Mass 6.419 X 10«3 kilograms
Mass of Mars/mass of Earth 0.107
Radius (equatorial) 3400 kilometers 2110 miles
Radius (polar) 3360 kilometers 2090 miles
Radius (average) 3380 kilometers 2100 miles
Radius of Mars/radius of Earth 0.531
Flattening (optical) 0.011
Flattening (dynamic) 0*00525
Density 4 grams per cubic centimeter
Gravitational acceleration (average) 375 centimeters per sec per sec
Day (sidereal) 24 h 37 min 22.67 sec
Speed of rotation 0.26 kilometer (0.16 mile) per second moons as black spots moving rapidly across the face of the Sun. Such a phenomenon is more correctly described as a transit than as an eclipse.
Attention was drawn in 1966 by S. F. Singer in the United States to the potential interest of Phobos and Deimos as subjects for space exploration. These satellites were either produced at the same time as Mars or they are captured asteroids. In the former case, they would probably represent the initial material from which the terrestrial planets, Mercury, Venus, Earth, and Mars, developed. Because of the small size of the satellites, and the high rate of loss of heat by radiation, no important changes in physical or chemical nature are expected as a result of an increase in temperature. If, on the other hand, Phobos and Deimos were originally asteroids, then an opportunity would be provided for studying these members of the solar system.
At the present time, little more is known about the Martian satellites than the characteristics of their orbits. Approximate estimates have been made of their size, but their shape, mass, density, composition, surface characteristics, and rotation are completely unknown. A spacecraft orbiting Mars could get close enough to one or the other (or possibly both) of the satellites to determine the size, shape, and rate of rotation. Surface features could be observed by means of television cameras and the color and reflectivity might provide information concerning the composition. In the words of Singer, "a direct examination of the Martian satellites will shed important light on the origin and early history of the solar system."
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