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"What happened? Any theories on that?"

One theory is that Mars lost its magnetic field, so the atmosphere was eventually stripped away by solar radiation.

[samtheman]

One theory is that Mars lost its magnetic field, so the atmosphere was eventually stripped away by solar radiation.

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Solar wind.

No molten iron core, no magnetic field, no protection from solar wind, which is mostly charged particles.

Solar wind is like sand paper on upper atmospheres of unprotected planets.

We are blessed to have a molten iron core.