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ASTRO 101
Principles of Astronomy
Instructor: Jerome A. Orosz
(rhymes with
“boris”)
Contact:
• Telephone: 594-7118
• E-mail: [email protected]
• WWW:
http://mintaka.sdsu.edu/faculty/orosz/web/
• Office: Physics 241, hours T TH 3:30-5:00
Homework/Announcements
• Homework due Tuesday, March 12: Question
4, Chapter 5 (Why is Earth’s surface not
riddled with craters as is that of the Moon?).
Comets!
•
http://earthsky.org/space/comet-panstarrs-possibly-visible-to-eye-in-march-2013
• Comet PANSTARRS should be viewable in the
western skies starting March 7. It will be close to
the crescent Moon on March 12.
•
http://earthsky.org/space/big-sun-diving-comet-ison-might-be-spectacular-in-2013
• Comet ISON might be very spectacular in
December, 2013, provided it survives its close
encounter with the Sun.
Coming up:
• Chapter 5 (The Earth)
• Chapter 6 (Other Planets and Moons)
Planetology
•
Some things we want to know about a planet:
1)
2)
3)
4)
5)
•
What are the surface features like?
What is the interior like?
What is the atmosphere like (if any)?
How did it form?
Is there (or was there) life?
An understanding of other planets may lead to
a better understanding of our own Earth.
Next:
The Terrestrial Planets
Two Types of Planets
• Planets come in two
types:
– Small and rocky.
– Large and gaseous.
Or
– Terrestrial
– Jovian
The Terrestrial Planets
• The terrestrial planets are
Mercury, Venus, Earth
(and Moon), and Mars.
• Their densities range
from about 3 grams/cc to
5.5 grams/cc, indicating
their composition is a
combination of metals
and rocky material.
The Earth
The Earth
The Earth
The Earth’s Atmosphere
•
The Earth’s atmosphere is useful in at
least three ways:
1) It keeps the Earth warmer than it would
otherwise be.
2) It keeps most of the harmful UV and X-ray
radiation from reaching the ground.
3) It allows us to breathe.
The Earth’s Atmosphere
•
Note that the Earth’s atmosphere is not in
“normal” chemical balance:
–
–
–
There is more methane, ozone, and oxygen
than expected
These species are very reactive, and would
normally disappear in a relatively short time
Biological activity keeps renewing these
gasses.
The Earth’s Atmosphere
• The temperature of the atmosphere has a
complex dependence on the height:
The Greenhouse Effect
• The Sun heats the Earth. Some of the energy is
scattered, and some heats the ground and water.
• The Earth tries to cool at night, but the atmosphere
traps much of the radiation.
The Interior of the Earth
• We can use earthquakes to
study the interior of the
Earth. There are two types
of waves:
– P-waves, which travel
through solids and liquids.
– S-waves, which only go
through solids.
Image from Nick Strobel (http://www.astronomynotes.com)
The Interior of the Earth
• We can use earthquakes to
study the interior of the
Earth. There are two types
of waves:
– P-waves, which travel
through solids and liquids.
– S-waves, which only go
through solids.
Image from Nick Strobel (http://www.astronomynotes.com)
The Interior of the Earth
• There is a solid inner core of iron, surrounded by a
liquid iron core, surrounded by the mantle composed
of silicates. A thin crust is on the outside.
The Interior of the Earth
• Natural radioactivity provides the energy
source that heats the Earth’s interior.
• The thin crustal “plates” float on top of the
liquid mantle.
• The motion of the crustal plates (a few cm
per century) causes earthquakes and
volcanoes.
The Interior of the Earth
• Convection causes motions of the crustal plates.
The Interior of the Earth
• Convection causes motions of the crustal plates.
• The spreading apart of two plates created this
mountain range under the Atlantic.
The Interior of the Earth
• Convection causes motions
of the crustal plates.
• The continental land
masses constantly move
relative to each other.
The Interior of the Earth
• Areas near plate boundaries are prone to earthquakes.
The Magnetic Field of the Earth
• The Earth has a “dipole” magnetic field, much like a
bar magnet.
The Magnetic Field of the Earth
• The Earth has a “dipole” magnetic field, much like a
bar magnet.
• This field helps protect us from the “solar wind”.
The Magnetic Field of the Earth
• The Earth has a “dipole” magnetic field, much like a
bar magnet.
• This field helps protect us from the “solar wind”.
• The interaction between the solar wind and the field
can produce the northern lights.
