To take your theory up a notch, what if Mars is the core of a gas giant. There has been talk about gas giants migrating inwards in other star systems. If a gas giant migrated to Mars orbit, would it loose it atmosphere after a while if it had a Mars size core surrounded by gas? For comparison, Uranus gravity is only 0.88 G.

As the lighter gasses burn off, like hydrogen and helium, and the atmosphere thins, water rains out, which might explain the early dendritic drainage channels. Given the number of craters we have seen , this would have happened during the late bombardment stage of the Solar System.

It is a thought, anyway.

Mars might have not needed to migrate to start off with a thicker atmosphere. Could Mars start out with a much thicker atmosphere approaching that of a gas giant before the Sun heated things up in the Early Solar System? Once the Sun really fired up, it would have driven out the primordial dust hanging around. Things would start heating up on Mars pretty fast, but if it started out with a thick atmosphere, it might have hung onto it for a while.

Later in Mars History we have the Volcanic out-gassing that would have caused a second age of thick atmosphere.

As crazy as it sounds- Mars being a core of a gas giant, it might be worth doing some modeling on this. The only thing that really tells me it couldn’t be so is that Mars is heavily cratered with ancient craters that were formed early in the Solar System’s history. And then there is the asteroid belt. I am not sure what effects a low mass gas giant (Mars size/mass core) migrating to, or forming at, 1.6 AU would do to the Asteroids Belt formation or the Terrestrial planets.

In our Solar System, the heaver elements are found more concentrated towards the Sun. Mercury is a dense metal choked planet with a huge iron Core. The outer worlds are mostly light elements that are frozen into ices. This makes perfect sense because the Sun’s energy drove the lighter elements farther away. My main question is, did the Sun drive these elements away during the beginning of the planets formation or later?

Using Mercury as an example.

1. Did Mercury form with more light elements, and then when the Sun heated up, it stripped Mercury of its light element?

2. Or did the Sun fire up while Mercury was forming, driving away the light elements before they could become part of Mercury? And any light elements we see today were brought by comets from the Outer System?

I tend to lead towards #2, and if that is the case, then Mars, if formed at its current location, could have never been a gas giant.