Lec 13 Flashcards
(30 cards)
4 criteria for the success of a solar system formation theory:
- It must explain the patterns of motion discussed in Chapter 7.
- It must explain why planets fall into two major categories: small, rocky terrestrial planets near the Sun and large, hydrogen-rich jovian planets farther out.
- It must explain the existence of huge numbers of asteroids and comets and why these objects reside primarily in the regions we call the asteroid belt, the Kuiper belt, and the Oort cloud.
- It must explain the general patterns while at the same time making allowances for exceptions to the general rules, such as the odd axis tilt of Uranus and the existence of Earth’s large Moon
nebular theory
The Nebular Theory is the leading scientific explanation for how our solar system formed, including the Sun, planets, moons, asteroids, and comets. It proposes that the entire solar system formed from a giant rotating cloud of gas and dust, called the solar nebula, about 4.6 billion years ago.
solar nebula
The nebular theory begins with the idea that our solar system
was born from the gravitational collapse of an interstellar cloud
of gas (and dust), called the solar nebula, that collapsed under
its own gravity.
This cloud gave birth to the Sun at its center and the planets in a spinning disk that formed around the young Sun
Where did the gas that made up the solar nebula come from?
According to modern science, it was the product of billions of years of galactic recycling that occurred before the Sun and planets were born.
Recall that the universe as a whole is thought to have been born in the Big Bang, which essentially produced only two chemical elements: hydrogen and helium.
Heavier elements were produced later, some through the nuclear fusion that makes stars shine, and most others through nuclear reactions accompanying the explosions that end stellar lives.
The heavy elements then mixed with other interstellar gas that formed new generations of star
What caused the orderly patterns of motion?
solar nebula began as a large and spherical cloud of low-density, cold gas
-initially, this material was so spread out that gravity alone may not have been strong enough to pull it together and start its collapse
-the collapse may have been triggered by a cataclysmic event(explosion of supernova)
–once the collapse started, gravity allowed it to continue and started shrinking
Because gravity pulls inward in all directions, it explains why the Sun and the planets are
spherical.
However, other physical laws also apply, and these explain how orderly motions arose in the solar nebula.
Heating effect
The temperature of the solar nebula increased
as it collapsed. Such heating represents energy conservation in action.
As the cloud shrank, its gravitational potential energy was converted to the kinetic
energy of individual gas particles falling inward.
These particles crashed into one another, converting the kinetic energy of their inward fall to the random motions of thermal energy
The Sun formed in the center, where temperatures and densities were highest.
spinning
Like an ice skater pulling in her arms as she
spins, the solar nebula rotated faster and faster as it shrank in radius.
This increase in rotation rate represents conservation of angular momentum in action
The rotation of the cloud may have been
imperceptibly slow before its collapse began, but the cloud’s shrinkage made fast rotation inevitable.
The rapid rotation helped ensure that not all the material in the solar nebula collapsed into the center:
The greater the angular momentum of a rotating cloud, the more spread out it will be
flattening effect
The solar nebula flattened into a disk.
This flattening is a natural consequence of collisions between particles in a spinning cloud.
A cloud may start with any size or shape, and different clumps of gas within the cloud may be moving in random directions at random speeds.
These clumps collide and merge as the cloud collapses, and each new clump has the average velocity of the clumps that formed it.
The random motions of the original cloud therefore become more orderly as the cloud collapses, changing the cloud’s original lumpy shape into a rotating, flattened disk.
Similarly, collisions between clumps of material in highly elliptical orbits reduce their eccentricities, making the orbits more circular
why are there 2 major types of planets?
Terrestrial planets formed
in the warm inner regions of the swirling disk, while jovian
planets formed in the colder outer regions.
condensation
the basic process of seed formation was
like the formation of snowflakes in clouds on Earth:
When the temperature is low enough, some atoms or molecules in a gas may bond and solidify.
The general process in which solid (or liquid) particles form in a gas is called
condensation—we say that the particles condense out of the gas.
(Pressures in the solar nebula were generally too low to allow the condensation of liquid droplets.)
These particles start out microscopic in size, but they can grow larger with time.
accretion
The process by which small “seeds” grew into planets is called accretion
Accretion began with the microscopic solid particles that condensed from the gas of
the solar nebula.
These particles orbited the forming Sun
with the same orderly, circular paths as the gas from which they condensed.
Individual particles therefore moved at
nearly the same speed as neighboring particles, so “collisions” were more like gentle touches.
Although the particles were far too small to attract each other gravitationally at this point, they were able to stick together through electrostatic forces—the same “static electricity” that makes
hair stick to a comb.
Small particles thereby began to combine into larger ones
planetesimals
As the particles grew in mass, they
began to attract each other through gravity, accelerating their growth into boulders large enough to count as planetesimals, which means “pieces of planets.”
Some models suggest that this stage of accretion may have been enhanced by pebble-sized particles gathering together due
to friction with the solar nebula gases.
