S
sapping
A process of erosion where water leaks to the surface through the pores of rocks; as the water flows away, it slowly removes material to form valleys and channel networks.
satellite
A body that revolves around a larger body.
scarp
A line of cliffs produced by faulting or erosion; a relatively straight, clifflike face or slope of considerable linear extent, breaking the general continuity of the land by separating surfaces lying at different levels.
scopulus
A lobate or irregular scarp.
semimajor axis
One-half of the longest dimension of an ellipse.
shatter cone
Striated conical fracture surfaces produced by meteorite impact into fine-grained, brittle rocks such as limestone.
shepherd satellite
A satellite that constrains the extent of a planetary ring through gravitational forces.
shield
Any of several extensive regions where ancient Precambrian crystalline rocks are exposed at the Earth's surface.
shield volcano
A volcano in the shape of a flattened dome, broad and low, built by flows of very fluid lava.
shock metamorphism
The production of irreversible chemical or physical changes in rocks by a shock wave generated by impact, or detonation of high-explosive or nuclear devices.
siderial
Of, relating to, or expressed in relation to stars or constellations.
siderial rotation
Rotation time measured with respect to the fixed stars rather than the Sun or body orbited.
siderophile elements
This phrase literally means iron-loving elements. It includes Iridium, Osmium, Platinum and Plladium, which are found in the metal-rich interiors of chemically segregated asteroids and planets; consequently, these elements are extremely rare on Earth's surface.
silicate
A rock or mineral whose structure is dominated by bonds of silicon and oxygen atoms (ie. olivine).
sinus
A bay.
solar cycle
The approximately 11-year, quasi-periodic variation in the frequency or number of solar active events.
solar nebula
The large cloud of gas and dust from which the Sun and planets condensed 4.6 billion years ago.
solar wind
A tenuous flow of gas and energetic charged particles, mostly protons and electrons -- plasma -- which stream from the Sun; typical solar wind velocities are almost 350 kilometers (217 miles) per second.
spatter cone
A low, steep-sided cone built up from fluid pyroclasts coating the surface around a vent.
spectroradiometer [SPEC-tro-RAY-dee-om-it-er]
A device that measures the amount of reflected or radiated energy from a surface in two or more wavelengths.
spectrum
The distribution of wavelengths and frequencies.
speed of light
Light speed equals 299,792,458 meters/second (186,000 miles/second). Einstein's Theory of Relativity implies that nothing can go faster than the speed of light.
spicules
The grass-like patterns of gas seen in the solar atmosphere.
stratosphere
The cold region of a planetary atmosphere above the convecting regions (the troposphere), usually without vertical motions but sometimes exhibiting strong horizontal jet streams.
stishovite
A dense, high-pressure phase of quartz that has so far been identified only in shock-metamorphosed, quartz-bearing rocks from meteorite impact craters.
subduction
The process of one lithospheric plate descending beneath another.
sublime
Sublimation occurs when a substance changes directly from a solid to a gas without becoming liquid.
sulcus
Subparallel furrows and ridges.
sulfuric acid
A heavy, corrosive, oily, dibasic strong acid H2SO4 that is colorless when pure; it is a vigorous oxidizing and dehydrating agent.
sunspot
An area seen as a dark spot on the photosphere of the Sun. Sunspots are concentrations of magnetic flux, typically occurring in bipolar clusters or groups. They appear dark because they are cooler than the surrounding photosphere.
superior planets
The planets Mars, Jupiter, Saturn, Uranus, Neptune and Pluto are superior planets because their orbits are farther from the Sun than Earth's orbit.
synchronous orbit radius
The orbital radius at which the satellite's orbital period is equal to the rotational period of the planet. A synchronous satellite with an orbital inclination of zero (same plane as the planet's equator) stays fixed in the sky from the perspective of an observer on the planet's surface. These orbits are commonly used for communications satellites).
synchronous rotation
A satellite's rotational period is equal to its orbital period; this causes the same side of a satellite to always face the planet. Synchronous rotation occurs when a planet's gravity produces a tidal bulge in its satellite. The gravitational attraction and bulge acts like a torque, which slows down the satellite until it reaches a synchronous rotation.
synthetic-aperture radar
SAR is a side-looking imaging system that uses the Doppler effect to sharpen the effective resolution in the cross-track direction.