— This composite image shows the massive mountain range running just south of Titan's equator, and its height-revealing shadows. It is about 150 kilometres long (93 miles), 30 kilometres (19 miles) wide, and about 1.5 kilometres (nearly a mile) high (Image: NASA)
These images snapped from 15,000 kilometres (9,300 miles) away, shows two views of the same mountain-riddled area. Near the bottom of the right-hand image, a band of bright clouds is especially clear. These clouds may be produced when gaseous methane in Titan's atmosphere cools and condenses into methane fog (Image: NASA)
Titan's mountains of ice may have been built following a violent impact that cracked the crust of Saturn's giant moon. The moon's tallest known mountains were revealed in the latest images from the Cassini spacecraft on Tuesday.
The mountains appear in images from the probe's VIMS instrument (Visual Infrared Mapping Spectrometer). The features have been seen before but this time, crucially, the instrument also spotted a hazy shadow stretching away from the range. The length of the shadow reveals that the mountains are about 1500 metres high (nearly a mile).
The range is about 150 kilometres (93 miles) long, but makes up just part of a larger mountainous area. "There are a whole bunch of these features, probably also mountain ranges," says Robert Brown at the University of Arizona, leader of the VIMS team.
They are all clustered close to a huge semicircular feature that Cassini scientists suspect to be an ancient impact basin, the scar left behind when something heavy hit Titan.
Brown suggests that the impact might have broken the thick ice crust, and that could have created the mountains: "We think it works like Earth's mid-ocean ridges, with plates pulling apart and magma welling up in the middle." And another odd feature, shaped like a vast letter H, might be a rift valley created by the same event.
The material that welled up to build Titan's mountains was not molten rock, but something much chillier probably a mixture of water and ammonia. This "cryomagma" would be at subzero temperatures, though still hot compared with the surface temperature of -180°C.
Some of the mountains appear to be snowcapped, with bright deposits lying along the ridge. It could be methane snow, or some other organic material.
The mountains also seem to be the source of a band of clouds that sometimes stretche almost all the way around Titan. These clouds had puzzled planetary scientists because they are fixed at a precise latitude about 40° south.
One suggestion was that the cloud-band might be puffed out of an ice volcano, but now it seems more likely to be generated simply by the height of the mountain range. A steady wind blows from west to east around Titan, and as it rises up over the mountains it should cool down, forcing methane droplets to condense out of the nitrogen atmosphere, forming clouds.
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