A dark spot on Mars - Syrtis Major

[LongTomH]

Great post over at the MarsDaily page at SpaceDaily with imagery from ESA's Mars Express spacecraft, along with history of observation of the Syrtis Major area. Syrtis Major was one of the few features that could be seen through Earth-bound, pre-space age telescopes, starting with the great Dutch astronomer Christiaan Huygens:

Syrtis Major was first observed in 1659 by the Dutch mathematician, physicist and astronomer Christiaan Huygens (1629-1695), who was also the first to recognise Saturn's rings.

The pronounced dark colouring of the region allowed Huygens to determine Mars' rotation through the change in position of Syrtis Major in the course of his telescopic observations; he was thus able to, for the first time, estimate the length of the planet's day - and he did so relatively precisely, arriving at a value of 24.5 hours (in fact it is 24 hours and 37 minutes). The name Syrtis Major is derived from the Roman name for the Gulf of Sidra on the coast of Libya.

Early astronomers just saw Syrtis Major as a dark spot on the Martian surface. At first, they thought it was a sea, later hypotheses were that it was vegetation.