happened once? because the gravity changed? it'd be an ongoing process.
and only hydrogen and helium are currently prone to jeans escape on mars. the current dominant mechanism for atmospheric loss is the solar wind. which eats away ~1/3 megaton/century. mars has teratons of atmosphere remaining. and it can be added faster than that rate of loss.
(once warmed up, oxygen may begin to escape the atmosphere again. not necessarily faster than it can be replaced, though.)
and only hydrogen and helium are currently prone to jeans escape on mars. the current dominant mechanism for atmospheric loss is the solar wind. which eats away ~1/3 megaton/century. mars has teratons of atmosphere remaining. and it can be added faster than that rate of loss.
(once warmed up, oxygen may begin to escape the atmosphere again. not necessarily faster than it can be replaced, though.)