Planetary Properties

Mars, the "Red Planet," is named after the Roman god of war because it commonly appears to have a (bloody) reddish tinge when viewed in our sky. It 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/cm3, which is considerably less than the 5.2-5.5 g/cm3 characteristic of Mercury, Venus, and the Earth. This density gives it a mass about 11 percent of that for Earth. The physical properties of Mars are summarized in this table and the orbital properties in this table.

Observation from Earth
Mars 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. Early Earth-based observations concluded that Mars:

1. Has a reddish hue over 60 percent of the planet, which we now known to be caused by red dust and rocks on the surface of the planet.
2. 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.
3. 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.
4. 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.
5. Has an atmosphere with clouds.

Many of these features, particularly the "canals," made Mars and conjectured life there a favorite of science fiction writers early in this century (see the box below).

Earlier Misconceptions of Mars

In 1895 a professor of astronomy, Samual Leland Phelps, wrote in a book called World Making that with a new 40 inch telescope being built by the University of Chicago,

"It will be possible to see cities on Mars, to detect navies in [its] harbors, and the smoke of great manufacturing cities and towns... Is Mars inhabited? There can be little doubt of it ... conditions are all favorable for life, and life, too, of a high order. Is it possible to know this of a certainty? Certainly." (Quoted in National Geographic, February, 1973)
There is no evidence for the things that Professor Leland thought we would see with an Earth-based telescope, but Mars is still of great interest to us, not the least reason being that it may once have harbored conditions favorable to the evolution of life.