Imagine humanity discovers a new habitable planet. It is pretty much identical to Earth, orbiting a star of one solar mass which is also the age of the Sun. Unfortunately the life there is lethal to Earth life (q.v. War of the Worlds). So we need to sterilise it by orbital bombardment before we can colonise it. Two questions arise:
How massive / energetic must an impact be to melt the crust to a depth of 15 km?
How long will it take for a new, habitable, crust to form? Are we talking about centuries, millennia, or millions of years?
Wouldn’t it be simpler to fire a ship’s fusion drive or something at the planet, and sterilize it with high-energy particles much like a nearby supernova might also do? then the only question would be whether you got into the depths of the oceans too. Plan B would be to bombard it with rocks or icebergs moved from further out in the system. 15km deep sounds like overkill. Raise the overall temperature above sterilization range
The Chicxulub meteor is estimated between 7 to 50 miles diameter, assume 7mi/sec and that and the after-effects supposedly killed anything larger than about 5lb. (and pretty much most other life too). Estimates are it created a years-long “nuclear winter”.
Do you mean 15 km at the point of impact, or 15 km around the whole earth? I’m thinking that an impact that would do that kind of damage on the opposite side of the impact site would do a fuckton more damage at the impact site itself…but my calculations might be off and I welcome any corrections.
As far as I am aware, there is nothing known to humans that can completely sterilize a space. Bacteria survive in nuclear reactors and there are extremophiles that live in ocean vents, ocean deeps, high pH …
So the complete extinction of all living things seems to be impossible. Is that acceptable for the OP’s criteria for sterilization?
Well, in the OP’s scenario, I don’t think there will be any life in the top 15 km of the crust. Nothing can survive in molten lava. As I read it they’re going to use multiple impactors, not just one, which makes it more feasible to evenly melt the entire surface of the planet. Nevertheless, to say this is a monumental undertaking is a astronomical understatement. I believe the Chicxulub impactor only melted an area of at most a couple hundred miles around the impact site, and it wasn’t 15 km deep. So you’d probably need tens of thousands of Chicxulub sized impactors.
Let’s not worry about whether something can survive lava or not. Your question is how much energy to melt 15 km into the crust. Let’s first assume that we’ve already burned off the water and it’s just a shell of rock. The earth is about 510 million square km, so simplifying the math those 15 km are about 7.6 billion cubic km of material you’re going to be melting. I’m going to refer you to this link http://home.earthlink.net/~jimlux/lava.htm which estimates about 840 kJ of energy to melt 1 kg of stone. The average density of the crust is about 2670 kg / m^3 so that works out to 2.67 trillion kg / cubic kilometer, so every cubic kilometer of stone we’re melting would require 2,242 trillion kJ of energy. We multiply that by our 7.6 billion number and we get a butt ton of energy or approximately 1.7x10^25 kJ of energy. A 1 megaton nuclear bomb releases about 4.18X10^12 kilojoules of energy, so again a quick and dirty math shows we would need to launch 4 x 10^12 nuclear bombs to completely melt the crust, so 4 trillion 1 megaton weapons.
This is of course, completely quick and dirty. It assumes that all of the energy goes into the crust and that the crust is on average room temperature (It of course isn’t.) I’m merely conjecturing that it’ll all balance out in the end.
Chixculub is certainly small beer compared with what is needed.
Yes, I’m envisioning an impact significantly more massive than Chixculub, melting the entire surface of the planet. Perhaps multiple carefully targeted such impacts.
If we can bombard a planet sufficiently hard to sterilize it of even single celled life we can also terraform an uninhabited planet, which we may as well do since we will need to fix all the damage we caused anyways so that the sterilized planet is ready for earth life; or (for significantly less energy and resources than either option) we could carve up the asteroid belt of whatever system this planet is in to form millions of artificial habitats, spinning to simulate gravity, with a combined livable area vastly larger than the surface of any given planet by several orders of magnitude.
Also, even once a new crust does form, an impact that big would eliminate the atmosphere, and any oceans would boil away and the water molecules would fly off to space. With fewer comets to deliver water and air to the planet in an older solar system, the planet would never become habitable of its own accord.
How close to the sun would you have to move this planet to get the desired result(providing this new planet’s source of heat/light is pretty much the same as our own)?
The desired result of melting the crust to 15 km down? Closer than Mercury, since the surface of Mercury isn’t molten 15 km down. In fact, you would need to orbit within the star’s “surface” – an area of superheated gas much more diffuse than our own atmosphere.
Sure, but if we’re going down that route of extreme terraforming, why even bother with this planet in the first place? There’s got to be an uninhabited rock out there in the right temperature zone that wouldn’t require eco-cide (is that even a word?) that we could essentially terraform from scratch without needing to kill anything first.
It’s not about the morality, it’s about practicality. It’s harder to take a living planet, sterilize it, and then bring it back to livable conditions than it is to take a sterile rock and terraform it.
And yes, the energy required to boil the oceans and put every atmospheric particle on an escape trajectory is orders of magnitudes lower than the energy required to melt the crust for 15 km. So you’d be missing anything that makes this rock earth-like long before you got to your goal.