This week’s image comes from Titan, Saturn’s largest moon,
and shows a river flowing across the surface of that distant moon. It was captured
by NASA’s Cassini spacecraft in orbit around Saturn. Unlike terrestrial rivers
the material flowing through this stream is not water, but rather a mix of
liquid hydrocarbons, such as ethane and methane, which would be gaseous were
they on Earth. The solid ground into which this river has carved it channel is
also not rock, but solid ice of the sort which covers much of Titan’s surface.
Despite the vastly different composition there is a striking similarity between
the morphology of this river system and its terrestrial counterparts.
This river system has been dubbed Titan’s “River Nile” due
to the similarities between the branching river and the morphology of one of
Earth’s longest rivers however this river system isn’t nearly as long, or
complex as that of the Nile. The full image shown to the right is approximately 400 km
long, and shows the entire catchment of the river, from the sources of its
various tributaries to the estuary where it flows into Ligeia Mare one of
Titans large polar seas. Earth’s Nile is over 6000 km long, but then Earth is a
much larger planet than Titan is!
Various characteristic features of terrestrial rivers can
clearly be seen here. The river has several tributaries which converge to form
a wider stream. The river starts off as a number of relatively narrow streams,
which get progressively wider the further downstream you go. They end in quite
a wide channel, which rapidly opens out into the lake. The river system also features
prominent meanders. This indicates that like a terrestrial river the liquid is
predominantly following topographic lows, carving a gently winding path as it
does. The description of this image over at the NASA website notes that the
river probably follows a pre-existing fault in the surface of the moon, as it
does not meander as much as would otherwise be expected, so is probably
constrained by the shape of the fault.
NASA also notes that the dark colour of the river in this
radar image indicates that this feature is filled with liquid. This indicates
that this isn’t the channel left by a past event. It is flowing today, and has
clearly done so for some time. This is very exciting as it shows that the
hydrology of Titan is active. Other worlds like Mars have relic channels but
these are usually attributed to sudden outflows of water rather than
persistently flowing streams. Titan clearly has a hydrological cycle, and it is
believe that hydrocarbon rain feeds this river valley just as water rain does
on Earth. Cassini has observed areas which repeatedly get darker, suggesting
that they are wetted by rainfall. This puts Titan and Earth in a very exclusive
club, and makes comparing the similarities and differences between the two
worlds a very useful field of study.
Image Credit:
Titan Rivers (NASA / JPL–Caltech / ASI) https://saturn.jpl.nasa.gov/resources/5700/
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