Valles Marineris
This enormous feature on Mars is a canyon on a scale we struggle to comprehend. It is 4000 kilometers long, comparable to the distance between New York and Los Angeles. It is up to 300 kilometers wide – comparable to the distance between New York and Boston. It is 11 kilometers deep at its greatest, more than 6 times the depth of the Grand Canyon and twice as deep as any measureable canyon on Earth. How in the world did this thing form?
By characterizing Mars through orbital mapping and spectroscopy, scientists have established that Valles Marineris is some expression of opening or tension in the martian crust. It is found on the edge of a gigantic volcanic province known as Tharsis, and somehow this volcanic province must have contributed to the stresses that formed it. Studying layers on each side of the canyon indicates that they’re not offset side-to-side by any strike slip motion, so the creation of this canyon involved just the canyon opening, nothing sliding past each other. While there are sedimentary deposits nearby, the canyon can’t have been eroded by water as the shape is wrong and there’s nowhere to put the large mass of material that would have had to be eroded. So how could this feature have formed?
There are two basic ideas for how it formed; a rift and a graben. If this was a rift, then that means one side pulled away from the other, leaving a gap in-between. If it’s a graben, then that means the area in the canyon dropped downwards as a set of large blocks. Both of these require extensional stresses, but each of them requires different properties of the martian crust.
The best model for forming these valleys requires several details. First, when the Tharsis province grew, it put a huge amount of weight on the crust, and the crust began bending to support it – putting the whole area under stress. Second, Tharsis grew at a unique spot on Mars – right on top of the so-called Dichotomy boundary – the boundary between lowlands in the northern hemisphere and highlands in the southern hemisphere. Third, Tharsis is volcanic, and molten rock trying to find its way upwards will find fractures wherever it can, and push them to open.
When Tharsis formed, it put the crust under intense stress, and the weakest spot was the edge of the province, right on top of the dichotomy boundary. This spot became a location of intense stress, leading to the formation of fractures and the beginning of valley opening. Once fractures formed, molten rock from the Tharsis province found these fractures and pushed into it, opening the valley further in the process and leaving a set of exposed volcanic dikes and other volcanic rocks at the bottom of each chasm.
In summary; fractures formed on the dichotomy boundary, and molten rock pushed those fractures open, causing a set of large grabens to drop down. The former surface of the planet is thus likely found at the bottom of those valleys, in many cases buried by later sediments and volcanic rocks. If scientists ever visited this location, that model can be tested by examining the faults and the rocks at the site.
Scientists are still working to understand how a feature like this could form in detail, so as additional rocks or structural features are discovered, this model may still undergo wholesale changes in the future. However, it fits many of the details – the stresses the rocks can take, the type of motion observed, and the relationship to the surrounding volcanic rocks.
-JBB
Image credit: ESA/DLR/FU Berlin (G. Neukum)
https://flic.kr/p/dnpoPa
Reference:
https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2012JE004059
https://agupubs.onlinelibrary.wiley.com/doi/full/10.1002/2016JE005231
