Dark Syrtis Major (CTX Frame - Natural Colors; credits: Dr Paolo C. Fienga - Lunexit Team)

On 13 October 1659, Dutch Astronomer Christiaan Huygens turned one of his telescopes towards a bright orange spot in the sky and produced what Percival Lowell would later call “the first drawing of Mars worthy of the name ever made by man”.

Huygens included a "dark spot" in his drawing that is thought to represent Syrtis Major, a small sliver of which is visible in this HiRISE image.

Syrtis Major is a Shield Volcano and its dark color comes from the dark Basaltic Rock present in the area, visible because it lacks the Dust that paints the rest of the Planet its distinct, rusty orangish color. By tracking this "dark spot" in repeated observations, Huygens concluded that Mars rotated every 24 hours: a time not too far off from its true rotation period of approx. 24 hours and 39,58 minutes.