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PTYS/ASTR 206 Mercury and Venus3/20/07
Announcements
• Reading Assignment– Chapter 13
• 4th Homework due today
• Quiz on Thursday (3/22)– Will cover all material since the last exam. This is Chapters 9-12
and the part of 13 covered in the lecture on Thursday of the textbook. Note that there were 3 lectures not covered in detail in the textbook (2/20, 2/22, 3/6)
• Next study-group session is next Wednesday (3/28) from 10:30AM-12:00Noon – in room 330.
PTYS/ASTR 206 Mercury and Venus3/20/07
Most of our detailed information about Mercury’s surface is from Mariner 10 (1974/75)
• Mariner only saw one side of the planet due to 3:2 spin-orbit resonance.
• There are presently three planned missions to Mercury with data return beginning in 2009.
PTYS/ASTR 206 Mercury and Venus3/20/07
Messenger
• Launched – Aug 3, 2004
• Science Goals:– Why is Mercury so dense?– What is the geological history
of Mercury?– What is the structure of
Mercury’s core?– What is the nature of
Mercury’s magnetic field?– What are the unusual
materials at Mercury’s poles?– What volatiles are important
at Mercury?
PTYS/ASTR 206 Mercury and Venus3/20/07
Mercury is heavily cratered, like the Moon, but it is also different from the Moon
• Mariner 10 found, surprisingly, a heavily cratered planet that resembles the Moon
• Mercury also has a regolith (the fine powder-like “top soil” due to constant bombardment of small asteroids and meteorites)
PTYS/ASTR 206 Mercury and Venus3/20/07
• Heavily cratered surface
• Less dense cratering than moon
• Gently rolling plains
• Scarps
• No evidence of tectonics
PTYS/ASTR 206 Mercury and Venus3/20/07
Note how much more densely the craters occur on the moon’s surface.
This is a photo of the Moon
PTYS/ASTR 206 Mercury and Venus3/20/07
Ray Craters
Lunar crater Tycho
Mercury ray crater
Craters show ejecta, like the moon
• But on Mercury the ejecta are confined to a smaller area
• Moon has smaller gravity !
PTYS/ASTR 206 Mercury and Venus3/20/07
Scarps are cliffs
This one is more than a km high
They probably formed as the planet cooled and shrank
PTYS/ASTR 206 Mercury and Venus3/20/07
Wrinkled ridges
• Thought to have been caused by the shrinking of the surface following the cooling of the core of the planet.
• As the core cooled and shrank, the surface was forced to shrink also, developing ridges which are scattered across the planet.
PTYS/ASTR 206 Mercury and Venus3/20/07
Caloris Basin
• The ONLY mare
• Thought to have formed from a huge impact
PTYS/ASTR 206 Mercury and Venus3/20/07
Caloris Impact
• Created seismic waves that traveled through the planet
• Created hilly terrain on the opposite side
PTYS/ASTR 206 Mercury and Venus3/20/07
Similar feature on the moon
Mare OrientaleLunar Orbiter 4 photo
PTYS/ASTR 206 Mercury and Venus3/20/07
Ice at Mercury’s poles
• Some craters provide permanent or near permanent shade from the Sun
• Reflective features within some of these craters– Water ice
PTYS/ASTR 206 Mercury and Venus3/20/07
The Interior of Mercury
• Metallic iron-nickel core – About 75 % of the planet’s
diameter – Density is similar to (but
less than) Earth– Smaller, so iron core takes
up more volume
• The core is likely to be hot and at least partially molten.
• Solid rocky layer topped with a thin crust about 100 km thick.
PTYS/ASTR 206 Mercury and Venus3/20/07
Iron in Earth vs. Mercury
• The percentage of Mercury’s overall volume occupied by the core is much more than Earth
Mercury core is 42% of overall volume
Earth core is 17% of overall volume
• Why then is Earth denser than mercury?– Earth is bigger, has a larger mass which
compressed the core even more, making it denser
PTYS/ASTR 206 Mercury and Venus3/20/07
Why does Mercury have such a High
Iron Content?
• Mercury formed nearer the protosun, where it was hotter and more volatile substances didn't condense as readily.
• Maybe a very powerful solar wind from the early Sun blew away some mantle.
• Maybe Mercury hit something that tore off a lot of its early mantle.
PTYS/ASTR 206 Mercury and Venus3/20/07
Mercury’s Magnetic Field
• Mercury has a weak magnetic field (1% of Earths) indicating part of the core is liquid
• This magnetic field causes a magnetosphere similar to Earth’s but weaker
• Some solar wind can penetrate Mercury’s magnetosphere and reach the surface
PTYS/ASTR 206 Mercury and Venus3/20/07
Mercury Summary
• Slightly larger than the moon• 3-to-2 spin-orbit resonance• Heavily cratered surface with • contraction features (scarps)• Iron-rich core• Weak magnetic field
PTYS/ASTR 206 Mercury and Venus3/20/07
Venus – Our “Sister” Planet
• 2nd planet from the Sun• Semi-major axis = 0.72 AU
• Radius = 6052 km (0.95Rearth)
• Mass = 4.9x1024kg (0.82Mearth)
• Density = 5240 kg/m3
• Rotational period = 243 days– Retrograde (why?)
