Transcript Chpt6

Chapter 6: Terrestrial Planets
Goals
Explain how Mercury's rotation has been influenced by its orbit around the Sun.
Describe how the atmospheres of Venus and Mars differ from one another and from
Earth's.
Compare the surface of Mercury with that of the Moon and the surfaces of Venus
and Mars with that of Earth.
Describe how we know that Mars once had running water and a thick atmosphere.
Discuss the similarities and differences in the geological histories of the four
terrestrial planets.
Explain why the atmospheres of Venus, Mars, and Earth are now so different from
one another.
Chapter 6: Terrestrial Planets
Chapter 6: Terrestrial Planets
Rotation Rates
Ground based telescopes cannot see much details
because of its small size and large distance.
So astronomers have used the Doppler
effect with radar pulses to measure the
rotation rate.
They find that one Mercury day lasts two
Mercury years.
This odd synchronous rate is due to
Mercury's eccentric orbit. It presents the
same face to the Sun every other time
around.
Chapter 6: Terrestrial Planets
After the Sun and Moon, Venus is the
brightest object in the sky. The same clouds
that make it bright also prevent us from
seeing surface detail and measuring its
rotation rate.
Once again Doppler radar revealed the answer, a 243-day rotation
rate and in the wrong direction! Its rotation axis is almost
perpendicular to its orbital plane.
The only explanations for this is that Venus had a close encounter
of the planetary kind early in its history.
Chapter 6: Terrestrial Planets
Mars, on the other hand, has very little
atmosphere and we can visually measure a
rotation rate of 24.6 hours with an inclination to
its orbit of 25.2o. Very similar to earth!
With its polar ice caps and dark regions near
the equator, we can see seasonal changes as
the planets orbits the Sun
Chapter 6: Terrestrial Planets
Chapter 6: Terrestrial Planets
Atmospheres (Mercury)
Astronomers have never observed, either from earth or from spacecraft,
any atmosphere on Mercury.
Because Mercury is so close to the Sun it has a high temperature (700K)
which, combined with its low gravity (1/3 Earth), means Mercury has
troubles holding onto an atmosphere.
Because it has no atmosphere it has no protection from the Sun and its
600K temperature range is the greatest for any planet in the solar system.
Chapter 6: Terrestrial Planets
Atmospheres (Venus)
While the cloud tops are a comfortable 240K,
radio observations of the surface reveal
temperatures of 600K!
Venus has the most massive atmosphere of all
the terrestrial planets, 90 times the pressure of
Earth’s.
Venus’ atmosphere is made of 96.5% CO2 and
3.5% nitrogen.
Given its similar size and distance from the Sun as Earth, it must have
started out similar to Earth.
Astronomers believe a run away greenhouse effect caused the high
temperatures which destroyed the water and the oxygen combined with
sulfur to form sulfuric acid clouds.
Chapter 6: Terrestrial Planets
Atmospheres (Mars)
The Martian atmosphere is only 1/150 that of Earth’s.
It contains 95.3% CO2, 2.7 % nitrogen, 1.6% argon plus small
amounts of H2O, O2 , and CO
Average temperature on Mars is 50K cooler than on Earth.
Chapter 6: Terrestrial Planets
Surfaces (Mercury)
The best comparison of the surface
of Mercury is the Moon
Heavily crated, no lava flows, or
evidence of clouds, water, or dust
storms
Craters on Mercury tend to have double
rings.
Chapter 6: Terrestrial Planets
Mercury is geologically dead.
As the planet cooled, it shrake and
cracked producing large scarps.
Evidently Mercury small size allowed to
to cool quickly and its thick crust
prevented any plate tectonic formation
This impact was so large that it disturbed
the back side of Mercury producing the
“weird terrain”.
Chapter 6: Terrestrial Planets
Surfaces (Venus)
Because the thick clouds abscise the
surface, most information has been
obtained by radar mapping by US
spacecraft and by landers from the
Soviets.
Chapter 6: Terrestrial Planets
Surfaces (Venus)
Volcanism appears
to resurface Venus
every few hundred
million years. Thus
we see no craters.
