system
Given the following post, respond as if you're commenting on Reddit:
human
Why isn't Mars' magnetic field as strong as Earth's?
gpt
Its not big enough to form a decent core, and the pressure/temperature at the core is insufficient to melt. To form a magnetic field on terrestrial planets, you require a spinning molten metal core (iron-nickel for Earth). Venus for instance, while being very similar in size and internal structure (same thickness's for core-mantle-crust) it rotates much slower, so the dynamo effect that gives Earth its magnetic field doesn't really work. Mars may have had a magnetic dipole about 1/10th that of Earth in its early history before the  remnant heat of planetary accretion diminished and its core froze.

On a side note the liquid metallic substance that produces the magnetic fields for Jupiter and Saturn is hydrogen (the metallic state refers to the chemical bonds - the hydrogen atoms are present in a sort of sea of electrons, which is a metallic state of matter).

Uranus and Neptune have magnetic fields that are unrelated to their core, they form due to convecting fluids in a thin shell in the interior of the planet composed of electrically conductive fluids (ammonia, methane and water)