astro 3
Fastest to slowest cooling
mercury, mars, venus, and earth.
What kind of material in the solar nebula could condense at temperatures as high as 1,500 K, such as existed in the inner region of the nebula?
metals
The terrestrial planet cores contain mostly metal because
metals sank to the center during a time when the interiors were molten throughout.
Compared to the distance between Earth and Mars, the distance between Jupiter and Saturn is ________.
much larger
Using only pencil and paper calculations, Arrhenius concluded that a doubling of the carbon dioxide concentration would raise global temperatures by about _____.
5° C
Every 4.5 billion years, half of the atoms in a sample of uranium-238 will undergo radioactive decay and become atoms of lead-206. Suppose you lived in another planetary system around a faraway star and found a meteorite that was originally made of uranium-238, but is now one-quarter uranium-238 and three-quarters lead-206. What would be your best estimate of that planetary system's age?
9 billion years
What percentage of the solar nebula's mass consisted of hydrogen and helium gases?
98 percent
On a cloudless day, what happens to most of the visible light headed toward Earth?
It reaches Earth's surface, where some is reflected and some is absorbed.
Earth has a relatively strong magnetic field, but Mars does not. Which of the following probably explains why Mars lacks a strong magnetic field today?
Its core is too cool for convection.
Why is Mars red?
Its surface rocks were rusted by oxygen.
How did the lunar maria form?
Large impacts fractured the Moon's lithosphere, allowing lava to fill the impact basins.
Which of the following planets has the thinnest atmosphere?
Mars
Which of the following show evidence of ancient river beds?
Mars
Which planet (besides Earth) has been visited by the largest number of robotic spacecraft?
Mars
All of the following statements about Mars are true. Which one might have led to a significant loss of atmospheric gas to space?
Mars lost any global magnetic field that it may once have had.
Mars' atmosphere is mostly carbon dioxide. Why does it not experience a runaway greenhouse effect like Venus?
Mars' atmosphere is too thin.
Which planet has the most extreme temperature difference between day and night?
Mercury
Spacecraft have landed on all the terrestrial worlds except
Mercury.
Rank the five terrestrial worlds in order of size from smallest to largest.
Moon, Mercury, Mars, Venus, Earth
When were the heat-trapping effects of gases that cause the greenhouse effect first measured by scientists?
More than 150 years ago.
Why do Earth rocks have much younger ages than most meteorites?
Most Earth rocks have been melted and reformed since Earth formed from the solar nebula.
According to our theory of solar system formation, why do we find some exceptions to the general rules and patterns of the planets?
Most of the exceptions are the result of giant impacts or close gravitational encounters.
Which of the following is not a major pattern of motion in the solar system?
Nearly all comets orbit the Sun in same direction and roughly the same plane
Which planet could an astronaut visit without the need for a spacesuit (and survive)?
None; an astronaut would need a spacesuit to survive a visit to any other planet in the solar system
The tallest mountain on Mars is called ___________________.
Olympus Mons
Which of the following Mars surface features provides dramatic evidence that volcanism has played a role in shaping the surface of Mars?
Olympus Mons
Based on everything you have learned about the formation of our solar system, which of the following statements is probably not true?
Only a tiny percentage of stars are surrounded by spinning disks of gas during their formation.
Which of the following observations indicates that Mars had a warmer, thicker atmosphere in the past?
There are dried-up riverbeds on Mars.
The long, deep canyon running along the equator is called _______________.
Valles Marineris
Which of the following worlds has the thickest atmosphere?
Venus
All the following statements about Venus are true. Which one offers evidence of a global repaving about a billion years ago?
Venus has relatively few impact craters and these craters are distributed fairly evenly over the entire planet.
What is the most likely reason Venus does not experience plate tectonics like Earth?
Venus's lithosphere is stronger and thicker than Earth's.
based on the total amount of infrared-absorbing greenhouse gases in their atmospheres, from greatest to least.
Venus, Earth, Mars, Mercury
Which planet has the highest average surface temperature, and why?
Venus, because of its dense carbon dioxide atmosphere
Which of the following is furthest from the Sun?
a comet in the Oort cloud
How fast do tectonic plates move on Earth?
a few centimeters per year
The heavy bombardment phase of the solar system lasted
a few hundred million years.
accretion
a growth in size; an increase in amount
How have we been able to construct detailed maps of surface features on Venus?
by using radar from spacecraft that were sent to orbit Venus
Within the frost line, planetesimals were composed entirely of rock, and outside the frost line, planetesimals were composed entirely of ice.
false
The earthquakes that occur in Southern California generally occur above a __________.
fault.
