Select the text from the paragraph that best helps you understand that Isotope means a variant of an element with a different number of neutrons. There may be more than one correct choice.
A 2015 study by NASA scientists revealed that up to one-fifth of Mars's surface may once have been covered by water. The researchers obtained this figure by comparing the water in Mars's atmosphere today with the water found inside a 4.5 billion-year- old Martian meteorite. Specifically, the scientists calculated the ratio of Hâ‚‚O to HDO in each water sample. HDO is a variant of water containing deuterium, which is an Isotope of hydrogen. Hâ‚‚O is light and easily escapes into space from Mars's atmosphere; HDO, meanwhile, is heavier due to the addition of a neutron in deuterium and is more likely to persist in the atmosphere as a result. The scientists used the difference between the two water samples' Hâ‚‚O:HDO ratios to calculate how much water Mars has lost in the last 4.5 billion years-an estimated eighty- seven percent.
