Gamora and Nebula, sisters in arms

"Gamora arrives on Torndune--a once-lush planet that has been strip-mined for the power source beneath its surface--with a mission: collect the heart of the planet. Her adopted sister Nebula is in hot pursuit, determined to claim the planet's heart and shame her sister. Each allies herself with a faction on the planet in order to gain access to the massive diggers capable of drilling to the center of the planet. It's a competition they can never win--unless they learn to trust each other"--Adapted from jacket.