CONCEPT SUMMARY- 5
Density -What is it, how is density of the planets determined, what does it tell us about the planets.
Anatomy of Planets - Core, mantle, crust, lithosphere
BIGGER PLANET =>
SMALLER PLANET =>
Moon - Surface largely saturated with craters except for the lunar mare - which are large crater basins that have been flooded with lava. This flooding episode must have happened about 3.8 billion years ago because dating of rocks from different regions tells us that the crating rate dropped considerably after about 3.8 billion years ago.
Mercury - Surface saturated with craters. This tells us that the surface has suffered very little reprocessing for several billion years.
Mars - The density of craters differs considerably in different regions of Mars - particularly between the north and southern hemispheres. The north is much less cratered (and hence is younger) than the south.
Venus - The crater density is fairly uniform - this tells us that most of the surface has about the same age. The density of craters allows us to estimate that the whole planet must have been re-surfaced about 800 million years ago.
Earth - On Earth most of the craters have been eroded or removed. But the remnants of about 300 craters have been found.
Moon - Very little volcanism except the lunar mare (only found on Earth-facing side) where lava leaked through and flooded large impact basins. This occurred about 3.5 billion years ago - AFTER the end of the heavy bombardment - so the mare have much less craters than the other lunar regions.
Mercury - Virtually no volcanism visible.
Mars - A few LARGE shield volcanoes (particularly Olympus Mons, the largest mountain in the solar system). These volcanoes are thought to be the result of isolated plumes of hot (relatively low viscosity) lava coming up through the mantle and eruption through a central vent onto the surface. These volcanoes have not been active for a billion years or so.
Venus - Many volcanic features are found on the surface of Venus - ranging from flooded plains (suggesting very runny lava), shield volcanoes to very strange domes and features where it seems that the crust was pushed up by a plume of viscous lava but the lava did not erupt onto the surface. From studying the distribution of craters on the volcanic terrains, it appears that Venus suffered from a very active period of volcanism about 800 million years ago (but since there seems to have been little activity)
Earth - The Earth is still undergoing volcanic activity. Most of the volcanic activity seems to be at boundaries between plates (see next section) where the crust is weaker and the lava can leak more easily through the crust. There are some place, notably the Hawaiian islands, where lava is erupting through the middle of a plate. It is believed that there is a "hot spot" - a plume of hot mantle material, that rising under Hawaii. As the Pacific plate moves westward the plume has produced the chain of islands - Kawaii being the farthest west and oldest, the Big Island being the farthest east and still volcanically active.
Moon - Very little tectonics - just a few faults, mostly along the edges of mare, where lava leaked through.
Mercury - Very little tectonics - just a few places where the crust seems to have shrunk as it cooled.
Mars - Considerable tectonic activity - one large feature, Valles Marinaris, stretches about 1/3 the way around the planet. But otherwise, the tectonic features seem to be associated with relieving stresses produced by volcanic activity.
Venus - Many tectonic features. Some are large scale, some are small scale. But there is no systematic location of mountains ranges or systems of long faultlines that would indicate that Venus has undergone plate tectonics.
Earth - The Earth is still undergoing both tectonic and volcanic activity. Most of the volcanic and tectonic activity seems to be at boundaries between plates. There are places where the plates are moving apart (ocean ridges), where the plates are colliding (mountain ranges), where plates sink down into the mantle (subduction zones) and where plates slide against each other (faultlines). The plates move relative to each other at a few inches per year, re-processing most of the lithosphere over time scales of about a billion years.
Moon & Mercury - No atmosphere, no erosion.
Mars - The current atmosphere of Mars is very tenuous (low pressure). And the surface is too cold for liquid water. Any water is frozen in the polar caps or in the soil. BUT there is evidence of erosion in the past - suggesting that at one point the atmosphere was denser, the surface warmer and water flowed in rivers, canyons. There is also evidence of occasional catastrophic flooding.
Venus - no water, very little evidence of erosion (though current, radar pictures may just not be of sufficient high resolution to see evidence of erosion.). It is probable that the sulfuric acid in the clouds rains out and causes some chemical erosion of the rocks.
Earth - Plenty of evidence of erosion - practically all surfaces are eroded - mostly by water, some ice (glaciers), some wind (deserts).