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EDG
2010-Feb-21, 05:57 PM
I'm having a bit of an imagination failure here.

Let's say we have a gas giant that's large enough to have satellites with earthlike density that are the size of Earth, or possibly bigger if they are icy. This is based on the mass ratios proposed by Canup and Ward in their paper "A common mass scaling for satellite systems of gaseous planets" published in Nature v441, pg 834-839, 2006. ( http://www.nature.com/nature/journal/v441/n7095/abs/nature04860.html )

But what would the surfaces of those large moons be like? We're used to small airless rocky bodies such as Mercury and the Moon in our own solar system, and small icy bodies in the outer system (ranging from Miranda to Europa to Ganymede), but what happens if we scale them up?

How would the surface tectonics change as we look at larger moons? We already see a pretty big difference in crater structure as we go from tiny moons like Mimas to our own moon and then to Mercury. What other structures could we see on an airless body the size of Venus or Earth? Would larger moons that had atmospheres that orbited gas giants beyond the snow line essentially be scaled-up versions of Titan?

danscope
2010-Feb-21, 06:56 PM
I should think much would depend on how far these satelites were away from
their local star. Otherwise you would have a cold and unchanging... or certainly slow to change. Water would be ice. Stuff like that.

Romanus
2010-Feb-21, 06:58 PM
I think the most important factor here is the distance of the primary from the central star. If our hypothetical Earth-sized moon orbits a planet within the habitable zone, I don't see why it wouldn't be amenable; it would probably have a pretty dense atmosphere as well. One thing to keep in mind is whether it's the only big moon in the system, or if there are others that could induce tidal stresses that greatly increase tectonic and volcanic activity.

Moving beyond the snow line though, I think things get a little more uncertain. Radiation is one thing to consider--a moon orbiting in a powerful magnetic field would be continually bombarded by high-energy particles that could erode the atmosphere--as well as formation environment. One can't simply model what Earth would be like if we moved it out to Jupiter or Saturn, because it didn't form there. Perhaps an Earth-sized moon that formed around a gas giant beyond the snow line would be a water-rich ocean world, or perhaps a more refractory one.

What I'm trying to get at in this rambling post is that you have a lot of possibilities to work with inside reasonable bounds. If it were up to me, I'd pick a body that gets an insolation equal to about 1/9th terrestrial, roughly equivalent to the middle of the asteroid belt in our Solar System. This would be a distance with a wide variety of potential volatiles, depending on gravity and atmospheric density: water, CO2, and ammonia.

astromark
2010-Feb-21, 07:42 PM
With nothing other than wit as my motive... :)...

It can not happen. Its just simply to random to expect the right balance of available free elements at the tempretures required. At the density required and the absence of deadly radiation forces.

But then there's Earth.... OH :eh:

EDG
2010-Feb-21, 08:59 PM
I'm not talking about orbits or anything though, and I'm not after a specific result here. I guess I was a bit waffly with the original question, but I think it boils down to these questions:

What would an earth-sized rocky, airless world look like? What features could we expect to see that are different to what we see on the smaller worlds we're familiar with?

Similarly, what would an earth-sized icy airless world look like? (granted these would probably be much rarer, since an earth-sized icy world would probably be able to hold onto an atmosphere).

Essentially, how would the tectonic and other surface processes that work on the smaller airless worlds in our solar system scale up if we're looking at an earth-sized version of them? Would an earthsized, airless rocky moon be tectonically active in the same way as Earth? Or would it be more like Venus?

galacsi
2010-Feb-21, 09:14 PM
I'm having a bit of an imagination failure here.

Let's say we have a gas giant that's large enough to have satellites with earthlike density that are the size of Earth, or possibly bigger if they are icy. This is based on the mass ratios proposed by Canup and Ward in their paper "A common mass scaling for satellite systems of gaseous planets" published in Nature v441, pg 834-839, 2006. ( http://www.nature.com/nature/journal/v441/n7095/abs/nature04860.html )

But what would the surfaces of those large moons be like? We're used to small airless rocky bodies such as Mercury and the Moon in our own solar system, and small icy bodies in the outer system (ranging from Miranda to Europa to Ganymede), but what happens if we scale them up?

