Download 5) Earth in space and time. The student understands the solar

5) Earth in space and time. The student
understands the solar nebular accretionary
disk model. The student is expected to:
(b) investigate thermal energy sources, including kinetic heat of
impact accretion, gravitational compression, and radioactive decay,
which are thought to allow protoplanet differentiation into layers;
(d) explore the historical and current hypotheses for the origin of the
Moon, including the collision of Earth with a Mars-sized planetesimal;
• Thea planetesimal collision creates Earth’s Moon
(f) compare extra-solar planets with planets in our solar system and
describe how such planets are detected.
• Observation with telescopes of other star systems based on the
stars radial velocity caused by the gravitational force between
the planets and the star (Wiggle Effect)
• Transit Method: Photometric method
The forces operating
during the formation
of the Solar System
were responsible for
the diversity of
matter in the Solar
System and also
responsible for
diversity of
planetary internalstructures.
As the nebula cools
1) The inner zone stays warm (>100 ° C) and only high temperature condensates
form - giving the terrestrial planets high density.
Five major elements; Fe, Mg, Si, O, and S; comprise at least 95% of the mass of each
of the terrestrial planets. These elements are high temperature condensates.
2) Outer zone cools more, so low-T materials condense into the outer - low
density planets with lots of, ice, and frozen gases like CH4, CO2 etc...
This model shows planetesimal
accretion in our solar system. The
total time frame for the process is
about 441 million years.
There were 11 inner planetesimals
after 79 million years, and six after
151 million years.
Suggestions are that the Earth
accreted in about 100 million years.
The terrestrial planets (inner rocky
planets) formed close to the sun,
because nothing else would accrete
there. The gases all vaporized
because of the temperature.
Gases have the chance to freeze past
the frost line (between Mars and
Jupiter), and the outer planets are
composed largely of frozen gases as a
result.
Planets can differentiate by mass and
density, the same way that solar systems do.
As you might remember:
Potential energy is stored, while kinetic
energy is possessed by objects in motion.
Early differentiation of the Earth involved the
separation of Fe-Ni rich (heavy) from silicate
material (light) to form the core and mantle.
High temperatures were necessary and
differentiation likely occurred in response to
large-scale melting, induced by high-energy
impacts. (kinetic heat of impact accretion)
Kinetic energy from these impacts caused the
melting.
"Earth's accretion history was dominated by
tens of high-energy collisions with Moon- to
Mars-sized bodies
Over time, differentiation
occurred based on temperature
and mass.
Silicates include minerals that
contain both oxygen and
silicon, and compose the vast
majority of the Earth’s crust.
High-density materials tend to sink through lighter materials. Iron, the
commonest element that is likely to form a very dense molten metal phase, tends
to congregate towards planetary interiors.
The main zones in the solid Earth are the very dense iron-rich metallic core, the
less dense magnesium-silicate-rich mantle and the relatively thin, light crust
composed mainly of silicates of aluminum, sodium, calcium and potassium.
Even lighter still are the
watery liquid hydrosphere
and the gaseous, nitrogenrich atmosphere.
low-density silicate rocks,
such as granite, are well
known and abundant in the
Earth's upper crust.
Temperature within the Earth increases with depth.
The Earth's internal heat comes from a combination of
residual heat from planetary accretion (about 20%) and heat
produced through radioactive decay (80%).
The major heat-producing
isotopes in the Earth are
potassium-40, uranium-238,
uranium-235, and thorium232. At the center of the
planet, the temperature may
be up to 7,000 K
(Water freezes at 273
Kelvins)
There have been many
explanations as to the
origin of Earth’s one
natural satellite, the Moon.
The Sister Theory
This theory states that the Moon
formed in orbit around the Earth.
Theories state that physical
structures and compositions of the
planets depend on their distance from
the Sun.
• Mercury, which is closer to the
Sun than we are, is considerably
richer in dense materials
• Mars, which is further from the
Sun, is considerably richer in less
dense materials.
We discount the sister theory now, because the Moon has a density like that of
Mars, and considerably lower than that of the Earth. It would be much easier to
understand this low density if the Moon were formed near the orbit of Mars.
The Capture Theory
This theory states that the Moon was
captured from somewhere else.
When an object comes by a planet, it
can either run into it, or pass by it in a
hyperbolic orbit, which carries it off
into space.
The capture theory states
that Earth was sufficiently
large enough to “capture” the
migrating moon in it’s
gravitational field.
Remember, fission is what
occurs when objects “split”.
