Deck 6: Formation of the Solar System

Question 1

There is very little hydrogen or helium in the inner part of the solar system today. Astronomers believe the reason for this is that

A) all the light elements went into the formation of the Sun itself and little were left over for the rest of the solar system.
B) the intense radiation from the early Sun drove the light elements out of the inner solar system.
C) heavier elements were attracted in from the outer part of the solar system, displacing the light elements originally in the inner part.
D) the light elements underwent chemical reactions and were locked up in chemicals in the inner solar system.

Answer 1

the intense radiation from the early Sun drove the light elements out of the inner solar system.

Question 2

Which of these physical parameters was MOST important in determining the contrasting properties of the planets that eventually formed in the solar system?

A) mix of chemical constituents
B) overall rotation of the nebula
C) temperature distribution in the nebula
D) density of hydrogen gas in the nebula

Answer 2

temperature distribution in the nebula

Question 3

The mass of the solar nebula that began contracting to form the solar system was estimated to be

A) 1 solar mass, the same as the mass of the solar system today.
B) only a fraction of a solar mass because additional mass has since been attracted from nearby objects.
C) a few solar masses.
D) thousands of solar masses to allow for the mass energy radiated away since the formation.

Answer 3

a few solar masses.

Question 4

In the solar system, the distribution of light and heavy elements is as follows

A) Within a few astronomical units of the center, most of the matter consists of heavy elements. Beyond that, most of the matter consists of light elements.
B) Within a few astronomical units of the center, most of the matter consists of light elements. Beyond that, most of the matter consists of heavy elements.
C) Most of the light and heavy elements are within a few astronomical units of the center.
D) Most of the light and heavy elements are generally farther than a few astronomical units from the center.

Question 5

The material from which our solar system formed is believed to be

A) the hot, dense gases at the center of a globular star cluster.
B) a vast cloud of pure hydrogen.
C) the convergence of the solar winds of several nearby stars.
D) a cold, dark cloud of gas and dust.

Question 6

What force ensured that once the solar system began collapsing it continued to do so?

A) angular momentum
B) magnetic interactions
C) electric attractions between charges
D) gravity

Question 7

Which of these time sequences for the formation of the solar system is MOST probable?

A) The Sun contracted first as a gas ball, and the planets and moons formed shortly afterward by accretion and condensation.
B) The Sun formed first, the planets were spun off from the Sun, and the moons in turn were spun off from the planets.
C) The planets formed first out of a cold nebula of gas and dust, followed by the Sun, which formed when the gas had become much hotter.
D) The Sun formed initially, and the planets and major moons were captured much later as they drifted by the Sun.

Question 8

What process heated the early solar nebula as it slowly condensed toward a central protosun?

A) radioactive decay of heavy elements originally formed in the Big Bang
B) release of heat as molecules formed and gases condensed into ices
C) thermonuclear fusion in the protosun, followed by radiative heating of the nebula
D) generation of heat by collisions of particles as they gained kinetic energy in falling toward the center of the nebula

Question 9

The standard model of the formation of the solar system assumes that the material that became the solar system began as a large spherical cloud of gas and dust, rotating slowly. As the solar system formed, most of this material was transformed into a compact, flattened disk, rotating more rapidly. What is the explanation for this change in shape and rate of rotation?

A) conservation of angular momentum
B) influence of nearby stars
C) differentiation of materials easily vaporized and not easily vaporized
D) alignment with the plane of the Milky Way Galaxy

Question 10

In the solar system, what is the snow line?

A) either one of a pair of lines on any planet, parallel to its equator, between which snow is never found
B) a boundary in the solar system such that ices of water, carbon dioxide, methane, and ammonia are found only outside of this line (i.e., farther from the Sun)
C) a boundary in the solar system such that all fluids outside of this line (i.e., farther from the Sun) must be frozen
D) a boundary within the asteroid belt which divides the asteroids into those composed of rock and those composed of ice

Question 11

The early phases of planetary formation into protoplanets were characterized by the

A) slow accretion of small particles by gravitational attraction and collision.
B) breaking apart of very large objects into planets.
C) condensation of hot gas clouds.
D) violent collapse of matter under gravity.

Question 12

The collapse of a gas cloud can be triggered by a number of mechanisms. Which one is currently believed to be the likely cause of the collapse that resulted in the formation of the solar system?

