astronomy picture of the day. mercury mass = 0.055 m earth radius = 0.38 r earth surface temp: 100...
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![Page 2: Astronomy Picture of the Day. Mercury Mass = 0.055 M Earth Radius = 0.38 R Earth Surface Temp: 100 - 700 K Moonlike: Surface craters, no atmosphere](https://reader036.vdocuments.us/reader036/viewer/2022062516/56649d7a5503460f94a5e751/html5/thumbnails/2.jpg)
Mercury
Mass = 0.055 MEarth
Radius = 0.38 REarth
Surface Temp: 100 - 700 K
Moonlike: Surface craters, no atmosphere
Orbital period = 88 days
Rotation period = 59 days (Exactly 2/3 of Mercury’s year!)
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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
Earth
Moon
If orbit period faster than spin period, bulge moves around surface of Moon => friction
What does this have to do with Mercury?
Top view of Moon orbiting Earth
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Mercury’s Rotation• Highly eccentric orbit => Mercury’s speed changes as it
orbits the sun => no synchronous rotation
• Next best thing - presents same side to sun every other time around.
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Venus
● About how far from the Sun (in AU) is Venus?
● How long is its orbital period (Venutian year)?
● How long is its rotational period (Venutian day)?
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Venus
Mass = 0.82 MEarth
Radius = 0.95 REarth
Average distance from Sun = 0.72 AU
Orbital period = 225 days
Rotation period = 243 days (longer than orbital period, and retrograde!)
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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
1) Water and CO2 evaporate from oceans into atmosphere.
2) Greenhouse effect more efficient.
3) Temperature rises.
4) More evaporation (back to #1).
=> complete evaporation of oceans. Thick atmospheric blanket.
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Atmospheric Comparison
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How does Venus lose its interior heat?
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How does Venus lose its interior heat?
• We don’t know!
• No plate tectonics => no interior convection
• Possible volcanic activity, but …
• Distribution of craters on surface is uniform
● Surface of Venus is same age everywhere: 500 Million yrs. old
• Cataclysmic resurfacing?
● Geologically dead rest of the time
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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
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The Martian Atmosphere
95% CO2
Surface Temperature 250 K.
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 MonsLargest 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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Where did the water go?
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1. Polar Ice Caps2. Evidence for "Permafrost" layer beneath surface
"Splosh" craters suggesting liquefied ejecta.
Where did the water go?
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Mars' Moons Phobos and Deimos
Phobos: 28 x 20 kmDeimos: 16 x 10 km
Properties similar to asteroids. They are probably asteroids captured into orbit by Mars' gravity.