Astronomy Picture of the Day

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!)

Question

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

Determining rotation rate of a planet• Use reflected radio waves to determine line of

sight doppler shifts.

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

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.

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)?

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!)

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

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?

"Radar Echo" technique measures altitude

space probe

time for signal to return tells you the altitude of surface feature.

Planet Surface

Venera 14 photo of surface. Lander destroyed after about an hour!

Venus' Atmosphere

- Hot at surface - 750 K! (Room temp. on Earth about 300K)

- Why so hot?

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.

Atmospheric Comparison

How does Venus lose its interior heat?

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

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

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?

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.

Olympus MonsLargest known volcano in Solar System – 3X the height of Everest!

Question

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

Evidence for Past Surface Water

"runoff channels" or dry rivers

"outflow channels"

teardrop "islands" in outflow channels

standing water erosion in craters?

Where did the water go?

1. Polar Ice Caps2. Evidence for "Permafrost" layer beneath surface

"Splosh" craters suggesting liquefied ejecta.

Where did the water go?

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.