Exiles of Gaia by Renna Olsen

http://www.amazon.com/Exiles-Gaia-New-Mother-Earth-ebook/dp/B00HCQCF1M/ref=sr_1_1?s=books&ie=UTF8&qid=1387337120&sr=1-1&keywords=exiles+of+gaia

Seven hundred years have passed since humanity was forced to leave the earth for the satellite colonies, collectively known as Skyweb. Civilization as it was once known continued unabated above the earth, overseen by the now powerful United Nations. Meanwhile, on Earth, powerful nano-aggregate machines referred to as the Guardians, were left on the planet to clean and restore the Earth to a habitable condition, replicating entire species from the DNA databank and paving the way for humanity’s eventual return.
In the first few centuries of the exodus, the orbital cities monitored the progress of the machines closely as they took on the monumental task of healing the dead planet. They observed the massive processing units as they consumed the decaying structures of the mega cities, sifted the oceans of poisons and debris and prepared to reintroduce the lost life on Earth from their vast DNA databanks. But then something changed.
Over the last three hundred years, the Guardians ceased all communication with Skyweb and the Earth had become ever more obscured by a shifting silvery layer of nanites. All subsequent efforts to penetrate the covering met with disaster.
With little information to go on and the orbital cities of Skyweb slowly degenerating, the United Nations prepares to launch another in a series of disastrous Earth exploration missions. The newly christened UNSS Persephone and its crew of scientific experts risk all as they confront the mysterious forces on their home world and the terrible secrets of the UN itself.
United Nations Secretary General Delphine Prentiss has enough to deal with when she gets word that one of the scientists assigned to the Earth exploration mission has been murdered. With the Kvara miners on Mars threatening civil war and the Ark Colonization Project bankrupt, her own reelection is in jeopardy and she needs this mission to succeed at all costs.
The United Nations Investigative Service Detectives assigned to the case have little to go on except a symbol for the Gaia Movement painted near the body and things get more and more complicated as they realize that this murder is just the beginning and goes deeper than just the highly anticipated Earth mission.