
Mercury found to be tectonically active - azazqadir
https://www.nasa.gov/feature/the-incredible-shrinking-mercury-is-active-after-all
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sidcool
Noob question. Aren't the moons of Jupiter and Saturn also tectonically
active? With the immense solar pull, Mercury would have some heavy sheer
forces causing plates to move.

I am sure I am missing something. Please enlighten me.

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mafribe
Io, Jupiter's innermost moon and the most volcanically active body in the
solar system, is interesting because its source of heat is believed to be
tidal heating: Jupiter's gravity pulls Io inwards while Jupiter's other moons
pull Io outwards. This leads to a weird, eccentric orbit, which in turn means
changing levels and directions of gravity leading to movement of Io's mass,
e.g. molten rock under the surface. This causes friction that heats Io, whence
volcanism.

[https://en.wikipedia.org/wiki/Tidal_heating_of_Io](https://en.wikipedia.org/wiki/Tidal_heating_of_Io)

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yathern
I'm no physicist, but I'm curious where the heat comes from,
thermondynamically speaking. Does this changing level of gravity gradually sap
the speed of Io?

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T-hawk
Yes. It comes from the orbital kinetic energy of Io and the other moons.

The same happens in the Earth-Moon system. The Moon causes tides on Earth's
oceans, which increase friction between water and land and therefore produce
heat. Thermodynamically speaking, that heat comes from the Moon's orbital
momentum, as it recedes from Earth and slows in its orbit.

~~~
mafribe
Earth has also got a small bit of tidal movement of the crust, but not
remotely as much as Io. There there is a theory that tidal movement of the
Earth's crust is one (of many) factors that can trigger earthquakes.

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dspillett
_> (of many) factors that can trigger earthquakes._

Or slowly feed in energy that is stored and released when something else
pushes the system far enough out of equilibrium that a quake happens.

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T-hawk
"Feed in energy" \- that energy has to come from somewhere. The point of this
subthread is where does that energy come from thermodynamically? The answer is
orbital or rotational kinetic energy via tidal forces, which of course do
affect both crust and water.

~~~
mafribe
Yes, but different celestial bodies get there energy from different sources.
For the earth it's mostly radioactive decay in the core of the planet.

In others, like Io, it's believed to be mostly tidal friction.

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logfromblammo
If Mercury is tectonically active, that spells trouble for Kim Stanley
Robinson's sci-fi city on rails, Terminator, continuously rolling away from
the dawn, powered by the thermal expansion from direct sunlight.

If you need the rails running around the equator to remain straight in order
to keep moving at a constant rate, all your railroad ties now have to
compensate for the shifting plates.

At least that story accounted for Mercury not being tidally locked to the Sun.

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sevenless
Why not put the city at a pole?

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logfromblammo
Didn't fit the story, apparently.

The poles are probably the best option for the first Hermean habitat, because
the low axial tilt means that the poles are always a survivable temperature,
and some polar craters are permanently shaded, with water ice inside.

~~~
sevenless
Or just live underground. There should be zones at around room temperature not
too far off the surface.

Colonizing Mercury makes more sense than colonizing Mars. _Much_ more abundant
energy, reasonable gravity, plenty of water at the poles, and to avoid
radiation you'd have to live underground in either place. The only drawback is
the higher delta-V to get there.

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jkot
It has highly eccentric orbit, tidal forces must be enormous.

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leojg
How much incidence has its proximity with the Sun? Can the Sun affect it in a
similar way Jupiter or Saturn affects its moons?

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Udo
I tried some calculations to answer that
([https://news.ycombinator.com/item?id=12589228](https://news.ycombinator.com/item?id=12589228)).
Just taking into account the parent body, Mercury should experience a much
more pronounced variation in its tidal gradient than does Io. However, I
suspect it's the interaction with the other moons that make up for it in Io's
case.

~~~
leojg
Yeah, now I recall that Io huge volcanic activity is thought to be because is
squashed and pulled by Jupiter in one side and the other moons in the other

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dogma1138
Pretty surprising for a tidally locked and that small of a body.

~~~
InclinedPlane
Mercury has a 3:2 rotation vs orbit resonance (it's "day" appears to last 2
years).

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jlebrech
so if planets can shrink, can they grow?

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leojg
Actually Earth adds a few tons of mass a day by stuff hitting it(meteorites,
space junk, etc)... of course is really little compared to the huge size of
the Earth!

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barking
Ridiculously, I never twigged until now that the planet and the element shared
a name despite being aware of both, forever.

~~~
inimino
Somehow I was also wondering what the element mercury could possibly have to
do with tectonics, until seeing the NASA domain... Perhaps because the element
is just more commonly mentioned than the planet?

~~~
cjhveal
Another possible contributing factor is that Mercury/mercury is a capitonym[0]
in that it changes its understood meaning via capitalization, which is
obviously obscured when it's used in a headline or as the first word in a
sentence.

[0]:
[https://en.wikipedia.org/wiki/Capitonym](https://en.wikipedia.org/wiki/Capitonym)

