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KTevolved
2012-Sep-19, 04:55 AM
So Earths crust is made up of basalt, igneous, sedimentiary and metamorphic rocks. So could there be an entirely different form of rock on an exoplanet crust? Something truly out of this world like maybe a metallic glass type of rock. Just over all alien to anything in the solar system

astrostu
2012-Sep-19, 05:39 AM
Hmm. Hard to speculate on this sort of thing. We have dozens of terrestrial-type objects in our own solar system and those are the rock types we see (sedimentary, igneous, metamorphic, conglomerate -- basalt is igneous). I'd say it's unlikely that a major form of rock exists elsewhere that we don't have here, but you can never prove a negative except maybe in math.

That's not to say you couldn't create it for sci-fi. Though, as with anything, how would you create your substance? How would "metallic glass" be created naturally, do you think? Maybe super-heating metal and silica and then cooling it in some way?

willstaruss22
2012-Sep-19, 05:52 AM
I was just interested because there are a huge number if different processses in solar system formation with different amounts of chemicals and all. Yea thats what i was thinking. Maybe think planet would form very close to the star and then it getting flung into the very outer part of the solar system.

astrostu
2012-Sep-19, 05:54 AM
I've been watching Stargate Universe and there's an episode with a hyper-advanced race (don't show them) that created an entire star system, or at least a planet. So that's another option if you were musing for writing purposes.

Ara Pacis
2012-Sep-19, 05:54 AM
Aren't most metals in crystal form if they aren't in some other sort of chemical bond? Do you want amorphous, like maybe metal foam?
http://en.wikipedia.org/wiki/Metal_foam

Or do you mean transparent, like Star Trek's Transparent Aluminum? Besides corundum (http://en.wikipedia.org/wiki/Corundum), there's Aluminum Oxynitride (http://en.wikipedia.org/wiki/Aluminum_oxynitride) and some new material created recently with similar properties:
http://en.wikipedia.org/wiki/Transparent_aluminium#Transparent_aluminum

Nereid
2012-Sep-19, 09:57 AM
Here's a somewhat left-field thought: in the solar system, oxygen dominates carbon (and nitrogen), so rocks are made up - almost entirely - of oxides, or compounds containing oxygen. Even the carbon is - mostly? often? - in the form of carbonates. Ignoring 'native' forms, such as iron and gold, and metal mixtures/solutions/compounds (e.g. nickel-iron). Oh, and the odd sulphide, etc.

But what if a protoplanetary nebula were (highly) deficient in oxygen (just suppose; maybe it was born from a particular kind of Type 1a supernova?), dominated by carbon (and nitrogen)? What would the geology of solid bodies in such planetary systems be like? Sure, there'd still be 'iron' cores, and 'native' minerals, sulphides, etc; but what if carbides and nitrides dominated?

For the icy bodies, in the solar system we have water, ammonia, and dry ice (other ices too?); in an oxygen-deficient system the icy bodies would be composed of ammonia, and ...? Not methane, because it doesn't get cold enough for it form ices, does it? Cyanogen?

As far formation processes are concerned:

- what would sedimentary rocks, formed in liquids other than water, be like?

- how common would - could - reverse sublimation be? I know that it's important for the martian polar caps (CO2), and sulphur forms this way in terrestrial volcanoes (and, presumably, on Io); does it produce dramatically different rock types?

- for igneous rocks, the energy source is interior to the surface (mostly); what sort of rock forms would result from an exterior energy source? I'm thinking of a planet very close to its homesun, tidally locked, maybe without an atmosphere; or perhaps in a highly elongated orbit

- planet-wide shock formation, e.g. due to the blast from the homesun going nova (not supernova, that'd vaporise the planet)

Nereid
2012-Sep-19, 12:27 PM
Must not have had enough coffee ... aeolian processes certainly modify rocks, and can, even on Earth, produce rock formations (dunes, loess, etc). They must be important on Mars, and may be on Titan too (how many other solid surface solar system bodies have dense enough atmospheres? Triton, maybe Pluto; Venus?). Perhaps there are planetary system bodies on which such processes dominate, in terms of "form of rock on an exoplanet crust"?