A) the shock wave produced by a supernova
B) density waves in our spiral galaxy
C) the gravitational pull of a passing star
D) a collision with another gas cloud

Question 13

The process of accretion accounts for how

A) elements are transformed into heavier elements by nuclear reactions.
B) giant planets differentiate into rocky cores and gaseous envelopes.
C) clouds of interstellar gas and dust contract to form protostars.
D) dust grains and ice crystals coalesce to form planetesimals.

Question 14

The MOST probable process for the formation or acquisition of the planets of the Sun is the

A) capture of planets from outer space by gravity.
B) relatively slow growth of smaller objects by collisions and mutual gravitational attraction.
C) freezing of immense gas clouds by the cold temperature of space.
D) breakup by tidal distortion of a single large companion body to the Sun.

Question 15

What name is given to the concentration of mass that formed at the center of the solar nebula and eventually became the Sun?

A) solar hub
B) antisun
C) pseudosun
D) protosun

Question 16

The process of accretion in planetary formation is the

A) slow condensation by gravity of gas atoms into large, dense gas clouds that are the preplanetary masses.
B) slow acquisition from deep space by the giant planets of their complement of moons by gravitational capture.
C) slow accumulation of solid particles by gravity and collision into larger, solid objects.
D) relatively rapid gravitational collapse (in less than 10⁶ years) of gas clouds to form planets.

Question 17

Suppose one were to go back in time and explore the solar nebula during the formation of the solar system. If she were to travel outward from the protosun, the first solid material encountered would be

A) dust-sized grains of rocky material.
B) dust-sized grains of frozen hydrogen, water ice, and rocky minerals.
C) snowflakes made of frozen water, methane, ammonia, and carbon dioxide.
D) snowflakes made of frozen hydrogen and helium.

Question 18

Why did the temperature start to rise at the center of the solar nebula?

A) Supernova explosions were stirring up the material there and causing turbulence.
B) The nebula was contracting, which increased collisions between atoms moving in it.
C) Fusion reactions were beginning in the core, releasing tremendous amounts of heat.
D) Massive stars nearby were heating the nebula with their ultraviolet radiation.

Question 19

The MOST probable theory for the formation of the solar system is

A) an encounter in which a passing star ripped off material from the Sun to form the planets.
B) a capture theory in which the Sun, after formation, captured objects moving through space to form the planets.
C) the condensation of a nebula of cold gas and dust into the Sun and planets.
D) the condensation of a nebula of hot gas into the Sun and planets.

Question 20

What are the three "common" substances that are believed to be important in planet formation?

A) electromagnetic radiation, electrical discharges (e.g., lightning), and water
B) rock, ices, and gas
C) solid, liquid, and gaseous hydrogen
D) hydrogen, helium, and neon gases

Question 21

The ice giants are

A) Jupiter and Saturn.
B) Neptune and Uranus.
C) Titan and Pluto.
D) Tatooine and Alderaan.

Question 22

The manner in which the large, outer planets formed in our solar system was MOST likely the

A) gravitational condensation of hydrogen, helium, and dust in eddies or vortices in the outer solar nebula.
B) gravitational condensation of hydrogen and helium gas, followed by the capture of planetesimals.
C) accretion of planetesimals to form a core, followed by the gravitational capture of hydrogen and helium gas.
D) accretion of cold planetesimals containing large quantities of hydrogen and helium.

Question 23

What is the Kuiper belt?

A) band of dust in the plane of the ecliptic, extending from near the orbit of Mars to beyond the orbit of Pluto
B) broadest band of asteroids in the asteroid belt, separated from other bands by Kirkwood gaps
C) relatively flat distribution of objects in the plane of the ecliptic, extending from around the orbit of Pluto to about 50 au from the Sun
D) approximate spherical distribution of comets centered on the Sun and extending out to about 50,000 au

Question 24

The large gravitational influences of the gas giants in our solar system forced some planetesimals in their neighborhood to move inward toward the Sun. As a consequence of this,

A) these planetesimals became moons around the inner planets.
B) the gas giants themselves were forced outward farther from the Sun.
C) the gas giants also fell inward toward the Sun.
D) these planetesimals fell into the Sun causing the Sun to flare up and expel the remaining hydrogen and helium from the inner solar system.

