Longest river system photographed on another world
Vid Fluminia
412 kilometre(s)
Not Applicable ()

The longest river on another world is the Vid Flumina, a 412-km-long (256 mi) river of liquid methane located on Saturn's moon Titan. The river flows north through a network of deep canyons to the southern shore of a methane sea called the Ligeia Mare. It was first spotted in radar scans of Titan's surface taken by the Cassini spacecraft on 26 September 2012.

With its relatively straight course and a current that flows almost directly north, the Vid Flumina is often compared to the Nile River on Earth. According to a 2016 study that used 2013 data from Cassini's synthetic aperture radar, the Vid Flumina runs for at least half of its length through a network of 500–600-m-deep (1,640-1,968-ft) canyons.

Intriguingly, the profile of the Vid Flumina's canyon suggests that it was formed by erosion rather than tectonic activity, and that it is flooded for a significant portion of its length. This means that the surface level of the Ligeia Mare has risen significantly since the process of erosion began, creating a fjord-like narrow inlet.

Titan has a hydrologic cycle similar to that of Earth, except the working fluid in this cycle is methane (with some ethane and other hydrocarbons mixed in) rather than water. This means that Titan has methane clouds, methane rain, and even methane rivers, lakes and seas. Liquid water cannot exist on the surface of Titan as temperatures average around -176°C (-284°F), with water (in the form of a kilometres-thick layer of ice) instead forming the moon's crust.

The river is named after a river of ice-cold, poisonous water mentioned in Norse mythology.