Download 3-to-2 spin-orbit coupling

Chapter 6
The Terrestrial Planets
Mercury
Mercury: Basic Characteristics
Mass = 3.302×1023 kg (0.055 Earth)
Radius = 2,440 km (0.383 Earth)
Density = 5,427 kg/m³
Albedo (reflectivity) = 0.11 (Earth = 0.39)
Average distance from Sun = 0.387 A.U.
Travel time from Earth = 110 days (each way)
Mercury: Key Concepts
(1) Mercury has a 3-to-2 spin-orbit coupling
(not synchronous rotation).
(2) Mercury has no permanent atmosphere
because it is too hot.
(3) Like the Moon, Mercury has cratered
highlands and smooth plains.
(4) Mercury has an extremely large iron-rich core.
(1) Mercury has a 3-to-2 spin-orbit coupling
(not synchronous rotation).
Every 2 years Mercury will
rotate 3 times.
Mercury’s Unusual Orbit
Orbital period (Revolution) = 87.969 Earth days
Rotation period =58.646 Earth days = (2/3) x 87.969 days
Mecury is NOT in synchronous rotation (1 rotation per
orbit).
Instead, it has 3-to-2 spin-orbit coupling (3 rotations
for 2 orbits).
Time between one noon
and the next is
176 Earth days.
Sun is above the horizon
for 88 Earth days at the
time.
Daytime temperatures reach as high as:
700 Kelvin (800 degrees F).
Nighttime temperatures drops as low as:
100 Kelvin (-270 degrees F).
Another Strange Mercurial Feature:
Mercury has scarps, or cliffs, as much as 3 km high,
500 km long:
They are probably the result of cracking as the crust
cooled.
Venus
Venus: Basic characteristics
Mass = 4.868×1024 kg (0.815 Earth)
Radius = 6,052 km (0.949 Earth)
Density = 5,204 kg/m³
Albedo (Reflectivity) = 0.65 (Earth = 0.39)
Average distance from Sun = 0.723 A.U.
Travel time from Earth = 110-150 days (each way)
Venus: Key Concepts
(1) The surface of Venus is hidden from us by
clouds of sulfuric acid.
(2) The atmosphere of Venus is hot because of
a runaway greenhouse effect.
(3) The surface of Venus shows volcanic activity
but no plate tectonics.
(4) The interior of Venus is similar to that of
the Earth.
Venus: Revolution and Rotation
Orbital period (Revolution) = 225 Earth days
Rotation period = 243 Earth days
Rotation of Venus is retrograde (backwards)
On Venus, the time from noon to noon = 117 days
Venus is sometimes called Earth’s twin:
Radius = 95% of Earth’s
Mass = 82% of Earth’s
Surface gravity = 90% of Earth’s
(2) The atmosphere of Venus is hot
because of a runaway greenhouse effect.
Dense and high-pressure atmosphere:
Air pressure at the surface of Venus = 90 times air
pressure on Earth (2/3 of a ton per square inch).
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96.5% carbon dioxide (CO2)
3.5% nitrogen (N2)
0.015% sulfur dioxide (SO2)
almost no water vapor
CO2 is a greenhouse gas: so Venus has a Enormous
greenhouse effect.
Surface temperature = 733 Kelvin
(860 degrees Fahrenheit)
Even hotter than Mercury!
Temperatures drop very little during the long night,
thanks to the thick atmospheric blanket.
Escape velocity for Venus = 10.4 km/sec
Venus: many volcanoes, few impact craters.
More than 1600 major volcanoes.
Estimated average age of crust = 500 million years
(older than Earth, much younger than Mercury or Moon).
Perspective view of Venusian volcanoes
(Warning: vertical scale is exaggerated by a factor of 10):
Mars
Mars: Basic Characteristics
Mass = 6.418×1023 kg (0.107 Earth)
Radius = 3,377 km (0.531 Earth)
Density = 3,934 kg/m³
Albedo = 0.15 (Earth = 0.39)
Average distance from Sun = 1.523 A.U.
