RepetoHow does a planet's rotation differ from its revolution around the Sun?
Rotation is spinning about its own axis; revolution is traveling around the Sun.
Astronomy: the Solar System
130 questions in English on the Sun, planets, moons, orbits and small bodies.
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What is the order of the eight planets outward from the Sun?
Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune.
Is the main asteroid belt a densely packed obstacle course with neighboring rocks almost touching?
No. Individual asteroids are usually separated by very large distances.
In the ideal Keplerian description of a planet's bound orbit, where is the Sun relative to the ellipse?
At one focus, approximately treating the Sun as stationary because it is much more massive.
What is unusual about Uranus's rotation axis?
It is tilted by about 98 degrees relative to the perpendicular to its orbital plane, so the planet rotates nearly on its side.
Under the IAU's 2006 Solar System definition, what three conditions identify a planet?
It orbits the Sun, its gravity makes it nearly round, and it has cleared its orbital neighborhood.
All 130 cards
Every card in the deck, in study order.
Question Answer
How does a planet's rotation differ from its revolution around the Sun? Rotation is spinning about its own axis; revolution is traveling around the Sun.
What is the order of the eight planets outward from the Sun? Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune.
Is the main asteroid belt a densely packed obstacle course with neighboring rocks almost touching? No. Individual asteroids are usually separated by very large distances.
In the ideal Keplerian description of a planet's bound orbit, where is the Sun relative to the ellipse? At one focus, approximately treating the Sun as stationary because it is much more massive.
What is unusual about Uranus's rotation axis? It is tilted by about 98 degrees relative to the perpendicular to its orbital plane, so the planet rotates nearly on its side.
Under the IAU's 2006 Solar System definition, what three conditions identify a planet? It orbits the Sun, its gravity makes it nearly round, and it has cleared its orbital neighborhood.
Which four planets are called terrestrial planets, and what do they have in common? Mercury, Venus, Earth and Mars; they have rocky surfaces and interiors containing rock and metal.
Is Venus's rotation period relative to the stars longer or shorter than its orbital period around the Sun? Longer: roughly 243 Earth days for a rotation versus 225 for an orbit. Its sunrise-to-sunrise day is a different interval.
If measurements suggest a subsurface ocean, does that alone demonstrate that the ocean contains life? No. Evidence for an environment and evidence for organisms are different scientific claims.
What creates the heliosphere around the Solar System? The solar wind and the Sun's magnetic influence form a bubble within the surrounding interstellar medium.
How can a spectrum help identify substances in a planet's atmosphere? Atoms and molecules absorb or emit characteristic patterns at particular wavelengths.
How can a planetary gravity assist change a spacecraft's speed relative to the Sun without an engine burn during the flyby? The moving planet's gravity redirects the spacecraft and exchanges orbital energy with it; the spacecraft can gain or lose heliocentric energy.
Near new Moon, is the Moon on Earth's Sun-facing side or on the opposite side? It is on the Sun-facing side, so most of the sunlit half faces away from Earth.
What does accretion mean in planet formation? Growth by collecting material, including particles or larger bodies that collide and become incorporated.
Why do interplanetary rovers need some autonomous control instead of continuous instant driving from Earth? Radio signals take time to travel between Earth and the rover, delaying commands and feedback.
What is a comet's nucleus? Its solid central body, containing frozen volatile substances mixed with dust and rock.
Why does a satellite in orbit not need gravity to vanish? It is continually falling under gravity while its sideways motion carries it around the central body.
How does atmospheric methane help give Uranus and Neptune their blue-green appearance? Methane absorbs much of the red light, leaving more shorter-wavelength light to be reflected.
What does plate tectonics describe on Earth? The movement and interaction of large pieces of the rigid outer shell, which continually reshape the surface.
Approximately how far is one astronomical unit, or AU, in kilometers? About 150 million kilometers, roughly the mean Earth–Sun distance.
What motion generates most of Earth's global magnetic field? Motion of electrically conducting liquid metal in its outer core.
Why could ice contribute more solid material in colder regions of the early disk than near the young Sun? Volatile substances could remain frozen in colder regions but were gaseous in hotter regions.
Why do sunspots look dark compared with the surrounding photosphere? Their strong magnetic fields inhibit heat transport, making them cooler and dimmer than nearby regions.
Why is radar useful for mapping Venus's solid surface from orbit? Radar wavelengths can penetrate its thick cloud cover, which hides the surface in visible light.