Question 25

A substantial fraction of the water found on Earth is believed to have come from

A) the impact that produced the Moon.
B) water-rich comets.
C) Earth's brief migration outside the snow line.
D) the combination of primordial hydrogen with oxygen produced by green plants.

Question 26

A small body orbiting the Sun has an orbit intersecting the ecliptic at an angle of 60 degrees. It is MOST likely a member of which of these?

A) plutinos
B) asteroid belt
C) Oort cloud
D) Kuiper belt

Question 27

The region outside the orbit of Neptune in which a large number of objects composed of rock and ice circle the Sun not far from the plane of the ecliptic is called the

A) outer solar system.
B) asteroid belt.
C) Kuiper belt.
D) Oort comet cloud.

Question 28

In the Nice model of solar system formation,

A) all of the outer planets formed at the same time.
B) Jupiter and Saturn formed first, followed by Uranus and Neptune.
C) Neptune and Uranus formed first, followed by Jupiter and Saturn.
D) the inner planets formed before the outer planets.

Question 29

According to current theories of planetary formation, why is Saturn less massive than Jupiter?

A) Saturn formed far out in the solar system where gas and dust were not as readily available as where Jupiter formed near the Sun.
B) Saturn formed in the inner part of the solar system where the rocky inner planets had already absorbed most of the material.
C) Saturn was originally more massive but has been repeatedly struck by large planetesimals that have stripped off some of its matter.
D) Jupiter formed first, using up or ejecting much of the material available in the outer solar system.

Question 30

The large gravitational influences of Jupiter and Saturn forced some planetesimals in their neighborhood to move outward away from the Sun. A consequence of this was

A) that conservation of angular momentum caused the giant planets to spin faster.
B) collisions that produced many fragments became the rings of the outer planets.
C) collisions that formed Uranus and Neptune.
D) the creation or at least enlargement of the Kuiper belt and the Oort cloud.

Question 31

One particularly important collision in the early inner solar system about 100 million years after Earth's formation resulted in

A) the extinction of 70 % of the species then in existence on Earth, including the dinosaurs.
B) the formation of a secondary asteroid belt between the orbits of Earth and Venus.
C) the destruction of the planet Vulcan, which had an orbit inside the orbit of Mercury.
D) the formation of our Moon.

Question 32

The present positions of the gas and ice giants are

A) significantly closer to the Sun than they originally were, because of interactions with inner planets.
B) significantly closer to the Sun than they originally were, because of resonant interactions.
C) significantly more distant from the Sun than they originally were, because of interactions with the inner planets.
D) significantly more distant from the Sun than they originally were, because of resonant interactions.

Question 33

The manner in which the terrestrial planets formed was the

A) gravitational condensation of hydrogen, helium, and dust in eddies or vortices in the solar nebula.
B) gravitational condensation of gas, followed by the capture of solid planetesimals.
C) accretion of solid planetesimals containing mostly rocky material.
D) accretion of planetesimals to form a core, followed by the gravitational capture of gas from the solar nebula.

Question 34

According to the consensus present theory of solar system formation, where were the four inner planets formed?

A) All were formed farther out in the solar system and migrated inward.
B) All were formed near their present locations in the inner solar system.
C) Mercury was formed near the Sun, but the others were formed farther out and migrated inward.
D) Our present theory has no clear answer to this question.

Question 35

The region at the farthest limits of the solar system in which a large number of objects composed of rock and ice circle the Sun in a roughly spherical region is called the

A) outer solar system.
B) asteroid belt.
C) Kuiper belt.
D) Oort cloud.

Question 36

When the orbits of two bodies are in resonance,

A) angular momentum is no longer conserved.
B) the bodies produce the same line radiation.
C) the gravitational interactions between the bodies reinforce each other.
D) the orbits quickly become hyperbolic.

Question 37

During the solar system's formation, Saturn's expanding orbit resulted in

A) Neptune, which initially formed closer to the Sun than Uranus, being forced past Uranus' orbit.
B) the inward migration of Jupiter.
C) the relatively small mass of Saturn.
D) the formation of the asteroid belt.

Question 38

As the forming planet Jupiter moved through the early outer solar system, it collided with huge numbers of gas and dust particles there. What effect did this have?