On Earth, ice is usually not regarded as a 'rock type', but it is - often - deposited by reverse sublimation, even directly (frost). Chill an Earth-like planet down a bit, and this process will likely dominate, in terms of the form of rock (ice in this case) on an exoplanet crust (a snowball planet ... aeolian processes may be important on such a planet too).

Then there's whatever 'rock type' the lunar regolith is. This 'rock type' dominates the surface of many airless solar system bodies, doesn't it? And it's also important on Earth, and possibly on Venus ... though the formation processes differ greatly. Could there be exoplanets (or exomoons or exoasteroids) where the composition, temperature, and pressure are such that regoliths become quite rock-like (high mechanical strength), other than by liquid-mediated transport and deposition (i.e. not sedimentary processes)? Perhaps some form of sintering, or chemical cementing ... or even a biologically-mediated process ...

grapes
2012-Sep-19, 12:52 PM
an exoplanet crust (a snowball planet ... aeolian processes may be important on such a planet too).The Earth itself may have gone through a snowball phase.


So Earths crust is made up of basalt, igneous, sedimentiary and metamorphic rocks. So could there be an entirely different form of rock on an exoplanet crust? Something truly out of this world like maybe a metallic glass type of rock. Just over all alien to anything in the solar systemAs astrostu mentions, basalt is igneous (but conglomerate is also probably sedimentary).

The crust does contain some (small) pure metallic deposits, and there are large glass (obsidian, for instance) fields, but they still fall into the igneous-sedimentary-metamorphic classification.

Nereid
2012-Sep-20, 10:56 AM
You have before you an image of a solid body surface, part of an object in orbit around a star, or a planet. It is - you are told - large, as in >100 km in diameter.

There are many circular (or nearly so) features, which you determine are depressions (perhaps you are told there's a uniform lighting source, stage left); there are also other depressions that are not at all circular. And there are positive topographic features too.

Can you tell if the solid surface is composed of rock? of ice? of glass? of metal? (any other form of solid??)

To what extent does the "igneous-sedimentary-metamorphic classification" require the solid surface to be composed of rock (or glass or metal)? Don't these three refer to how the solid was formed (solidified from the liquid state; deposited in, or by, a liquid and later compacted/compressed/cemented; mashed, smashed, squished, softened, etc)? If so, then for completeness there should also be something like aeolian, and something to capture reverse sublimation, shouldn't there?

One more: a solid surface may be composed of loose material - dust, grains, 'sand', 'pebbles', etc - and such a surface may be very deep. The transition to a more rigid surface may be abrupt, or gradual; think of a layer of volcanic ash over a smooth granite surface, vs powder snow gradually becoming solid ice. How to fit such a surface into the above type of classification scheme?

grapes
2012-Sep-20, 12:53 PM
To what extent does the "igneous-sedimentary-metamorphic classification" require the solid surface to be composed of rock (or glass or metal)? Don't these three refer to how the solid was formed (solidified from the liquid state; deposited in, or by, a liquid and later compacted/compressed/cemented; mashed, smashed, squished, softened, etc)? If so, then for completeness there should also be something like aeolian, and something to capture reverse sublimation, shouldn't there?
Such broad classifications serve a small purpose--getting one to think hard how some outlier may fit into the accepted paradigm. :)

Still, aeolian and precipitates are generally accepted as sedimentary rock--even pyroclastic flow products can be:
http://en.wikipedia.org/wiki/Sedimentary_rock#.22Other.22_sedimentary_rocks


One more: a solid surface may be composed of loose material - dust, grains, 'sand', 'pebbles', etc - and such a surface may be very deep. The transition to a more rigid surface may be abrupt, or gradual; think of a layer of volcanic ash over a smooth granite surface, vs powder snow gradually becoming solid ice. How to fit such a surface into the above type of classification scheme?Is sand a solid? :)

Kidding, sorta. A small grain of sand could be said to be a rock, but taking it farther down removes it from the realm of the classification. The igneous/sedimentary/metamorphic classification scheme is used for coherent and rigid rock, more or less.

cran
2012-Oct-17, 08:59 PM
For the icy bodies, in the solar system we have water, ammonia, and dry ice (other ices too?); in an oxygen-deficient system the icy bodies would be composed of ammonia, and ...? Not methane, because it doesn't get cold enough for it form ices, does it? Cyanogen?It gets cold enough on Earth to form submarine methane ice. There should be plenty in ice planets and satellites.