Does calling Uranus and Neptune ice giants mean that they are solid balls of ice? No. Their interiors contain a large fraction of heavier materials such as water, ammonia and methane under high-pressure conditions, beneath gaseous atmospheres.
Which giant planets are classified as gas giants, and which as ice giants? Jupiter and Saturn are gas giants; Uranus and Neptune are ice giants.
How does remote sensing differ from an in situ measurement? Remote sensing measures from a distance; an in situ instrument measures material or conditions where it is located.
For small bodies orbiting the same central mass, what does Kepler's third law relate? The square of orbital period is proportional to the cube of orbital semimajor axis.
Which planet has the greatest mass and diameter in the Solar System? Jupiter.
Why does the Southern Hemisphere have summer when the Northern Hemisphere has winter? The Southern Hemisphere is tilted toward the Sun while the Northern Hemisphere is tilted away.
Which spacecraft made flybys of Uranus in 1986 and Neptune in 1989? Voyager 2.
Why do many impact craters remain visible on the Moon for long periods? It lacks Earth's weather-driven erosion and plate tectonics, so many surface features are preserved.
Near full Moon, where is Earth relative to the Sun and Moon? Earth lies roughly between the Sun and Moon, so the Moon's sunlit half faces us.
What keeps Saturn's ring particles circling the planet? Each particle follows an orbit governed mainly by Saturn's gravity.
Where is the main Kuiper Belt relative to Neptune? Beyond Neptune's orbit, extending roughly from 30 to 50 AU from the Sun.
What makes Earth's surface water unusual among the eight planets today? Earth has extensive, persistent oceans of liquid water at its surface.
What produces Jupiter's visible bands and zones? Clouds and powerful atmospheric winds form alternating bands at different latitudes.
What is the main cause of Earth's seasons? Earth's axial tilt changes the angle and duration of sunlight received by each hemisphere during the year.
In the standard formation model, what was the solar nebula? The cloud of gas and dust from which the Sun and surrounding planet-forming disk developed.
Does Europa's icy surface rule out liquid water beneath it? No. Several lines of evidence support a global liquid-water ocean beneath the ice shell.
What is an asteroid in introductory Solar System usage? A relatively small rocky or metallic body orbiting the Sun, usually without the active coma typical of an active comet.
Ignoring obstruction and other losses, how does sunlight per unit area change from 1 AU to 2 AU? It falls to one-quarter because sunlight spreads over an area proportional to distance squared.
What do ancient river channels and lake deposits on Mars indicate? Liquid water once flowed or collected at the surface under conditions different from today's.
Between which two planetary orbits does the main asteroid belt lie? Mars and Jupiter.
Using P² = a³ for a small body orbiting the Sun, with P in Earth years and a in AU, what is P when a = 4 AU? 8 Earth years, since P² = 64.
For two fixed point masses, how does gravitational force change if their separation doubles? It becomes one-quarter as strong, because force varies inversely with separation squared.
Why are real planetary orbits not exact, permanently fixed Keplerian ellipses? Other bodies exert gravitational forces, and additional effects can alter the orbit over time.
Is all water on Mars necessarily liquid? No. Much known Martian water is frozen, including polar and subsurface ice.
Under the IAU classification adopted in 2006, what kind of Solar System body is Pluto? A dwarf planet.
What process supplies most of the Sun's present energy? Nuclear fusion in its core converts hydrogen into helium and releases energy.
Why is there no Earth-like solid surface on Jupiter where a lander could simply park? Its gaseous outer layers grade into increasingly dense fluid at depth; there is no accessible rocky ground beneath the clouds.
What causes the Moon's phases as seen from Earth? As the Moon orbits Earth, we see different fractions of its sunlit half.
What is the solar corona? The Sun's very hot, thin outer atmosphere, extending far beyond the visible disk.
Why is Venus hotter at its surface than Mercury even though Venus is farther from the Sun? Venus's dense atmosphere produces an extremely strong greenhouse effect.
What is the solar wind? A continuing outflow of charged particles from the Sun into space.
What kind of landform is Olympus Mons on Mars? A giant shield volcano built by repeated lava flows.
What does Triton's retrograde orbit around Neptune mean? It orbits in the opposite direction to Neptune's rotation.
What do Enceladus's south-polar plumes eject into space? Water vapor and icy particles from activity connected to a subsurface ocean.
Why do many comets develop a coma when they approach the Sun? Solar heating releases gas from volatile material and carries dust away from the nucleus.
What is a meteoroid before it enters an atmosphere? A small natural solid object traveling through space.
