What Martian dark feature was the first surface feature of another planet seen through a telescope?

The Martian dark feature first seen through a telescope as a surface feature of another planet was Syrtis Major Planum. Dutch astronomer Christiaan Huygens drew the marking in 1659 while observing Mars, making it the earliest documented planetary surface feature beyond Earth.

Huygens used repeated observations of Syrtis Major to estimate Mars's rotation period. His result was close to the modern Martian solar day of about 24 hours 37 minutes. The feature appeared as a dark albedo marking because its relatively dark basaltic terrain contrasts with dustier, brighter surroundings.

The name has changed over time. Earlier observers called it the Hourglass Sea and used other imaginative names, while modern planetary mapping identifies it as Syrtis Major Planum, a broad volcanic plateau near the boundary between Mars's northern lowlands and southern highlands. It is sometimes shortened to Syrtis Major, which refers to the same prominent feature in ordinary usage.