How would the surface tectonics change as we look at larger moons? We already see a pretty big difference in crater structure as we go from tiny moons like Mimas to our own moon and then to Mercury. What other structures could we see on an airless body the size of Venus or Earth? Would larger moons that had atmospheres that orbited gas giants beyond the snow line essentially be scaled-up versions of Titan?

It's all depends . . . . !

Of the distance from the sun : A big moon ,accreted around a Jovian planet near its sun ,could not have much of water , to the contrary ,beyond the snow line you will got big icy balls.

Of the distance to the gas giant itself in several ways :

in its youth the gas giant is very hot ,is shine like a star and too near moons will be fried and will finish desert body with very few water and atmosphere.

if the Jupiter example can be extrapolate , the inner moons like IO , EUROPA & GANYMEDE will be baked in radiations belts . there is a risk of them being stripped of their atmosphere. (Or not ? )

As the moons will very probably be tidally locked as are the Jupiter's moons , for the farther moons you will have very long days with alternating great colds and great heaths . very tough climate and very strong erosion with great winds and big storms.

Surface tectonics can also be affected by tidal heating and some of these moons can have gig volcanic episodes in their life , like IO today in our solar system .(Mainstream interpretation) Maybe no plate tectonic but big volcanoes like Olympus Mons on Mars or Yellowstone caldera on earth.

And there are many other possibilities like variants of Europa , only bigger and hotter and so on and so on . . .

Edit : Sorry , I forgot Avatar moons , full of unobtainium !

astromark
2010-Feb-22, 04:02 AM
I think the numbers can be found misleading. A larger than Jupiter could have a as large as Earth, moon. What it might look like is up for hundreds of maybe's... In fiction you can make your own rules. As long as the reader can comprehend the environment from your description, then almost anything can work. When building a imagined system. You can have what ever you can explain... Remember that scene from 'Avatar' where the Islands are floating. They incredibly made it expectable. You know its not. See my point ?

EDG
2010-Feb-22, 05:19 AM
Right, but can we make any predictions as to what they'd look like?

It's easy to just say "craters and impact basins" for a rocky earth-sized world, but what about other stuff? e.g Mercury has scarps, would a larger airless planets be likely to have them too? What about the fold belts of Venus? Would we get big volcanoes like on Mars, or chains of smaller ones and fissure zones like we do on Earth? Could we get the sort of crustal dichotomy we see on Earth without liquid water? Would tidal stretching be worse on larger moons than on smaller ones? I'm figuring that large impact basins on the same relative scale as Valhalla on Callisto or the Aitken basin on the Moon would be rare because you'd be less likely to have an impactor big enough to make that kind of scar on an earth-sized world.

As for icy worlds, would the processes that make the bright sulci and other tectonic features on Ganymede scale up with the planet? I'd expect all the icy earthsized worlds to have sub-surface oceans, but what could we possibly see on the surface of such worlds if the ice shell is hundreds of km thick?

I think these worlds would look different from the worlds we know of in our own system (and as it is, each of those are unique anyway), but I'm trying to get an idea of how different they might be if we extrapolated the processes that we know happen on the worlds in our own system upwards to more massive worlds that we haven't seen yet.

(maybe the Orion's Arm guys might have some ideas?)

Ara Pacis
2010-Feb-22, 08:52 AM
Could we get the sort of crustal dichotomy we see on Earth without liquid water?

IIRC, some suggest that the reason for the Earth's crustal dichotomy is due to another planet hitting earth, the Giant Impact hypothesis. The idea is that the impact launched a lot of the lighter mineral layers (felsic material) into an orbit that created the moon and gave the earth an unusually large core. The resulting earth didn't have enough felsic material to cover the planet effectively, hence the mafic rock of the ocean abyssal plains.

cran
2010-Feb-22, 12:29 PM
Right, but can we make any predictions as to what they'd look like?

It's easy to just say "craters and impact basins" for a rocky earth-sized world, but what about other stuff? e.g Mercury has scarps, would a larger airless planets be likely to have them too? What about the fold belts of Venus? Would we get big volcanoes like on Mars, or chains of smaller ones and fissure zones like we do on Earth? Could we get the sort of crustal dichotomy we see on Earth without liquid water? Would tidal stretching be worse on larger moons than on smaller ones? I'm figuring that large impact basins on the same relative scale as Valhalla on Callisto or the Aitken basin on the Moon would be rare because you'd be less likely to have an impactor big enough to make that kind of scar on an earth-sized world.

