DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

mare
mare Nameforaflatareaonthemoon. Maria
are now known to be dust-covered cooled melt
flows from meteorite impacts.
Marianas Trench Undersea trench running
roughly south from Japan at about 140 ◦ E.
Formed by the subduction of the Pacific tectonic
plate below the Philippine plate. The deepest
ocean waters in the world occur in the Challenger
deep, part of the Marianas Trench.
marine snow In oceanography, particles of
organic detritus and living forms whose downward
drift, in a dense concentration, appears
similar to snowfall.
Markarian galaxies Galaxies showing an
excess of blue and near UV emission, identified
by B.E. Markarian through an objective prism
survey with the 1-m Schmidt telescope of the
Byurakan observatory. The lists Markarian published
in the 1970s include approximately 1500
objects, of which ≈ 10% are Seyfert galaxies,
≈ 2% are quasars, ≈ 2% are galactic stars,
and the wide majority are galaxies with enhanced
star formation, such as star-forming
dwarf galaxies and starburst galaxies. See dwarf
galaxy, Seyfert galaxies, starburst galaxy.
Mars The fourth planet from the sun. Named
after the Roman god of war, Mars has a mass
of M = 6.4191 × 10 26 g, and a radius R =
3394 km, giving it a mean density of 3.94 g cm −3
and a surface gravity of 0.38 that of Earth. The
rotational period is 24 h 37 m 22.6 s around an axis
that has an obliquity of 23 ◦ 59’. This rotation
is, in part, responsible for the planet’s oblateness
of 0.0092. Mars’ orbit around the sun is
characterized by a mean distance of 1.5237 AU,
2.28 × 10 8 km, an eccentricity of e = 0.0934,
and an orbital inclination of i = 1.85 ◦ . Its sidereal
period is 687 days, and its synodic period
is 779.9 days. An average albedo of 0.16 gives
it an average surface temperature of around 250
K, varying from 150 to 300 K. Its atmosphere is
more than 90% CO2, with traces of O2, CO, and
H2O. The atmospheric pressure at the surface is
3.5 mbars. Mars continues to be well studied in
part because there is evidence of past liquid water
on the surface, and thus Mars may have once
harbored life. Mars has a highly varied terrain
© 2001 by CRC Press LLC
304
of mountains, canyons, and craters that are kilometers
in height and depth. Mars has a silicate
mantle and core which is probably a mixture of
Fe and S. Its moment of inertia has recently been
measured to be I = 0.365MR 2 . Mars has two
satellites (Phobos and Deimos), which orbit the
planet in synchronous rotation.
Mars Climate Orbiter (MCO) An orbiting
spacecraft that belongs to the Mars Surveyor
’98 program along with the Mars Polar
Lander (MPL). MCO was launched on December
11, 1998, to reach Mars 9 1/2 months later.
The spacecraft was destroyed on September 23,
1999 when a navigational error pushed it too
far into the Martian atmosphere. The error was
attributed to a confusion between metric and English
units of force.
Mars Global Surveyor (MGS) A spacecraft
launched on November 11, 1996, that signifies
America’s successful return to Mars after
a 20-year hiatus. Surveyor took 309 days to
reach Mars, and entered into an elliptical orbit
on September 12, 1997. During its first year
at Mars, the Orbiter Camera observed evidence
of liquid water in the Martian past, extensive
layered rock, boulder-strewn surfaces, volcanism
and new volcanic features, the Martian fretted
terrain, the polar layered deposits, and the
work of wind on the Martian surface. The orbit
has since been circularized through aerobraking.
During this period, the Global Surveyor
was able to acquire some “bonus” science data,
such as a profile of the planet’s northern polar
cap.
The primary mapping phase of the mission
began on March 15, 1998. It ended 687 Earth
days later on January 31, 2000. In addition to
making a photographic map of the entire planet,
Mars Global Surveyor studied the planet’s topography,
magnetic field, mineral composition,
and atmosphere. Surveyor is also used as a communications
satellite to relay data to Earth from
landers on the surface of Mars. This phase of
the mission is scheduled to last until January 1,
2003, or until the spacecraft’s maneuvering propellant
runs out.
Mars Microphone A microphone developed
for the Planetary Society by the University of