13 November 2008
Mars is often touted as the “most earth-like” planet, but if you take a look at its surface there are some aspects that are decidedly alien. Sure, there are dry river beds and canyons and volcanoes. But there are also craters. Everywhere. So many that, when Mariner 9 sent back the first spacecraft images of Mars, people were dismayed to see a surface that looked just like the moon!
Is Mars just astronomically unlucky? Nah, we get hit with just as much stuff as Mars does, only Mars is old. More specifically, its surface is old. The planet itself is about the same age as earth. However because Mars is smaller, it cooled faster and that means that it is much less geologically active.
What do you notice? Yes, state the obvious. (if there’s one thing that I’ve learned from years of professors asking questions in class it’s that often they are looking for a really obvious statement) In this case we have a lot of obvious statements to choose from, such as: “The oceans are lower than the continents!” and “There are no craters” and “Mountains form in long ridges!”
All of this is because earth’s crust slides around as plates “floating” atop layer of softer rock. The plates pull apart in places like the middle of the Atlantic, and new crust is formed as the hot rock from underneath is extruded upward. The plates crash together in places like the Himalayas and western South America, forming huge mountain ranges as the crust buckles and folds.
Plate tectonics is a tremendously important process on the Earth. It recycles our crust, processes the rocks, and even moderates our atmosphere. There are some scientists who think that plate tectonics may be a requirement for a planet with thriving life!
What observations can we make here? The northern hemisphere is lower and smoother. There is a giant canyon and giant volcanoes! There are lots of craters! Also note what mars lacks. There are no long linear mountain ranges and no mid-ocean-ridges. There is not a really sharp distinction between “low” and “high” like there is on the Earth. It just sort of slopes gradually from the southern highlands to the northern lowlands.
All of this is because Mars lacks plate tectonics. Its crust is, as far as we can tell, one big slab of rock. The only hint that there was ever plate tectonics on Mars comes from its magnetic field, shown below.
Red on this map is magnetic “north” and blue is magnetic “south”. Clearly astronauts can leave their compasses at home. Notice that the magnetic field forms stripes. On earth this happens at a very much smaller scale at the mid-ocean ridges: as the magma wells up between the spreading plates, it freezes and traps the current magnetic field of the planet. As the magnetic pole switches (which it does occasionally) the changes get trapped in the rocks. Some scientists have looked at the map of Mars’s magnetic field and said “aha! stripes! there must have been plate tectonics!” I’m a bit skeptical of that though. Where are the spreading centers and mountain ranges you would expect?
It’s possible that the magnetic field we’re seeing is the remnant of such ancient plate tectonics that there are no surface features left, but I don’t think we can definitively say “yes, Mars had active plate tectonics. It is interesting to note that the presence of water in the crust and mantle seem to make plate tectonics much easier to maintain. It turns out that water makes rocks melt at lower temperatures, effectively “lubricating” the undersides of the plates on earth. Could Mars have lost its plate tectonics because it lost it’s water? Maybe, but nobody really knows for sure.
Mars is/was certainly tectonically active though. One of the biggest events in Mars history was the formation of the Tharsis rise (the huge area of high topograpphy, capped by four giant volcanoes). Notice what is directly to the east of Tharsis? A giant crack in the crust, otherwise known as Valles Marineris. It’s likely that Valles Marineris formed when the stress of the growing Tharsis rise of the crust became too great and the martian crust just ripped like the seam on a pair of too-tight pants. In fact, there are faults and cracks encircling Tharsis. Here’s a map of one type of fault, marked with the black lines:
An even closer look shows that there are places on Mars where the surface is pretty much all faults! For example, check out this view of the volcano Alba Patera, north of the main Tharsis rise. It is shot through with cracks!
Mars may have had plate tectonics, but if it did, all evidence was erased except for the magnetic stripes preserved in the ancient southern crust. Mars did have lots of other tectonics though. All over the planet there are cracks and faults indicating that once, long ago, the surface quaked and split. We don’t know how active Mars is today. Someday, we may send a lander, or even better a network of landers, carrying seismometers that could listen to the tremors of the red planet’s crust and tell us what is going on deep inside. Until then all we can do is look at the surface, trace the faults and imagine what must have caused them, billions of years ago.