SFRPG

The forum for Science Fiction Role Playing Game inspiration and information! So Say We All!
It is currently Tue Dec 18, 2018 10:59 am

All times are UTC




Post new topic  Reply to topic  [ 14 posts ]  Go to page 1 2 Next
Author Message
PostPosted: Thu Sep 22, 2016 3:57 pm 
Offline

Joined: Thu Sep 18, 2008 12:41 pm
Posts: 409
Location: Northern California
So recently, I was playing around with one of the programs that discusses what would happen if Earth were to be hit by a body of a given size, traveling at a given velocity.

I've recently been wondering what happens to the planet afterwards (this isn't a discussion about if there's any life on the world afterwards, I don't care about that).

For the sake of simplicity, we'll start with Earth. Again, it doesn't matter if it is actually Earth or just some Earth-mass planet. Things like water or life don't matter.

So if this hypothetical Earth were to be hit by something with sufficient mass and velocity, there'd be sufficient energy to simply blow the planet apart into fragments. Now, here is the part I am curious about. Assuming that the world had a molten core (like ours) a lot of the fragments would probably be sufficiently soft to form into planetoids of various sizes. What would happen to the more firm crust? Would there have been sufficiently energy imparted by the collision (but before the breaking) that the crust would have been rendered molten as well? Or would there be jagged pieces of former-surface floating around (or more likely, flying around)?

The next idea is, what happens if the planet is hit by something pretty huge and energetic, but not quite large enough to simply outright destroy the planet? I've read a theory that Mars was likely may have been hit by something like that and similarly that the Earth was once hit by something like that, the mass was which later become our moon. I'm not sure about how valid these theories remain among scientists. However, my curiosity is if an Earth was hit by something like that around our present time (in regards to having a hard crust, molten center, and so on) ... what would happen to the Earth? Say the collision was energetic enough so that the Earth would lose about 10% of its mass (assuming it could without being blown apart completely). I'd imagine that such a collision would still render the entire surface molten once more. I'm curious if that'd really change the surface of the planet in that case - if the Earth has a molten surface, wouldn't that simply smooth out the planet back into a more-or-less spherical shape or would the crust cool too soon and the planet left lopsided?


Top
   
PostPosted: Thu Sep 22, 2016 6:34 pm 
Offline
Administrator
User avatar

Joined: Thu Jul 03, 2008 1:22 am
Posts: 5329
Keep in mind that the crust is generally pretty thin, so when it comes to a huge (planetary-impactor scale) collision it's essentially not that different from the mantle. What you'd end up with is probably a cloud of debris that mostly would pull itself back together again (without the parts that flew off at greater than escape velocity) and re-melt and re-differentiate to an extent.

If it doesn't break up the planet, then the impact would render the entire surface molten (and probably a few hundred km down as well). Think about the Giant Impact that made the moon, for example. It would impart so much energy into the protoearth that the entire surface layer would melt (and eventually it'd cool down a new crust would form, atmosphere would be outgassed, water vapour would condense out of the superheated steam atmosphere etc).

It also wouldn't end up being 'lopsided' - the mantle isn't solid, it's 'plastic' so it can flow to smooth things out. And the molten surface above it obviously would as well. So what you'd end up with is just a normal smaller, spherical planet.

_________________
SFRPG Owner/Admin
This post (or any other post I made here) may not be quoted or copied beyond the SF RPG boards without my explicit permission.
evildrganymede.net - visit the The Worldbuilding Hub
Check out my Youtube channel for all my streamed gaming videos!


Top
   
PostPosted: Thu Sep 22, 2016 9:25 pm 
Offline
User avatar

Joined: Tue Jul 08, 2008 8:42 pm
Posts: 2732
Location: Texas, USA
Regarding your theory about the Moon being formed by such a planetary collision, this is the prevailing theory. A Mars-sized body struck the proto-Earth a glancing blow. Much of the mantle was blown off and formed a ring around the new Earth. The Mars-sized body was absorbed into the new Earth and the ring of material condensed into the Moon. The rocks brought back from the Moon confirm this theory as they match the material from the Earth's mantle.

