PTYS/ASTR 206Mercury and Venus 3/20/07 Mercury and Venus.

PTYS/ASTR 206 Mercury and Venus3/20/07

Mercury and Venus


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.


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.


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?


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


Note how much more densely the craters occur on the moon’s surface.

This is a photo of the Moon


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 !


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 OrientaleLunar 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’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


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

• 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


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.)


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

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


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 CO2 and SO2, 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 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


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


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 !

PTYS/ASTR 206Mercury and Venus 3/20/07 Mercury and Venus.

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Transcript of PTYS/ASTR 206Mercury and Venus 3/20/07 Mercury and Venus.