There are no auroras on Venus because it
lacks a strong magnetic field.
According to our theory of solar system formation, what are asteroids and comets?
leftover planetesimals that never accreted into planets
Human civilization is responsible for what astronomers call ___________.
light pollution.
Suppose you view the solar system from high above Earth's North Pole. Which of the following statements about planetary orbits will be true?
ll the planets orbit counterclockwise around the Sun.
Larger planets are supposed to stay warm inside __________ than smaller planets.
longer
Thus, these worlds experience volcanism and tectonics for much ___________ than smaller worlds do.
longer
Which two factors are most important to the existence of plate tectonics on Earth?
mantle convection and a thin lithosphere
In the context of planetary geology, what do we mean by outgassing?
the release by volcanism of gases that had been trapped in a planetary interior
Rank the stars based on the Doppler shift that we would detect on Earth.
the star moving fastest toward Earth will have the greatest blueshift, the star moving across our line of sight will have no shift at all, and the star moving fastest away from us will have the greatest redshift.
When we see a region of a planet that is not as heavily cratered as other regions, we conclude that
the surface in the region is younger than the surface in more heavily cratered regions.
Which of the following best describes erosion?
the wearing down or building up of geological features by wind, water, ice, and other phenomena of planetary weather
Sunsets are red because sunlight must pass through more atmosphere when the sun is low in the sky, and the atmosphere scatters even more of the shorter wavelengths, leaving mostly red light to color the sky.
true
The current atmospheric concentration of carbon dioxide on Earth is at its highest in over a million years.
true
The jovian planets have no solid surface.
true
The more massive planets in the solar system tend to be less dense than the lower mass planets.
true
The process in which one plate slides under another is called subduction and occurs where seafloor crust descends into the mantle below continental crust.
true
The sky is blue because molecules in the atmosphere scatter blue light more effectively than red light.
true
Today, Mars has no planetary magnetic field.
true
Venus has a thicker atmosphere than Earth.
true
We are confident that rising CO2 levels are caused by the burning of fossil fuels because atmospheric CO2 now shows the same carbon isotope ratio as fossil fuels.
true
With the exception of Venus, planetary surface temperatures decrease at greater distances from the sun.
true
Within 4 AU of the Sun, all planets have solid surfaces.
true
Without greenhouse gases, water on Earth's surface would be completely frozen.
true
Which of the following best describes the lunar maria?
relatively smooth, flat plains on the Moon
The Tharsis Bulge on Mars is most likely the result of
rising material in the mantle of Mars.
_________________ found in the Atlantic Ocean between North America and Europe is composed of dense and relatively young rock.
seafloor crust
The slowly increasing distance between South America and Africa is due to ______________.
seafloor spreading
Layered _________________ exposed by erosion can be seen when looking at the Grand Canyon in the United States.
sedimentary rock
Mars has two moons that are most similar in character to ________.
small asteroids
The extremely deep ocean Marianas Trench is a result of ____________.
subduction
Sunsets and sunrises often appear red because
sunlight must pass through more atmosphere, which scatters the shorter wavelengths more than the longer wavelengths, allowing more red light to reach your eye.
Which of the following statements does not apply to the formation of gas giants like Jupiter, compared to terrestrial planets?
surface dramatically altered during bombardment
According to our theory of solar system formation, which law best explains why the solar nebula spun faster as it shrank in size?
the law of conservation of angular momentum
According to our theory of solar system formation, which law best explains why the central regions of the solar nebula got hotter as the nebula shrank in size?
the law of conservation of energy
Red light is
the longest wavelength visible light, and longer wavelength means lower frequency (because wavelength×frequency=speed of light ).
The age of the solar system can be established by radiometric dating of
the oldest meteorites
Which of the apparent exceptions to the solar nebular theory cannot be explained by a giant impact event?
the orbit of Triton in the opposite direction to Neptune's rotation
One of the two laws of thermal radiation (Wien's law) states that
the peak wavelength of a spectrum is directly related to an object's temperature. A peak at yellow-green wavelengths corresponds to a temperature of about 5800 K.
