Astro 101 RQ Chapter 8
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Astro 101
Rachel Goodin
RQ Chapter 8
3/08/2024
3. What do we mean by the
solar nebula
? What was it made of, and where did it come
from?
The solar nebula refers to the piece of interstellar cloud from which our own solar system
formed. The nebula theory, which is a theory of our solar system’s birth, begins with the idea
that our solar system was born from the gravitational collapse of an interstellar cloud of gas
(called the solar nebula) that collapsed under its own gravity. The gas that made up the solar
nebula was the product of billions of years of galactic recycling that occurred before the Sun and
planets were born. The gas that made up the solar nebula contained (by mass) about 98%
hydrogen and helium and 2% all other elements combined. The Sun and planets were born
from this gas, and Earth and the other terrestrial worlds were made primarily from the heavier
elements mixed within it.
5. List the approximate condensation temperature and abundance for each of the four
categories of materials in the solar nebula. Which ingredients are present in terrestrial
planets? In jovian planets? In comets and asteroids?
-
Hydrogen and helium gas: 98% of the solar nebula. These gases never condense in
interstellar space and therefore do not have a condensation temperature.
-
Hydrogen compounds: 1.4% of the solar nebula. Materials such as water (H
2
O),
methane (CH
4
), and ammonia (NH
3
) can solidify into ices at low temperatures (below
about 150 K under the low pressure of the solar nebula).
-
Rock: 0.4% of the solar nebula. Rocky material is gaseous at high temperatures, but
condenses into solid form at temperatures below 500 K to 1300 K, depending on the
type of rock.
-
Metal: 0.2% of the solar nebula. Metals such as iron, nickel, and aluminum are also
gaseous at high temperatures, but condense into solid form at temperatures below 1000
K to 1600 K, depending on the metal.
The solid seeds of metal and rock from the inner solar system ultimately grew into the terrestrial
planets, while the seeds of ice as well as metal and rock from the outer solar system ultimately
grew into the jovian planets. Asteroids are the rocky leftover planetesimals of the inner solar
system and are therefore made up of metal and rock, whereas comets are the icy leftover
planetesimals of the outer solar system and are therefore made up of ice, metal, and rock.
6. What was the
frost line
? Which ingredients condensed inside and outside the frost
line? What role did it play in the formation of two distinct types of planets?
The frost line was the boundary in the solar nebula beyond which ices could condense; only
metals and rocks could condense within the frost line. The frost line lays between the
present-day orbits of Mars and Jupiter and marked the key transition between the warm inner
regions of the solar system where the terrestrial planets formed and the cool outer regions
where the jovian planets formed. Inside the frost line, only metal and rock could condense into
solid “seeds”. Outside of the frost line, the solid seeds were built of ice along with metal and
rock. Due to the fact that hydrogen compounds were nearly three times as abundant in the
nebula as metal and rock combined, the total amount of solid material was far greater beyond
the frost point than within it. Thus the stage was set for the birth of two types of planets: planets
born from seeds of metal and rock in the inner solar system (terrestrial planets) and planets
born from the seeds of ice as well as metal and rock in the outer solar system (jovian planets).
7. Briefly describe the process by which terrestrial planets are thought to have formed.
The process by which the small seeds of metal and rock from which the terrestrial planets
eventually grew is called accretion, which begins with the microscopic solid particles that
condensed from the gas of the solar nebula. These particles orbited the Sun with the same
orderly, circular paths as the gas from which they condensed. Although the particles were far too
small to attract each other gravitationally at this point, they were able to stick together through
electrostatic forces. Small particles thereby began to combine into larger ones, attracting each
other through gravity and accelerating their growth into boulders large enough to count as
planetesimals (pieces of planets) as they grew in mass. These planetesimals would continue
growing rapidly into the terrestrial planets we now know today, but would eventually halt growth
when reaching their final largest size as further growth became more difficult.
8. How was the formation of jovian planets similar to that of the terrestrial planets? How
was it different? Why did the jovian planets end up with so many moons?
Accretion as it occurred with the terrestrial planets should have occurred similarly in the outer
solar system, but the condensation of ices meant both that there was more solid material and
that this material contained ice in addition to metal and rock. Additionally, the leading model for
jovian planet formation holds that the largest ice-rich planetesimals became sufficiently massive
for their gravity to capture some of the hydrogen and helium gas that made up the vast majority
of the surrounding solar nebula. This added gas made their gravity even stronger, allowing them
to capture even more gas. Ultimately, the jovian planets accreted so much gas that they wound
up bearing little resemblance to the icy seeds from which they grew. Thus, the formation of
jovian planets was alike to that of terrestrial planets in that it utilized accretion, but different in
that it did so to a much larger scale with the use of hydrogen and helium gases. During this
process, each jovian planet came to be surrounded by its own disk of gas, spinning in the same
direction as the planets rotated. This created ideal conditions for the maximum formation of
moons, for moons that accreted from ice-rich planetesimals within these disks ended up with
nearly circular orbits going in the same direction as their planet;s rotation and lying close to their
planet’s equatorial plane.
