Solution
= Solution
Hydrostatic balance makes the atmospheric column mass $(P_0-P_{\rm top})/g$. Multiplying by the surface area gives the <mass of a thin hydrostatic atmosphere>
$$
\boxed{M_{\rm atm}\simeq
\frac{4\pi R_p^2(P_0-P_{\rm top})}{g}
\simeq\frac{4\pi R_p^4P_0}{GM_p}},
$$
where the second expression neglects the top pressure and atmospheric self-gravity.