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Lecture 12

AST101H5 Lecture Notes - Lecture 12: Dayside, Escape Velocity, Permafrost


Department
Astronomy
Course Code
AST101H5
Professor
John Lester
Lecture
12

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Chapter 9
The Terrestrial Planets
Mercury
Mercury is hard to study from earth because
It is the planet nearest to the Sun, so the Sunlight makes it hard to observe
It's the smallest major planet
It doesn't have a moon
Test how strong its gravity is to find its mass
Therefore, it was not until we sent a satellite to mercury that we started to learn about it
Mercury's radius can be measured from earth using its angular size and distance -
although its small size limits the accuracy - 2440 km
Mercury's mass cannot be measured from earth because it does not have a moon
orbiting it
Finally, in 1974, our Mariner 10 space probe measured mercury's mass using the
method from chapter 3 = 3.3 x 1023 kg = 0.055 MEarth
Using the mass and radius we find Mercury's average density = 5.43 gm/cm3, very
similar to earth's average density
Mercury's rotation
Short distance from the sun, 0.387 AU, orbits in short time, 87.97 days
Because Mercury is so close to the Sun's powerful gravity, it is similar to our Moon being
so close to Earth
Therefore, we expected Mercury’s rotation to be synchronized to its orbit = both 87.97
days -- WRONG
Mercury's rotation CANNOT synchronize because its orbit is too elliptical
Mercury's distance from the sun varies by ~21% while our moon varies by only
~5%, we knew this but did not pay attention to it
Kepler's 2nd law, found that a planet's orbit speed varies with distance from the
body it is orbiting
The rotation cannot speed up and slow down to match its changing orbit speed
Therefore, Mercury's rotation cannot synchronize to it sorbit
Our 1974 space probe discovered that Mercury's rotation does the next easiest
thing, which is to do 3 spins every two orbits
Mercury’s Temperature
On Mercury’s surface, the sun rises once every 176 days, creating very long, hot days
and very long, cold nights
Daytunem temperature reaches 710 K
Nighttime temperature drops to 80 K
Mercury's Atmosphere
Because Mercury's mass is only 0.055 x Earth's mass, its escape velocity is only
4.3km/s
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Because mercury's dayside is 710 K, the speed of any atmospheric gas is so fast that it
escapes to space
Therefore there's no atmosphere on mercury
Mercury's Surface
Without atmosphere, there's no erosion --- its surface is very heavily cratered, like our
moon
There is evidence of massive impacts such as those that created the Moon's maria
Estimates of their age ~ the ages of the moon's maria
Mercury has one unique surface feature: enormous “scarps” = cliffs
Up to 3000 m high, measured from its shadow (CN tower is 500 m)
Up to 1000 km long
Possibly created by the solid surface cracking as it rapidly cooled and shrank
Mercury's Interior
We have no seismic measurements to study Mercury's interior
We know mercury's average density of 5.4 gm/cm3, which is almost the same as earth
Therefore, we conclude that mercury has a metallic core and a rocky mantle like Earth
However, because Mercury is less massive than Earth, its gravity is weaker and its
interior is less compressed than Earth’s
This tells us that Mercury's metal core must be a larger fraction of its volume
Maybe Mercury has a larger metallic core because Rock could not
condense as easily so close to the sun
Or, maybe Mercury has a collision that blasted of some of its rocky
mantle
Mercury's Magnetic Field
Recall Earth's magnetic field is explained by a combination of:
Metallic core
Molten core
Rotation
Mercury
Has a metallic core
Has a small radius so its core should have cooled and solidified, but some
observations suggest it might still be partly molten
Our space probes to Mercury have found that is does have a magnetic field, about 1%
as strong as Earth's
Either:
Mercury's large metallic core compensates for its slow rotation
It rotated much faster long ago, then the magnetic field “froze” in when it slowed
down
Venus
Venus is the planet that comes closest to Earth and it is the brightest planet
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find more resources at oneclass.com
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