Basic Facts
PTYS/ASTR 206 Mercury and Venus3/20/07
Venus – Our “Sister” Planet
• Atmosphere
– Mostly C02
– Runaway greenhouse effect– Very dense and massive
• 90 times the pressure at Earth’s ruface
– Russian Venera probes were either crushed during descent or survived only an hour due to high pressures
• Surface temperature = 750K– Earth = 300K, Mercury=700K
• No significant intrinsic magnetic field
Basic Facts (cont.)
PTYS/ASTR 206 Mercury and Venus3/20/07
2 Russian spacecraft landed on Venus and took the only pictures from its surface
(they then failed shortly after landing due to extreme pressure and temperature)
PTYS/ASTR 206 Mercury and Venus3/20/07
Venus surface temperature – 750K
• Hotter than Mercury!
• So hot, that rocks almost glow!
• A plastic construction helmet subjected to brief exposure of the surface temperature of Venus
PTYS/ASTR 206 Mercury and Venus3/20/07
Three reasons why Venus is so bright as viewed from Earth
1. It is close to Earth
2. It is close to the Sun
3. It has a high albedo– The planetwide
cloud cover reflects most of the sunlight striking it
PTYS/ASTR 206 Mercury and Venus3/20/07
Venus has a Retrograde Rotation
• Most planets and moons have prograde rotation, Venus is opposite
• The reason is not known for certain, but may be due to a large impact, or tidal forces
PTYS/ASTR 206 Mercury and Venus3/20/07
Venus rotates very slowly
• Rotation rate was not known until fairly recently
– Cannot see the surface
– determined from radar imaging
PTYS/ASTR 206 Mercury and Venus3/20/07
Venus clouds rotate faster than the planet
• The clouds of Venus rotate in the same direction as the surface
• But MUCH faster– It takes 4 days for
them to go around planet
– 220 mph
PTYS/ASTR 206 Mercury and Venus3/20/07
Summary of Venus Atmospheric Components
Earth Venus
CO2absorbed in rock free in atmosphere
(96.5%)
Nitrogen free in atmosphere (78%) free in atmosphere (3.5%)
Water mostly condensed on surface decomposed long ago, and hydrogen escaped
Oxygen product of life (21%) no life to produce it
PTYS/ASTR 206 Mercury and Venus3/20/07
Venus’s Runaway Greenhouse effect
• Venus MAY have had oceans in the distant past.– high pressure of the atmosphere could prevent evaporation
(even though it is close to the Sun).
• As the Sun became brighter, Venus got hotter and the water evaporated (just got too hot)
• The increase in water vapor (a greenhouse gas) led to a further increase in temperature
• Also, since there was no more water to dissolve CO2 and SO2, these gases were free in the atmosphere
– These are also greenhouse gases !
• It got even hotter !
PTYS/ASTR 206 Mercury and Venus3/20/07
On Venus, greenhouse gases are NOT recycled
On Earth, greenhouse gases are recycled
PTYS/ASTR 206 Mercury and Venus3/20/07
Atmospheric Circulation
• The circulation of the Venusian atmosphere is dominated by two huge convection currents in the cloud layers, one in the northern hemisphere and one in the southern hemisphere.
• Nearly equal day and night temperatures
• And equal pole and equator temperatures
PTYS/ASTR 206 Mercury and Venus3/20/07
Volcanic eruptions are probably responsiblefor Venus’s clouds
• Pioneer Venus Orbiter recorded very high levels of SO2 which then steadily declined over the next several years– Similar Earth-based
observations have been made since the 1950’s
• Venus’s clouds consist of droplets of concentrated sulfuric acid
• Mostly shield volcanoes– One may be active
PTYS/ASTR 206 Mercury and Venus3/20/07
Venus’s Surface
• Two continents
• Surface consists of recently solidified basalt and few impact craters– Major resurfacing
event about 500 million years ago
– No small craters
• No plate tectonicsRadar image of Venus – from Magellan
PTYS/ASTR 206 Mercury and Venus3/20/07
Venus’s Surface (cont.)
• The surface of Venus is surprisingly flat, mostly covered with gently rolling hills
• There are a few major highlands and several large volcanoes
PTYS/ASTR 206 Mercury and Venus3/20/07
NO Plate Tectonics on Venus
• The surface of Venus shows no evidence of the motion of large crustal plates, which plays a major role in shaping the Earth’s surface
• The surface may be too hot for the crust to move as rigid plates
• The lack of plate tectonics on Venus may be due to its lack of water (which is critical for plate tectonics on Earth)
PTYS/ASTR 206 Mercury and Venus3/20/07
Venus Surface Features
Pictures of Venus' surface taken by Magellan's radar imaging equipment. The left picture shows a group of volcanoes, the middle picture is a volcano which appears to have sank in the hot magma below it, and the right picture show mysterious volcanoes which look like pancakes on the planet's surface!