928 Encyclopedia of the Solar System
Lagrangian points The five locations in the circular
restricted three-body problem at which the net gravitational and
centrifugal forces in the frame rotating with the massive bodies
is zero. The first three Lagrangian points,L 1 ,L 2 , andL 3 , lie on
the line connecting the massive bodies; all three colinear
Lagrangian points are unstable. TheL 4 andL 5 Lagrangian
points each make equilateral triangles with the two massive
bodies; orbits about the triangular Lagrangian points are stable
to small perturbations provided the ratio between the masses of
the two bodies is≥27.
Landforms Natural physical features of a planet’s surface.
Langmuir probe Instrument used to measure electron and
ion densities. The external sensor is usually a stiff wire and the
current is measured as different voltages are applied.
Laplace resonance Occurs when three or more orbiting
bodies have a simple integer ratio between their orbital periods.
Last universal common ancestor The hypothetical latest
living organism from which all currently living organisms
descend.
Latent heat Heat that is released or absorbed during a phase
change, i.e., vapor or liquid or ice to ice or liquid or vapor.
Latent heat contributes to heating and cooling the atmosphere
in regions where ice and liquid clouds form and dissipate. It also
contributes to the heat capacity of a parcel of gas/cloud and
therefore influences the adiabatic temperature gradient.
Libration A small oscillation around an equilibrium
configuration, such as the angular change in the face that a
synchronously rotating satellite presents toward the focus of its
orbit.
Lightcurve A graph of an object’s brightness versus time.
Since asteroids and cometary nuclei are usually not perfect
spheres, the observed projected area of the object varies as the
object rotates. The time difference between the peaks of the
lightcurve provide a measure of the object’s rotation rate and the
shape of the lightcurve can be statistically modeled to derive the
object’s shape.
Limb-darkening The darkening of the observed edges a
planetary disk or a star. This may be due to the scattering
properties of the surface (if, for example, it is a strongly
backscattering surface, like an icy one) or more usually to the
presence of an optically thick atmosphere. It is often
characterized by an exponentk, the Minnaert exponent, for a
scattering law of the formI=I 0 μkμk 0 −^1 , whereμandμ 0 are
the cosines of the angle between the normal at a given point and
the observer and sun respectively andI 0 is the brightness of the
center of the disk.k= 0 .5 corresponds to a flat disk (rather like
the moon), whilek=1 is a Lambertian disk with strong
limb-darkening.k< 0 .5 corresponds to limb-brightening,
typical of a scattering but optically thin region above an
absorbing (dark) region in the atmosphere.
Lithophile Material made of elements that are commonly
found in rocks, such as Si, O, Al, Ca, and Fe; derived from the
Greek, meaning “rock-loving.”
Lithosphere The rigid outer shell of a planetary body,
generally including a chemically distinct crust and part of the
upper mantle; the lithosphere is rheologically defined and so its
thickness depends strongly on temperature.
Lobate scarp A long sinuous cliff (seeThrust fault).
Longitude of perihelion (ω ̄) The sum of the longitude of
the ascending node and the argument of perihelion.
Longitude of the ascending node (Ω) The nodes of an
orbit are the points where the orbit crosses some reference
plane, usually the ecliptic. The ascending node is where
the orbit crosses the reference plane from south to north.
The longitude of the ascending node is the angle between the
location of the ascending node and some standard direction in
the reference frame, usually the direction of the vernal equinox.
Long-period comet A comet with an orbital period of more
than 200 years. Some long-period comets have orbital periods of
millions of years.
Lorentz force Force exerted by a magnetic field on a
moving charged particle. This force is always perpendicular to
the motion of the particle.
Lyapunov exponent Measure of the rate of divergence of
two nearby trajectories in a system. A positive Lyapunov
exponent is associated with chaotic motion, and its inverse gives
an estimate of the timescale for exponential separation of nearby
orbits.
Macroscopic cross section The product of the number
density (number per cm^3 ) of the target nuclei and the
microscopic cross section. The macroscopic cross section has
units of cm−^1 and gives the probability per unit path length that
a particle will undergo an interaction, for example, in a planetary
surface.
Mafic Dense, Fe- and Mg-rich silicate minerals, such as
those that dominate the mantle; usually refers to basalts and
other refractory igneous rock types.
Magnetic reconnection A magnetic instability that can be
triggered in the solar corona, where the topology and
connectivity of magnetic field lines change; believed to be the
primary cause of flares and coronal mass ejections.
Magnetic storm A prolonged interval of intense
geomagnetic activity often lasting for days.
Magnetopause The outer boundary of a magnetosphere
between the solar wind region and a planets’ magnetic field
region, where a strong thin current generally flows.
Magnetosheath The region between a planetary bow shock
and magnetopause in which the shocked solar wind plasma
flows around the magnetosphere.
Magnetosphere The region of space around a planet or
satellite dominated by its intrinsic magnetic field and associated
charged particles.
Magnitude A logarithmic unit of brightness. Large magnitude
values correspond to faint objects. The Sun, the faintest star
visible with the unaided eye, and the faintest Kuiper belt object