What observational evidence supports the idea that Mercury shrank after it formed?
the presence of many long, tall cliffs
Current CO2 levels are higher than they have been at any other time during the history of the human species.
True
What is the primary reason that we launch X-ray telescopes into space rather than building them on the ground?
X-rays from space do not reach the ground.
Australia is composed of relatively old and thick _____________.
continental crust.
What drives the motion of the continental plates on Earth?
convection cells in the mantle
As a general rule, smaller planets __________ than larger planets.
cool more rapidly
From center to surface, which of the following correctly lists the interior layers of a terrestrial world?
core, mantle, lithosphere, crust
Earth is the only terrestrial planet to have experienced tectonic stresses and volcanic activity.
false
Erosion is the most dominant geological process on Venus.
false
Historically, there is no link between increased CO2 levels and higher average temperatures.
false
In the inner solar system, the largest surface features (such as volcanoes or valleys) are found on the largest planets.
false
The Moon probably formed at the same time that Earth formed, rather like the formation of a double planet.
false
The high level of oxygen in Earth's atmosphere can be explained by chemical processes.
false
The rise of life on the planet Earth some 4 billion years ago was made possible, in part, by the plentiful oxygen in Earth's early atmosphere.
false
Approximately how deep is an impact crater compared to its width?
10-20%
How much more light does an 8-meter telescope gather than a 2-meter telescope
16 times as much
According to current climate models and given the current emission rate of carbon dioxide, how much will the Earth's average temperature increase by the end of this century?
2-5°C
According to modern scientific dating techniques, approximately how old is the solar system?
4.5 billion years
Since the time Arrhenius made his prediction, the carbon dioxide concentration has risen by about 30% and the global average temperature has risen by about 1°C. Which statement below best describes how scientists should evaluate Arrhenius's prediction?
Although Arrhenius's prediction was not exact, the actual change is similar enough to the predicted change to validate the general idea that a rising carbon dioxide concentration would lead to global warming.
Why can we assume that a rock sample containing argon-40 had none of this isotope when the rock initially formed?
Argon-40 is a gas that does not combine with other elements, so it would not be present in a rock when it formed.
Why did the solar nebula heat up as it collapsed?
As the cloud shrank, its gravitational potential energy was converted to thermal energy.
According to our basic scenario of solar system formation, why do the jovian planets have numerous large moons?
As the growing jovian planets captured gas from the solar nebula, the gas formed swirling disks around them, and moons formed from condensation accretion within these disks.
In what way is Venus most similar to Earth?
Both planets are nearly the same size.
Which of the following best describes the geological histories of the Moon and Mercury?
Early in their histories, they suffered many impacts and experienced some volcanism and tectonics, but they now have little geological activity at all.
The following statements are all true. Which one counts as an "exception to the rule" in being unusual for our solar system?
Earth has a large moon.
Longest to shortest volcanic activity
Earth, Mars, Mercury,
Which of the following are relatively unchanged fragments from the early period of planet building in the solar system? A. Kuiper belt comets B. Oort cloud comets C. asteroids D. all of the above
D. all of the above
What kind of surface features may result from tectonics? A. valleys B. mountains C. cliffs D. volcanos E. all of the above
E. all of the above
Arrhenius was not particularly alarmed by his discovery at the time. What changed that made later scientists become much more concerned about the climate effects of carbon dioxide?
Emissions of carbon dioxide rose much faster than Arrhenius had anticipated.
Earth emits thermal radiation characteristic of its surface temperature, which means it is almost entirely infrared
For Earth, the surface temperature is too low to emit any visible, ultraviolet, or X-ray light, so those are all ranked equally
What motivated Arrhenius to calculate how changes in carbon dioxide might affect the climate?
He was seeking to understand the causes of ice ages.
Why would we expect to find a higher percentage of heavy elements in a young star, as opposed to an older star?
Heavy elements are produced by stars living and dying, so more heavy elements get incorporated into future generations of stars
The large, roundish, low-elevation region in the southern hemisphere is called ______________.
Hellas Basin
Why does the Moon have a layer of "powdery soil" on its surface?
It is the result of countless tiny impacts by small particles striking the Moon.
How does the amount of heat released in a planet's interior by radioactive decay change with time?
It drops off with time.
