Using the the theory of FS op modules, we study the asymptotic behavior of the homology of M g,n , the Deligne-Mumford compactification of the moduli space of curves, for n ≫ 0. An FS op module is a contravariant functor from the category of finite sets and surjections to vector spaces. Via maps that glue on marked P 1 's, we give the homology of M g,n the structure of an FS op module and bound its degree of generation. As a consequence, we prove that the generating function n dim(H i (M g,n ))t n is rational, and its denominator has roots in the set {1, 1/2, . . . , 1/p(g, i)} where p(g, i) is a polynomial of order O(g 2 i 2 ). We also obtain restrictions on the decomposition of the homology of M g,n into irreducible S n representations.