9. What is the
solar wind
, and what roles did it play in the early solar system?
Solar wind is a stream of charged particles such as protons and electrons that are ejected
outward from the Sun in all directions. The vast majority of the hydrogen and helium gas in the
solar nebula never became part of any planet due to it being blown away by these
aforementioned solar winds. Observations show that stars tend to have much stronger winds
and emit much more high-energy radiation when they are young, so the young Sun should have
had a strong enough combination of radiation and wind to clear the solar system of its remaining
gas. This clearing of the gas sealed the compositional fate of the planets, for if the hydrogen gas
had remained any longer, it might have continued to cool until hydrogen compounds could have
condensed into ices even in the inner solar system. In that case, the terrestrial planets might
have accreted abundant ice, and perhaps hydrogen and helium gas as well, changing their
basic nature. This clearing of the nebula due to solar winds also helps to explain the
unexpectedly slow rotation of the Sun, for the gas cleared from the nebula into interstellar space
carried away with it much angular momentum, leaving the Sun with greatly diminished angular
momentum and the slow rotation that we see today.
10. How did planet formation lead to the existence of asteroids and comets?
As stated earlier, asteroids are the rocky leftover planetesimals of the inner solar system, while
comets are the icy leftover planetesimals of the outer solar system. Thus, all the leftover
planetesimals that were not pulled to and combined with other planetesimals through
gravitational attraction became comets or asteroids. Evidence that asteroids and comets are
leftover planetesimals comes from analysis of meteorites, spacecraft visits to comets and
asteroids, and theoretical models of solar system formation. The asteroids and comets that exist
today likely represent only a small fraction of the leftover planetesimals that roamed the young
solar system. The rest are now gone, some of them being flung into deep space by gravitational
encounters, while many others collided with planets, leaving behind impact craters in their wake.
12. What is the leading hypothesis for the Moon’s formation, and what evidence supports
this hypothesis?
The leading hypothesis for the formation of the Moon suggests that it formed as the result of a
giant impact between Earth and a huge planetesimal. According to models, a few leftover
planetesimals may have been as large as Mars. If one of these Mars-sized objects struck a
young planet, the blow might have tilted the planet’s axis, changed the planet’s rotation rate, or
completely shattered the planet. The giant impact hypothesis holds that a Mars-sized object hit
Earth at a speed and angle that blasted Earth’s outer layers into space. According to computer
simulations, this material could have collected into orbit around our planet, and accretion within
this ring of debris could have formed the Moon. Strong supporting evidence for the giant impact
hypothesis comes from two features of the Moon’s composition. Firstly, the Moon’s overall
composition is quite similar to that of Earth’s outer layers. Secondly, the Moon has a much
smaller proportion of easily vaporized ingredients (such as water) than Earth does. This fact
supports the giant impact hypothesis because the heat of the impact would have vaporized
these ingredients. As gases, they would not have participated in the subsequent accretion of the
Moon.
13. Describe the technique of
radiometric dating
. What is a
half-life
?
Radiometric dating refers to the process of determining the age of a rock (i.e. the time since it
solidified) by comparing the present amount of a radioactive substance to the amount of its
decay product. 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.
Radiometric dating relies on the careful measurements of a rock’s proportions of various atoms
and isotopes, and the key to this technique lies in the fact that some isotopes are radioactive,
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which is essentially 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. While the decay of any single nucleus is an instantaneous event,
laboratory studies show that a modest amount of any radioactive parent (original) isotope will
gradually transform itself into the daughter (resulting) isotope at a very steady rate. Once this
rate of transformation is measured, we can use it in order to calculate the half-life, which is the
time it would take for half of the parent nuclei to decay. Every radioactive isotope has its own
unique half-life, which may be anywhere from a fraction of a second to many billions of years.
14. How old is the solar system, and how do we know?
In order to use radiometric dating to find the age of the solar system, we must find rocks that
have not melted or vaporized since they first condensed in the solar nebula. The meteorites that
have fallen to Earth are our source of such rocks, for many of them 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 years after that, Earth and the other planets
had formed by about 4.5 billion years ago. In other words, our solar system is approximately 4.5
billion years old, making it only about a third of the 14-billion-year age of our universe.