According to our theory of solar system formation, what three major changes occurred in the solar nebula as it shrank in size?
It got hotter, its rate of rotation increased, and it flattened into a disk.
What do we mean by the frost line when we discuss the formation of planets in the solar nebula?
It is a circle at a particular distance from the Sun, beyond which the temperature was low enough for ices to condense.
How does the Sun's mass compare with that of the planets?
It is about a thousand times more massive than all the planets combined.
In what way is Pluto more like a comet than a planet?
It is made mostly of rock and ice.
What happens to the energy that the ground absorbs in the form of visible sunlight?
It is returned upward in the form of infrared light.
Which of the following best describes convection?
It is the process in which warm material expands and rises while cool material contracts and falls.
About 2% of our solar nebula consisted of elements heavier than hydrogen and helium. However, the very first generation of stars in the universe probably consisted only of hydrogen and helium. Which of the following statements is most likely to have been true about these first-generation stars?
Planets were likely very rare or non-existent around these first-generation stars.
On average, a cupful of which planet would weigh the least?
Saturn
How does seafloor crust differ from continental crust?
Seafloor crust is thinner, younger, and higher in density.
Why does the interior of a large planet take longer to cool than the interior of a smaller planet?
Small planets have a larger surface area-to-volume ratio than larger planets.
____________ refers to telescopic observations in which we separate an object's light so we can measure its intensity at different wavelengths.
Spectroscopy
Which of the following is not a major difference between the terrestrial and jovian planets in our solar system?
Terrestrial planets contain large quantities of ice, and jovian planets do not.
________________ is a high-elevation region dotted by tall volcanoes.
Tharsis Bulge
Which of the following is not evidence supporting the idea that our Moon formed as a result of a giant impact?
The Pacific Ocean appears to be a large crater-probably the one made by the giant impact.
How is Einstein's famous equation, E = mc2, important in understanding the Sun?
The Sun generates its energy by converting 4 million tons of mass to energy each second.
Which of the following statements about our Sun is not true?
The Sun's diameter is about 5 times that of Earth.
What makes us think that Mars must once have had an atmosphere that was warmer and thicker?
The atmosphere is too cold and thin for liquid water today, yet we see evidence that water flowed on the surface in the past.
According to our present theory of solar system formation, which of the following statements about the growth of terrestrial and jovian planets is not true?
The jovian planets began from planetesimals made only of ice, while the terrestrial planets began from planetesimals made only of rock and metal.
According to our theory of solar system formation, why do all the planets orbit the Sun in the same direction and in nearly the same plane?
The laws of conservation of energy and conservation of angular momentum ensure that any rotating, collapsing cloud will end up as a spinning disk.
Suppose that two stars are separated in the sky by 0.01 arcsecond, and you observe them with a telescope that has an angular resolution of 1 arcsecond. What will you see?
The light from the two stars will be blended together so that they look like one star.
Many meteorites appear to have formed very early in the solar system's history. How do these meteorites support our theory about how the terrestrial planets formed?
The meteorites' appearance and composition is just what we'd expect if metal and rock condensed and accreted as our theory suggests.
Under what circumstances can differentiation occur in a planet?
The planet must have a molten interior
Based on the information provided, why was the landing site,called Meridiani Planum, chosen for the Opportunity mission?
The region was already known to have minerals likely to have formed in liquid water.
What is the primary reason we divide the ingredients of the solar nebula into four categories (hydrogen/helium gas; hydrogen compound; rock; metal)?
The temperature at which these materials condense into a solid varies considerably.
The region of our solar system between Mercury and Mars has very few asteroids, while the region between Mars and Jupiter has many asteroids. Based on what you have learned, what is the most likely explanation for the lack of asteroids between Mercury and Mars?
There were very few planetary leftovers in this region because most of the solid material was accreted by the terrestrial planets as the planets formed.
Which of the following statements about our solar system's terrestrial worlds are true?
They are all made primarily of rock and metal. Their interiors all have a core, mantle, and crust. They are all of nearly the same age.
How do scientists "know" what Earth's temperature would be without greenhouse gases?
They calculate this temperature from Earth's reflectivity and distance from the Sun.
Which of the following is not a characteristic of the outer planets?
They have relatively high densities.
Why do we think Mercury has so many tremendous cliffs?
They were probably formed by tectonic stresses when the entire planet shrank as its interior cooled.
