Why is one hemisphere of Mars believed to be older than the other?
One hemisphere is darker.
One hemisphere is more heavily cratered.
One hemisphere is lower.
One hemisphere leans more away from the Sun
Which One? Please explain
Most of these don't make sense to me, but I don't really understand what is meant by "lower".
You know, parts of the earth's crust are older than others. If you removed all of the water from the oceans and tested the rocks that make up the solid ground (continents as well as ocean floors), you would find that the Atlantic ocean is young, and the Pacific ocean is old.
This is because of plate tectonics. The Earth is made up of a bunch of plates that are moving around on what is sort of a liquid. In the middle of the Atlantic Ocean is the MID-ATLANTIC RIDGE, where there are two plates that are sliding away from one another, and lava comes up to fill the void (Iceland is right on this ridge). Lava makes rocks that are "young". On the fringes of the Pacific, all of the plates are sliding toward each other, and you get what is called a SUBDUCTION ZONE, where old rock from the Pacific Ocean floor are pushed under the continental rock, melted down, and then come back in western North America and eastern Asia as lava from volcanoes.
This tectonic activity may be present on Mars, but we're not sure. Barring that, I would be inclined to guess that the cratering may have an effect, as this would expose younger rock from beneath.
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