Glossary 935
Solidus Line or surface in a phase diagram below which the
system is completely solid.
Space debris Man-made particulates littered in space.
Space ultraviolet That part of the ultraviolet
electromagnetic spectrum that can only be observed from space
because the Earth’s atmosphere is opaque at those wavelengths;
commonly thought of as the region below wavelengths of
3000 A. ̊
Space weather The variable level of geomagnetic activity
controlled by the conditions in the solar wind.
Space weathering A process acting on the surface of
planetary and asteroidal bodies that changes their surface
optical properties over time.
Spectral absorption A particular wavelength of light that is
selectively absorbed by a particular material. Patterns of
absorptions can serve as “fingerprints” to remotely identify
surface or atmospheric materials.
Spin–orbit resonance Simple numerical relationship
between the spin period of a planet or satellite and its orbital
period. Most natural satellites in the solar system are in the 1:1
spin–orbit resonance, also called the synchronous spin state.
Sputtering An atmospheric loss process that occurs when
ions that have been picked up by the magnetic field embedded
in the solar wind impact a planetary atmosphere and undergo
charge exchange. Charge exchange neutralizes the ions, which
can impart their large energies to surrounding particles by
collision. Upward-directed energetic particles can then escape.
This process may have been important on Mars after it lost its
magnetic field and its upper atmosphere was no longer shielded
from the solar wind. Sputtering can also occur when energetic
particles from the solar wind or a planetary magnetosphere
strike the surface of an airless planet or satellite and cause atoms
of the surface materials to escape.
Stellar metallicity The amount of chemical elements,
heavier than hydrogen and helium, contained in a star.
Observations indicate that stellar metallicity is a critical factor in
the efficiency of the formation and/or orbital migration of
extrasolar planets.
Stellar occultation When a planet or asteroid passes in front
of a star and the star is briefly hidden from view. Such events
can be used to probe the size and also the atmospheric structure
of the planet (or asteroid) doing the occulting, or the structure
of rings around a planet.
Stellar reflex motion The movement of a star along its orbit
around the barycenter of a star/companion system. If a star has
no companions (stars or planets), the barycenter coincides with
the star’s own center of mass, and no reflex motion exists.
Strain Forces acting in opposite directions, pulling materials
apart.
Stratigraphy Study of rock layers.
Stratosphere Region in an atmosphere overlying the
troposphere that is strongly stabilized against convection by
heating because of the absorption of ultraviolet radiation from
the Sun.Stratumis Latin for “layer.”
Stratosphere, mesosphere Region (also together called
middle atmosphere) whose temperature is controlled by
radiative balance. On Earth, it extends from about 10 to 95 km,
and on Venus, from 65 to 95 km.
Stream structure Pattern of alternating flows of low- and
high-speed solar wind.
Stromatolite A geological feature formed by the conversion
of loose, unconsolidated sediment into a coherent layer, as a
result of the growth, movement, or activity of microorganisms;
e.g., blue-green algae. Microfossils associated with stromatolite
formation are an important form of evidence for early life on
Earth, and thus a search for stromatolites could undertaken on
other planets in sites where liquid water might have
accumulated.
Subaerial Referring to landscapes, such as islands or
continents, that are exposed to the air.
Sublimation The phase change of a solid directly to gas, as in
the conversion of ice directly into vapor.
Substorm The elementary disturbance of the magnetosphere
that produces geomagnetic activity.
Sulfur allotropes Sulfur cooled rapidly from different
temperatures, resulting in different colors.
Superior geocentric conjunction The point in a planetary
satellite’s orbit where it is directly opposite Earth, such that the
satellite lies on a straight line connecting Earth, the planet, and
the satellite.
Surface of section Means of studying the regular or chaotic
nature of an orbit by plotting a sequence of points in two
dimensions that can represent all or part of the coordinates of
the orbit in phase space.
Synchronous orbit An orbit whose period is equal to the
rotation period of the primary.
Synchronous rotation Dynamical state caused by tidal
interactions in which a satellite presents the same face toward
the primary, because the satellite’s rotation period is equal to its
orbital period.
Synodic period For an inferior planet, the time between
successive conjunctions. For a superior planet, the interval
between successive oppositions. For the Sun, the time taken for
one revolution of the Sun as seen from Earth.
Synodic rotation period Apparent rotation period of a
target that is moving relative to the observer (who may also be
moving), to be distinguished from the sidereal rotation period
measured with respect to the fixed stars.
Tadpole orbits Orbits that librate about the stableL 4 orL 5
triangular Lagrangian points in the restricted three-body
problem. These orbits appear to be shaped like tadpoles in the
frame rotating with the mean-motion of the massive bodies.
Taxon Grouping of organisms or bodies with similar
characteristics.