As for icy worlds, would the processes that make the bright sulci and other tectonic features on Ganymede scale up with the planet? I'd expect all the icy earthsized worlds to have sub-surface oceans, but what could we possibly see on the surface of such worlds if the ice shell is hundreds of km thick?

I think these worlds would look different from the worlds we know of in our own system (and as it is, each of those are unique anyway), but I'm trying to get an idea of how different they might be if we extrapolated the processes that we know happen on the worlds in our own system upwards to more massive worlds that we haven't seen yet.

(maybe the Orion's Arm guys might have some ideas?)

As I remember it, tidal effects arise from the difference (of gravitational attraction) between the nearest and furthest points of the affected body - so, larger diameter should >> larger effect ...

I'd be surprised if an Earth-sized body could not retain some sort of atmosphere; the density and composition would depend on other variables ...

If the Earth-sized body contains significant fluids (again, composition can vary), then something like active tectonics is plausible; though perhaps reduced in activity to orbital interactions after tidal locking -
the nature of the tectonic responses again would depend somewhat on the composition of the solid surface, and the nature of the fluids involved ...

impact craters should be less noticeable over time than on smaller bodies, due to a combination of whatever passes for surface erosion and tectonic processes - large, sharp surface craters would likely be recent ...

basically, to pursue most of your questions, you first want to determine what your Earth-sized body is made of ... and that does suggest first deciding where in the stellar-planetary system it was made ...

.........

Ara Pacis - do you have a source for that utterly incredible idea?

EDG
2010-Feb-22, 04:20 PM
IIRC, some suggest that the reason for the Earth's crustal dichotomy is due to another planet hitting earth, the Giant Impact hypothesis. The idea is that the impact launched a lot of the lighter mineral layers (felsic material) into an orbit that created the moon and gave the earth an unusually large core. The resulting earth didn't have enough felsic material to cover the planet effectively, hence the mafic rock of the ocean abyssal plains.

The difference is caused by chemical evolution (the granitic material of the continental crust forms as a result of subduction of basaltic ocean material and the partial melt rising to the surface), the concept of "not enough material" (aside from making no sense at all) doesn't factor into it at all.

galacsi
2010-Feb-22, 04:34 PM
The difference is caused by chemical evolution (the granitic material of the continental crust forms as a result of subduction of basaltic ocean material and the partial melt rising to the surface), the concept of "not enough material" (aside from making no sense at all) doesn't factor into it at all.

Is , this explanation of the origin of the granititic crust by progressive accretion of partially melt material , able to explain all the granitic crust ? I am septic , because there seems to have been a granitic crust from the beginning (Or am I wrong ? ) And also the moon has a thick crust and has no plate tectonic.

EDG
2010-Feb-22, 05:43 PM
Initially the crust of Earth was basic (komatiites, basalts etc). Granitic Island arcs (like Japan) formed at the subduction zones, then as the plates moved stuff around, the island arcs grew into continents. All the continental material is made of that partial melt that I described above.

(this is probably somewhat over-generalised, but the general sequence of events is correct).

Ara Pacis
2010-Feb-22, 11:36 PM
The difference is caused by chemical evolution (the granitic material of the continental crust forms as a result of subduction of basaltic ocean material and the partial melt rising to the surface), the concept of "not enough material" (aside from making no sense at all) doesn't factor into it at all.

That which you describe is the prevailing theory, I think. I don't recall where I read the hypothesis related to the continents. It was probably several years ago and related to research into the Rare Earth hypothesis. I have found a couple mentions of this, but not from a reputable site, and I am searching for better sources. It may be unrelated, but I did come across a hypothesis arguing against the Giant Impact hypothesis that suggests zircon crystals used to date the continents to an earlier period may be due to an impact event on the far side of the moon instead.

eburacum45
2010-Feb-22, 11:46 PM
Why do you think an Earth-sized moon would be airless? If it has any tectonic and/or volcanic activity, whether caused by tidal effects or not, it will retain an atmosphere. It probably wouldn't be like the atmosphere of Earth or even Venus, but it would be there.

EDG
2010-Feb-23, 12:08 AM
Why do you think an Earth-sized moon would be airless? If it has any tectonic and/or volcanic activity, whether caused by tidal effects or not, it will retain an atmosphere. It probably wouldn't be like the atmosphere of Earth or even Venus, but it would be there.

