Mercury

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Astronomy Picture of the Day
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Mercury

• Mass 0.055 MEarth
• Radius 0.38 REarth ?
• Surface Temp 100 - 700 K
• Average distance from Sun .39 AU
• Moonlike Surface craters, no atmosphere
  (escaped long ago due to high surface temp low
  mass)?
• Orbital period 88 days
• Rotation period 59 days (Exactly 2/3 of
  Mercurys year!)?

Composite image from Mariner 10 1970s
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Question

• How do we determine the rotation rate of a planet?

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Determining rotation rate of a planet

• Use reflected radio waves to determine line of
  sight doppler shifts.

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We always see the same face of the Moon. So
period of orbit period of rotation Why?
Tidal Locking
If orbit period faster than spin period, bulge
moves around surface of Moon gt friction What
does this have to do with Mercury?
Moon
Top view of Moon orbiting Earth
Earth
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Mercurys Rotation

• Highly eccentric orbit gt Mercurys speed changes
  as it orbits the sun gt no synchronous rotation
• Next best thing - presents same side to sun every
  other time around.

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Venus
Mass 0.82 MEarth Radius 0.95 REarth
Atmospheric Pressure 90.0 APEarth Average
distance from Sun 0.72 AU Orbital period 225
days Rotation period 243 days (longer than
orbital period, and retrograde!)?
Atmosphere 96.5 CO2 the rest Nitrogen.
(recall Earths' atmosphere 78 Nitrogen, 21
Oxygen, 0.9 Argon, and ONLY 0.03 CO2 , with
water vapor making up 0.1-3)?
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Anomalous rotation of Venus

• Extremely slow - Venutian day longer than
  Venutian year!
• Retrograde - Sun rises in the west and sets in
  the east!
• Most likely due to a collision during solar
  system formation

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Venus

• Thick clouds prevent viewing of surface. (UV
  Image)?
• High temperatures and pressures, acidic gases
• Led to much speculation.
• How did we get info about surface?

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"Radar Echo" technique measures altitude
space probe
time for signal to return tells you the altitude
of surface feature.
Planet Surface
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Venera 14 photo of surface. Lander destroyed
after about an hour!
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Venus' Atmosphere
- Hot at surface - 750 K! (Room temp. on Earth
about 300K)? - Why so hot?
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Runaway Greenhouse Effect Venus started out
somewhat hotter due to its closer proximity to
the Sun then Earth, but THEN 1) Water and CO2
evaporate from oceans into atmosphere. 2)
Greenhouse effect more efficient. 3) Temperature
rises. 4) More evaporation (back to 1).
gt complete evaporation of oceans. Thick
atmospheric blanket.
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Atmospheric Comparison
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Mars
Mass 0.11 MEarth Radius 0.53 REarth
Average distance from Sun 1.52 AU Rotation
Period 24.6 hours Orbital Period 687 days
Two distinct regions The southern highlands are
much older and more heavily cratered than the
northern lowlands (which were created by enormous
volumes of lava) and, in fact, are even older
than most portions of the Earth's crust.
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The Martian Atmosphere

• 95 CO2
• Average Surface Temperature 220 K ( colder than
  Earth's by about 70K).
• Surface Pressure 0.006 that of Earth's
  atmosphere (thin air!)?
• Why so cold with such a thin atmosphere?

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The Martian Atmosphere

• 95 CO2
• Surface Temperature 250 K.
• Surface Pressure 0.006 that of Earth's
  atmosphere (thin air!)?
• "Reverse Runaway Greenhouse Effect"
• Temperature - important factor in planetary
  evolution!
• Heat loss mechanisms and differentiation,
  Greenhouse effects, etc.

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Olympus Mons
Largest known volcano in Solar System 3X the
height of Everest!
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Question

• What is the main reason that many scientists
  think Mars may have once harbored life?

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Evidence for Past Surface Water
"runoff channels" or dry rivers
"outflow channels"
teardrop "islands" in outflow channels
standing water erosion in craters?
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Mars' Moons Phobos and Deimos
Deimos 16 x 10 km
Phobos 28 x 20 km
Properties similar to asteroids. They are
probably asteroids captured into orbit by Mars'
gravity.
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Nice summary of rotations, directions, and tilt
of the four Terrestrial Planets
One final note none of the other three
terrestrial planets show plate tectonic movement.