By the mid-17th century, telescopes had improved enough to make visible the seasonally growing and shrinking polar ice caps on Mars, and features such as Syrtis Major, a dark patch thought to be a shallow sea.

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“By the mid-17th century, telescopes had improved enough to make visible the seasonally growing and shrinking polar ice caps on Mars, and features such as Syrtis Major, a dark patch thought to be a shallow sea.”