Mars, itself is somewhat insufficient to hold and retain an atmosphere. It would be a challenge. However, it is thought to be doable.
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If I remember right (what I've read in the past) Mars has enough mass to hold down an atmosphere with significant pressure, but it doesn't have the strong electromagnetic protection that the earth does and the solar wind from the sun gets through and strips Mars of any 'air'
Yes, as I recall, it would constantly be “losing” atmosphere and so would need constant “topping off”....which makes it “doable” but not perfect.