How Rough is Rough?

Friday morning Pamela and I both went to the session on comparative planetology. While we’d worked pretty hard to make sure we were never both in the same session, this was sort of necessary. We had to leave mid-session in order to get to the airport in time for my 2.15 flight home – so the logistics were made much easier if we just sat next to each other.

Pamela already covered a lot of that session, but my favourite talk was comparing the terrestrial planets’ topographic roughness (and when they say terrestrial planets, apparently they also include the Moon).

Large-Scale Topographic Roughness of Terrestrial Planets: A Comparison (Kreslavsky M.A., Head J.W. III, Harmon J.K.)

The data collection was simple, though the most limited data set is from Mercury. They only had a set of Arecibo radar profiles in a very narrow equatorial region – from latitudes of about 5 degrees south to 12 degrees north.

For the Moon they used the Lunar Prospector’s LIDAR profiles. Venus, Earth and Mars they had gridded topography from Magellan RA, SRTM and MOLA (respectively).

On difficult part of this whole study was choosing a measure of roughness. The different data sets have different properties – so they had to choose a measure that would be tolerant of those, and not introduce too much bias or error.

They ended up looking for all the differences in elevation in all the planets. In places where there is a high roughness variability, they chose large (100,000 square km) “geologically homogeneous regions”. Their basic criterion for a good region, roughness measure and a sufficient data set was if we were to remove some random half of the data, the result is the same.

So, in looking at the relative roughness, Mercury’s data set is very good – it averages well. The far side of the Moon is significantly rougher than Mercury.

To compare with the Earth, they chose two regions of different roughnesses. One was the western US (mountains) and the other was the Russian plaines – these areas’ roughnesses differ by an order of magnitude.

The western US is very similar to Mercury, the Martian highlands. The Martian lowlands are very similar to the Russian plains.

Venus’ tesserae roughness was graphed nearly identical to Mercury – which is rougher than the Earth and Martian plains by one order of magnitude. Typical plaines on Venus, on the other hand, were much rougher than Earth’s plains but not as rough as Earth’s mountains.

What does this mean? Well, the Moon is the roughest which makes sense – it has the last gravity. For the Earth and Venus, impacts are less effective at changing their surfaces than plate tectonics are (as compared to the Moon). Mercury could be experiencing gravitational relaxation (thinning of the lithosphere) and infilling of volcanic basins. Mars’ topography is probably due to sedimentary basin infill and erosion. Mars’ cratered terrains are smoother than Mercury, which suggests it does not have a thinner lithosphere. It could be that there’s a lower effective cohesion in the crust because of water content – this would be consistent with smaller simple or complex crater diameters.

That was pretty cool – I can see how it would be good to know how the different terrestrial planets are similar and different from each other. That will affect our theories of planet formation as well as any plans for manned missions to those particular bodies.

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   March 21st, 2008 at 2:11 pm

   […] How Rough is Rough? […]

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