
UCLA scientist discovers plate tectonics on Mars - kposehn
http://www.washingtonpost.com/blogs/innovations/post/ucla-scientist-discovers-plate-tectonics-on-mars/2012/08/10/d03186aa-e306-11e1-98e7-89d659f9c106_blog.html
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allardschip
The scientist in the article claims that Mars still has tectonic movements
which strikes me as odd. I thought the consensus on Mars tectonics was that it
has stopped a long time ago. If there is movement than the core must be liquid
and moving. If its core is liquid and moving it would generate a magnetic
field. Just like our liquid core creates the magnetic field around earth.

The thing is.... Mars has no significant magnetic field like we have around
our home planet. It's also the reasons why more of the sun's radiation gets
through to the surface on Mars than on earth. Our magnetic shield deflects
radiation, Mars does not have that shield.

It's believed that Mars lost it's liquid core because it's smaller than earth
and therefore has a higher surface to volume ration. That caused it to cool
off a long time ago.

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mbell
On Earth the tectonic plates are only 'crust thick', except for areas where
they are subducted into the mantle. A liquid substrate is not a requirement
for plate movement.

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allardschip
Interesting but I am not getting it. How does plate movement happen without a
liquid substrate?

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mbell
Two materials with different densities and strengths will move differently
under the similar forces. As for more detail, wikipedia provides a nice start:
<http://en.wikipedia.org/wiki/Plate_tectonics>

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allardschip
Thank you. So different densities are sufficient, not liquidity? From the same
article on Wikipedia: "Dissipation of heat from the mantle is acknowledged to
be the original source of energy driving plate tectonics, through convection
or large scale upwelling and doming.". Mars is thought not to have the
dissipation of heat any more hence the lack of tectonic movement?

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mbell
I'm not a geologist much less an astro-geologist so I can only offer what I've
picked up on this topic through curiosity along the way but here is my
understanding:

Tectonic movements are caused by temperature, density and other material
differences, upwelling as you've mentioned and I've also heard that
gravitational influences are important, effects of the moon, sun and earth
gravity vs the density layout causing strain.

My understanding of the Mars issue is that the planet cooled off much faster
than the Earth did, as a result there isn't as much thermal induced stress and
upwelling going on. As for why Mars cooled off faster, I don't think anyone
knows definitively. Mars has very little atmosphere so it doesn't retain much
heat. It is also much smaller than the Earth so it would be expected to cool
off faster under equal conditions. On the other hand Venus is also smaller
than the Earth but is one seriously hot place, and not completely due to its
proximity to the Sun.

So why doesn't Mars have an atmosphere?

Well, it has little to no magnetic field so it is very poorly protected from
solar winds, a massive solar ejection could have just ripped the atmosphere
off. On the other hand maybe Mars used to have a magnetic field and it was
only lost due to cooling effects that occur even with an atmosphere. Using
Venus again as a counter point is also has nearly no magnetic field yet it
maintains an extremely dense atmosphere so there is no 1:1 correlation with
not having a magnetic field and not having an atmosphere.

I don't know of any consensus on what it takes during planet formation to end
up with an atmosphere to retain heat and a magnetic field to protect that
atmosphere, I've heard lots of theories but its still an open question to be
answered.

