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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
Text:
“Discovering the Essential Universe,
Fifth Edition”
by
Neil F. Comins
Course WWW Page
http://mintaka.sdsu.edu/faculty/orosz/web/ast101_fall2012.html
Note the underline: … ast101_fall2012.html …
Also check out Nick Strobel’s Astronomy Notes:
http://www.astronomynotes.com/
Homework/Announcements
• Homework due Thursday, October 25:
Question 6, Chapter 7 (What role did Charon
play in enabling astronomers to determine
Pluto’s mass?).
Next:
Chapter 5 and Chapter 6: 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.
• 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
• 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.
NEXT:
Mars
• Named for the Roman god of war, owing to
its red color.
• Its mass is 10% of the Earth’s mass, its
radius is 50% of Earth’s radius, and its
gravity is 38% of Earth’s gravity.
• Mars has usually been considered to be the
place to look for extraterrestrial life.
Mars
• In some ways, Mars is a bit like Earth:
 Its rotation rate is 24 hours and 37 minutes.
 The inclination of the axis is 25.2o.
 The seasonal variations of the solar heating are
similar to that on Earth.
Mars
• Mars is different from the Earth in some
important ways:
 The surface temperature ranges from about -125 oC
to 25 oC, with typical temperatures below 0 oC.
 The atmospheric pressure is about 1% of that on the
Earth, which is too thin to maintain a significant
greenhouse effect.
Why the Interest in Mars?
• Every few years Mars passes relatively close to
Earth.
• During these close passages, it is possible to see
surface features through a good telescope.
Early Observations of Mars
• Early observations were by eye (through a telescope).
The observer had to draw what he saw.
• Here is drawing from 1800
(image from William Sheehan, http://www.uapress.arizona.edu/online.bks/chap03.htm)
Early Observations of Mars
• Here is drawing from 1877.
(image from William Sheehan, http://www.uapress.arizona.edu/online.bks/chap05.htm)
Early Observations of Mars
 The Italian astronomer Schiaparelli made detailed
observations in the 1870s.
 He noted the presence of “canali.”
(image from William Sheehan, http://www.uapress.arizona.edu/online.bks/chap05.htm)
Early Observations of Mars
• Schiaparelli meant “channels.”
• English translations used “canals,”
implying that they are of intelligent design.
Note, however, that beavers dig canals too.
Early Observations of Mars
• Schiaparelli meant “channels.”
• English translations used “canals,”
implying that they are of intelligent design.
Note, however, that beavers dig canals too.
• Near the turn of the 20th century, many
people were convinced that there was life
on Mars.
Life on Mars
• This famous movie
made in 1953 was
based on H.G. Well’s
classic novel of 1898.
• On Oct 30, 1930,
Orson Wells broadcast
a fake news report of
invading Martians,
causing widespread
panic.
Modern Views of Mars
• We know now that the
early observations of
“canals” were in error.
• There are no cities or
civilizations.
Modern Views of Mars
• The surface of Mars has been mapped extensively by
orbiting spacecraft.
• One sees plains, deep canyons, numerous craters, and
volcanoes, which are extinct.
Modern Views of Mars
• The surface of Mars has been mapped extensively by
orbiting spacecraft.
• One sees plains, deep canyons, numerous craters, and
volcanoes, which are extinct.
Modern Views of Mars
• The surface of Mars has been mapped extensively by
orbiting spacecraft.
• One sees plains, deep canyons, numerous craters, and
volcanoes, which are extinct.
Searching for Life
• How do you detect primitive life remotely?
Searching for Life
• How do you detect primitive life remotely?
 Organisms on Earth breathe and have certain
metabolisms, so look for signs of respiration
and other biological processes.
Searching for Life
• How do you detect primitive life remotely?
 Organisms on Earth breathe and have certain
metabolisms, so look for signs of respiration
and other biological processes. What if life on
Mars is different than that on Earth?
Searching for Life
• How do you detect primitive life remotely?
 Organisms on Earth breathe and have certain
metabolisms, so look for signs of respiration
and other biological processes. What if life on
Mars is different than that on Earth?
 Life on Earth is carbon-based, so look for
organic compounds.
Searching for Life
• How do you detect primitive life remotely?
 Organisms on Earth breathe and have certain
metabolisms, so look for signs of respiration
and other biological processes. What if life on
Mars is different than that on Earth?
 Life on Earth is carbon-based, so look for
organic compounds. What if life on Mars is
different than that on Earth? Also, organic
compounds can be made by other means.
Searching for Life
• In 1976 the Viking
spacecraft landed on
Mars. Soil was
analyzed by three
experiments, but no
signs of life were
found.
Searching for Life
• In 1984 a meteorite from
Mars was discovered in
Antarctica.
• Evidence for microscopic
fossils in the meteorite
was announced in 1996.
• Most scientists today
remain unconvinced.
Recent Events: Mars Rover
• Recently, NASA landed
twin rovers on Mars.
These probes are vastly
more superior than the
Viking landers.
Recent Events: Mars Rover
• The cameras deliver extremely high quality images.
Recent Events: Mars Rover
• The cameras deliver extremely high quality images.
Evidence for Water on Mars
• These images and others suggest the presence of
liquid water on Mars, at least in the past.
Evidence for Water on Mars
• These images and others suggest the presence of
liquid water on Mars, at least in the past.
Life on Mars
• There is no evidence that there is currently
life on Mars. The recent Mars rovers will
perform additional experiments along these
lines.
• It is possible that there was primitive life in
the early history of Mars, but the only way
to be sure would be to go there.
Comparing the Inner Planets
Next: The Giant Planets