A) The additional mass made the young Jupiter move faster, causing it to migrate inward to the region where, according to Kepler's third law, periods are shorter and speeds faster.
B) The additional mass made the young Jupiter move more slowly, causing it to migrate outward to the region where, according to Kepler's third law, periods are longer and speeds slower.
C) The additional mass made the young Jupiter move faster, giving it more energy and causing it to migrate outward.
D) The additional mass made the young Jupiter move more slowly, giving it less energy and causing it to migrate inward.

Question 39

The formation of terrestrial-type planets around a star is most likely to have occurred by what process?

A) accretion, or slow accumulation of smaller particles by mutual gravitational attraction
B) breakup of a large disk of matter that formed around the star
C) capture of objects traversing the depths of space by the star
D) condensation of gas from the original star nebula

Question 40

The reason for the vast amount of hydrogen in the interior of Jupiter is probably that

A) nuclear fission of atoms in Jupiter's interior split all nuclei down to hydrogen nuclei early in its history.
B) Jupiter became so hot in its interior that all kinds of atoms and molecules were melted down to the fundamental atom, hydrogen.
C) Jupiter formed from the initial gravitational contraction of hydrogen gas.
D) the mass of the initial condensation of rocks at Jupiter's orbit was sufficient to attract vast amounts of gas to it.

Question 41

How much total mass is contained in the asteroid belt?

A) Much less than that of Earth's Moon
B) Roughly that of Earth's Moon
C) Roughly that of Earth
D) Roughly that of Neptune

Question 42

The asteroid Oumuamua, discovered in 2017, is unusual because it

A) is not spherical.
B) is on a hyperbolic orbit and may have originated around another star.
C) passed through the inner solar system.
D) collided with Jupiter.

Question 43

What process had the greatest influence on the features of the Moon during the first billion years of its existence?

A) erosion by an early, short-lived atmosphere
B) volcanoes
C) impacts from space
D) mountain-building from geological activity

Question 44

Which one of these statements about asteroids and the asteroid belt is NOT true?

A) The asteroid belt contains several dwarf planets.
B) All of the asteroids together would make a mass much less than the mass of the Moon.
C) Some asteroids have their own moons.
D) The gravitational influence of Jupiter helps prevent the asteroids from coalescing into a larger body.

Question 45

All of these are consequences of the period of Late Heavy Bombardment EXCEPT that

A) our Moon has large maria.
B) Mercury lost its relatively light outer layer, leaving a planet with an unusually large density.
C) our Moon was formed after a collision between Earth and another body.
D) the number of asteroids in the asteroid belt decreased.

Question 46

Today, the asteroid belt does NOT contain a planet because

A) the planet that originally formed there was scattered into the Sun by Jupiter.
B) the planet that originally formed there was captured as one of Jupiter's moons.
C) the planet that originally formed there was scattered to the Kuiper belt by Jupiter.
D) interactions with Jupiter prevented one from forming.

Question 47

The asteroid belt exists between the orbits of which planets?

A) Jupiter and Saturn
B) Venus and Earth
C) Mars and Jupiter
D) Earth and Mars

Question 48

The asteroid belt is composed of

A) irregularly shaped bodies composed primarily of ices.
B) several planet-sized objects with dense methane atmospheres.
C) large, rocky bodies typically about the size of our Moon.
D) rocky bodies with diameters from less than a kilometer to hundreds of kilometers.

Question 49

Pluto is in a 2:3 resonance orbit with Neptune. What does this mean?

A) During the time Neptune orbits the Sun 3 times, Pluto orbits it twice.
B) During the time Neptune orbits the Sun twice, Pluto orbits it 3 times.
C) Neptune rotates on its axis 3 times while Pluto rotates twice about its axis.
D) Neptune's mean orbital radius is 2/3 of Pluto's mean orbital radius.

Question 50

A Kuiper belt object whose orbit is tilted with respect to the orbits of the planets is MOST likely to have

A) originated in the Oort cloud.
B) formed in place during the early history of the solar system.
C) been a scattered moon of a giant planet,
D) been scattered to its current location by interactions with the giant planets.

Question 51

Which of these is NOT a trans-Neptunian object?

A) A plutino
B) An ice giant moon
C) A comet in the Oort cloud
D) A cubewano

Question 52

What is a "plutino"?

