10. The impact crater record on Venus is unusual because…a) Venus is the only planet with no craters at allb) The age of the surface seems larger than the age of the solar

systemc) All the impact craters were erased half a billion years agod) Only craters smaller than a few kilometers in size can be

founde) No new craters have formed for 3.5 billion years

• Craters on Venus tell a strange story– Only ~1000 craters - Randomly distributed– Imply that the surface is only 700-800 Myr old

• Catastrophic resurfacing• Continual removal• Occurred over the entire planet

– What happened for the first 3.8 billion years on Venus?

Balch crater – 40 km

Heloise crater – 38 km

• Most people thought the answer was ‘D’– Only craters smaller than a few km are found

• The dense atmosphere screens out craters less than 20 km in size

All craters > 3km in size Atmosphere stops any smaller impacts from reaching

the surface ‘Burn’ marks on the surface (smooth areas) maybe

the result of impactors exploding in the atmosphere

16. Mars has the largest volcanoes in the solar system. Olympus Mons is over 20km high; nothing on the Earth (volcanic or otherwise) even comes close to this. The main reason Mars can have such high mountains is…

a) Due to its weaker gravity which lowers the weight of the mountainb) Due to its colder temperature which strengthens its surface rocksc) Due to its thinner atmosphere, so the pressure on the mountains is lessd) Due to its different rock composition which happens to be stronger than

Earth’se) All of the above

• Mars is made from the same rocky stuff as the Earth• Rocks on Earth and Mars are not near the melting

point – so temperature doesn’t matter

So is it the smaller weight of the atmosphere or the smaller weight of the rocks?

How much does Earth’s atmosphere weigh?• Pressure is 105 pascals (which is weight per square meter)• That’s a mass of ~ 10,000 kg m-2 (divide by Earth’s gravity)• A cubic meter of rock has a mass of 3000 kg• So the weight of the atmosphere is about same as a layer

of rock 3.3m thick

Olympus Mons is 26,000m highWould adding an extra 3m make much difference?

• It’s the gravity

• 26km of rock on Mars weighs about as much as 9km of rock on the Earth

• Earth’s highest mountain is 8.8km high

20. Methane has been discovered recently in the martian atmosphere which prompted some people to wonder if currently-living things on Mars produced it. What are the alternative explanations?

a) Methane can be delivered to Mars by impacts of cometsb) This methane may have formed billions of years ago so it doesn’t mean life

exists todayc) The active volcanoes on Mars pump out methaned) Methane is trapped in the polar ice which melts and releases it to the

atmospheree) Alteration of rocks deep underground by warm water can release methane

• Methane on Mars– CH4

– Detected by telescopesand Mars Express

• Why we care…– Methane is destroyed quickly – Lifetime of only a few centuries– Implies continuous production

• So what’s making it?– Life

• Cows? Unlikely…• Chemolithotrophic microbial ecosystems

– Hydration of rocks• Serpentinization – a geologic process• Warm volcanic rock combines with water – releases

methane

MethaneMethane

25. Continents currently cover less than one third of the surface of the Earth. The area of this continental crust is…

a) Shrinking with timeb)Expanding with timec) Holding steady for the past few billion yearsd)Can go up or down depending on the situatione) It depends on sea-level

• What happens when plates collide?– All depends on density– Oceanic crust has a higher density

than the continents

• Three possible collisions– Ocean-continent – South America and the Nazca plate

– Ocean-ocean– West pacific region

– Continent-continent– India collides with Eurasia

Ocean

Mantle

Oceanic crust

Continen-tal

crust

Press & Siever, 2nd edition

• Famous Mariana trench– Almost 11km below seas level– Pressure > surface of Venus

– You could fit Mt. Everest in there and still have a few km to spare!

New continental crust is created Continental crust doesn’t subduct

Area of continents growing with time