Dropnauts - Liminal Sky: Redemption Cycle, #1

Over a century after the end of the Earth, life goes on in Redemption, the sole remaining lunar colony, and possibly the last outpost of humankind in the Solar System. But with an existential threat burrowing its way to the Moon's core, humanity must recolonize the homeworld. Twenty brave dropnauts set off on a mission to explore the empty planet. After training for two and a half years, four of them-Rai, Hera, Ghost and Tien-are bound for Martinez Base, just outside the Old Earth city of San Francisco.
But what awaits them there will turn their assumptions upside down-and in the process, either save or destroy what's left of humanity.