I’ve read a few (brief) articles online that suggest the Moon was formed very early in the solar system’s development when Mars, or what would one day become Mars, struck the Earth a glancing blow and knocked the future Moon away from it. The Moon then somehow got to a stable orbit around the Earth and solidified.
My question is, how could the orbit have become stable? As I said the articles are very brief and aimed at a mass audience, and they don’t go into this question. But disregarding atmospheric friction, suppose I’m standing on the ground next to a cannon that can fire projectiles at 19000 mph. I aim the cannon fairly low and fire away. I know that the projectile will reach space, possibly attaining a great altitude over the opposite side of the earth. But since there would be no stabilizing burn at apogee, the projectile will then start to descend, and eventually return to earth. Possibly it would hit me in the backside.
How do planetologists theorize that the Moon could have reached a stable orbit if it originated from a ground-level impact? Is there some principle of orbital mechanics that would tend to stabilize the orbit as time goes on?
I can’t fully answer this, but I’ll note a couple of points. First, it wasn’t Mars that impacted the Earth, it was a body usually called Theia. Theia is often said to be the size of Mars, which may be where your confusion on that point arose.
Second, saying that Theia “knocked the future Moon” away from Earth isn’t a very accurate description of the event. It was more a giant “splash”, that caused large amounts of both Earth and Theia to be ejected from the impact site. Some of this material would have been ejected completely from the system, some would have fallen back to Earth, and some would have coalesced into the Moon.
Yes, “tidal circularization”. Tidal forces tend to force a satellite into a more circular orbit over time, if it’s tide-locked. And the Moon is tide-locked.
Circularization occurs when the tidal effect allow excess orbital energy to be dissipated (angular momentum is conserved, but for a given angular momentum, total energy can vary - a circular orbit has the least orbital energy). Tidal effects vary with the inverse cube of the distance between the two objects. Dissipation of energy occurs when the deformation of the objects due to tidal effects leads to conversion of kinetic energy to heat. For a molten Moon close to Earth, the tides are going to be constantly squeezing molten rock in different directions, which would (in geological terms) quickly circularize the orbit
Don’t forget the Earth wasn’t solid yet and the energy of the impact softened it up but good too.
So you have two big splashes of goop, one much larger than the other, co-rotating about a common center of gravity somewhere in the space between them. Each of which settles into a more or less spherical shape and cools while meanwhile the tidal forces circularize their co-orbit.
AIUI tidal force dissipation is faster in softer more maleable bodies versus magically absolutely rigid bodies. Kneading the two semi-molten bodies will quickly round out the orbits.
Absolutely correct. The more inelastic the objects (the more deformation turns into heat), the quicker they circularize (objects that couldn’t deform, or objects that deformed elastically wouldn’t circularize at all)
Which also suggest the orbital circularization process will be mostly over before the two co-orbiting objects have an opportunity to sphericize themselves and then cool to their current relativley high (but still not infinite) rigidity.
A lot of good points were made in this thread, but there’s one idea I want to clarify. The common claim that “you can’t just shoot a projectile into orbit” is only strictly true in a two-body system. Once you introduce more than two bodies, orbital dynamics change dramatically. With multiple gravitational influences, it is possible for material to end up in stable orbit if the timing, velocity, and geometry line up.
In the giant-impact model of the Moon’s formation, a huge amount of debris, representing a significant fraction of Earth’s mass, was ejected into space. Naturally, some of that material fell back to Earth, some escaped entirely, but a portion remained in orbit. And because this wasn’t a single projectile, but countless fragments interacting gravitationally, they were able to exchange momentum, collide, merge, and effectively “average out” into a more stable orbital configuration. Over time, this debris accreted into what eventually became the Moon.
It should be noted that, when you’re looking at planet-scale objects, there mostly isn’t any such thing as “rigidity”. Look at the size of Mount Everest, compared to the size of the Earth as a whole: That’s about how much non-sphericalness rock can support.
Are you sure it was Mars? The only such theory I have heard of was the one noted above, that it was a proto-planet that we now refer to as Theia. In which case, far from the bulk glancing off Earth to become Mars and leaving the Moon behind as a remnant, the bulk was actually absorbed into proto-Earth itself.
Conceptually, it seems a lot more plausible that this would result in fragments reaching a stable orbit around Earth (with the bulk being absorbed to form Earth along with proto-Earth) than the idea that the bulk would still have sufficient energy to break away as Mars in a completely different orbit, and yet leave a substantial fragment behind as our moon.
I’ll just note that Triton, biggest moon of Neptune, is believed to have been a captured dwarf planet, since it orbits opposite Neptune’s rotation and the direction of the rest of her moons. Despite being captured the orbit is the MOST circular of all the big moons in the s.s.–IOW it had to have been captured in a very eccentric orbit but managed to circularlize it over time. [Its orbit is significantly out of plane with Neptune’s equator note.]
Are there any other moons in our solar system that were created by such a violent impact? The other “double planet” example is Pluto/Charon theorized as a “kiss and capture” system where two spheres just bumped into each other and evolved from the resulting snowman shape. The rest seem to either be gravitational capture or accretion of ring debris.
Most moons of asteroids are thought to be the result of collisions.
This is more-or-less correct, but with a slightly misplaced emphasis. After the initial collision, it was mostly collisions between the various fragments that kept some of them from falling back within a single orbit. The majority of fragments that didn’t escape fell back to Earth within a single orbit. Gravity between those fragments wouldn’t have been strong enough to circularize their orbits in a single orbit. But some got enough sideways mometum via collision to avoid hitting the Earth. Accretion and circularization would happen to those that stayed in orbit, but over a much longer period than a single orbit.
No, so long as the size of the Moon is small compared to the distance to the Earth, tidal effects are close to symmetric. Basically, you’ve got gravity pulling stuff in towards the Earth, and centrifugal force pulling it away. Right in the middle of the Moon, those balance, but on the ends, they tend to stretch it out both ways.
The reason for this vast hemispheric differences could be the result of a literal imbalance in the Moon. According to research published in Nature, there’s about a 2 to 3 percent mass asymmetry between the sides, mostly in the mantle layer. The driver seems to be heat from deep within the Moon, which also causes there to be moonquakes — more in some areas than others.
