DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

mascon
cally identical to the composition of the Martian
atmosphere. The 13th meteorite is much
older than the other 12 (dating from 4.5 × 10 9
yrs ago) and may contain evidence of fossilized
Martian life. These meteorites are believed to
have been ejected from the planet’s surface duringtheformationofimpactcratersonMars.
The
subsequent discovery of lunar meteorites indicates
that this is a common process, at least for
the smaller solar system bodies.
mascon “Mass concentration”; concentration
of density beneath the surface of the moon,
detectablebecauseofitseffectonsatelliteorbits.
Mascons associated with lunar craters typically
lie under the center of the craters; there is usually
a mass deficit in a ring associated with the
crater wall.
maser Acronymformicrowaveamplification
by stimulated emission of radiation: amplification
of radiation coming from excited states of
molecules. While a maser can be a laboratory
instrument, especially interesting are the natural
astrophysical masers. Amplified lines have
been observed at frequencies in the range of 1
to 100 Ghz, in association with dense molecular
clouds associated to star forming regions, or in
circumstellar envelopes of cold late-type stars,
such as giant and supergiant M stars, carbon
and S stars, where diatomic or more complex
molecules are not dissociated by the radiation
from the star. Masers are produced by stimulated
emission: a photon of frequency matching
the frequency of a particular transition between
two states induces the emission of a second photon,
whose frequency and phase are identical to
the first. Net amplification of the line radiation
is achieved by population inversion, a condition
realized when the higher energy level is
more populated than the lower by a “pumping”
source of energy, like a background radiation
field. Astrophysical masers have been observed
at several frequencies corresponding to rotational
transition of di- and tri-atomic molecules.
The most luminous masers have been observed
in external galaxies at the frequency of 22.235
Ghz corresponding to a rotational transition of
the water vapor molecule; they are known as
water mega-masers.
© 2001 by CRC Press LLC
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mass coefficient See inertia coefficient.
mass-defect The binding energy Eb of a system
expressed in terms of Einstein’s famous formula
Eb = Mbc 2 .(Mb is taken positive if the
system is bound.) In Newtonian gravity, and in
Einstein’s general relativity,
Mgrav = M0 − Mb
where M0 is the sum of the masses of the system’s
constituents if it were infinitely dispersed,
and Mgrav is the active gravitational mass that
determines, e.g., planetary orbits. In other relativistic
theories of gravity this simple relation
does not hold, and different kinds of binding energy
make different contributions to the active
gravitational mass.
mass extinctions Extinctions of a large percentage
of flora and/or fauna have occurred
throughout Earth’s history. The definition of
a mass extinction is that more than 25% of
the existing species disappear from the Earth
within a short time period (generally < 10 5 yrs).
Since the boundaries between terrestrial geologic
epochs are defined by changes in the fossil
record, many of these boundaries are coincident
with mass extinctions. Mass extinctions result
from major environmental changes that occur
rapidly enough that many species do not have
time to adapt. Several mass extinctions are now
associated with large impact events on the Earth,
the most famous being the extinction of the dinosaurs
and other species at the end of the Cretaceous
Period (65×10 6 yrs ago) by the creation of
the approximately 200-km-diameter Chicxulub
impact crater in Mexico. Debris tossed into the
atmosphere by these large impacts decreases the
amount of sunlight reaching the surface, causing
a collapse of the food chain and accompanying
starvation, which leads to the mass extinctions.
Mass extinctions are also suspected
to occur when plate motions cause changes in
ocean currents, which leads to climate change,
and perhaps by periods of enhanced volcanic activity.
massive ghost A theoretical massive particle
with unphysical properties which spoils the
physical content of a quantum particle theory.