Travel time from Earth = 150-300 days (each way)
Mars: Key Concepts
(1) Mars has a tenuous (thin) atmosphere,
with little water vapour and few clouds.
(2) Mars has large volcanoes and a huge rift
valley, but no plate tectonics.
(3) Robotic ―rovers‖ have given us a close-up
look at Mars.
(4) Mars has two small irregular moons,
Phobos and Deimos.
(5) Life on Mars??
Mars: Revolution and Rotation
Orbital period (Revolution) = 1.9 years
Rotation period = 24 hours, 37 minutes
Rotation axis of Mars is tilted by 25 degrees.
Martian seasons similar to Earth seasons (only longer).
Low density atmosphere implies:
(1) small greenhouse effect
(2) large temperature swings.
Summer on Mars:
High = 270 Kelvin (0oC)
Low = 200 Kelvin (-70oC)
Summer in Earth’s deserts:
High = 310 Kelvin (40oC)
Low = 290 Kelvin (20oC)
(2) Mars has large volcanoes and a
huge rift valley, but no plate tectonics.
Two hemispheres of Mars are very different from
each other.
North:
recent volcanic activity (like Earth and Venus).
South:
heavily cratered highlands (like Moon and Mercury).
BIGGEST VOLCANO in the Solar System:
Olympus Mons
About the size of Arizona or Poland (~600 km across).
Last eruption about 300 million years ago.
(3) Robotic ―rovers‖ have given
us a close-up look at Mars.
Spirit and
Opportunity
(still going
since January
2004!) are
taking images
and spectra of
Mars rocks.
Success!!
(4) Mars has two small irregular
moons, Phobos and Deimos.
Phobos (―fear‖):
28 kilometers long.
Deimos (―panic‖):
16 kilometers long.
Irregular in shape,
undifferentiated.
Probably captured
asteroids.
The Moons of Mars
Phobos
1410 km
Deimos
86 km
Compositions of main-belt
Asteroids.
Comets & Asteroids
Comet
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Definition:
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Wild 2 Comet
Stardust NASA mission
Dirty snowball (long haired star)
Origin:
–
Theory: During spherical nebular phase, ice chunks
formed in outer part of cloud.
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These became comets in Oort cloud (outside solar system)
Theory: Some formed later from plantesimals in outer
solar system, now in Kuiper Belt
Evidence for Comet
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Comets can have a short or long
period
–
Short: < 200 yrs
–
Long: 1 – 30 million years
Comets can have orbit in the plane of
the planets or not.
Composition: Water ice, CO,
ammonia, methane, rock
Which type of comet are in the Oort
cloud? Long or short period?
Parts of a Comet
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Comets glow only as they near the
sun
Ice chunks in the Kuiper belt aren’t
luminescent—too cold
Nucleus: solid part, a few km across
Coma: luminous ball around
nucleus
Tails of Comet
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Comets have 2 tails
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Dust Tail
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Ion Tail
–
Range in size up to 1 AU!
Tails always point away from the Sun, due to
solar wind
Do comets melt?
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Estimates are that comets lose
~0.1% of initial mass each trip
around the Sun.
Particles from comet can
continue to orbit even after
comet disintegrates
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―Meteor‖ Showers
Asteroids
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Asteroid belt was discovered in 1800, when Ceres
was first thought to be a planet.
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After other similar objects in same orbit discovered, it was
reclassified.
Asteroids have similar composition to rocky
planets
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Origin:
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Planet that never formed
Planet that broke apart
What’s the difference between a
meteoroid and asteroid?
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Size!
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Usually > 300 m -> asteroid
Asteroid ―Ida‖ and her moon
Meteor’s name depends on where it is:
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Meteoroid is in space
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Meteor is falling through Earth’s atmosphere
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Meteorite is on the ground.