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How does Venus's rotation direction compare with that of most planets? It rotates in the opposite, or retrograde, direction.
Which of the eight planets has the largest orbital semimajor axis? Neptune.
Are Saturn's bright rings a single solid sheet? No. They consist of enormous numbers of separate particles, mostly water ice with some rocky material.
Are the magnetic axes of Uranus and Neptune closely aligned with their rotation axes? No. Both have strongly tilted, offset magnetic fields.
What extra capability does a rover have compared with a stationary lander? It can travel across the surface to investigate multiple locations.
Why does the Sun not need oxygen to keep shining? Its energy comes from nuclear fusion, not chemical combustion.
What is the main source of the visible light by which we see planets in ordinary sky observations? Sunlight reflected by their atmospheres or surfaces.
Why does the Moon keep approximately the same face toward Earth? It rotates once in about the same time that it completes one orbit relative to the stars.
Is Saturn the only giant planet with rings? No. Jupiter, Uranus and Neptune also have ring systems, though they are much less conspicuous.
What are the Moon's dark maria? Broad plains formed by ancient basaltic lava flows, not seas of liquid water.
Must every meteor shower originate from a comet? No. Most are associated with comets, but some are associated with asteroids.
Why are ancient meteorites useful for studying Solar System formation? Their minerals, chemistry and ages preserve evidence of early materials and processes.
What blocks or shadows what in a solar eclipse versus a lunar eclipse? In a solar eclipse the Moon blocks the Sun from some observers on Earth; in a lunar eclipse the Moon enters Earth's shadow.
Has the distant Oort Cloud been directly imaged as a complete shell of objects? No. It is an inferred reservoir, supported especially by the orbits of long-period comets.
How does a solar flare differ from a coronal mass ejection? A flare is a burst of radiation; a coronal mass ejection expels a large cloud of magnetized solar plasma. They can occur together.
What is a protoplanetary disk? A rotating disk of gas and solid material around a young star, within which planets can form.
What does the roughly 11-year solar activity cycle describe? The rise and fall of activity such as sunspots and eruptions; individual cycles vary in length and strength.
Does an active comet's tail simply trail behind it along its direction of travel? No. Solar radiation and the solar wind generally drive tails away from the Sun; their shapes also depend on particle properties and motion.
What do perihelion and aphelion mean for an orbit around the Sun? Perihelion is the closest point to the Sun; aphelion is the farthest.
Where does the dwarf planet Ceres orbit? In the main asteroid belt between Mars and Jupiter.
Why do some meteor showers recur at similar times of year? Earth repeatedly passes through streams of debris along its annual orbit.
Which two gases make up most of Earth's dry atmosphere? Nitrogen and oxygen.
When is incoming space material called a meteorite? When a surviving solid piece reaches the ground.
In an elliptical Keplerian orbit, where does a planet move fastest? At perihelion, its closest approach to the Sun.
What is the photosphere of the Sun? The layer from which most visible sunlight escapes; it appears as the Sun's visible surface but is not solid ground.
Why does every new or full Moon not produce an eclipse? The Moon's orbit is tilted relative to Earth's orbital plane, so most alignments pass above or below the required line.
What gives much of the Martian surface its reddish color? Iron-bearing minerals have oxidized, producing rust-like material in the soil and dust.
Is Neptune's atmosphere motionless because it is far from the Sun? No. It has active weather, including very strong winds and large storms.
What role did planetesimals play in Solar System formation? They were small solid bodies that helped build larger planetary embryos and planets through growth and collisions.
A spacecraft is 90 million kilometers from Earth. At 300,000 kilometers per second, what is the approximate one-way signal travel time? 300 seconds, or 5 minutes, before processing delays.
What is a comet's coma? The diffuse cloud of gas and dust surrounding an active nucleus.
What binds the planets and most other Solar System bodies into a system around the Sun? Gravity; the Sun contains most of the system's mass and dominates its large-scale orbital motion.
How do the shapes of the Kuiper Belt and the proposed Oort Cloud differ? The Kuiper Belt is a broad, disk-like region beyond Neptune; the much more distant Oort Cloud is modeled as roughly spherical.
Why can regions near Uranus's poles experience years of continuous daylight or darkness? Its extreme axial tilt, combined with a long orbital period, keeps a pole toward or away from the Sun for a long time.
What liquids fill Titan's stable surface lakes and seas? Mainly methane and ethane, rather than liquid water.
Which Solar System moon is largest in diameter? Ganymede, which orbits Jupiter.
