Would it, in theory, be possible to get a planet like Mars's inner core spinning by crashing another planet or moon into it?

I'm not sure if enough heat would be generated, but this might be a simplest way to get Mars' Magnetic field going, by crashing a large body into it (perhaps Ceres?) - and, I realize the entire idea is bonkers/crazy. I'm just curious if, theoretically, this might generate enough heat to get Mars 1) completely molten, and leave it with a solid Iron core with a liquid Iron core spinning around it - enough to create a protective magnetic field.

If Ceres is too small, maybe Callisto (the furthest out of Jupiter's large moons).

• Isn't Mars 'inner core solid? I don't think that'll work if the core has cooled... Commented May 29, 2015 at 8:38
• I think I asked it badly - probobly should edit, cause obviously Mars inner core spins with the planet. I meant, heat it up enough so that the outer core liquifies around a solid inner core, which would create a magnetic field. Commented May 29, 2015 at 9:16
• I will be somewhat pedantic (not that I don't like that :-D) and point that in the XIXth Lord Kelvin calculated that Earth was 20 million year old, or otherwise it would be way cooler. The difference (that keeps us warm and the core fluid after 4 billion years) is due to heat caused by radioactive decay. So, if you want the warming of the core to last, maybe you want to send a radioacive moon. Commented May 29, 2015 at 11:33
• I think, Lord Kelvin's calculations were rather badly estimated. John Perry's were much better, and that was before they knew about radioactivity. en.wikipedia.org/wiki/… Now, I'm not smart enough to check their math, but Wiki suggests that Perry's method was better and Kelvin's was flawed. Commented May 31, 2015 at 6:05
• @userLTK good point, thanks for the info. Commented Sep 22, 2015 at 23:49

YES - NO - IT DEPENDS

The answer to your question depends entirely upon why you want Mars' core spun up. If you wish to make it inhabitable, then your proposal doesn't work. A collision big enough to melt & spin Mars' core will liquify & vaporize up to 2/3 of the planet. Eventually it will reform and will have a stronger magnetic field but we're talking 100,000s or millions of years.

It's a long term fixer upper.

Another possibility is to begin a bombardment of Mars along its equator to spin it up some more (but not too much or the days become too short). This would require a series of much smaller (mile wide or less?) asteroids over a long period of time (say 100 years?).

If Mars' core still has some liquid left, this ought to induce a stronger field. If not, you won't see much if any effect.

If you're end goal is just to maintain a stable atmosphere, then bombarding Mars with icy bodies (moonlets from Saturn?) would provide both much needed volatiles and some angular momentum for the planet.

Once it had that atmosphere, it would begin leaking out. However, atmospheric loss would occur over very long periods of time. So your average person wouldn't worry about it much.

Some numbers:
- Mars' atmospheric loss rate ~ 100 tons/day
- Mass of Mars' atmosphere ~ 25 Teratonnes
- Mars' atmospheric pressure ~ 0.6% of Earth's
- So you need $\mathrm{166 \times 25 \times 10^{12}\ tonnes = 4.2 \times 10^{15}\ tonnes}$ of more gases
- Assuming loss rate is a function of atmosphere mass, loss rate $= \mathrm{166 \times 100 = 16,600\ tons / day}$
- Mars atmospheric half life will be about 500,000,000 years.

So nothing urgent that we'd need to work on.

the answer to this question suggests that a moon with an unusually strong magnetic field could strengthen mars' own magnetic field.

If that moon was heavy enough, tidal forces should help liquefying the core, thus creating the dynamo necessary to maintain the field over astronomical timescales.

The benefits of this method over the crash you suggest would be the "instant" habitability of the planet (as compared to a boiling blob of magma waiting to cool enough...), and, much more important, the beautiful view a large moon provides.

• Speaking of the view... if you're precise enough to insert the new moon in just the right orbit, you could duplicate one of the most interesting coincidences in the solar system. Commented May 31, 2015 at 3:53
• @2012rcampion while it's can't produce a full eclipse, phobos is close enough to Mars' surface to cause regular partial ones and block out part of the sun. It orbits so fast they only last for 30 seconds or so. en.wikipedia.org/wiki/Solar_eclipses_on_Mars Commented Sep 22, 2015 at 23:55
• @userLTK The coincidence I mention is that the angular size of the Moon as seen from Earth so perfectly matches that of the Sun that we get both total and annular eclipses, depending on how close the Moon is to periapsis. Phobos can't have a nice annular eclipse due to its irregular shape, and is too far from Mars to make a total eclipse. Commented Sep 23, 2015 at 1:21
• I got what you meant. I just thought the Phobos eclipses are kinda neat, cause you don't expect an eclipse from a 20 mile diameter asteroid, but it's close enough to mars to sort of give you one. Commented Sep 23, 2015 at 2:06

To melt the core without destroying the planet in the process, you should find a means of introducing a quantity of antimatter into the core. The amount needs to be carefully calibrated, since too small of a slug of antimatter would not liquify the core, while too much could cataclysmically destroy the planet.

The other factor would be to induce the rotation of the liquid core to generate the magnetic field. I suspect that liquifying the core through a sudden infusion of energy might create a very chaotic initial condition, with multiple swirling vortexes of liquid iron going in multiple directions. If you are not in a real hurry, the core could just be left alone and eventually the inertia of the planet's rotation should get everything going in the proper direction, and hopefully a stable magnetic field will develop. (Of course this will also probably result in a lot of violent tectonic activity as the various vortexes and eddies shed energy or combine and develop more energy). How to stir several tens of millions to tones of molten iron in the proper direction from the "outside" will probably involve some sort of induction effect.

Expanding on this idea a bit, if you surrounded Mars with a cage of wire in orbit, energized the "cage" and spun it rapidly, the reaction of the electromagnetic field with the iron core might give you the induction needed to melt the core (far more slowly than inducing antimatter into the core), while also "stirring" the melt in the proper direction. Since much of the Martian regolith is also iron oxide, there will be a few complications with this idea.

A possibly beneficial side effect would be the reawakening of the Tharsis volcanoes, including Olympus Mons, which should pump a lot of extra gasses into the atmosphere to assist in the terraforming effort.

I was thinking about this today while watching a documentary about terraforming mars. I was surprised it did not mention Mars' lack of a magnetic field as an obstacle.

My thought was that some sort of external force field / magnetic shield would need to be built in space using satellites.

• I have some doubts considering the feasibility of such an approach. While not having anything near enough knowledge to prove it, i would assume that you would need a really, heally huge sattelite or array of sattelites to create anything like a reasonable magnetic field around a planet? Commented May 29, 2015 at 14:46
• I am sure if we were advanced enough that we could smash planets into one another we could build some sort of satellite array shield. I mean are we trying to make this place habitable or just throw things at it to see what happens? If it is the former then smashing a large body into mars would cause a lot of problems including increased tectonic activity (both from the impact and the warmer mantle) and no doubt push mars out of its current orbit. Some sort of human made shield seems like a relatively simple solution. Commented May 30, 2015 at 2:40
• If you can give Mars an atmosphere like Earth's, it will protect the inhabitants from radiation and last a while before the solar wind blows it off, probably more than long enough for us to figure out how to induce a magnetic field capable of keeping the atmosphere in. It isn't a near-term problem. Commented Jan 20, 2017 at 22:48