You have before you an image of a solid body surface, part of an object in orbit around a star, or a planet. It is - you are told - large, as in >100 km in diameter.

There are many circular (or nearly so) features, which you determine are depressions (perhaps you are told there's a uniform lighting source, stage left); there are also other depressions that are not at all circular. And there are positive topographic features too.

Can you tell if the solid surface is composed of rock? of ice? of glass? of metal? (any other form of solid??)Just by a standard light image? Possibly not, especially as naturally formed objects tend to be composed of more than one mineral. Spectral imaging should give some clues, as would mass and density data.


To what extent does the "igneous-sedimentary-metamorphic classification" require the solid surface to be composed of rock (or glass or metal)? Don't these three refer to how the solid was formed (solidified from the liquid state; deposited in, or by, a liquid and later compacted/compressed/cemented; mashed, smashed, squished, softened, etc)? If so, then for completeness there should also be something like aeolian, and something to capture reverse sublimation, shouldn't there?

One more: a solid surface may be composed of loose material - dust, grains, 'sand', 'pebbles', etc - and such a surface may be very deep. The transition to a more rigid surface may be abrupt, or gradual; think of a layer of volcanic ash over a smooth granite surface, vs powder snow gradually becoming solid ice. How to fit such a surface into the above type of classification scheme? Commonly called "loose cover", surficial cover, alluvium or regolith. Desublimation is simply another form of deposition.

Unconsolidated particles like sand and ash are considered granular fluids in behaviour and loose or surficial cover in stratigraphic classifications; not a rock type.

And yes, rock classifications apply regardless of the minerals involved. So, the solid ice surface of Europa is igneous in origin, and the visible surface of Io is loose cover.

IreneAnt
2012-Oct-23, 06:15 PM
I can't believe I missed this thread the first time around. Thank you cran for reviving it.

Here is some perspective from a planetary geologist's point of view.
- There really are only 3 processes for forming surface materials on a planet; igneous, sedimentary, and metamorphic.
- The composition of the "rocks" (metal, silicate, water, etc.) is irrelevant. If it crystallized from a liquid, it's igneous. If it formed from a bunch of sediments or precipitated from a liquid/vapour, it is sedimentary. If it was heated or compressed enough to recrystallize (but not be melted), it is metamorphic.
- That said, classification of these processes can be quite complex and intertwined. For example:
- Volcanic ash can be both igneous (in that the ash crystallized from a liquid melt) and sedimentary (in that some ash particles solidify in flight and are then cemented after they land).
- Impacts can produce both sedimentary materials (ejecta) and igneous materials (impact melt).
- Sedimentary materials can grade into metamorphic materials as the pressure of the overlying sediments allows for recrystallization.
- The ambient temperature (and pressure) at the surface of the planet is very important. On Earth, water is liquid and so is involved in sedimentary processes. On Europa, water is a solid and so any evidence of liquid water implies igneous processes. One could conceive of a planet that is so close to the sun where metal is liquid and so is involved in the sedimentary process by moving around unmelted silicate sediments.

