“Mars can never be colonized, because the inner core has cooled and unlike the Earth’s core, it is solid and unmoving. Because of this, the gravity is not strong enough to hold the atmosphere to the planet.”
Close, but no cigar.
A molten core produces a planetary magnetic field which shields a planet from the solar wind of its sun. Without that protection, a planet’s atmosphere will be gradually blown away.
That’s what happened to Mars’ atmosphere.
People have speculated for decades about terraforming Mars, but unless we can figure out how to reheat the planet’s core, it ain’t happening.
Thank you for clarifying.