Announcements Reading Assignment Chapter 13 4 th 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.
Mercury continued
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.
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? Messenger
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)
Heavily cratered surface Less dense cratering than moon Gently rolling plains Scarps No evidence of tectonics
This is a photo of the Moon Note how much more densely the craters occur on the moon s surface.
Ray Craters Craters show ejecta, like the moon But on Mercury the ejecta are confined to a smaller area Moon has smaller gravity! Mercury ray crater Lunar crater Tycho
Scarps are cliffs This one is more than a km high They probably formed as the planet cooled and shrank
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.
Caloris Basin The ONLY mare Thought to have formed from a huge impact
Caloris Impact Created seismic waves that traveled through the planet Created hilly terrain on the opposite side
And on the opposite side of Caloris basin
Similar feature on the moon Mare Orientale Lunar Orbiter 4 photo
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
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.
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
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.
Mercury has a weak magnetic field (1% of Earths) indicating part of the core is liquid Mercury s Magnetic Field This magnetic field causes a magnetosphere similar to Earth s but weaker Some solar wind can penetrate Mercury s magnetosphere and reach the surface
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
Venus Our Sister Planet Basic Facts 2 nd planet from the Sun Semi-major axis = 0.72 AU Radius = 6052 km (0.95R earth ) Mass = 4.9x10 24 kg (0.82M earth ) Density = 5240 kg/m 3 Rotational period = 243 days Retrograde (why?)
Atmosphere Venus Our Sister Planet Basic Facts (cont.) Mostly C0 2 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
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)
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
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
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
Venus rotates very slowly Rotation rate was not known until fairly recently Cannot see the surface determined from radar imaging
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
Summary of Venus Atmospheric Components Earth Venus CO 2 absorbed 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
Greenhouse effect: Earth vs. Venus
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 CO 2 and SO 2, these gases were free in the atmosphere These are also greenhouse gases! It got even hotter!
On Venus, greenhouse gases are NOT recycled On Earth, greenhouse gases are recycled
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
Volcanic eruptions are probably responsible for Venus s clouds Pioneer Venus Orbiter recorded very high levels of SO 2 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
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 tectonics Radar image of Venus from Magellan
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
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)
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!
Venus s Pancake Volcanoes
Venus Surface Features
Venus Summary Size is like Earth Geology is similar to Earth (except for water) no plate tectonics (may be related to lack of water) no magnetic field Atmosphere Runaway greenhouse effect Not a good place to visit!