PLANETARY SCIENCE36 The Moon That Would Be a Planet By Ralph Lorenz and Christophe SotinTitan, Saturn’s largest natural satellite, has an atmosphere thicker than Earth’s and a surface that is almost as varied.NEUROSCIENCE44 The Brain’s Dark Energy By Marcus E. RaichleBrain regions active when we allow our minds to wander may hold a key to understanding neurological disorders and even consciousness itself.ENERGY50 Fusion’s False Dawn By Michael MoyerAs a historic milestone nears, skeptics question whether it will ever be possible to build a working fusion reactor that could supply virtually unlimited clean energy.GEOLOGY58 Evolution of Minerals By Robert M. HazenLooking at the mineral kingdom through the lens of deep time leads to a startling conclusion: most mineral species owe their existence to life.MEDICINE66 Toxic Gas, Lifesaver By Rui WangThough lethal, hydrogen sulfide turns out to play key roles in the body—a finding that could lead to new treatments for heart attack victims and others. animal behavior72 Worm Charmers By Kenneth CataniaFollow an intrepid scientist as he learns why earthworms rush to the surface when bait hunters rub a piece of metal across a stick poked into the ground.ENVIRONMENT78 Climate Change: A Controlled Experiment By Stan D. Wullschleger and Maya StrahlScientists have manipulated grasslands and forests to see how precipitation, carbon dioxide and temperature changes will affect the future of the biosphere.