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Mercury found to be tectonically active
- dogma1138 10y agoPretty surprising for a tidally locked and that small of a body.
- InclinedPlane 10y agoMercury has a 3:2 rotation vs orbit resonance (it's "day" appears to last 2 years).
- jkot 10y agoIt has highly eccentric orbit, tidal forces must be enormous.
- sidcool 10y agoNoob 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.
- mafribe 10y agoIo, 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
- parr0t 10y agoFascinating. Thanks for the link.
- yathern 10y agoI'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?
- T-hawk 10y agoYes. 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 10y agoEarth 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.
- dspillett 10y ago>(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.
- T-hawk 10y ago"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 10y agoYes, 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.
- Udo 10y agoTo answer the other half of your question (mafribe already said why the moons are tectonically active): what's needed for heating due to tidal friction is not only strong gravity, but a strong gravity gradient that changes over time. I was doing some back-of-the-envelope calculations just now to show how Io experiences much more tidal stress than Mercury due to a stronger change in gradient, when I discovered my assumptions were wrong, but I'm posting the results anyway: Mercury's orbit has a huge eccentricity, and its distance to the sun varies between 0.47 AU at Aphelion and 0.307 AU at Perihelion. Taking Mercury's size into account, that should work out to a difference in solar gravity between its near and far side of about 0.000004 m/s^2 at Aphelion and 0.000013 m/s^2 at Perihelion! That's actually quite the squeeze! Now comparing that to Io, which has often been reported as having this wild orbit, upon looking up the actual numbers I saw that Io's orbit around Jupiter is actually nearly circular: 420000km at Periapsis and 423400km at Apoapsis. The moon itself has a radius of 1800km, that means its orbit is so circular its variation falls within the size of the actual moon. Furthermore, plugging in the numbers, Io's gravity gradient between its near and far side remains pretty much stable at 0.000014 m/s^2 no matter where it is, varying only by a factor of about -1.37x10^-08. So the gradient change itself is not dramatic in Io, but reasonably pronounced in Mercury - didn't expect that. Taking a look at the orbital period of both objects, Io completes about 50 orbits in the same time it takes Mercury to orbit once, but even a factor of 50 isn't enough to blow up Io's tidal action to make up for the observed difference in tidal heating. My only conclusion at this point is that I either made a careless mistake somewhere (which is likely) or that Io's squeezing action must come almost entirely from its interaction with the other Jupiter moons. In the end, a heat source is not required for Mercury to experience tectonic activity though. As described in the article, the cooling and shrinking of the planet itself are sufficient to cause disturbances on the surface.
- mcguire 10y agoShouldn't Mercury have reached thermal equilibrium already?
- Arnt 10y agoIf Mercury is in an eccentric orbit, then the sun's gravity changes depending on where in its orbit is is. Adding to that, when it'a closer, the difference between the gravity on the bright side and dark side is more pronounced than when it's farther away.
- mcguire 10y agoMercury is tidally locked to face the sun; I don't know about the various moons. If they're rotating, tidal flexing from rotation would also heat them up.
- Eerie 10y agoMercury is NOT tidally locked to face the sun. Jeez, are you banned from Google?
- mcguire 10y ago"As seen from the Sun, in a frame of reference that rotates with the orbital motion, it appears to rotate only once every two Mercurian years. An observer on Mercury would therefore see only one day every two years." Apparently, my memory is faulty.
- dsp1234 10y agoThere is a novel by Kim Stanley Robinson whose plot revolves around a city that moves along tracks that circle mercury. With the heating and cooling of the tracks, due to the sun, being the force the drives the city, rather than electricity. It also depicts people walking on the dark side staying just shy of the sun side. https://en.wikipedia.org/wiki/2312_(novel) https://en.wikipedia.org/wiki/2312_(novel)
- astrange 10y agoThat's a very classical approach to scifi, like those Larry Niven stories (Neutron Star etc) where a lone hero man with no friends, but a spaceship, defeats a high school physics class problem. It turns out reading only books about lone men with no friends doesn't prepare you for real life.
- Steuard 10y agoMy understanding is that scientists believed it was tidally locked for quite some time (until the mid-1960s, and not confirmed until Mariner 10 in 1974-5): the resonance between its rotation and its orbit (together with limitations of where in its orbit Earth had a good view of its surface) meant that observers routinely saw the same surface features in the same places. Some of that history is discussed here: https://en.wikipedia.org/wiki/Mercury_(planet)#Ground-based_telescopic_research https://en.wikipedia.org/wiki/Mercury_(planet)#Ground-based_...
- barking 10y agoRidiculously, I never twigged until now that the planet and the element shared a name despite being aware of both, forever.
- inimino 10y agoSomehow 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?
- barking 10y agoI think it's because the context going by the title is earth based for me. Whenever I've heard mention of tectonics it's related to earth and and the only mercury on earth is the element, so my brain zoomed in on that. I remember once asking a german friend if she didn't think that english was better for only having one form of my, the, etc. She said that in german there was less room for confusion because the listener would immediately know whether 'my friend', for example, was a boy or a girl. German is more strongly typed, we rely more on the context in english. It seems that in my experiences Mercury the planet and Mercury the element have never before come up in a context where there could be ambiguity.
- cjhveal 10y agoAnother 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
- theandrewbailey 10y agoNot to mention elements named after planets: Uranium and Neptunium. Even minor planets, too: Plutonium and Cerium.
- T-hawk 10y ago
- leojg 10y agoHow much incidence has its proximity with the Sun? Can the Sun affect it in a similar way Jupiter or Saturn affects its moons?
- Udo 10y agoI 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 10y agoYeah, 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
- jlebrech 10y agoso if planets can shrink, can they grow?
- logfromblammo 10y agoIf 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.
- sevenless 10y agoWhy not put the city at a pole?
- logfromblammo 10y agoDidn'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 10y agoOr 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.