Instead we see lots
of volcanic
formations.
The most common
are Shield
volcanoes much
like Hawaii.
Chapter 6: Terrestrial Planets
Surfaces (Mars)
From the Earth and space Mars shows
many interesting surface features.
Polar ice caps which grow and shrink
with the seasons, and dark patches
which also appear to wax and wane
with the seasons.
So convinced were some astronomers
that life existed on Mars that
observatories were built and detailed
maps produced to study the canals.
Chapter 6: Terrestrial Planets
Percival Lowell built an observatory
in Flagstaff, Arizona where he
mapped out several canals.
Chapter 6: Terrestrial Planets
Surfaces (Mars)
(a) Mars's northern hemisphere
consists of rolling volcanic plains
(false color image.)
(b) The southern Martian highlands
are heavily cratered
(true color). Both photographs show
roughly the same scale, nearly 1000
km
across.
Chapter 6: Terrestrial Planets
Most of our information about the
surface comes the Viking landers
and Mars Pathfinder (Sojourner)
They reveal a dry, rocky desert
plain.
The red color is due to iron oxide
(rust) on the surface.
Chapter 6: Terrestrial Planets
Surfaces (Mars)
While Mars may now be geologically dead, in the past
volcanoes played an important role
Olympus Mons is the largest volcano.
At 700 km across and 25 km tall it is
about three times larger than similar
features on Earth.
There is evidence that water once
flowed on Mars.
Chapter 6: Terrestrial Planets
Surfaces (Mars)
Just beneath the surface of Mars
may be extensive permafrost.
Evidence includes the mud-like
response to meteor impacts.
Chapter 6: Terrestrial Planets
Geology (Mercury)
Mercury’s magnetic field is 100 times
smaller that Earth’s
Still, we were surprised because
Mercury does not rotate very fast (one
of the necessary conditions). On the
other hand, its density implies lots of
iron/nickel in the core (the other
condition).
Its core is much bigger (in a relative sense) than Earth’s
Perhaps the magnetic field is a left over frozen from when it was
created.
Chapter 6: Terrestrial Planets
Geology (Venus)
Has no detectable magnetic field. Since it is other wise very
Earthlike this must be caused by the slow rotation rate.
We have no seismic data to examine the interior.
While Venus is geologically active it does not have any plate
tectonics.
It may be that the high temperatures have prevented the crust
from being solid enough to support plates.
Chapter 6: Terrestrial Planets
Geology (Mars)
No Martian magnetic field has ever been detected.
Since Mars rotates as fast as the Earth, this must imply that
Mars has very little molten iron/nickel core.
While Mars once had active volcanoes, because of its small
size and lack of atmosphere, it has cooled to the point where
it too has become dead.
Chapter 6: Terrestrial Planets
Atmospheric Evolution of Venus, Earth, and Mars
Most planets start off with an initial or primary atmosphere which
formed at the same time the planet did.
The atmospheres later evolve into a secondary atmospheres which
may be caused by volcanoes, living systems, or changing
temperatures.
The primary atmosphere is usually made up of light gasses like
hydrogen, helium, methane, ammonia, and water vapor.
Chapter 6: Terrestrial Planets
Earth’s Atmosphere
Most of the light gasses in Earth’s were lost into space because of
its high temperatures and low gravity.
Earth’s secondary atmosphere came from volcanoes and perhaps
comets.
Oxygen is so reactive that it must be replaced for it to be at its
present concentration.
Green plants continually produce oxygen to keep the
concentrations high.
Chapter 6: Terrestrial Planets
Venus’ Atmosphere
Venus’ current atmosphere was caused by the run away greenhouse
effect
Venus is too hot to maintain water and without green plants to
produce oxygen, 99% of Venus is CO2.
Chapter 6: Terrestrial Planets
Martian Atmosphere
While it appears that Mars once had large amounts of H2O
probably produced by volcanoes, its temperature is too low to
allow running water now.
Most of the CO2 was absorbed into the rocks and minerals
with little left over to keep the planet warm.
Most of the water may be stored in permafrost just beneath the
surface.