Download jun30 - Astronomy

The Outer Layers of the Sun
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The Photosphere
The Chromosphere
The Corona
The Heliosphere
The Chromosphere
2,500 km think irregular region above
photosphere of hot thin gas.
Density is about a million times less than the top
of the photosphere and the temperature rises to
10,000 K.
Discussion
If the chromosphere is so much hotter than the
photosphere, why does it not shine brighter than
the photosphere?
Why do you think it is red?
Calcium K line at 393.4 nm
What makes the chromosphere
so hot?
The chromosphere is heated from above by the
solar corona.
Plages
Bright patches surrounding sunspots. Plages are
associated with concentrations of magnetic fields.
Filaments & prominences are the
same thing!
Prominences
Two types:
1. Quiescent – can last for months
2. Active – last only for hours. Lie above
sunspots and have violent motions.
Hedgerow Prominence
Flares
Sudden violent explosions in magnetically active
regions. Last from 1 to 20 minutes and release
as much energy as 2.5 million 100-megaton
nuclear bombs in an area about the size of Earth.
This is enough energy to cause thermonuclear
fusion to take place on the Sun’s surface.
The Transitional Region
A thin region (can be as thin as a few 10’s of km)
above the chromosphere where the
temperature rises from 10,000 K to 1 million K
typical of the corona.
Corona
Outer part of the Sun’s atmosphere. About as
bright as the quarter phase Moon, it is visible
only when the light of the photosphere is
blocked.
The corona is so thin that on Earth we would
consider it a vacuum.
The Corona is surprisingly hot
The temperature of the corona is over 1 million K.
Discussion
At the distance of the Earth’s orbit the corona,
although cooler than near the Sun, is still 0.14
million K. Why don’t astronauts burn up when
they go on space walks?
Discussion
What causes the helmet shaped structures
in the corona? Why isn’t the corona spread
out uniformly around the Sun?
Coronal Mass Ejection
Giant magnetic bubbles that can hurl 5 to 50
billion tons of matter at speeds of 400 km/sec.
70% of coronal mass ejections are associated
with, or followed by, erupting prominences.
While 40% are accompanied by solar flares that
occur at about the same time and place.
The Sun-Earth connection
Coronal mass ejections and solar flares can be
directed at Earth. Luckily for us, Earth has a
magnetic field and an atmosphere to protect us.
Aurora
When high speed particles from the Sun collide
with atoms in Earth’s upper atmosphere. The
electrons are knocked into higher energy
orbitals and emit light when returning to the
ground state.
Solar Wind
Although the Sun’s surface gravity is much
higher than the Earth’s, it is not able to
contain particles with a temperature on over a
million K. Thus the hot corona spews matter
constantly (not just during flares, CME, and
explosive prominences) into space at a rate of
about 1 million tons per second.
Exam Next Wednesday
essay & multiple choice questions
Covers chapters 1-8, S1 & 14
Allowed one standard sheet of notes with
writing on one side only
Terrestrial planet geology
Terrestrial Planets
All the terrestrial planets are more or less
differentiated, i.e. the densest materials have
sunk to the core and the lighter materials have
floated to the surface.
The terrestrial planets were all completely
molten at some time in the past.
Discussion
Why do you think all the terrestrial planets were
so hot in the past? Isn’t space rather cold?
Terrestrial planets interior
structure
Core – highest density material, mostly iron
and nickel
Mantle – high density silicate rocks
Crust – lower density silicate rocks, granite
and basalt.
Discussion
What’s a silicate? Give and example of a
silicate.
Earth’s internal structure
1. Solid crust – 5 km thick under the oceans,
made of basalt: silicates of aluminum,
magnesium and iron with a density of about
3.5 g/cm3. Under the continents the crust is
35 to 70 km thick and is made mostly of
granite: silicates of aluminum, sodium and
potassium with a density of 3.0 g/cm3. The
continents float on the basalt.
2. Mantle – solid, but top layer is plastic called
the asthenosphere. Is about 2800 km thick
and made of compounds rich in iron and
magnesium. Density increases from 3.5
g/cm3 at top to 5.5 g/cm3 near the bottom.
3. Outer core – liquid. Is about 2200 km thick
and is made of iron, nickel and sulfur.
4. Inner core – solid. Is about 1300 km thick
and is composed of nearly pure crystalline
iron with a density of 13 g/cm3.
Why is inner core solid while
outer core is liquid?
Isn’t the inner core hotter than the outer core?