Also, I wanted to tie up some concepts that have been bouncing around this thread (kudos to grapes and cran for addressing some aspects of these very well).
- The origin of the energy for these processes is irrelevant. Impacts provide an external energy source and can produce igneous impact melts.
- Sedimentary processes don't require liquid. Aeolian and impact processes are very much sedimentary. It doesn't matter how the material is moved around, just that it is.
- Sediments themselves (sand, lunar regolith, snow, etc.) are not rocks. They must be lithified into a coherent mass in some way to be considered rocks. This is usually accomplished by the weight of the overlying sediments (so sediments will grade into sedimentary rocks just like sedimentary rocks grade into metamorphic rocks) and is fairly common on all planetary surfaces (so no need to invoke exotic planets with unusual pressure/temperature regimes).
- Chemical cementation and sintering are sedimentary processes. So are most biological cementation processes.
- A 100 km diameter planetoid is pretty small for most internal processes to function. At this size, you wouldn't expect much except impact processes to be evident on the surface. To get a really good comparison to Earth-like planets, you need to consider something on the order of 10,000 km in diameter.
- It is often no possible to tell, based on morphology alone, whether the surface you are looking at is ice or silicate or even metal, since all these materials behave fairly similarly at temperatures where they are "hard". So, an impact into a solid icy crust (like Europa's) tends to look very similar to an impact into a solid silicate crust (like the Moon's).

I hope this was helpful. If anyone has more questions on this topic, do not hesitate to ask....

Icefox
2012-Nov-01, 08:59 PM
On Earth, ice is usually not regarded as a 'rock type'...

But, technically speaking, it IS a rock. Ice is the most common mineral on Earth's surface. People just don't usually think of it that way because it behaves differently from other rocks, what with melting at 0C instead of 1200C. :-)



Here's a somewhat left-field thought: in the solar system, oxygen dominates carbon (and nitrogen), so rocks are made up - almost entirely - of oxides, or compounds containing oxygen. Even the carbon is - mostly? often? - in the form of carbonates. Ignoring 'native' forms, such as iron and gold, and metal mixtures/solutions/compounds (e.g. nickel-iron). Oh, and the odd sulphide, etc.

But what if a protoplanetary nebula were (highly) deficient in oxygen (just suppose; maybe it was born from a particular kind of Type 1a supernova?), dominated by carbon (and nitrogen)? What would the geology of solid bodies in such planetary systems be like? Sure, there'd still be 'iron' cores, and 'native' minerals, sulphides, etc; but what if carbides and nitrides dominated?)

I'm backtracking a bit, but this caught my interest... This idea is not as far in left field as you may think! Astronomers and geologists have long known that the element abundances on Earth (and other terrestrial planets, as far as we know) are roughly similar to the Sun, and that some stars have element abundances very different from the Sun. So, if these stars with weird amounts of elements have planets, those planets should also have weird amounts of elements. So from what we know, there SHOULD be planets out there with just as many rocks made of silicon carbides as silicon oxides, or with drastically lower amounts of magnesium (a major part of Earth's mantle), or possibly other weird stuff going on! You're still limited to the things that might be easily cooked up by supernovae, so you're unlikely to get a planet extremely rich in lithium or something, but still.

Anyway, People are working on looking for such stars which have planets around them, and trying to figure out what such planets might look like. Right now, the answer is "We have no idea!", since people have only known such planets exist at all for a few years.

Some paper abstracts by people working on this problem:
http://iopscience.iop.org/2041-8205/747/1/L2
http://adsabs.harvard.edu/abs/2009DPS....41.0501B

cjameshuff
2012-Nov-01, 11:22 PM
Aren't most metals in crystal form if they aren't in some other sort of chemical bond? Do you want amorphous, like maybe metal foam?

Metallic glasses (http://en.wikipedia.org/wiki/Amorphous_metal) are amorphous, but something completely different from foams.

Glassy phases of materials that are usually seen in crystalline form might be common on the surface of a volcanically active terrestrial planet that is far from its sun. Think blobs of lava falling onto nitrogen ice.

cjameshuff
2012-Nov-02, 12:31 AM
Also, vapor phase processing could give some interesting new results. On Earth, it's pretty much limited to sulfur and water, but you might get a much more diverse range on a hot planet in close orbit, particularly if it spiraled in or the sun is entering a red giant phase. (it may be rather depleted in volatile elements if it formed close to the sun...look at Earth's loss of tellurium, for example)

There may be interesting things in the cooler regions near Mercury's poles...and we've seen much hotter planets out there. This is also part of why the idea of surface exploration of Io interests me.