Glossary 929
seen with theHubble Space Telescopehave magnitude values of
−26.74,+6, and+28, respectively. For every change by five
magnitudes, the brightness changes by a factor of 100. One
magnitude equals a factor of 100^1 /^5 or∼2.5119 in brightness. All
magnitudes are scaled to the flux of Alpha Lyrae, also named
Vega, which is designated as magnitude 0.
Main sequence When stars are plotted on a graph of their
luminosity versus their surface temperature (or color), most stars
fall along a line extending from high-luminosity, high–surface
temperature stars, to low- luminosity, low–surface temperature
stars. This plot is known as the Hertzsprung–Russell diagram,
and the line is known as the main sequence. Stars spend the
majority of their lifetimes on the main sequence, during which
they produce energy by hydrogen fusion within their cores.
Mantle Portion of the interior of a body between the core
and the lithosphere that is hot enough to allow material to flow
by solid-state creep, allowing fluid-like behavior to occur on
time scales of 10^8 –10^9 years.
Mantle convection Movement of material within the Earth’s
mantle occurs because the density of constituent rock is
related to temperature. Buoyant hot material thus tends to rise
and cooler material tends to sink over time. In our experience,
convection is most commonly associated with liquids or gases,
however, deep within the Earth, plastic and even solid rock
under pressure can convect, and thus transport heat away
from the core, through the mantle, and ultimately toward the
surface.
Mare (pl., maria) Latin word for “sea,” used first by Galileo
to refer to the dark patches on the lunar surface, now known to
be basaltic lava flows.
Mascons Regions of the Moon of excess mass concentrations
per unit area, identified by positive gravity anomalies and
associated with basalt-filled multiring basins.
Mass wasting The downslope movement of rock, regolith,
and soil under the influence of gravity.
Maxwellian distribution The distribution of particle
velocities for a gas in thermal equilibrium.
Mean-motion resonance An orbital resonance in which the
orbital periods of the bodies involved are in a simple integer
ratio. For example, Pluto is in a 2:3 mean-motion resonance
with Neptune; it completes two orbits around the Sun for every
three of Neptune.
Meridional circulation Motions of the atmosphere in the
plane defined by the vertical and latitudinal coordinates.
Atmospheric motions in the vertical and north–south directions
participate in the meridional circulation.
Meridional mixing Mixing of the atmosphere along
meridians (lines of constant longitude), for example, between
polar regions and mid-latitudes.
Metallic hydrogen High-pressure (≥1.4 Mbar) metallic
form of hydrogen found in the interiors of Jupiter and Saturn.
Meteor ight phenomenon that results from the entry of a
meteoroid from space into Earth’s atmosphere.
Meteorite Meteoroid that has reached the surface of Earth or
another planet without being completely vaporized.
Meteoroid A small fragment of an asteroid or comet that is
in interplanetary space. When a meteoroid enters a planetary
atmosphere and begins to glow from friction with the
atmosphere, it is called ameteor. A fragment that survives
atmospheric entry and can be recovered on the ground is called
ameteorite.
Micrometeoroid Meteoroid smaller than about 0.1 mm in
size.
Micron, micrometer, orμμm One millionth of a meter.
Microscopic cross section An effective area that gives the
probability that a particle (for example, a neutron or gamma ray)
will undergo a reaction with a target nucleus (or atom). The
microscopic cross section has units of barns per nucleus. One
barn is 10−^24 cm^2.
Mie scattering The scattering of sunlight by atmospheric
particles such as aerosols.
Miller–Urey experiments Laboratory experiments in which
mixtures of gases representing the composition of planetary
atmospheres or the Earth’s early atmosphere were placed in
sealed vessels and exposed to various forms of energy such as
UV and energetic particles. In general the experiments tended
to produce more complex combinations of molecules from the
initial gases.
Mineral Naturally-occurring substance of specified chemical
composition and physical properties having a characteristic
atomic structure and/or crystalline form.
Minor planet Another term for an asteroid.
Mixing ratio Fractional mass of a particular component of an
intimate mixture.
Molecular cloud Cold, dense, region of the interstellar
medium containing molecular hydrogen: H 2 , often the site of
star formation.
Moment of inertia Quantity that is the measure of the
density distribution within a planet, specifically the tendency for
an increase of density with depth. It has a value of 0.400 for a
sphere of uniform density.
Moon A body in orbit around another larger body, known as
the primary, such as a planet, dwarf planet, or asteroid. Also
called asatellite.
Morphology Study of the shape of landforms on a planetary
surface.
Near-Earth object (NEO) Any object, such as an asteroid or
comet, orbiting the Sun with a perihelion distance less than 1.3
AU.
Nearly isotropic comet (NIC) A population of comets with
orbits that are randomly inclined to the ecliptic plane. By
definition, the Tisserand parameter of NICs is less than 2. Also
known as along-period comet.
Neutron A neutral particle with mass similar to that of the
proton. Neutrons and protons are the primary constituents of