Does the label near-Earth asteroid mean an impact with Earth is inevitable? No. It describes an orbital category; impact risk requires evaluating the particular orbit and its uncertainty.
Does the broad disk-and-accretion model establish every detail of how each planet formed? No. The contributions of different growth processes, collisions and orbital migration remain active research topics.
What elements dominate Jupiter and Saturn by mass? Hydrogen and helium.
Under the IAU’s 2006 definition, can a nearly round natural satellite qualify as a dwarf planet? No. The dwarf-planet definition explicitly excludes satellites.
What is a major energy source for Io's intense volcanic activity? Tidal flexing driven by Jupiter's gravity and orbital interactions with other moons heats Io's interior.
Approximately how long is one Neptune orbit around the Sun in Earth years? About 165 Earth years.
Why cannot Earth's changing distance from the Sun explain the opposite seasons of its two hemispheres? Both hemispheres are essentially the same distance from the Sun at a given time; their different sunlight angles and day lengths matter.
If sunlight travels about 150 million kilometers at about 300,000 kilometers per second, approximately how long does it take to reach Earth? About 500 seconds, or 8 minutes 20 seconds.
What mainly causes the regular alternation of day and night at most locations on Earth? Earth's rotation carries those locations into and out of sunlight.
Approximately when did the Solar System begin forming? About 4.6 billion years ago.
Are all asteroids fragments of one planet that exploded? No. Many are remnants of early planet-building material, subsequently modified or broken by collisions.
What point do two mutually gravitating bodies orbit in the ideal isolated two-body model? Their common center of mass, or barycenter.
Why is Saturn noticeably flattened at its poles and wider at its equator? Its rapid rotation produces an equatorial bulge.
What physical process concentrated material toward the center of the young Solar System? Gravity caused part of a gas-and-dust cloud to collapse, building the young Sun near its center.
What does Mercury's heavily cratered surface reveal about its history? It preserves evidence of many impacts, with much less resurfacing than Earth's active surface.
How do Uranus and Neptune differ compositionally from Jupiter and Saturn? They contain a larger proportion of heavier constituents and a smaller proportion of hydrogen and helium.
Compared with Earth's, what are the main character and dominant gas of Mars's atmosphere? It is much thinner and consists mostly of carbon dioxide.
Why return asteroid material to laboratories on Earth instead of relying only on spacecraft images? Earth laboratories can analyze the material with a wider range of instruments and methods than a spacecraft can carry, including techniques developed after the mission.
How do a comet's ion tail and dust tail typically differ in shape? The ion tail is usually straighter and points away from the Sun; the dust tail is often broader and curved.
What is plasma, the state of much of the Sun's material? A gas containing freely moving charged particles, including ions and electrons.
Is the Moon's far side permanently dark? No. Both near and far sides receive sunlight during the lunar day-night cycle.
Under the IAU's 2006 definition, how does a dwarf planet differ from a planet? It orbits the Sun and is nearly round but has not cleared its orbital neighborhood; it is not a satellite.
What does it mean that Saturn's mean density is lower than that of liquid water? Its total mass divided by its overall volume is less than water's density; it does not describe every layer inside Saturn.
How can water ice persist in some craters on Mercury despite the planet's proximity to the Sun? Deep, permanently shadowed areas near the poles can remain extremely cold.
What gas makes up most of Venus's atmosphere? Carbon dioxide.
Why can Mercury have very large differences between daytime and nighttime surface temperatures? Its extremely thin exosphere provides little heat redistribution or insulation.
How does a flyby spacecraft differ from an orbiter at its target? A flyby passes the target without entering a bound orbit around it; an orbiter remains gravitationally bound to make repeated observations.
Why do the planets orbit in approximately the same plane and direction? They largely inherited the motion and flattened arrangement of the rotating disk in which they formed.
What is the meteor in the phrase shooting star? The light-producing atmospheric event caused by incoming space material, not a star falling from space.
About how many Earth days does Mercury take to orbit the Sun? About 88 Earth days.
How do greenhouse gases help keep Earth's surface warmer than it would otherwise be? They absorb and emit infrared radiation, altering how energy escapes to space and warming the surface and lower atmosphere.
Which of the eight planets is smallest in diameter? Mercury.
Does a planet pull on the Sun gravitationally as well as the Sun pulling on the planet? Yes. The gravitational forces are equal in magnitude and opposite in direction.
What is Jupiter's Great Red Spot? A large, long-lived atmospheric storm.
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