Next:
The Moon
The Moon
• The moon is relatively
small:
– Mass = 1/81 of Earth’s
– Radius = 1/4 of Earth’s
– Gravity = 1/6 of Earth’s
• It appears large in the sky
because it is so close,
about 240,000 miles,
compared to 93,000,000
miles to the Sun.
The Moon
• The albedo of the Moon is less than 10%.
The Moon
• The albedo of the Moon is less than 10%.
• The surface is composed of very dark rock
(as dark as coal).
The Moon
• The albedo of the Moon is less than 10%.
• The surface is composed of very dark rock
(as dark as coal).
• But why is the Moon so bright?
The Moon
• The albedo of the Moon is less than 10%.
• The surface is composed of very dark rock
(as dark as coal).
• But why is the Moon so bright?
• The Moon is nearby, and we view it
contrasted against a dark sky.
The Moon
• The surface gravity on the Moon is 1/6 of
the Earth’s.
The Moon
• The surface gravity on the Moon is 1/6 of
the Earth’s. This is too weak to retain an
atmosphere.
The Moon
• The surface gravity on the Moon is 1/6 of
the Earth’s. This is too weak to retain an
atmosphere.
• There is no water on the Moon, nor is there
weather of any kind.
The Moon
• The surface gravity on the Moon is 1/6 of
the Earth’s. This is too weak to retain an
atmosphere.
• There is no water on the Moon, nor is there
weather of any kind. The surface features
we see are very old.
What do we see on the Moon?
• We always see basically the same side of the Moon
facing us, although there is some wobble.
What do we see on the Moon?
• The rotational period is equal to the orbital period.
What do we see on the Moon?
• Craters and mountains have been seen since the
time of Galileo. We know they are craters by
looking at the illumination patterns.
What do we see on the Moon?
• Craters are best seen
during the crescent
phases since surface
features cast shadows.
What do we see on the Moon?
• More craters as seen
from a NASA probe.
How did the Craters Form?
• The craters on the
Moon were caused by
impacts of large
bodies, not by
volcanoes.
• In many cases you can
see trails of lightercolored matter thrown
out by the impacts.
How did the Craters Form?
• The lunar craters are caused by impacts, and not by
volcanoes.
What do we see on the Moon?
• We also see large
darker areas called
“maria” (Italian for
“seas”).
What do we see on the Moon?
• We also see large
darker areas called
“maria” (Italian for
“seas”).
What do we see on the Moon?
• We also see large
darker areas called
“maria” (Italian for
“seas”).
How did the Maria Form?
• An impact of a very large body and subsequent lava
flows may have formed the maria.
The Lunar Interior
• The inside of the Moon can be
studied using seismic equipment
left by the Apollo astronauts,
and by tracking the orbits of
spacecraft near the moon.
• The moon has an uneven
interior, with mass
concentrations near the maria.
• The interior is cold, and there
are no active volcanoes today.
The Earth and Moon
• The Earth has an
atmosphere
• The Earth is hot
inside
• The Earth has lots
of water
• The Earth has lots
of iron
• The Moon does not
have an atmosphere
• The Moon is cold
inside
• The Moon has very
little water
• The Moon has little
iron
Next
• The formation of the Moon
The Formation of the Moon
• The density of the Moon is about 3.3
grams/cc, somewhat less than the Earth.
The Formation of the Moon
• The density of the Moon is about 3.3
grams/cc, somewhat less than the Earth.
 This density is similar to the density of the
Earth’s Mantle.
The Formation of the Moon
• The density of the Moon is about 3.3
grams/cc, somewhat less than the Earth.
 This density is similar to the density of the
Earth’s Mantle.
 The Moon is deficient in iron.
The Formation of the Moon
• The density of the Moon is about 3.3
grams/cc, somewhat less than the Earth.
 This density is similar to the density of the
Earth’s Mantle.
 The Moon is deficient in iron.
 The surface composition of the Moon is
similar, but not exactly like that of the
Earth.
The Formation of the Moon
•
In the late 1960s, there were three main
theories on how the Moon formed:
The Formation of the Moon
•
In the late 1960s, there were three main
theories on how the Moon formed:
1) Fission. The Earth somehow spun up and flung
off the Moon, leaving behind a depression that
would become the Pacific Ocean.
The Formation of the Moon
•
In the late 1960s, there were three main
theories on how the Moon formed:
1) Fission. The Earth somehow spun up and flung
off the Moon, leaving behind a depression that
would become the Pacific Ocean.
