Mars has a rotational period of 24 hours and 37 minutes, a period for revolution about the sun of 687 days, and a diameter of 6800 km (about half that of Earth). Its average density is 3.9 g/cc, which is considerably less than the 5.2-5.5 g/cc characteristic of Mercury, Venus, and the Earth. This density gives it a mass about 11% of that for Earth. It is most easily observed from Earth when it is at opposition. Even then, it was difficult in the past to observe from Earth because of turbulence in the Martian atmosphere and ours.

The animation to the right shows a sequence of Hubble Space Telescope images of the Martian surface. The image at the bottom of the page shows several still views from the same source.

Earth based observations concluded that Mars

• Has a reddish hue over 3/5 of the planet, which we now known to be caused by red dust and rocks on the surface of the planet.
• Has polar ice caps waxing and waning with the seasons that we now know to be composed both of dry ice (frozen carbon dioxide) and water ice.
• Has surface markings that some originally thought looked like "canals" from Earth. These are now known to be features like the edges of mountain ranges.
• Has areas of changing color that were once thought to correspond to vegetation. We now believe these regions of changing color to be due to blowing sand, not vegetation.
• Has an atmosphere with clouds.

Three views of Mars. Click on the image for a larger version and here is a further description of the images

Increased understanding of Mars had to await the results of space probes, beginning with Mariner 4, 6, 7, and 9 in the period 1965-1971, and the Viking 1 and 2 probes in 1976.

Information supplied by: http://csep10.phys.utk.edu