930 Encyclopedia of the Solar System
the atomic nucleus. Neutrons liberated from the nucleus by
nuclear reactions decay by beta emission with a mean lifetime of
900 seconds.
New comet A nearly isotropic or long-period comet that is
entering the planetary region for the first time since it was
placed in the Oort cloud. Dynamically new comets are usually
taken to be nearly isotropic comets with original semimajor axes
greater than 10,000 AU.
Newton’s laws Three laws of motion and one of gravity that
describe aspects of the physical world: (1) A body remains at
rest or in uniform motion unless it is acted upon by an external
force. (2) The acceleration of a body is directly proportional to
the force acting upon it and inversely proportional to its mass.
(3) For every action, there exists an equal and opposite reaction.
(Gravity) The gravitational attraction between any two
spherically symmetric objects is proportional to the product of
their masses and inversely proportional to the square of the
distance between their centers.
Node One of the two points where a body’s orbit crosses a
reference plane, such as the ecliptic.
Nongravitational force A force not due to gravity that acts
on comets and asteroids and that can significantly alter their
orbits. The most important nongravitational force for comets is
the reaction force due to the outgassing of volatile materials
from the day side of the nucleus.
Nonsynchronous rotation The state of a satellite whose
rotation period is not equal to its orbital period.
Nonthermal escape Atmospheric escape of gases in
processes that do not depend on the temperature of the bulk
upper atmosphere. Nonthermal escape can occur when a neutral
species is photoionized by solar extreme ultraviolet radiation
and recombines with an electron to form a fast neutral atom.
This is called photochemical escape. Alternatively, a fast ion can
impart its charge to a neutral atom through collision or charge
exchange, and become a fast neutral atom with escape velocity.
Today, photochemical escape is important for the loss of carbon,
oxygen, and nitrogen from Mars.
Nonthermal radio emission Radio emission produced by
processes other than those which produce thermal emission.
In particular, in planetary science we are concerned with
cyclotron and synchrotron radiation. Cyclotron radiation is
emitted by (nonrelativistic) electrons, often in the auroral
(near-polar) regions of a planet’s magnetic field at the frequency
of gyration around the magnetic field lines (cyclotron
frequency). The emission is like a hollow cone pattern.
Synchrotron radiation is produced by relativistic (i.e., particle
velocity approaches the speed of light) electrons. This radiation
is strongly beamed in the direction in which the particle is
moving. There are other types of nonthermal radio emissions
that involve coupling from various plasma wave modes to radio
waves.
Nucleosynthesis The creation of stable and unstable isotopes
in stars.
Nucleus The central cometary body that is the source of all
the other cometary features. The nucleus is composed of volatile
materials, primarily water ice, and dust particles composed of
both silicates and organics. Sublimation produces molecular
gases and releases the dust particles. The nucleus has a typical
radius of 1–10 km.
Oblateness A measure of the amount to which the shape of a
planet or other body differs from a perfect sphere.
Obliquity The angle between a planet’s equator and its
orbital plane. Earth’s current obliquity of 23.5◦is sufficient to
cause seasons.
Observational bias The effect that some astronomical
objects are easier to discover and observe than others, generally
because they are brighter. The observed sample of comets
suffers from severe bias because, in general, only those comets
that pass well within the orbit of Jupiter become bright enough
to be observable.
Occultation Obscuration of a body brought about by the
passage of another body in front of it. Occultations of stars by
planets or by planetary rings allow the observer to probe the
atmospheric structure of the planet or the structure of the rings.
Occultation of stars by asteroids allow the observer to determine
the size of the asteroid.
Oersted Unit of magnetic intensity in the
centimeter–gram–second system, equivalent to the gauss.
Oligarchic growth Self-regulated stage of planet formation
that follows runaway growth.
Oort cloud A large reservoir of several times 10^12 cometary
nuclei surrounding the planetary system and extending from a
few thousand AU to about 100,000 AU from the Sun. The outer
Oort cloud, beyond∼10,000 AU, is roughly spherical; the inner
Oort cloud is flattened toward the ecliptic plane. The existence
of the Oort cloud is inferred from the semimajor axis
distribution of long-period comets.
Opacity The ability of an atmosphere to absorb (or
sometimes scatter) radiation. Also called optical depth. A beam
of monochromatic radiation passing through an atmosphere with
an optical depth of one will have its intensity reduced by a factor
ofe(= 2 .718), while an optical depth of 4 absorbs or scatters
98% of the radiation. Opacity is a function of wavelength as well
as of the pressure, temperature, and composition of the region
of the atmosphere under consideration.
Opposition effect Nonlinear surge in brightness as a
celestial object approaches being viewed at zero phase angle.
Opposition The position of a superior planet, a comet or an
asteroid when it is opposite the Sun in the sky, i.e., when its
elongation approaches 180◦.
Optical depth Measure of the integrated extinction of light
along a path through a medium, such as an atmosphere or the
disk of particles forming a ring. Normal optical depth refers to
the extinction along a path perpendicular to the ring plane.
Orbit The path of a planet, asteroid, or comet around the
Sun, or of a satellite around its primary. Most bodies are in