A couple of possible reasons:

- it's deep within the very powerful radiation belts of its (borderline-brown dwarf) superjovian parent. The charged particles therein sputter off the atmosphere faster than it's formed (exacerbated if the jovian is close to its primary star).

- the jovian is too close to the star for even its earth-mass moons to be able to retain an atmosphere anyway.

But true, in most cases they'd probably at least be able to have a thin atmosphere.

IsaacKuo
2010-Feb-23, 05:30 PM
A couple of possible reasons:

- it's deep within the very powerful radiation belts of its (borderline-brown dwarf) superjovian parent. The charged particles therein sputter off the atmosphere faster than it's formed (exacerbated if the jovian is close to its primary star).
Why do you expect this will have a significant effect on the atmosphere of an Earth-sized moon? Sputtering in the Jovian system wasn't strong enough to deplete water from even the small moons.

- the jovian is too close to the star for even its earth-mass moons to be able to retain an atmosphere anyway.
This doesn't seem likely. Even if it were so close that the volatiles were depleted, that just means you'd end up with an atmosphere of non-volatiles (something with higher melting points).

But true, in most cases they'd probably at least be able to have a thin atmosphere.
All of the planets with Earth-like mass or greater which we know of have thick atmospheres. This includes Venus, which is a lot hotter than Earth.

EDG
2010-Feb-24, 12:20 AM
Why do you expect this will have a significant effect on the atmosphere of an Earth-sized moon? Sputtering in the Jovian system wasn't strong enough to deplete water from even the small moons.

We've been through this already, and it seems your opinion about this is less conservative than mine.


This doesn't seem likely. Even if it were so close that the volatiles were depleted, that just means you'd end up with an atmosphere of non-volatiles (something with higher melting points).

There's a huge gap in molecular weights between gases like CO2 and SO2 and "non-volatiles" like rock or metals, and a huge range of temperature between the boiling point of water and rock. If it can't hold onto the heavy gases, the surface isn't suddenly going to start vapourising to form atmospheric material.



All of the planets with Earth-like mass or greater which we know of have thick atmospheres. This includes Venus, which is a lot hotter than Earth.

That in no way implies that every planet with earth-like mass will have thick atmospheres, it just means that the ones in our own system do. If Venus was closer to the sun and was so hot that it couldn't retain CO2, it wouldn't have a thick atmosphere of CO2 and there probably wouldn't be enough of anything heavier (e.g. SO2) to result in a thick atmosphere either. If it's close enough to the sun then it won't retain any gas that would originate from normal planetary de-gassing, and wouldn't have much of an atmosphere at all.

IsaacKuo
2010-Feb-24, 07:03 AM
We've been through this already, and it seems your opinion about this is less conservative than mine.
What do you base your opinion on? Mine is based on the observation that the sputtering effect in Jupiter's example is too weak to deplete volatiles from even the small moons. If it can't even deplete water from Amalthea, how is it supposed to deplete volatiles from an Earth-like body?

Anyway, according to Amalthea - Implications of the temperature observed by Voyager (http://adsabs.harvard.edu/abs/1983Icar...54..524S), Amalthea radiates slightly more heat than it receives from the Sun because of heat radiation from Jupiter (<9° K), sunlight reflected off of Jupiter (<5° K), and charged particle bombardment (<2° K). All of these effects put together are weak compared to the already weak effect of direct sunlight (at that distance), and charged particle bombardment is weakest of all.

So I really don't see it as being a big factor in causing atmosphere loss in an Earth-like body. A brown dwarf would have somewhat more internal heat, and might have somewhat more of a charged particle bombardment effect, but its radius will still be about Jupiter sized.

And again, we're talking about effects which don't even put a serious dent into the volatile content of small Jovian moons. Even if you scale up the effects to brown dwarf levels...the Earth just plain has a much stronger gravity well than small Jovian moons.

There's a huge gap in molecular weights between gases like CO2 and SO2 and "non-volatiles" like rock or metals, and a huge range of temperature between the boiling point of water and rock. If it can't hold onto the heavy gases, the surface isn't suddenly going to start vapourising to form atmospheric material.
The point is that an Earth mass body is able to hold onto heavy gases. The escape velocity is over 10km/s. In order to make those gases escape on the scale of gigayears, you'd need to heat up the atmosphere to a heck of a lot hotter than Venus, which is already hot enough to melt lead. So yeah, if it's hot enough to make carbon dioxide escape, the surface is going to be hot enough to form atmospheric material out of other substances.