The melting point of substance depends on both
temperature and pressure. In general, the
melting point goes up with pressure. The inner
core is hotter than the outer core but is under a
greater pressure and thus has a higher melting
temperature.
Discussion
How can we know anything about the core of
the Earth when our deepest mines and bore
holes haven’t even made it through the curst?
Earthquakes
Earthquakes produce three types of waves that
travel through the Earth.
1. Surface waves
2. Primary waves, or P waves
3. Secondary waves, of S waves
Wave speed
The speed of seismic waves depends primarily
on the density of the material through which
they travel. If the density changes, the waves
will be refracted, just as light is refracted in a
glass lens.
Discussion
Which type a wave do you think will travel
better through the interior of a substance
which is liquid and why?
S waves cannot travel far through a liquid. On
the opposite side of the Earth from and
epicenter of an earthquake, seismographs only
detect P waves.
The molten core causes the P waves to bend is
such a way that the density of the core can be
determined.
Discussion
How do we know that the Earth’s inner core is
solid?
Discussion
Study of seismic waves on Earth indicate that
the inner solid core is rotating faster than the
rest of the Earth. How do we know the core is
rotating faster?
Discussion
What do you suspect would be the result of
this difference in rotation rate between the
solid and liquid cores?
Planetary seismic data?
We know the internal structure of the
Moon because astronauts placed nuclear
powered seismic stations at the Apollo
landing sites. But this information is not
available for any of the other planets.
Planets internal structure
Two tools without seismic data:
1) Mean density
2) Gravitational mapping – mascons
Density
Mass of the planet divided by the volume
of the planet.
Higher density implies a larger
percentage of high density materials,
such as iron and nickel, lower density
implies more silicates.
Mapping the gravitational field
By carefully tracking an orbiting space probe,
concentrations of denser materials below the
surface can be mapped. Space probes are
accelerated more toward higher density
regions.
How the planets got hot
1) Heat of accretion
2) Heat of differentiation
3) Heat from radioactive decay
Discussion
Which of the terrestrial worlds (including the
Moon) was likely the hottest during its
formation? Why?
Heat and planets
All the terrestrial planets started out hot
and have been losing heat over time by
radiating it into space from their surfaces.
2nd law of Astronomy 201
Larger planets lose heat more slowly than
do smaller planets.
Discussion
Larger planets have larger surface areas, and
a larger surface area should radiate more
energy into space? So shouldn’t larger
planets cool faster? Why do larger planets
cool more slowly than smaller planets?
Volume and heat
The greater the surface area, the faster heat
will be radiated. But, it is the volume that
stores the heat. The greater the volume of a
planet the more internal heat it can retain.
Also, more massive planets have more
radiative material.
It’s geometry
The surface area increases as the square of
the radius. But the volume increases as
the cube of the radius. Thus, a larger
sphere has less square miles to radiate the
heat per cubic mile of material.
This is why people get fat!
Geologic activity
Internal heat drives geologic activity on the
planets’ surfaces.
Discussion
Because heat is radiated from the surface
of a planet, the surface is cooler than the
interior.
How does the heat from the core of the
planet get to the surface?
How does the heat get out?
Convection in the mantle.
Note that the mantle is not liquid but it is
plastic, meaning that it can flow like silly
putty or glass.
Plate tectonics
Lithosphere
The convective cells in the planets do not
make it to the surface as on the Sun, but are
stopped at the base of the lithosphere. The
lithosphere includes the crust and the upper
mantel region of cooler, stronger rock which
does not flow as easily as the warmer, lower
mantel rock.
Geologic processes
1) Impact cratering
2) Volcanism
3) Tectonics
4) Erosion
3rd law of astronomy 201
The more impact craters on a surface, the
older that surface is.
“Age” referring to the time since the
surface was last molten
Discussion
Which area on the Moon is older, the light
region to the left or the dark region in the
center of the picture?
Why?
All the terrestrial planets probably receive
about the same number and size distribution
of impacts. All the other geologic processes
(volcanism, tectonics, and erosion) tend to
erase impact craters on the surface.
Discussion
Rank the terrestrial planets (include the
Moon) in terms of the age of their surfaces
from youngest to oldest to try and predict
which planets will have the most craters.
1.
2.
3.
4.
5.
Earth
Venus
Mars
Mercury
Moon
Smaller planets retain less heat and
therefore have less geologic activity.
Discussion
Why do you think Earth’s oceans have so
few impact craters as compared to Earth’s
continents?
Venus radar map
Mars laser altimeter map