imaging Mapping Spectrometer) on board Cassini took pictures of the eastern and southern sections of Ontario Lacus on 5 December 2007, sections of the lake which showed up on earlier images from the ISS (Imaging Science Subsystem) camera of Cassini as a striking dark, smooth plateau near the South Pole of the Satellite of Saturn. The VIMS pictures, taken at a resolution of 500 metres per pixel were overlaid on the ISS image to create a 'negative'. For the bottom picture, infrared wavelengths of 4.99 micrometres (µm), 2.03 µm and 1.29 µm on the three colour channels, red, green and blue were used; because liquids absorb certain colours significantly at 5 µm, this being particularly the case for liquid ethane at 2.03 µm, Ontario Lacus appears as a dark blue area: it is filled with liquid ethane. It is possible that the sea also contains liquid methane, but this is difficult to ascertain, as Titan's atmosphere contains relatively high levels of methane. The sea is surrounded by a narrow, bright band, which is probably a transitional zone, a sort of 'beach'. VIMS obtained the top picture was taken using several infrared filters, a technique which creates a colour ratio similar to a negative. That allows further details to be revealed in the south-eastern corner of Ontario Lacus. However, it remains difficult and speculative to interpret the information: the slightly brighter area may be an island, or a trough. In the red channel, a ratio was created out of the 2.03 µm/1.27 µm wavelengths, for the green channel 1.59/1.27 µm, and for the blue channel 5.0 µm/1.27 µm.

Titan's mysterious dense atmosphere

Mysteriöser, von einer dichten Atmosphäre eingehüllter Saturnmond Titan

Cassini delivers this stunning vista showing small, battered Epimetheus and smog-enshrouded Ti-tan, with Saturn's A and F rings stretching across the scene. The colour information in the colorised view is completely artificial: it is derived from red, green and blue images taken at nearly the same time and phase angle as the clear filter image. This colour information was overlaid onto the previously released clear filter view in order to approximate the scene as it might appear to human eyes.