Ancient plate tectonics on Mars? Hmmm...

As a contribution to this edition of the Accretionary Wedge, I’d like to scrape off the following: The hemispherical dichotomy and the idea of ancient Martian plate tectonics make me go hmmm….



This is a topographic map of Mars created using data from the NASA Mars Orbital Laser Altimeter (MOLA). The dark blues and cool colors represent topographic lows; the reds and warm colors represent topographic highs. The ‘hemispherical dichotomy’ refers to the difference between the topography of the northern 1/3 of the planet and the southern 2/3 of the planet. The northern hemisphere of Mars sits approximately 2-3 km topographically lower than the southern hemisphere, and the south pole is estimated to be ~6 km higher than the north pole.

In addition, the surface geology of each hemisphere is radically different. Craters and terrain heavily mark the southern hemisphere, but the northern lowlands are relatively smooth and flat. If crater density is used as an indicator of absolute age (i.e., crater counts), then the southern hemisphere may be ancient crust formed during the accretion of Mars and the northern hemisphere may be much younger.

It’s generally agreed that Mars lacks a self-sustaining magnetic field and is not tectonically active, BUT… did plate tectonic processes operate on Mars in the past? I submit, for your enjoyment and wonder (hmmm…) the following observations:



1.) The Martian surface is characterized by both extensional features (e.g., grabens and rifts) and contractional features (e.g., wrinkle ridges), but evidence of strike-slip movement is rare. To the right is a photo of a heart-shaped graben taken by the Mars Global Surveyor Orbital Camera. Are these types of features telling us something about Martian tectonic history?

2.) Numerous volcanic features on the Martian surface span a wide range of geologic ages, which are estimated using cross-cutting relationships and crater counts. Studies of the Tharsis region, a volcanic plateau with several shield volcanoes, provide information on the petrology and structure of the Martian interior. For example, Olympus Mons (photo below), a shield volcano twice the size of Mauna Loa in Hawaii is characterized by ancient voluminous basalt flows. What does this type of feature imply about isostatic compensation and the interior structure of Mars?



3.) Finally, the Mars Global Surveyor made repeated low-altitude passes through the martian atmosphere to circularize the spacecraft’s orbit using atmospheric drag. Magnetic data was collected every time the spacecraft dropped below 200km; the closest approach was at 101km. From this data, Connerney et al. (1999) present a martian magnetic field map.


Red and blue coloring represent positive and negative radial field measurements, and correlation between adjacent MGS tracks is possible. Are these magnetic stripes on the martian crust? Are martian anomalies a result of tectonism?

Hmmm…