A) any object in a 2:3 orbital resonance with Pluto
B) any object with the same orbital resonance as Pluto
C) any object in an orbit outside that of Pluto
D) any object beyond the orbit of Neptune that has enough mass to assume a spherical shape

Question 53

Cratering is common on all of the inner solar system bodies EXCEPT

A) asteroids.
B) Mercury
C) the Moon
D) Venus

Question 54

What caused the Late Heavy Bombardment in our solar system?

A) Debris from the impact that formed the Moon scattered throughout the inner solar system.
B) The changing orbits of the giant planets disturbed the orbits of many asteroids.
C) The close passage of Scholz's star disturbed the orbits of bodies in the Oort cloud and Kuiper belt.
D) Gravitational resonances between Neptune and Kuiper belt objects caused many of the latter to collide with the inner planets.

Question 55

The solar system formed about 4.5 billion years ago. What has been the rate of bombardment by space debris in the inner part of the solar system since then?

A) There has been a relatively steady bombardment by both large and small pieces of debris since the formation.
B) The rate of bombardment was very heavy just after the solar system's creation, but the rate has decreased steadily since then.
C) The rate decreased steadily for about 400 million years after the solar system's creation. Then a very heavy bombardment began and lasted about 300 million years. The rate has been greatly reduced since then.
D) The rate has been highly erratic with no clear pattern.

Question 56

The asteroid Gaspra (see Figure 6-4) is typical of MOST asteroids in that it is

A) potato-shaped with a smooth, metallic surface.
B) spherical and densely covered with craters.
C) potato-shaped with large and small craters.
D) spherical and deformed by thermal activity.

Question 57

The only known solar system body believed to have formed OUTSIDE of our solar system is

A) an asteroid.
B) a Kuiper belt object.
C) part of the Oort cloud.
D) a meteorite discovered in Antarctica.

Question 58

A cubewano is

A) an instability in a black hole accretion disk.
B) a Kuiper belt object in a fairly circular orbit that likely formed in place.
C) an object from the Oort cloud that has an orbit carrying it through the inner solar system.
D) a Kuiper belt object in an orbital resonance with Neptune.

Question 59

Where are the asteroids located?

A) Their orbits are distributed more or less uniformly throughout the solar system.
B) All asteroids have orbits between the orbits of Mars and Jupiter.
C) Most asteroids have orbits between the orbits of Mars and Jupiter, but some have orbits that cross those of the inner planets.
D) Most asteroids have orbits between the orbits of Mars and Jupiter, but some have orbits carrying them outside of Neptune's orbit.

Question 60

What is the origin of many of the bodies in the Kuiper belt and the Oort cloud?

A) These distant objects were ejected outflows from the Sun during the early history of the solar system.
B) These distant objects have been captured from interstellar space by the Sun's gravitational pull.
C) The gravitational forces from Jupiter and Saturn flung planetesimals from regions closer to the Sun out into these regions.
D) Early in the history of the solar system it is believed that these objects were pulled out of the Sun by a close encounter with a passing star.

Question 61

What is the basic difference between comets and asteroids?

A) Comets always move in open orbits around the Sun and hence visit the Sun only once in their lifetime, whereas asteroids move in closed orbits.
B) Comets always emit their own light, whereas asteroids only reflect sunlight.
C) Comets are spherical, whereas asteroids are mostly irregular in shape.
D) Comets are mostly composed of roughly equal mixtures of rocks and ices, whereas asteroids are mainly composed of rocks.

Question 62

Pluto was originally classified as a planet, but new criteria for the definition of a planet were adopted, and Pluto failed to meet one of them. Which one?

A) Pluto does not orbit the Sun directly.
B) Pluto does not have enough mass to pull itself into a roughly spherical shape.
C) Pluto does not spin fast enough to produce its own magnetic field.
D) Pluto does not have enough gravity to clear its orbit.

Question 63

Meteorites are useful for studying the earliest history of the solar system because

A) many arrive from other solar systems, to which we can directly compare our own.
B) most were ejected from other planets while in the process of formation.
C) most arrive from the outer solar system, where objects are difficult to study directly.
D) most are unchanged since their formation shortly after the solar system began.

Question 64

Viewed from a position above the north pole of the Earth, the direction of the motion of the planets in their orbits is

A) clockwise.
B) counterclockwise.
C) clockwise except the direction of Venus.
D) counterclockwise except the direction of Mercury.

