Astrogeology I-- Rocky Planets and Moons Quiz

Answer: The interior of the planet shrank and cooled

Small bodies tend to cool faster than larger bodies. As the smallest rocky planet, Mercury was the first to cool. As the interior cooled it contracted. The crust hardened first, and so when it shrank, it cracked to form the large thrust faults. Some are 500 km long and 2 to 4 km high!

Answer: Both of these

Micrometeorites chip tiny dust grains off of rocks. Expansion and contraction of rocks due to extreme temperature differences between day and night (227 C to -173 C on Mercury) causes grains to flake off. Notice I did not include Venus in this question-- the thick atmosphere causes micrometeorites to burn up before they can hit the ground.

The dust layer, on the moon at least, is called regolith.

Answer: Oceans

Of course water can hold dissolved CO2-- that's how soft drinks work! Venus and the Earth probably began with similar amounts of H2O and CO2, given they are about the same size and have about the same composition. But, Venus was closer to the sun than the Earth, so Venus's oceans evaporated, and that was the end of things. Please don't take me to be a lover of belching smokestacks-- too much CO2 can cause big problems even though the problem would never become as extreme as on Venus!

Answer: The crust is lightly and evenly cratered

The surface is evenly and randomly cratered-- it is impossible to distinguish a map of the crater distribution on Venus from a map with a randomly generated crater distribution. From the total number of craters, it seems that the crust is only 200 million years old (On Earth at this time, dinosaurs were hot stuff!) So, what is going on here? With no plate tectonics to gradually get rid of heat, perhaps the heat built up to a point that either the entire crust melted, or it melted just enough to trigger a huge volcanic resurfacing of the entire planet.

Answer: True

In one of those great accidents of science, the gravimeter included in the Apollo 17 instrument package had a design flaw, so it actually functioned as an ultra-sensitive seismometer! There are numerous small moonquakes. This is really helpful-- it's how we know anything about the structure of the moon's interior.

Answer: Basalt

The maria are made of basalt, just like seafloor rock on the Earth. However, Earth's highland areas, namely the continents, are mostly made up of granites. Anorthosites are very rare on the Earth, though they can be found in the Adirondack Mountains. Sandstones and carbonates are both sedimentary rocks, and so are not found on the moon.

Answer: Shield Volcano

The difference is in the composition of the lava. Cinder cone volcanoes are made of calcium- and potassium-rich silicate rocks, and shield volcanoes are made of iron- and magnesium-rich silicate rocks. This is a really nifty way to tell something about the composition of the rocks when you can't get a sample from the surface!

Answer: Some parts of the crust are magnetized

Some kinds of igneous rock contain tiny crystals of magnetite, a magnetic iron mineral. If the rock cools in the presence of a magnetic field, all the magnetite crystals line up in the direction of the field, like iron filings line up around a magnet. So, it seems Mars had an appreciable magnetic field. Why do we care? Life-- magnetic fields block some cosmic rays, which erode atmospheres and cause nasty mutations in organisms.

Answer: A rift valley, like Africa's Rift Valley

Rift valleys form from tectonic activity. A bubble of warm mantle rock wells up under some part of the crust. The crust there warms and stretches, then rips open to form a valley. If the process progresses far enough, lava wells up at the bottom of the valley, hardens, and begins to push the two pieces of crust apart.

The end result is two continents separated by an ocean basin. Examples on Earth would be the Atlantic Ocean, or the much younger Red Sea. Africa's Great Rift Valley is an ocean basin that never made it.

The Valles Marineris is the same thing-- Mars never quite had the size or heat to really get the plate tectonics crankin'.

Answer: Limestones

Asteroids in the inner half of the asteroid belt are mostly silicate minerals and metals. 4 Vesta is made up of silicate rock. Asteroids in the outer half of the asteroid belt are made up of dark, hydrated carbonaceous material. 1 Ceres, the largest asteroid, is one of these. How do we know this? First, from the spectra of the different asteroids. Second, from the meteorites that land on the Earth-- stony and iron meteorites, and carbonaceous meteorites.