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In one BBC article called "Etna escape: 'Pelted with the deadly, hot debris'", there was an image with the following description:

Europe's Sentinel-2a satellite pictured Thursday's lava flow from space

Europe's Sentinel-2a satellite pictured Thursday's lava flow from space

Can somebody explain the blue region of the picture? It could be a blend of thermal and visible satellite map?

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According to this article,

The red hot lava flowing from Mount Etna can be seen clearly in the image from Sentinel-2A. The surrounding snow has been processed in blue to distinguish from the clouds.

The Sentinel satellites use multispectral imagers:

13 spectral channels incorporating four visible and near-infrared bands at 10 m resolution, six red-edge/shortwave-infrared bands at 20 m and three atmospheric correction bands at 60 m.

So it's not so much a blend of visible and infrared channels, but a visible-light image with a false-color overlay based the composition of the soil (obtained by taking spectrograms).

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