Long tracks of elevated nitrogen dioxide (NO2) levels show up along certain shipping routes when viewed from NASA's Aura satellite. For more than a decade, scientists have observed “ship tracks” in natural-color satellite imagery of the ocean. These bright, linear trails amidst the cloud layers are created by particles and gases from ships. They are a visible manifestation of pollution from ship exhaust, and scientists can now see that ships have a more subtle, almost invisible, signature as well, according to 'NASA Earth Observatory'.