Gregory Benford 
Sunborn 
Martian Race Book 2

The first manned mission to Mars has been a resounding success, and excitement grows as more new discoveries are made. However, one phenomenon continues to defy rational explanation – the 'marsmat’ – a complex anaerobic life-form found in the planet’s honeycomb of tunnels. This raises questions about the nature and meaning of life itself which will lead the curious and the driven to Pluto and beyond, to the cold void at the fringes of the solar system.