934 Encyclopedia of the Solar System
kis Boltzmann’s constant,Tis temperature,mis the mean mass
of the gas, andgis the acceleration of gravity.
Scattered disk A collection of∼ 109 icy planetesimals in
high-eccentricity, low-to-moderate inclination orbits beyond
Neptune. Scattered disk objects typically have semimajor axes of
order 50-100 AU. The scattered disk is probably the primary
source of the Jupiter-family comets. Scattered disk objects may
have escaped the Kuiper belt billions of years ago, and/or may
be scattered Uranus-Neptune planetesimals.
Scattering law Function giving the dependence of a surface
element’s radar cross section on viewing angle.
Scintillator A transparent material that coverts the kinetic
energy of charged particles, such as electrons produced by
gamma ray interactions or alpha particles and recoil protons
produced by neutron reactions, into flashes of light detectable
by a photomultiplier tube or photodiode. A wide variety of
organic and inorganic materials scintillate and can be used for
radiation detection and spectroscopy.
Secondary crater A crater produced by the impact of blocks
of ejecta from a primary impact by a comet or asteroid.
Secular Continuing or changing over a long period of time.
Secular perturbations Long-term changes to the orbit of a
body caused by the distant gravitational perturbations of the
planets and other bodies.
Secular resonance Near-commensurability among the
frequencies associated with the precessions of the line of nodes
and/or apsides.
Seeing Blurringof the image of an astronomical object
caused by turbulence in the Earth’s atmosphere Atmospheric
seeing at the very best observatory sites, such as Mauna Kea, is
about 0.5 arcsec and can be as good as 0.25 arcsec. Seeing can
be improved usingadaptive optics.
Semiconductor Semiconductors, such as germanium,
silicon, and CdZnTe, can be used to detect gamma rays. Swift
electrons produced by Compton and photoelectric interactions
ionize the semiconductor, producing electron-hole pairs. The
electrons and holes drift under the influence of an applied
electric field to electrical contacts. As they drift, the electrons
and holes induce charge on contacts, which can be measured by
a charge-sensitive preamplifier. The amplitude of the charge
pulse is proportional to the energy deposited by the gamma ray,
which enables semiconductors to be used for spectroscopy.
Semimajor axis (a) Commonly thought of as the mean
distance of the orbit of a body from its primary. More formally, it
is one half of the longer of the two axes of an ellipse describing
the orbit, that passes through both foci of the ellipse.
Semiminor axis (b) One half of the minor axis (short
diameter) of an elliptical orbit.
Sensitivity to initial conditions A situation in which a tiny
change in an object’s initial state (position and/or velocity) will
make a big change in its final trajectory. Sensitivity to initial
conditions is a necessary condition for chaos.
Separatrix Boundary of a resonance, separating resonant or
librating motion inside the resonance from nonresonant or
circulating motion outside.
Shield volcano Broad volcano with a large summit pit
formed by collapse and gently sloping flanks, built mainly from
overlapping, fluid, basaltic lava flows.
Shock A discontinuous, nonlinear change in pressure
commonly associated with supersonic motion in a gas, plasma,
or solid.
Shock metamorphism Permanent physical, chemical, and
mineralogical changes in rocks, resulting from the passage of a
shock wave.
Shock wave Compressional wave, resulting from an impact
or explosion, which travels at supersonic velocities.
Short-lived isotopes Radioactive isotopes with half-lives
much shorter than the age of the solar system.
Short-period comet A comet with an orbital period< 200
years. Short-period comets includeJupiter-familyand
Halle-typecomets.
Sidereal Relative to the fixed stars. A sidereal period is the
orbital period of a planet around the Sun relative to the stars. A
sidereal year is the orbital period of the Earth around the Sun
relative to the stars.
Siderophile element Element that tends to join with iron
and is predominantly found in a planet’s core.
Silicate A compound containing silicon and oxygen.
Smooth plains The youngest plains on Mercury with a
relatively low impact crater abundance.
SNC meteorites Group of meteorites
(Shergotty–Nakhla–Chassigny) believed to be derived from
Mars because of their young ages, basaltic composition, and
inclusion of gases with the same composition as the Martian
atmosphere.
Solar activity cycle Cycle of∼11 year duration
characterized by waxing and waning of various forms of solar
activity such as sunspots, flares, and coronal mass ejections.
Solar corona The hot, tenuous outer atmosphere of the Sun
from which the solar wind originates.
Solar energetic particles Ions (usually protons) and
electrons generated in solar flares and in the corona and solar
wind by shock waves, with energies above hundreds of keV for
electrons and above an MeV per nucleon for ions.
Solar flare A disturbance in the solar atmosphere
characterized by a sudden, localized enhancement in
electromagnetic emission from visible to x-ray wavelengths.
Solar nebula The cloud of dust and gas out of which the Sun
and planetary system formed.
Solar phase angle Seephase angle.
Solar wind A magnetized, highly ionized plasma that flows
radially out from the solar corona at supersonic and
super-Alfv ́enic speed.