Question 65

Which of these solar system bodies is NOT likely to have hosted liquid water in the past?

A) Ceres
B) Ganymede
C) Titan
D) Mars

Question 66

What is the difference between an asteroid and a meteoroid?

A) Composition: asteroids contain a good deal of ice while meteoroids are rock and metal.
B) Size: asteroids are larger and meteoroids are smaller.
C) Location: asteroids are found throughout the solar system while meteoroids exist only in the inner solar system.
D) Endpoint: asteroids circle the Sun but meteoroids, by definition, have sailed through Earth's atmosphere and have reached the ground without burning up.

Question 67

Suppose Earth existed in a solar system containing two stars, relatively close together. What major change would result from such an arrangement compared to the situation at present?

A) No substantial change would result.
B) There are no stable orbits for such a system, so Earth would be doomed to fall into one of the stars.
C) We would be in an orbit in which one star or the other would always be in the sky, and, without a night, our biosphere would have developed differently.
D) We would have to be in a highly elliptical orbit giving us extremes in temperature.

Question 68

At what point in time is it said that the protosun became the Sun?

A) when thermonuclear fusion reactions began at its center
B) when it became hot enough to emit light and heat
C) when the temperature began to rise at its center
D) when planetary formation was complete

Question 69

MOST meteoroids are

A) rock-sized pieces of ice chipped off comets.
B) space debris that has fallen to Earth.
C) space debris composed of rocks or metals.
D) small pieces of debris ejected from planetary rings.

Question 70

MOST of the planets orbit the Sun on or close to the

A) plane of the Milky Way Galaxy.
B) ecliptic plane.
C) plane containing both north and south celestial poles and the zenith at Greenwich, England.
D) equatorial plane.

Question 71

The timescale over which material in the solar nebula accreted to form the inner planets was about _____ years.

A) 100 million
B) 4.6 million
C) 4.6 billion
D) 100,000

Question 72

Which planets do NOT have any natural satellites?

A) Mars and Venus
B) Mercury and Mars
C) Mercury, Venus, and Mars
D) Mercury and Venus

Question 73

Each of these characteristics is included in the formal definition of a planet EXCEPT

A) orbits a star directly.
B) has enough mass to pull itself into a roughly spherical shape.
C) spins fast enough to produce its own magnetic field.
D) has enough gravity to clear its orbit.

Question 74

An object in the solar system orbits the Sun and has enough mass that its gravity has pulled it into a spherical shape, but it does not have enough gravity to clear its neighborhood of orbiting debris. This object is classified as

A) a comet nucleus.
B) an asteroid.
C) a meteoroid.
D) a dwarf planet.

Question 75

All of these are methods believed to have formed planetary moons in the solar system EXCEPT

A) as a result of the planet being struck by a large object.
B) the capture of planetesimals.
C) the spontaneous splitting of a planet due to intense internal radioactivity.
D) condensation within a miniature nebula formed around the planet.

Question 76

What is the origin of the various moons around the planets in the solar system?

A) All are captured asteroids.
B) All formed by collision and condensation like the planets they orbit.
C) All were formed from material ejected when their planets were struck by large objects.
D) All these mechanisms are believed to have occurred.

Question 77

Which of these solar system bodies is MOST likely to have hosted liquid water in the past?

A) Ceres
B) Venus
C) Europa
D) Triton

Question 78

A theory of the origin of the solar system must take into account all important general properties of the planets, which include three of the four properties listed. Which one is NOT an important general property of the planets?

A) The planets are grouped by size and composition into three general groups.
B) The magnetic fields of the planets are produced by a variety of mechanisms.
C) The terrestrial planets all orbit much closer to the Sun than do any of the Jovian planets.
D) All the planets orbit the Sun in the same direction and nearly in the same plane.

Question 79

Comets are typically composed of

A) mostly gases, some of which are pushed out by the Sun to form a long tail.
B) slushy mixtures of liquid and ice.
C) chunks of rock that are generally a few tens of kilometers in diameter.
D) mixtures of ice and rock

Question 80

Which of these is NOT true of motions in the solar system?

A) The orbital motion of the planets is in the same direction as the rotation of the Sun.
B) All the planets orbit in the same direction as the solar nebula from which they formed.
C) The orbital period of the planets increases with distance from the Sun.
D) All the planets orbit the Sun in the same direction, except one.