Astronomy Picture of the Day

Mercury Mass = M Earth Radius = 0.38 R Earth  Surface Temp: 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 Mercury’s year!)‏ Composite image from Mariner s

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 Mass = 0.82 M Earth Radius = 0.95 R Earth Atmospheric Pressure= 90.0 AP Earth Average distance from Sun = 0.72 AU Orbital period = 225 days Rotation period = 243 days (longer than orbital period, and retrograde!)‏ Atmosphere: 96.5% CO 2 the rest Nitrogen. (recall Earths' atmosphere: 78% Nitrogen, 21% Oxygen, 0.9% Argon, and ONLY 0.03% CO 2, with water vapor making up 0.1-3%)‏

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 K! (Room temp. on Earth about 300K)‏ - Why so hot?

Runaway Greenhouse Effect Venus started out somewhat hotter due to its closer proximity to the Sun then Earth, but THEN: 1) Water and CO 2 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

Mars Mass = 0.11 M Earth Radius = 0.53 R Earth 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.

The Martian Atmosphere  95% CO 2  Average Surface Temperature 220 K ( colder than Earth's by about 70K). Surface Pressure that of Earth's atmosphere (thin air!)‏  Why so cold with such a thin atmosphere?

The Martian Atmosphere 95% CO 2 Surface Temperature 250 K. Surface Pressure 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 Mons Largest 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?

1. Polar Ice Caps 2. Evidence for "Permafrost" layer beneath surface Where did the water go? Well, it got Cold!

Mars' Moons Phobos and Deimos Phobos: 28 x 20 km Deimos: 16 x 10 km Properties similar to asteroids. They are probably asteroids captured into orbit by Mars' gravity.

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.