Because every planet's equatorial radius is longer than its polar radius, the surface of the planet at its equator is farther from the resembles the Earth on its surface (deserts, volcanic flows, dry river valleys, ice caps), the small volume and resulting small internal pressures of Mars make its internal processes considerably different from the Earth's, as can be seen in the upper color insert on page C-1.

Mars's average internal density is only about 71 percent of the Earth's, since its smaller mass does not press its internal matter into forms as dense as they are in the deep Earth. Similarly, Mars's gravity field is only about 40 percent as strong as the Earth's. These and other physical parameters for Mars are listed in the table at left.

Each planet and some other bodies in the solar system (the Sun and certain asteroids) have been given its own symbol as a shorthand in scientific writing. The symbol for Mars is shown below.

Many solar system objects have simple symbols; this is the symbol for Mars.

planet's center than the surface of the planet at the poles. To a lesser extent, the distance from the surface to the center of the planet changes according to topography such as mountains or valleys. Being at a different distance from the center of the planet means there is a different amount of mass between the surface and the center of the planet. What effect does the mass have? Mass pulls with its gravity (for more information on gravity, see the sidebar called "What Makes Gravity?" on page 68). At the equator, where the radius of the planet is larger, and therefore the amount of mass between the surface and the center of the planet is relatively larger, the pull of gravity is actually stronger than it is at the poles. Gravity is not a perfect constant on any planet:Variations in radius, topography, and the density of the material underneath make the gravity vary slightly over the surface. This is why planetary gravitational accelerations are generally given as an average value on the planet's equator.

Just as planets are not truly spheres, the orbits of solar system objects are not circular. Johannes Kepler, the prominent 17th-century German mathematician and astronomer, first realized that the orbits of planets are ellipses after analyzing a series of precise observations of the location of Mars that had been taken by his colleague, the distinguished Danish astronomer Tycho Brahe. Kepler drew rays from the Sun's center to the orbit of Mars and noted the date and time that Mars arrived on each of these rays. He noted that Mars swept out equal areas between itself and the Sun in equal times, and that Mars moved much faster when it was near the Sun than when it was farther from the Sun. Together, these observations convinced Kepler that the orbit was shaped as an ellipse and not as a circle, as had been previously assumed. Kepler defined three laws of orbital motion (listed in the table on page 7), which

The ellipticities of the planets differ largely as a function of their composition's ability to flow in response to rotational forces.

All Planets: Planetary Mass v. Orbital EHipcidty

4.5 x 1027 (1028) jj 4.5 x 1026 (1027) c 4.5 x 1025 CIO26)

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