Related by context. Frequent words. (Click for all words.) 55 interglacial period 53 dinosaurs roamed 52 Opportunity rover 51 interglacial 51 retreating glaciers 51 Cretaceous Period 51 Meridiani 50 comet Wild 49 Titan Saturn 49 extrasolar planet 49 Meridiani Planum 49 Gusev Crater 49 Saturn moon 48 martian 48 Red Planet 48 marine sediments 48 rover Opportunity 48 equatorial regions 48 Weddell Sea 48 volcanically active 48 Gliese #g 47 moon Europa 47 asteroid 47 Mars 47 Martian surface 47 Saturn moons 47 Martian arctic 46 volcanism 46 glaciation 46 moon Titan 46 permafrost 46 Cabeus crater 46 supernova explosion 46 Mars orbiter 46 biodiversity hotspots 46 Mars rover Opportunity 46 Global Surveyor 46 geologically active 46 astrobiologists 46 Mars Pathfinder 45 sedimentary rock 45 microfossils 45 comet dust 45 Medieval Warm Period 45 gas hydrates 45 rovers Spirit 45 Mars rover Spirit 45 continental crust 45 glacial melting 45 Tunguska 45 gigatons 45 rocky planets 45 super Earths 45 Polar Regions 45 megafauna 45 Saturn moon Enceladus 45 NASA Lunar Reconnaissance 45 volcanic crater 45 zircons 45 cosmic microwave background 45 mass extinctions 45 Voyager spacecraft 44 twin rovers 44 lavas 44 olivine 44 fossilized remains 44 Itokawa 44 plesiosaur 44 Little Ice Age 44 methane hydrates 44 glaciers 44 atmospheric carbon dioxide 44 Ice Age 44 Martian 44 Carboniferous 44 extremophiles 44 HD #b [002] 44 NASA Cassini spacecraft 44 NASA Mars Odyssey 44 ESA Mars 44 Martian soil 44 archaea 44 Phoenix lander 44 Lutetia 43 Cassini Huygens 43 Clyde Tombaugh 43 methane hydrate 43 greenhouse warming 43 Younger Dryas 43 Martian landscape 43 river deltas 43 geologic 43 Antarctica 43 PASADENA Calif. NASA 43 polar regions 43 Len Zacks 43 lake sediments 43 woolly mammoths 43 Mars lander 43 volcanic activity 43 comets