The terrestrial planets are made almost entirely of elements heavier than hydrogen and helium. According to modern science, where did these elements come from?
They were produced by stars that lived and died before our solar system was born.
What can we conclude from the fact that Neptune's large moon Triton orbits in a direction opposite to the direction in which Neptune rotates?
Triton must have been captured by Neptune.
Why would the weather become more severe as the greenhouse effect increased?
Warming would increase the evaporation of the oceans, leading to more water in the atmosphere and more frequent and severe storms.
Which of the following statements about Mars is not true?
We could survive on Mars without spacesuits, as long as we brought oxygen in scuba tanks.
In the context of plate tectonics, what is a subduction zone?
a place where a seafloor plate is sliding under a continental plate
Suppose we represent Earth with a basketball. On this scale, most of the air in Earth's atmosphere would fit in a layer that is ________.
about the thickness of a sheet of paper
Greenhouse gases in the atmosphere, such as carbon dioxide and water vapor, make Earth warmer than it would be otherwise because these gases __________.
absorb infrared light emitted by the surface
What process initially caused the terrestrial planets to be hot enough to be entirely molten?
accretion
The two principal sources for the internal heating of terrestrial planets are
accretion and radioactivity.
Scientists have confidence in the predictions of today's best climate models because they __________.
agree well with actual data for recent decades
Which planet has a ring system?
all the jovian planets
The Sun's spectrum is
an absorption line spectrum, which is produced when continuous light from a hot source (the Sun's interior) passes through a cooler cloud of gas (the gas that makes up the Sun's visible surface).
The Cassini mission to Saturn consists of ________.
an orbiter that orbits Saturn and a probe that descended to the surface of Titan
What are the two most important properties of a telescope?
angular resolution light-collecting area
The familiar twinkling of the stars is caused by ___________, which also blurs telescopic images.
atmospheric turbulence
Why is the sky blue (on Earth)?
because molecules scatter blue light more effectively than red light
According to our present theory of solar system formation, why were solid planetesimals able to grow larger in the outer solar system than in the inner solar system?
because only metal and rock could condense in the inner solar system, while ice also condensed in the outer solar system
Larger planets are also ____________ able to retain atmospheres, making erosion more important there.
better
Where are most of the known asteroids found?
between the orbits of Mars and Jupiter
When we say that jovian planets contain significant amounts of hydrogen compounds, we mean all the following chemical compounds except ________.
carbon dioxide
Earth's atmosphere contains only small amounts of carbon dioxide because
carbon dioxide dissolves in water, and most of it is now contained in the oceans and carbonate rocks.
The core, mantle, and crust of a planet primarily have differences in their
density.
How is carbon dioxide initially removed from the atmosphere?
dissolving carbon dioxide in water
Also, planets that are warm enough to have liquid water will have much more __________ than those that are not that warm because water can cause a lot of __________ (assuming everything else is the same, of course).
erosion
All the planets in the solar system have at least one moon.
false
All the planets in the solar system rotate (spin) in the same direction as they orbit the Sun.
false
The northern hemisphere of Mars has fewer craters than the southern hemisphere. This indicates that
geologic processes have resurfaced much of the northern hemisphere.
Volcanism is more likely on a planet that
has high internal temperatures.
Which region of Earth has warmed the most in recent decades?
he Arctic
Which of the following does not provide evidence that Mars once had abundant liquid water on its surface?
he presence of canali, discovered in the late 1800s by Giovanni Schiaparelli and mapped by Percival Lowell
A planet is most likely to have tectonic activity if it has
high internal temperature.
Old Faithful Geyser at Yellowstone National Park in the United States acquires its energy from a ___________.
hot spot
What is Jupiter's main ingredient?
hydrogen and helium
According to our present theory of solar system formation, which of the following lists the major ingredients of the solar nebula in order from the most abundant to the least abundant?
hydrogen and helium gas; hydrogen compounds; rock; metal
A camera is an example of an instrument used for __________ observations.
imaging
The processes responsible for all surface geology on the terrestrial planets are ________.
impact cratering, volcanisms, tectonics, and erosion
The processes responsible for virtually all surface geology are _________.
impact cratering, volcanisms, tectonics, and erosion
Scientists can estimate the age of a planetary surface by counting __________.
impact craters
Where is most of the water on Mars?
in its polar caps and subsurface ground ice.
