[DJtex]

Mars lost its atmosphere, including anything that evaporated off the surface, when it lost its magnetic field. That allowed the solar wind to whip to the surface and slowly carry away everything that wasn't nailed down.

How did Mars lose its magnetic field? Being half the size of Earth, their iron core cooled and hardened. Without a liquid core, no iron rotation, hence no magnetic field.

[RightWhale]

Earth's mag field is generated by the atmosphere. Mars has next to no atmosphere and especially no water vapor. No water vapor, no magnetosphere.

[Calvin Locke]

I thought surveys from the last few years detected a magnetic field, much stronger (relatively speaking) than any one had theorized?

[Jedi Master Pikachu]

You two seem so sure of that. How about: some suggest Mars' core stopped rotating, and it lost its magnetosphere. Without the magnetosphere, the solar wind stripped away Mars atmosphere, and the temperature dropped?