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Maia

then used as calibration standards in measurements. Such calibration is typically accurate about 0m.01.

One may also assign an absolute magnitude M to any star, defined as the apparent magnitude the star would have if it were 10 parsecs from the observer on the Earth.

Maia Magnitude 4.0 type B9 star at RA 03h 45m, dec +24◦21 ; one of the “seven sisters” of the Pleiades.

main field Main magnetic field of the Earth, the magnetic field originating inside the Earth.

main sequence star A star whose primary energy source is hydrogen burning in a core of about 10% of its total mass. Stars begin this phase of evolution when fusion begins in the core, terminating the proto-star era of stellar formation. The name arises because such stars make up a very large fraction of all the ones we see (in a volume-limited sample), and they occupy a diagonal stripe across a Hertzsprung– Russell diagram. The physics of main sequence stars are relatively simple, and qualitative relationships can be derived among their masses, luminosities, radii, temperatures, and lifetimes. The range of masses is 0.085 to 100 solar masses (for a composition like the sun). Lower masses are brown dwarfs and configurations with larger masses are quite unstable and quickly blow off enough surface material to come down to 100 solar masses or less. The relationships are:

LM4

(steeper at low mass, flatter a high mass)

1

1

R M 2

T M 2

lifetime M−2 to M−3 .

For masses less than about 1.5 solar masses, the main energy source is the proton-proton chain (whose rate depends on central temperature as about T 4.5, so that the stars have radiative cores). These lower mass stars have atmospheres in which the hydrogen is not totally ionized, so that their envelopes are convective (see convection). Conversely, stars of larger masses derive their energy from the CNO cycle and have convective cores, but the hydrogen

in their atmospheres is already ionized, so that they have radiative envelopes. No star of more than about 0.3 solar masses is fully convective after its formation stages, and so a core of helium gradually builds up at the center as a star ages from the the zero age main sequence toward the Hertzsprung gap. The main sequence phase takes up roughly 90% of a star’s (pre-white- dwarf or pre-neutron-star) life, and so 90% of the stars in a given volume will be main sequence stars. See brown dwarf, CNO cycle, convection, Hertzsprung Gap, Hertzsprung–Russell diagram, hydrogen burning, proton-proton chain.

major axis In an ellipse, the distance corresponding to the maximum length measured along one of the two symmetry axes, or this axis itself.

Majumdar–Papapetrou space-times (Majumdar, 1946; Papapetrou, 1947) In relativity, static space-times containing only electric fields in which the level surfaces of the gravitational potential V and electric potential φ coincide.

Solution of the gravitational equations yields

V = A + Bφ + (1/2)Gφ2 where A and B are arbitrary constants and G is the gravitational constant.

Manning coefficient A measure of roughness of an open channel. Denoted by the symbol n and appears in the Manning equation for calculation of velocity or flowrate in a channel with a free surface, such as a river, canal, or partially full pipe. The Manning coefficient increases as roughness increases.

Manning’s equation An equation that can be used to compute the average velocity of flow in an open channel, based on channel geometry and roughness:

U= κ RH 2/3S1/2

n

where U is the average velocity, n is the dimensionless Manning coefficient with typical values of 0.02 for smooth-bottom streams to 0.075 for coarse, overgrown beds, RH is the hydraulic radius and is defined as the ratio of the channel cross-sectional area to the wetted perimeter, S is the slope or energy gradient or the channel,

© 2001 by CRC Press LLC

March 5th event

and the constant κ is equal to 1 m1/3s−1 in SI units.

manometer A device for measuring pressure or pressure differences. It consists of a u-shaped tube, preferably of glass, partially filled with a liquid (mercury, oil, or water are commonly used). The difference in height, h, of the liquid in the two arms of the tube gives the pressure difference as 'p = ρgh, where ρ is the fluid density, and g is the local acceleration of gravity. If the meter-kilogram-second units are used, the pressure has units of Pascal (Pa). Pressure difference may also be indicated by giving h and the fluid being used, as in “mm Hg”, “inches of Mercury”, “inches of water”.

mantle The terrestrial planets are differentiated into layers, generally consisting of an outer crust, interior mantle, and central core. The mantle is usually the largest (by volume) of these layers and is composed of minerals that are intermediate in density between the iron comprising the core and the lighter materials (such as feldspars) which make up the crusts. The Earth’s mantle is composed primarily of magnesium, iron, silicon, and oxygen; the mantles of the other terrestrial planets are believed to be similar. Seismology indicates that there are several transition zones within the mantle, the result of changes in the densities and other properties of the component minerals with depth. Seismic studies of the Earth’s mantle indicate it is composed of solid rock, but due to the high temperatures and pressures, the mantle rocks can deform and flow over thousands to millions of years. This ductile flow of the mantle rocks is called mantle convection and is an efficient process by which heat is transported from the hot interior of the Earth to the cooler surface. This convection is believed to be the driving mechanism for the plate tectonics operating on the Earth. Plumes of hot material, not necessarily associated with the mantle convection cells, also occur and produce the hot spots that are responsible for intra-plate volcanism such as the Hawaiian Islands.

mantle convection On geologic time scales the Earth’s mantle behaves as a fluid due to solidstate creep processes. Because of the heat loss from the interior of the Earth to its surface, the

deep rocks are hot and the shallow rocks are cool. Because of thermal contraction the cool near surface rocks are more dense than the hot deep rocks. This leads to a gravitational instability with the near surface dense rocks sinking and the deeper light rocks rising. Plate tectonics, with the subduction of the cool, dense lithosphere, is one consequence.

mantle transition zone A zone that is located between depths of 410 and 660 km where seismic velocity and density increase markedly with increasing depth. Sometimes the mantle transition zone is taken to include the uppermost portion of the lower mantle down to depths of about 750 to 800 km. Depth distributions of seismic velocity and density, and their discontinuities at depths of 410 and 660 km can be explained by pressure and temperature dependency, and phase transformations of major composite minerals (olivine) in pyrolite composition.