2) The Moon was captured by the Earth’s gravity.
The Formation of the Moon
•
In the late 1960s, there were three main
theories on how the Moon formed:
1) Fission. The Earth somehow spun up and flung
off the Moon, leaving behind a depression that
would become the Pacific Ocean.
2) The Moon was captured by the Earth’s gravity.
3) The Moon was formed nearby the Earth at the
same time.
The Formation of the Moon
•
In the late 1960s, there were three main
theories on how the Moon formed:
1) Fission. The Earth somehow spun up and flung
off the Moon, leaving behind a depression that
would become the Pacific Ocean.
2) The Moon was captured by the Earth’s gravity.
3) The Moon was formed nearby the Earth at the
same time.
•
All three ideas are probably not correct.
The Formation of the Moon
• The currently most accepted model is the
ejection of material caused by a giant
impact.
The Formation of the Moon
• The currently most accepted model is the
ejection of material caused by a giant
impact.
• A large body (perhaps bigger than Mars)
collided with the young Earth and ejected a
considerable amount of material from the
Earth’s upper layers.
The Formation of the Moon
• The currently most accepted model is the
ejection of material caused by a giant
impact.
• A large body (perhaps bigger than Mars)
collided with the young Earth and ejected a
considerable amount of material from the
Earth’s upper layers.
• This material condensed and formed the
Moon.
The Formation of the Moon
• Computer simulations and chemical analysis of Moon
rocks supports the collision/ejection theory.
The Formation of the Moon
• Computer simulations and chemical analysis of Moon
rocks supports the collision/ejection theory.
• This collision could have caused the Earth’s rotation
axis to become tilted.
Newton’s Laws and Tides
• If the tides are caused
by the Moon pulling on
the oceans, then why is
there usually two high
tides per day?
Newton’s Laws and Tides
• If the tides are caused
by the Moon pulling on
the oceans, then why is
there usually two high
tides per day?
• Actually tides are
caused by differences
in the gravitational
forces.
Newton’s Laws and Tides
• Spring tides are when
the Sun and Moon are
roughly aligned (e.g.
new and full moon).
The tides tend to be
higher at these times.
• Local conditions can
also effect the height of
the tides.
Next:
Chapter 5: Other Planets and
Moons.
Mercury
• Mercury is the closest planet to the Sun.
• It is never seen against a dark sky, and it is never
far above the horizon.
Mercury as Seen From the Earth
• Here is the best
ground-based image
of Mercury.
Mercury as Seen From Earth
• Mercury is hard to study from the ground since it
is close to the Sun.
Mercury as Seen From Earth
• Mercury is hard to study from the ground since it
is close to the Sun.
• We can measure its average density. We find
the density is 5.4 grams/cc, much like the Earth.
Mercury as Seen From Earth
• Mercury is hard to study from the ground since it
is close to the Sun.
• We can measure its average density. We find
the density is 5.4 grams/cc, much like the Earth.
• We can measure the albedo, and we find it is
about 10%, much like the Moon.
Mercury as Seen From Earth
• Mercury is hard to study from the ground since it
is close to the Sun.
• We can measure its average density. We find
the density is 5.4 grams/cc, much like the Earth.
• We can measure the albedo, and we find it is
about 10%, much like the Moon.
• Mercury mass is 5.5% of the Earth’s mass, and
its gravity is 38% of the Earth’s.
Mercury as Seen From Earth
• Mercury is hard to study from the ground since it
is close to the Sun.
• We can measure its average density. We find
the density is 5.4 grams/cc, much like the Earth.
• We can measure the albedo, and we find it is
about 10%, much like the Moon.
• Mercury mass is 5.5% of the Earth’s mass, and
its gravity is 38% of the Earth’s.
• We expect Mercury to be similar to the Moon.
Mercury Seen up Close
• In 1974 NASA sent a probe to Mercury.
• It really does look like the Moon.
Mercury Seen up Close.
• Mercury is covered with craters.
Mercury Seen up Close
• In many cases you can
see rays of material
ejected by the impacts.
Mercury’s Interior
• Mercury has a large
iron core.
• This core is relatively
cold.
• There is very little
evidence of presentday geological
activity.
Mercury’s Interior
• Mercury has a large iron core.
• It is possible that a collision early in the history of
Mercury could have stripped off less dense material
near the surface, leaving behind the heavier material.