Regarding Mars: It IS a bit lobsided - there are highlands on one side of the planet and lowlands on the other with a CRACK in the middle. IF Mars had been solid when struck, the mantle would have been solid and so would the core. Could that situation make a world SLIGHTLY lobsided? I don't know.

_________________
My friends call me Richard. You can call me Sir.
www.XmasDragon.com


Top
   
PostPosted: Thu Sep 22, 2016 10:31 pm 
Offline
Administrator
User avatar

Joined: Thu Jul 03, 2008 1:22 am
Posts: 5329
The 'crack' (Vallis Marineris) came way after the giant impact on Mars that caused the northern lowlands (if indeed that is what caused it). And I think even Mars' mantle is plastic enough to flow (pretty sure it's not all completely frozen solid in there, and there'd be a geotherm anyway).

_________________
SFRPG Owner/Admin
This post (or any other post I made here) may not be quoted or copied beyond the SF RPG boards without my explicit permission.
evildrganymede.net - visit the The Worldbuilding Hub
Check out my Youtube channel for all my streamed gaming videos!


Top
   
PostPosted: Thu Sep 22, 2016 11:47 pm 
Offline
User avatar

Joined: Sun Jul 06, 2008 1:04 pm
Posts: 1092
Location: the Retirement Coast, NSW, Australia
EDG wrote:
The 'crack' (Vallis Marineris) came way after the giant impact on Mars that caused the northern lowlands (if indeed that is what caused it). And I think even Mars' mantle is plastic enough to flow (pretty sure it's not all completely frozen solid in there, and there'd be a geotherm anyway).

Being frozen solid makes some difference, but it's not an absolute. Even the strongest structural materials have compressive strengths of of only hundred of meganewtons per square metre. Whereas crustal material has a weight of 7–10 kN per square metre even under only 0.38 gee. Below on the order of 10 kilometres the pressure exceeds the compressive strength of any material, all minerals flow, and on a scale of kilometres everything is effectively a liquid. A viscous liquid, yes. That flows slowly, yes. But on a timescale of tens of millennia all planets are liquid drops.

Earth's mantle material is solid in the sense that transverse and shear waves will propagate through it, in contrast with Earth's outer core, which is liquid in the sense that only compressive waves will propagate through it. Do not be mislead by lava flows, nor by post-glacial rebound, nor by mantle convection. Earth's mantle is not molten; it is a solid that flows because the stresses exceed its compressive strength.

_________________
— Brett Evill

My SFRPG setting, Flat Black

© My posts on SFRPG must not be reproduced beyond the board except with explicit permission from me.


Top
   
PostPosted: Fri Sep 23, 2016 1:22 pm 
Offline

Joined: Thu Sep 18, 2008 12:41 pm
Posts: 409
Location: Northern California
Agemegos wrote:
A viscous liquid, yes. That flows slowly, yes. But on a timescale of tens of millennia all planets are liquid drops.


That's what I'm effectively wondering about.

In a setting, I imagine there's a solar system where there's a world that gets smashed (as planets sometimes do) by some enormous collision. It's probably a glancing collision rather than a cosmic "bullseye."

The reason why this is interesting to 'ordinary spacefarers' as opposed to astrophysicists is that it happened "just yesterday" as far as time is concerned - within the last few thousand years instead of millions of years ago.

My intention is to create a system that is dangerous to simply navigate around in, but is also insanely lucrative. While the intruder body that passed through the system probably has some long-term effects on orbits, that's not going to concern people too much.

What is going to be more interesting is that this collision has exposed and fragmented the core of the planet. Due to the way that heavier materials sink in during planetary formation, this stuff is usually not accessible to humans. However, in this case, the event is recent enough that there's still large chunks (at least in human terms) of elements which are scarce normally that you can exploit of a purity and quantity sufficient to make it worth hauling between solar systems (this setting has FTL travel).

The danger is that where the choicest items are, there's also a lot of debris, some of it quite fast moving. Much of it is eccentric or unstable orbits since the event was soon enough that various natural forces (such as gravity) that clean things up and make them more sane for starships have only begun to work on thinning out the debris.

So I'm mostly curious how a system like this would look.