The greenhouse effect raises Earth's surface temperature (from what it would be otherwise) because the infrared light radiated by Earth's surface __________.
is absorbed by greenhouse gases, slowing the escape of this energy to space
How does Earth's magnetosphere affect our atmosphere?
it deflects solar wind particles that would otherwise strip away the atmosphere.
Given the rate at which human activity is increasing CO2 levels, why can we not depend on the natural CO2 cycle to prevent global warming?
it operates too slowly.
Based on all we know about the terrestrial worlds, what single factor appears to play the most important role in a terrestrial planet's geological destiny?
its size
What is the main reason that Mars, compared to Earth, has become so geologically inactive?
its size
Which of the following is the main reason why Venus has so little wind erosion?
its slow rotation
The existence of a core-mantle-crust interior structure tells us that a world __________.
once had a molten interior
Deep trenches in the ocean mark places where
one plate slides under another, returning older crust to the mantle.
In essence, the solar nebular theory states that ________.
our solar system formed from the collapse of an interstellar cloud of gas and dust
Which of the following gases best absorbs ultraviolet light?
ozone
What percentage of the mass of the solar nebula consisted of elements other than hydrogen and helium?
percent
What process has shaped Earth's surface more than any other?
plate tectonics
In what part of the solar system is Pluto found?
the Kuiper belt
Which of the following can act like a long-term "thermostat" for the Earth's average temperature?
the carbon dioxide cycle
What type of stresses broke Earth's lithosphere into plates?
the circulation of convection cells in the mantle, which dragged against the lithosphere
Which of the following best describes tectonics?
the disruption of a planet's lithosphere by internal stresses
Which of the following does the solar nebular theory not predict?
the equal number of terrestrial and jovian planets
Which of the following best describes volcanism?
the eruption of molten rock from a planet's interior to its surface
Which of the following best describes impact cratering?
the excavation of bowl-shaped depressions by asteroids or comets striking a planet's surface
Which moons are sometimes called the Galilean moons?
the four largest moons of Jupiter: Io, Europa, Ganymede, and Callisto
What do we mean by accretion in the context of planet formation?
the growth of planetesimals from smaller solid particles that collided and stuck together
If we want to confirm that a star's brightness alternately dims and brightens, we need _______________observations of the star.
time monitoring
A key clue to how the solar system formed is the observation that all the planets orbit close to the same plane.
true
All four of the giant outer planets-Jupiter, Saturn, Uranus, and Neptune-have rings.
true
All planets orbit the Sun in the same direction (counterclockwise as viewed from above Earth's North Pole).
true
Based on our theory of how our own solar system formed, we would expect that other solar systems would be quite common.
true
Earth is habitable in part due to the greenhouse effect.
true
Earth is the only planet in the solar system known to have active plate tectonics across its entire surface.
true
Earth outgassed as much carbon dioxide as Venus, but on Earth most of it is locked up in the oceans and rocks.
true
Impacts were extremely common in the young solar system but are less common today.
true
Most of Earth's water likely resulted from impacts with icy planetesimals that originated in the outer regions of the solar system.
true
Oceans cover more of Earth's surface than land.
true
Outside the frost line, planetesimals could form into larger objects due to the presence of both rock and ice.
true
Considering only the tilt of their axis, which planet listed below would have the most extreme seasons?
uranus
Rank the atmospheric pressure at the surface, from highest to lowest.
venus, earth, mars, mercury
Rank the total amount of gas in their atmospheres, from most to least.
venus, earth, mars, mercury
The energy that warms Earth's surface comes primarily in the form of __________.
visible light from the Sun
Earth's temperature remains fairly steady, which means that Earth must return nearly the same amount of energy to space that it receives from the Sun. In what form(s) does Earth return most of this energy to space?
visible light reflected by clouds infrared light emitted by the surface and atmosphere visible light reflected by the surface
The closer a planet is to the Sun, the __________ it will be and therefore the __________ weather it should have.
warmer, more
Observations of young stars (as well as theory) tell us that when the Sun was young, the solar wind
was stronger than it is today.
The relatively few craters that we see within the lunar maria
were formed by impacts that occurred after those that formed most of the craters in the lunar highlands.
What are the circumstances under which convection can occur in a substance?
when the substance is strongly heated from underneath