What sort of conditions are you thinking of, anyway? What sort of temperature range?

EDG
2010-Feb-24, 08:25 AM
What do you base your opinion on? Mine is based on the observation that the sputtering effect in Jupiter's example is too weak to deplete volatiles from even the small moons.

Maybe I'm getting confused about the terminology here. It's not so much the sputtering itself that's the problem, it's the sputtered molecules being broken up/dissociated by the incoming charged particles, which would make them easier to be lost to space.



The point is that an Earth mass body is able to hold onto heavy gases. The escape velocity is over 10km/s. In order to make those gases escape on the scale of gigayears, you'd need to heat up the atmosphere to a heck of a lot hotter than Venus, which is already hot enough to melt lead.

Hm, doing a bit of number-crunching shows that Earth would have to be moved to about 0.015 AU from the sun before it was unable to hold onto CO2 over gigayears, at which point its surface temperature would be over 2200 K. So it looks like you have a point there.

cran
2010-Feb-24, 09:49 AM
Damn! had the reply ready, and a local blackout blew it away ...


... There's a huge gap in molecular weights between gases like CO2 and SO2 and "non-volatiles" like rock or metals, and a huge range of temperature between the boiling point of water and rock. If it can't hold onto the heavy gases, the surface isn't suddenly going to start vapourising to form atmospheric material...


Not so huge a gap as you seem to think; it's not necessary to melt or vaporise whole rock to contribute to an atmosphere - a partial melt of the lower temp minerals and/or native elements is sufficient ... even on cool old Earth ... another thing to keep in mind is that the whole surface doesn't need to be hot enough, as long as some locations are (eg, we don't need an average surface temp of 100C to pump steam into the atmosphere) ...

of the not-rare elements, sodium (a la Mercury) has a very low melting point for a metal; on Venus, lead sulfide appears to have the equivalent of a minor water cycle; sulfur generally would have a role in the atmospheres of hot Earths and, if you push the temp up another ~250K on a Venus-like world, silicon would also be a player ... and CO2 is pretty resilient, as well as relatively heavy for a volatile ...

cran
2010-Feb-24, 09:58 AM
[off-topic]


That which you describe is the prevailing theory, I think. I don't recall where I read the hypothesis related to the continents. It was probably several years ago and related to research into the Rare Earth hypothesis. I have found a couple mentions of this, but not from a reputable site, and I am searching for better sources. it sounds like something linked to the old "Moon was erupted from where the Pacific Ocean now is" fantasy ...


It may be unrelated, but I did come across a hypothesis arguing against the Giant Impact hypothesis that suggests zircon crystals used to date the continents to an earlier period may be due to an impact event on the far side of the moon instead.oh? ... this gets better and better ...
but pursuing it here would not only be somewhat OT, but also very ATM ...
but I await the grand announcement with interest ... [/off-topic]

Ara Pacis
2010-Feb-25, 12:54 AM
[off-topic]

it sounds like something linked to the old "Moon was erupted from where the Pacific Ocean now is" fantasy ...

Possibly, but while the conclusions may be similar, the proposed mechanisms are not.


oh? ... this gets better and better ...
but pursuing it here would not only be somewhat OT, but also very ATM ...
but I await the grand announcement with interest ... [/off-topic]

What grand announcement?

Not sure it would be OT, the Original Poster asked about tectonic and surface issues besides atmospheric issues.

chornedsnorkack
2010-Feb-26, 04:20 PM
How hot can a planet be and still have solid surface?

Mercury´s Caloris Basin and chaotic terrain opposite heat over 400 Celsius at perihelion inside 0,3 AU of Sun seem to have remained for milliards of years. Venus, about 500 Celsius under atmosphere, has mountains and relief - how old?

At which surface temperature would the surface rocks be soft enough for mountains to flow away through plastic creep/glacier flow? Not just lava flow in matter of minutes, but over millions of years (the timescale for tectonics)?

EDG
2010-Feb-26, 10:54 PM
REALLY hot... the melting point of basalts is around 1200C (so about 1500 K), you'd probably have to well get over 1000K for them to start flowing (atm pressure could make a difference to the melting point though, I'm not sure how that would work exactly though).