My (mostly uneducated) guess is that there might be a mostly molten (?) remainder of the world, possibly in a new, skewed (and unstable) orbit, surrounded by a cloud of debris ranging from dust to large pieces, with the cloud gradually thinning out the further from the collision site you go. However, a lot of debris is in weird, eccentric orbits where they won't be a few thousand years.


Top
   
PostPosted: Fri Sep 23, 2016 5:13 pm 
Offline
User avatar

Joined: Tue Dec 28, 2010 5:54 am
Posts: 215
If you've ever played Star Wars: Knights of the Old Republic II, you can get an idea of what your system would look like. The opening planet in the game is Peragus II, and the asteroid belt surrounding the planet was created as a result of an explosion brought about attempting to mine starship fuel.

_________________
The Gray Book: My homage to E.G.G.

Star Trek: Alpha Quadrant: The game Mongoose failed to make. (File Updated 5/19)


Top
   
PostPosted: Fri Sep 23, 2016 8:15 pm 
Offline
User avatar

Joined: Sun Jul 06, 2008 1:04 pm
Posts: 1092
Location: the Retirement Coast, NSW, Australia
Read up on the Theia Hypothesis. People have done computer simulations on impacts with various geometries, and have established timescales for the formation of the Moon from the debris of an Earth-Theia collision. If you look at some of the modelling you should get an impression of the intermediate stages between the giant collision and the coalescence of the debris into a moon.

I understand that different models suggest that the formation of the Moon after the Theia impact took somewhere between a month and a century.

There will be fragments of crustal material and fragments of mantle material from different depths, with systematically diffent compositions, but none with recognisable remains of the original surface. I think you might only get chunks of exposed core material if the impact was either shallow or so high velocity that its energy considerably exceeded the garvitational binding energy of the planet and disassembled yhe whole thing. Whether the original material was molten or not doesn't matter much: everything will have been strongly heated by the collision, and besides the sphericity of astronomical bodies doesn't depend on their being liquid: if they are big enough to have appreciable gravity they can't be strong enough to retrain any other shape; if they are small enough to have appreciable mechanical strength they have such small gravity that they will cool before they flow. Big thing are spheres, small thing are irregular.

As for the fragments being insanely valuable, remember that the giant impact isn't going to actually produce any new elements, just move around what is already there, i.e. a large and therefore typical sample of material of the system. If there is anything valuable because of rareity, there must be not much of it scattered thinly through an astronomical preponderance of useless junk.

Remember too that many asteroids are collisional fragments of differentiated bodies, of the mantles and cores of planetismals that collided during the accretion of the Solar System. I don't see any particular reason for the debris of a giant collision to be different from ordinary asteroidal material except perhaps in being poor in core material (iron and siderophile elements).

_________________
— Brett Evill

My SFRPG setting, Flat Black

© My posts on SFRPG must not be reproduced beyond the board except with explicit permission from me.


Last edited by Agemegos on Sat Sep 24, 2016 8:41 am, edited 1 time in total.

Top
   
PostPosted: Sat Sep 24, 2016 3:32 am 
Offline
Administrator
User avatar

Joined: Thu Jul 03, 2008 1:22 am
Posts: 5329
And then there's this: http://www.space.com/34044-moon-birth-m ... earth.html

_________________
SFRPG Owner/Admin
This post (or any other post I made here) may not be quoted or copied beyond the SF RPG boards without my explicit permission.
evildrganymede.net - visit the The Worldbuilding Hub
Check out my Youtube channel for all my streamed gaming videos!


Top
   
PostPosted: Sat Sep 24, 2016 8:42 am 
Offline
User avatar

Joined: Sun Jul 06, 2008 1:04 pm
Posts: 1092
Location: the Retirement Coast, NSW, Australia
EDG wrote:

Wow! Yeah, there's aren't going to be any recognisable chunks of surface survive through being evaporated.

_________________
— Brett Evill

My SFRPG setting, Flat Black

© My posts on SFRPG must not be reproduced beyond the board except with explicit permission from me.


Top
   
Display posts from previous:  Sort by  
Post new topic  Reply to topic  [ 14 posts ]  Go to page 1 2 Next

All times are UTC


Who is online

Users browsing this forum: No registered users and 1 guest


You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot post attachments in this forum

Search for:
Jump to:  
cron
Powered by phpBB® Forum Software © phpBB Limited