The fission theory states that
Earth's Moon probably formed
out of material splashed into
orbit by the impact of a large
body into the early Earth.
Differentiation on Earth had
probably already separated many
lighter materials toward the
surface already, so that the
impact removed a
disproportionate amount of
silicate material from Earth,
(lighter) and left the majority of
the dense metal behind.
.
The Moon's density is
substantially less than that of
Earth, due to its lack of a
large iron core.
We now embrace a Big Crunch, or Big Collision, theory in which an object
about the size of Mars runs into the Earth, knocking off a part of its mantle.
The pieces blasted out into space orbit the Earth, and form into the Moon.
This theory is attractive
because it solves all the
problems of previous theories.
• The low density of the
Moon is explained by its
being made up mostly of
mantle material.
•The similarity with Earth
rocks is explained by the
Moon having been part of
. the Earth.
•The low abundance of volatile materials is explained by having them
escape into space when the pieces of the Earth which are to
become the Moon are blasted out of the Earth.
Any planet is an extremely faint
light source compared to its
parent star.
In addition to the incredible
difficulty of detecting such a
faint light source, the light
from the parent star causes a
glare that washes it out. For
those reasons, only a very few
extrasolar planets have been
observed directly.
Instead, astronomers have generally had to resort to
indirect methods to detect extrasolar planets. At the
present time, several different indirect methods have
yielded success.
A star with a planet will move in its own small orbit in response to the planet's
gravity. This leads to variations in the speed with which the star moves toward
or away from Earth. I.e. the variations are in the radial velocity of the star with
respect to Earth
This has been by far the most productive
technique used by planet hunters. It is
also known as Doppler spectroscopy. It is
generally only used for relatively nearby
stars out to about 160 light-years from
Earth.
It easily finds massive planets that are
close to stars, but detection of those
orbiting at great distances requires many
years of observation. One of the main
disadvantages of the radial-velocity
method is that it can only estimate a
planet's minimum mass.
A pulsar is a neutron star: the small, ultra-dense
remnant of a star that has exploded as a
supernova. Pulsars emit radio waves extremely
regularly as they rotate.
Because the rotation of a pulsar is so regular,
slight changes in the timing of its observed
radio pulses can be used to track the pulsar's
motion.
Like an ordinary star, a pulsar will move in its
own small orbit if it has a planet. Calculations
based on pulse-timing observations can then
reveal the size and shape of that orbit, and the
probable planet.
The main drawback of the pulsar-timing method
is that pulsars are relatively rare, so it is unlikely
that a large number of planets will be found
this way. Also, and perhaps more importantly,
life as we know it could not survive on planets
orbiting pulsars since high-energy radiation
there is extremely intense.
When a planet crosses in front of a
star, then the star’s brightness dims
by a small amount.
This is the detection method used
by space telescopes that were
launched in the last decade.
One limitation of this method is
that it is only possible to detect
these crossings, also known as
transits, when the planet and the
star are perfectly aligned with the
line of sight of the detection
instrument.
This method, also called the
photometric method can determine
the radius of a planet.
The amount the star dims depends on
the relative sizes of the star and the
planet.
About 10% of planets with small orbits
have such alignment, and the fraction
decreases for planets with larger
orbits.
The transit method also
makes it possible to study
the atmosphere of the
transiting planet.
When the planet transits
the star, light from the star
passes through the upper
atmosphere of the planet.
By studying the stellar
spectrum carefully, as it
filters through that
planet’s atmosphere, one
can detect elements
present in that
atmosphere.
Missing frequencies through the spectroscope are clues, indicating elements or
compounds that absorb light at those frequencies are present in the
atmosphere. For example, if the light frequencies corresponding to methane
and carbon monoxide are missing from an analysis of the starlight, the
atmosphere contains methane and carbon monoxide, which absorbed the
missing light.
We use similar methods to determine atmospheric components around planets
in our own solar system.
The extrasolar planets that have
been easiest to detect are large,
and in small, close orbits.
Many of these are gas giants like
the planet Jupiter but orbiting as
close to their sun as planet
Mercury.
This type of planet has been called
a “hot Jupiter”.
Why couldn’t planets like hot
Jupiters have formed close to the
sun in our solar system?
At this stage, it appears to be fairly common for Sun-like stars to have
planets. Our current estimates are that it is possible as many as 40% of Sun-like
stars have some type of orbiting planet. This would mean there is an enormous
number of planets in our galaxy, given there are at least 200 billion stars.