Mercury’s Rotation
• Since Mercury is so close to the Sun, tidal forces
have forced it into a 3-to-2 spin-orbit coupling.
• As a result, a day on Mercury is 2 Mercury years
long!
Mercury
•
•
•
•
Mercury has a very thin atmosphere.
There is no water.
There is essentially no erosion.
It is relatively hot on the day side (up to 800oF)
since it is near the Sun. However, on the night
side it can be as low as -280oF
• It looks a lot like the moon on the surface, but it
is different in its interior.
Venus
• Venus has a mass and radius similar to that
of the Earth.
Venus
• Venus has a mass and radius similar to that
of the Earth. Its gravity is strong enough to
retain a substantial atmosphere.
Venus
• Venus has a mass and radius similar to that
of the Earth. Its gravity is strong enough to
retain a substantial atmosphere.
• The albedo is very high, more than 75%.
Venus
• Venus has a mass and radius similar to that
of the Earth. Its gravity is strong enough to
retain a substantial atmosphere.
• The albedo is very high, more than 75%.
Venus
• Venus has a mass and radius similar to that
of the Earth. Its gravity is strong enough to
retain a substantial atmosphere.
• The albedo is very high, more than 75%.
We do not see the surface, but rather the
tops of the clouds.
Venus
• Venus is the second closest planet to the Sun.
• It is never seen against a very dark sky, and it is
never far above the horizon.
Venus
• No surface features
are seen from Earth.
Venus
• The cloud patterns are
changing over several hours.
Venus
• The surface temperature is about 475o C,
compared to about 25o C for Earth.
Venus
• The temperature at the surface of Venus is high.
Venus
• The surface temperature is about 475o C,
compared to about 25o C for Earth.
• The atmospheric pressure at the surface of
Venus is 90 times that of the Earth.
Venus
• The surface temperature is about 475o C,
compared to about 25o C for Earth.
• The atmospheric pressure at the surface of
Venus is 90 times that of the Earth.
• The composition of the atmosphere is about
96% CO2, compared to mostly N and O on
the Earth.
Venus
• The surface temperature is about 475o C,
compared to about 25o C for Earth.
• The atmospheric pressure at the surface of
Venus is 90 times that of the Earth.
• The composition of the atmosphere is about
96% CO2, compared to mostly N and O on
the Earth.
• ??????
The Greenhouse Effect
• Venus has a “runaway” greenhouse effect
that heats the planet an extra 375o C.
The Greenhouse Effect
The Greenhouse Effect
• Venus has a “runaway” greenhouse effect
that heats the planet an extra 375o C.
• Some visible light from the Sun reaches the
surface and heats it.
The Greenhouse Effect
• Venus has a “runaway” greenhouse effect
that heats the planet an extra 375o C.
• Some visible light from the Sun reaches the
surface and heats it.
• The surface radiates the energy in the
infrared, which the CO2 in the atmosphere
absorbs.
The Greenhouse Effect
• Some visible light from the Sun reaches the
surface and heats it.
• The surface radiates the energy in the
infrared, which the CO2 in the atmosphere
absorbs.
• The extra heat “bakes out” more CO2 from
the rocks.
The Greenhouse Effect
• The surface radiates the energy in the
infrared, which the CO2 in the atmosphere
absorbs.
• The extra heat “bakes out” more CO2 from
the rocks.
• The extra CO2 leads to more trapping of the
surface infrared radiation.
The Greenhouse Effect
• The extra heat “bakes out” more CO2 from
the rocks.
• The extra CO2 leads to more trapping of the
surface infrared radiation.
• The extra trapped heat bakes out more CO2,
and so on…
The Surface of Venus
• Soviet spacecraft have landed on Venus and
recorded close-up pictures.
• These images show basalt, which is quite similar
to lava rock.
The Surface of Venus
• The Venusian surface has been mapped with radar
by the Magellan spacecraft.
• These maps reveal gently rolling hills, two
“continents”, and many volcanoes.
The Surface of Venus
• The Venusian surface has been mapped with radar
by the Magellan spacecraft.
• There are relatively few impact craters. Perhaps
melting of the surface has erased earlier craters.
Venus Summary
• Although Venus has a similar mass and radius as
the Earth, it is a very different place owing to the
runaway greenhouse effect:
– The temperature at the surface is about 475 oC.
– The atmospheric pressure is about 90 times that on
the Earth.
– The atmosphere is mostly CO2.
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