March 5th event A gamma ray transient, observed with 12 spacecraft in 1979, that caused great controversy. Its properties demonstrated that it could be a distinct, new class of highenergy transient. The March 5th event was clearly identified with the supernova remnant N49 in the Large Magellanic Cloud (from 55 kiloparsecs away). Its intensity, however, made it the brightest “gamma ray burst” to date, and led many to conclude that its apparent N49 source location must be accidental and that it came instead from a few parsecs away in the nearby interstellar region, then thought typical of gamma ray bursts. A few much smaller events (see soft gamma repeaters) were seen from the same direction over the years, contrasting to the apparent lack of repetition of usual gamma ray bursts, adding to the confusion. Now it is known, ironically, that gamma ray bursts originate from cosmological sources, considerably more distant than even the neighboring galaxies, and that the March 5th event and the four known soft gamma repeaters do originate in “magnetar” galactic and LMC supernova-remnant neutron stars, including N49. Only one other transient similar to the March 5th event has been detected in three decades of space-age monitoring, seen on August 27, 1998, confirming all aspects of this interpretation.

© 2001 by CRC Press LLC

mare

mare Name for a flat area on the moon. 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 × 1026 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 24h37m22.6s 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 × 108 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

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 = 0.365MR2. 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

© 2001 by CRC Press LLC

Martian meteorites

California, Berkeley Space Sciences Laboratory, for the Mars Polar Lander (launched on January 3, 1999, and destroyed on landing in Mars on December 3, 1999).

Mars Microprobe The Mars Microprobe Mission, also known as Deep Space 2 (DS2) was launched aboard the Mars Polar Lander on January 3, 1999.

The microprobes were two basketball-sized aeroshells designed to crash onto the Martian surface at a velocity of about 200 m per second and release a miniature two-piece science probe into the soil to a depth of up to 2 m. The microprobes were to have separated from the Mars Polar Lander on December 3, 1999 prior to entry into the Martian atmosphere, and return signals relayed through the Mars Global Surveyor. However, no signal was received from them. Subsequent studies indicated severe design defects in the probes. The Mars Polar Lander also crashed on Mars on December 3, 1999. See Deep Space 2, Mars Polar Lander.

Mars Pathfinder Mission The second of NASA’s low-cost planetary Discovery missions, originally called Mars Environmental Survey, or MESUR, Pathfinder. It was launched on December 4, 1996, arrived at Mars on July 4, 1997, and continued to operate for 4 months. The mission consisted of a stationary lander and a surface rover (Sojourner), which was controlled by an Earth-based operator. The rover survived for 3 months, significantly longer than its designated lifetime of 1 week. After landing, Mars Pathfinder returned 2.6 billion bits of information, including more than 16,000 images from the lander and 550 images from the rover (which infer that some of the rocks may be sedimentary, thus implying a significant fluvial history), as well as more than 15 chemical analyses of rocks and extensive data on winds and other weather phenomena.

The spacecraft entered the Martian atmosphere without going into orbit and landed on Mars with the aid of parachutes, rockets and airbags, taking atmospheric measurements on the way down. It impacted the surface at a velocity of about 18 m/s (40 mph) and bounced about 15 m (50 ft) into the air, bouncing and rolling another 15 times before coming to rest

approximately 2.5 minutes after impact, about 1 km from the initial impact site. The landing site, in the Ares Vallis region, is at 19.33◦N, 33.55◦W and has been named the Sagan Memorial Station.

Mars Polar Lander (MPL) A spacecraft launched on January 3, 1999, that crashed on landing on Mars on December 3, 1999. The lander was designed to directly enter the atmosphere, deploy a parachute, then fire rockets and soft land on the surface. Subsequent studies have indicated that a coupled software/hardware error led to premature shutdown of the landing rocket. Instruments on the Mars Polar Lander included cameras, a robotic arm, soil composition instruments, a light detection and ranging instrument (LIDAR), and two microprobes which were presumably deployed before the lander entered the atmosphere but were also lost.

Martian geophysical epochs Similar to assignments on Earth, names have been assigned to geophysical epochs on the planet Mars. The names and their approximate age correspondences are: Amazonian, 0 to 1.8 Gy BP; Hesperian, 1.8 to 3.5 Gy BP; Noachian, more than 3.5 Gy BP.

Martian meteorites Currently 13 meteorites display characteristics that indicate they are probably from Mars. Twelve of these meteorites have very young formation ages (ranging from 1.8 × 108 yr to 1.3 × 109 yr) and have compositions indicative of formation in basaltic lava flows. These young meteorites are called the Shergottites, Nakhlites, and Chassigny (or SNC meteorites), after the three meteorites which display the major chemical characteristics of each group. The young, basaltic compositions suggested a body that was volcanically active very recently. This process of elimination resulted in the Earth, Venus, and Mars as the likely parent bodies for these meteorites. Other chemical evidence (primarily from oxygen isotopes) eliminated the Earth from consideration and orbital dynamic considerations eliminated Venus as a possibility. The Martian origin of these meteorites was largely confirmed by the discovery of trapped gas within some of the meteorites; the isotopic composition of this gas is statisti-

© 2001 by CRC Press LLC

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