The planetesimals grew rapidly at first. As they grew larger, they had both more surface area to make contact with other planetesimals and more gravity to attract them.
Some planetesimals probably grew to hundreds of kilometers in size within a few million years (or even less)—a
long time in human terms, but only about one-thousandth of the present age of the solar system.
However, once the planetesimals reached these relatively large sizes, further
growth became more difficult
making the jovian planets
Accretion should have occurred similarly in the outer solar system, but condensation of ices meant both that there was more solid material
and that this material contained ice in addition to metal and rock.
-the leading model says that the largest ice-rich planetesimals became massive for their gravity to capture some H and He gas, making up the solar nebula
–this added gas made their gravity stronger, allowing them to capture more gas
model also explains large moons of jovian planets
-the same processes of heating, spinning, and
flattening that made the disk of the solar nebula also affected the gas drawn by gravity to the young jovian planets
-each jovian planet came to be surrounded by its own disk of gas, spinning in the same direction as the planet rotated
impact craters
When impacts occur on solid worlds, they leave behind impact craters as
scars.
Impacts have thereby transformed planetary landscapes and, in the case of Earth, altered the course of evolution.
For example, an impact is thought to have been responsible for the death of the dinosaurs
how did earth get the water that makes up our oceans and the gases that first formed our atmosphere?
The likely answer is that water, along with other hydrogen compounds, was brought
to Earth and other terrestrial planets by the impacts of water-bearing planetesimals that formed farther from the Sun.
As these planetesimals became part of the forming Earth, their gaseous content became trapped on or within our planet
giant impact
The Moon’s density is considerably lower than Earth’s, indicating that it has a very
different average composition.
So how did we get our Moon?
Today, the leading hypothesis suggests that it
formed as the result of a giant impact between Earth and a huge planetesimal
giant impact hypothesis
Strong support for the giant impact hypothesis comes from two features of the Moon’s composition.
1) the Moon’s overall composition is quite similar to that of Earth’s outer layers—just as we would expect if it were made from material blasted away from those layers
2) the Moon has a much smaller proportion of easily vaporized ingredients (such as water) than Earth.
–this fact supports the hypothesis because the heat of the impact would’ve vaporized these ingredients
process of solar system formation according to the nebular theory
Contraction of Solar Nebula: As it contracts,
the cloud heats, flattens, and spins faster,
becoming a spinning disk of gas and dust.
Condensation of Solid Particles: Hydrogen and
helium remain gaseous, but other materials can condense into solid “seeds” for building planets.
Accretion of Planetesimals: Solid “seeds”
collide and stick together. Larger ones attract
others with their gravity, growing bigger still.
Clearing the Nebula: The solar wind blows
remaining gas into interstellar space.
how do we measure the age of a rock?
some atoms
undergo changes with time that allow us to determine how long they have been held in place within a rock’s solid
structure.
In other words, the age of a rock is the time since its atoms became locked together in their present arrangement, which in most cases means the time since the rock
last solidified
radiometric dating
The method by which we measure a
rock’s age is called radiometric dating, and it relies on careful measurement of the rock’s proportions of various atoms and isotopes.
Recall that each chemical element is
uniquely characterized by the number of protons in its nucleus, and that different isotopes of the same element differ in their number of neutrons
The key to radiometric dating lies in the fact that some isotopes are radioactive, which is just a fancy way of saying that their
nuclei tend to undergo some type of spontaneous change (also called decay) with time, such as breaking into two
pieces or having a neutron turn into a proton.
half-life
time it would take for half of the parent nuclei to decay
how do we know the age of the solar system?
Radiometric dating tells us how long it has been since a rock solidified, which is not the same as the age of a planet as a whole.
For example, we find rocks of many different
ages on Earth.
Some rocks are quite young because they
formed recently from molten lava; others are much older.
The oldest Earth rocks are about 4 billion years old, and some small mineral grains date to almost 4.4 billion years ago, but even these are not as old as Earth itself, because
Earth’s entire surface has been reshaped through time
how do we determine when the planets first began to form?
To go all the way back to the origin of the solar system, we must find rocks that have not melted or vaporized since they first condensed in the solar nebula.
Meteorites that have fallen to Earth are our source of such rocks.
Many meteorites appear to have remained unchanged since they condensed and accreted in the early solar system.
Careful analysis of radioactive isotopes in these meteorites shows that the oldest ones formed about 4.56 billion years ago, so
this time must mark the beginning of accretion in the solar nebula.
Because the planets apparently accreted within about 50 million (0.05 billion) years after that, Earth and the other planets had formed by about 4.5 billion years ago.
In other words, the age of our solar system is only about a third of the 14-billion-year age of our universe
How did we arrive at a theory of solar system formation?
A successful theory must explain four major features of our solar system: patterns of motion, the existence of two types of planets (terrestrial and jovian), the presence
of asteroids and comets, and exceptions to the rules.
Developed over a period of more than two centuries, the nebular theory explains all four features and also can account for other planetary systems.
