The crux of the problem with Mars is that its magnetic field is smaller than its atmosphere. All water gets blown away by the solar wind. Venus has a similar si

The crux of the problem with Mars is that its magnetic field is smaller than its atmosphere. All water gets blown away by the solar wind. Venus has a similar situation, and it's reacting somewhat similarly, having lightweight particles blown away by solar wind. Indeed, the Earth is the only one of the 3 with a larger magnetic field than its atmosphere. We look like a small gas giant by comparison - our magnetic field reaches far out into space, more than half as big as Neptune or Uranis' magnetic fields. This protects our water and other environmental factors. Sadly, this means that seeding the planet with water, perhaps by towing an ice-asteroid into orbit and knocking it to the planet, would be a temporary fix at best. Eventually all of that water would just evaporate away and join the Heliopause.