Mass (10^{24}kg) Mass of the body in 10^24 kilograms Volume (10^{10}km^{3}) Volume of the body in 10^10 km^3 Equatorial radius (km) Radius of the body at the equator in kilometers Polar radius (km) Radius of the body at the poles in kilometers Volumetric mean radius (km) Radius of a sphere with the same volume as the body Core radius (km) Radius of the planet core in kilometers Ellipticity The ratio (equatorial - polar radius)/(equatorial radius) also called the flattening (dimensionless) Mean density (kg/m^{3}) Average density of the body (mass/volume) in kilograms/(meter^3) Surface gravity (m/s^{2}) Gravitational acceleration at the surface of the body or the 1 bar level in meters/(second^2) Escape velocity (km/s) Initial velocity required to escape the body's gravitational pull in kilometers/second GM (x 10^{6}km^{3}/s^{2}) Gravitational constant times the mass of the body in 10^6 kilometers^3/seconds^2 Visual geometric albedo The ratio of the body's brightness at a phase angle of zero to the brightness of a perfectly diffusing disk with the same position and apparent size, dimensionless. Bond albedo The fraction of incident solar radiation reflected back into space without absorption, dimensionless. Also called planetary albedo. Visual magnitude V(1,0) The visual magnitude of the body if it were one AU (1.496 x 10^8 kilometers) from the Earth at a phase angle of zero, dimensionless. Solar irradiance (W/m^{2}) Solar energy on the body in Watts/(meter^2) Black-body temperature (K) Equivalent black body temperature is the surface temperature the body would have if it were in radiative equilibrium and had no atmosphere, but the same albedo, in degrees Kelvin. Topographic range (km) Difference in elevation between the highest and lowest points on the planet's surface, in kilometers. Moment of inertia (I/MR^{2}) The moment of inertia of the body expressed as the rotational inertia divided by the body's (mass x radius^2). A hollow shell has a moment of inertia of 1, a homogeneous sphere 0.4 J_{2}(x 10^{-6}) The ratio of the difference in the moments of inertia to the mass of the body times the radius^2, (C-A)/(M R^2), x 10^-6, dimensionless

Semimajor axis (10^{6}km) Mean distance from the Sun in 10^6 kilometers Sidereal orbit period (days) The time it takes the body to make one revolution about the Sun relative to the fixed stars in days. Tropical orbit period (days) The time for the body to make one revolution about the Sun from equinox to equinox in days. For Earth, this equals exactly 1 year. Synodic period (days) The time interval between similar configurations in the orbit (e.g. opposition) of the body and Earth, in days. Perihelion (10^{6}km) The point in a body's orbit closest to the Sun, in 10^6 kilometers. Aphelion (10^{6}km) The point in a body's orbit furthest from the Sun, in 10^6 kilometers. Mean orbital velocity (km/s) The average speed of the body in orbit, in kilometers/second. Orbit inclination (deg) The inclination of the orbit to the ecliptic, in degrees Orbit eccentricity A measure of the circularity of the orbit, equal to (perihelion - aphelion distance)/(2 x semi-major axis) For a circular orbit, eccentricity = 0. Dimensionless. Sidereal rotation period (hrs) The time for one rotation of the body on its axis relative to the fixed stars, in hours. Equatorial inclination (deg) The tilt of the body's equator relative to the ecliptic, in degrees.

Surface Pressure: Atmospheric pressure at the surface, in bars, millibars (mb = 10^-3 bar), or picobars (10^-12 bar). Surface Density: Atmospheric density at the surface in kilograms/meters^3. Scale height: The height interval in which the atmospheric pressure changes by a factor of e = 2.7183 Average temperature: Mean temperature of the body over the entire surface in degrees Kelvin. Diurnal temperature range: Temperature range over an average day in degrees Kelvin. Wind speeds: Near surface wind speeds in meters/second Atmospheric composition: Relative composition by volume of gasses in the atmosphere. Mean molecular weight: Average molecular weight of the atmospheric constituents in grams/mole Atmospheric composition (by volume): Relative volume of constituents in the atmosphere, by percentage or ppm (parts per million).

Bar - A measure of pressure or stress. 1 bar = 10^5 Pascal (Pa) = 10^5 kg m^-1 s^-2 Ecliptic - An imaginary plane defined by the Earth's orbit. Equinox - The point in a body's orbit when the sub-solar point is exactly on the equator. Gravitational Constant - Relates gravitational force to mass, = 6.6726 x 10^-11 meters^3 kilograms^-1 seconds^-2 Opposition - An orbital configuration in which two bodies are on exact opposite sides of the Sun Phase Angle - The angle between the Earth and Sun as seen from the body.

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Questions and comments about this page should be addressed to:

Dr. David R. Williams, dwilliam@nssdc.gsfc.nasa.gov, (301) 286-1258

NSSDC, Mail Code 633, NASA/Goddard Space Flight Center, Greenbelt, MD 20771

Last Updated: 07 March 1996, DRW