What is Pluto's 'heart' made of?

The spectacular, flat landscape that dominates the left side of Pluto's icy "heart" can now be explained, say scientists.

Sputnik Planum is the most prominent feature on the diminutive world, covering 900,000 square km.

Broken into an array of polygons, it is devoid of any impact craters.

Reporting in the journal Nature, researchers say that roiling cells of nitrogen ice remove any blemishes, maintaining a super-smooth appearance.

Here, leading Pluto scientist Bill McKinnon explains how this takes place - and how it was discovered.