Photo of Mimas, Saturn's moon, moving across the planet.

Mimas transits across Saturn's northern latitudes. © NASA/JPL-Caltech/Michael Benson, Kinetikon Pictures 

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Planet Saturn

Blue skies on a golden planet, and two of our solar system's greatest extremes – Saturn is a planet of contrasts and surprises.

This image, one of 77 photographs that appeared in Otherworlds: Visions of our Solar System, shows the diminutive moon Mimas transiting across Saturn's northern latitudes, with long shadows cast by the planet's rings.

Michael Benson created the image using data returned by NASA's Cassini spacecraft in 2005. Saturn usually appears golden in images, as its thick clouds are predominantly yellow. So scientists were surprised when Cassini returned images of its northern hemisphere showing a blue sky.

The blue sky is caused by a process known as Rayleigh scattering: the scattering of sunlight by molecules in a planet's atmosphere. The effect is more pronounced with short-wavelength light, and increases with the angle of scattering, giving Saturn a blue hue at its northernmost latitudes. 

Two extremes

Benson's image also captures the two extremes of size for planet-like bodies.

Saturn's satellite Mimas is the smallest known planetary-mass object: a body large enough to have become rounded under the force of its own gravity. If Mimas were any smaller, its gravity would be too weak to shape it into a sphere.

In contrast, Saturn is one of our solar system's giants - of all the bodies orbiting our sun, only Jupiter is larger.

If Saturn were 40% larger, it would be similar in size to the smallest star discovered, OGLE-TR-122.

Stars begin to burn when the dense hydrogen inside them, pressurized under gravity, begins to turn into helium through nuclear reactions.

Audio commentary extract

Eleonora Rosatone, exhibition designer for Otherworlds, discusses the staggering scale of space and the inspiration behind her design for the exhibition.