Vanity.

From 'The Verge':

"NASA's original photo is somewhat obscured by a dust cover on the MAHLI camera used to capture the picture, but The Planetary Society's Emily Lakdawalla made the necessary touchups to reveal the memorable portrait above. That large "eye" you're looking at is actually one of Curiosity's most vital tools: it's a powerful laser capable of vaporizing Mars rocks and instantly analyzing their chemical composition as they vanish from sight."

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