Tycho Brahe

1609
Johannes Kepler (1571 - 1630), a student of Tycho Brahe, publishes Astronomia Nova (New Astronomy), which contain his first two laws of planetary motion. Kepler's first law assumes that Mars has an elliptical orbit, which was a revolutionary idea at the time. Until then, the classical belief held that a circle was perfect, and therefore all orbits must be circular.

1609
Galileo Galilei (1564 - 1642) observes Mars with a primitive telescope, becoming the first person to use it for astronomical purposes.

1659
The Dutch astronomer Christiaan Huygens (1629 - 1695) draws Mars using an advanced telescope of his own design. He records a large, dark spot on Mars, probably Syrtis Major. He notices that the spot returns to the same position at the same time the next day, and calculates that Mars has a 24 hour period.

1666
Giovanni Cassini (1625 - 1712) observes Mars and determines that the rotational period, or length of one Mars day is 24h, 40m.

1672
Huygens is the first to notice a white spot at the south pole, probably the southern polar cap.

1698
Huygens publishes Cosmotheros, which discusses what is required of a planet to support life, and speculates about intelligent extraterrestrials. This is one of the first published expositions of extraterrestrial life.