Well, the gravity is enough to hold the atmosphere should we make it but without the magnetosphere you will constantly get it slowly stripped away but the loss is not rapid. You can keep up with it by continuing to put O2 and nitrogen in the atmosphere. If you can get the CO2 up enough to sustain something green like lichen then you can eventually get enough to sustain trees yada which will make your O2. But you do have to make it faster than the solar winds strips it away. I don't have the numbers on that but its somewhere. How accurate the numbers are I don't know.
Did you know there are only 5000 Snow Leopards in the wild now and they are confined to Central Asia? However, the effective global population (those likely to reproduce) is less than half that number.