In 2007, the Department of Water Protection in Los Angeles detected high levels of bromate, a carcinogen that forms when bromide and chlorine react with sunlight, in Los Angeles's Ivanhoe Reservoir. Bromide is naturally present in groundwater and chlorine is used to kill bacteria, but sunlight is the final ingredient in the potentially harmful mix
According to MSL scientists based at NASA's Jet Propulsion Laboratory (JPL) in Pasadena, Calif., the ball isn't as big as it looks — it's approximately one centimeter wide. Their explanation is that it is most likely something known as a "concretion." Other examples of concretions have been found on the Martian surface before — take, for example, the tiny haematite concretions, or "blueberries", observed by Mars rover Opportunity in 2004 — and they were created during sedimentary rock formation when Mars was abundant in liquid water many millions of years ago.