The similarities are striking. Each planet has roughly the same amount of land surface area. Atmospheric chemistry is relatively similar, at least as Earth is compared to the other planets in the solar system. Both planets have large, sustained polar caps and the current thinking is that they're both largely made of water ice. The sibling planets also show a similar tilt in their rotational axises, affording each of them strong seasonal variability. The neighbors also present strong historic evidence of changes in climate.
Some Basic Facts about Mars: Ave. Solar Distance: 227,940,000 km (1.52 AU) Diameter: 6,794 km - Rotational Period (one day): 24.622 hours Mean Surface Temp: -63_ C - Orbital Period (one year): 686.98 days - Moons: 2 (Phobos and Demios) - Gravity: 38% Earth. Some Basic Facts about Earth - Ave. Solar Distance: 149,600,000 km (1AU) Diameter: 12756.34 km - Rotational Period (one day): 23 hours, 57 minutes Mean Surface Temp: 14_ C - Orbital Period (one year): 365.242 Earth days Moons: 1 (Luna) - Gravity: 9.78 (m/s2) Earth