While orbiting Saturn, the Cassini spacecraft has spotted lakes containing ethane on Titan, the planet's largest moon. Titan is so far the only planetary object other than Earth that is known to have liquid bodies on its surface. Titan is the largest satellite of the giant planet Saturn. Its diameter of 5,150 kilometres makes it bigger than Mercury and only 25% smaller than Mars. It is the second-largest satellite in the Solar System after Jupiter's Ganymede based on diameter alone, and would claim first place if its dense atmosphere, which extends more than 1,000 kilometres above its surface, were included. Fly-bys of Titan by the Voyager spacecraft in the early 1980s helped determine the main composition, temperature and pressure of this atmosphere. Although much colder than Earth's (around 90-94 kelvin at the surface), and lacking molecular oxygen (O_2), it shows many similarities to the atmosphere of our planet. It consists mainly of molecular nitrogen (N_2), with a surface pressure of 1.5 bar and, like Earth, it has a structure comprising a troposphere and a stratosphere. On page 607 of this issue, Brown et al.
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