Does Olympus Mons cause Mars to wobble?

05 June 2011

Question

I've been an avid listener for only the past month or so when I've been listening to the podcasts on my walk to and from the library everyday to revise, and am absolutely loving it so far and have managed to get through a good 30 or 40 of the latest podcasts in this short period of time... Anyway just a few things I was wondering if you could help me with:

Since Mars is comparatively small to Earth and yet has the largest mountain in the solar system; is Olympus Mons relatively large enough to cause Mars to wobble on its axis?

Thanks very much for your time

Dom Parker

Answer

Dominic F. - It certainly does make Mars wobble on its axis. Olympus Mons is a massive volcano. It's, I think, three times higher than Mt. Everest. What's important when Mars is rotating on its axis is where Mars' centre of mass is. So that's the average point where all of Mars' little bits of mass average towards. If Mars has a large mass on one side of the planet, then that will knock Mars' centre of mass off the centre of the spherical planet. So it will be rotating around offpoint which is slightly off the centre of the sphere. And that means, as it rotates, the sphere will shift slightly in space.

It's a bit like having two figure skaters on ice and one of them is really massive, it's spherical Mars, and the other one is not very massive and that's the volcano. And as they spin around on the ice, the massive figure skater won't move very much, but you'll see a little wobble each time he goes around.

Now, that's not quite what we're talking about when we normally talk about volcanoes on the Earth, causing a shift in the Earth's axis. What we're talking about there is movements of rock, either because volcanoes erupt or because we have earthquakes, and those movements shift at the centre of the mass of the earth, and that changes the way in which the earth rotates.

Chris - If you would like a good reference on what Dominic was saying, I was very lucky to speak with someone at Harvard a few years ago called Taylor Peron who actually did some interesting modelling on this and showed that when the sea would've been on Mars millions of years ago, this ancient ocean, it produced a shoreline. And if you trace the outline of that shoreline today, you see that in some places, it's maybe a kilometre higher than other bits of the shoreline which suggests either that the water had some very strange tidal movements, which seems unlikely, or more likely, the surface of Mars has been buckled in some way. When they modelled it, they found the buckling was directly explained by the migration of Olympus Mons in exactly the way you say Dominic, pulling the planet surface to put Olympus Mons on the equator of the planet because of that huge aggregation of mass there. So it's Taylor Peron and it was a paper in Nature, I think from 2006 issue if you want to look it up.

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