The martian atmosphere is made of Carbon Dioxide (CO2). On Earth we have what is known as the carbon cycle. It is something like the water cycle. Water on earth cycles between the ground and the air. Scientist want to understand more about the carbon cycle of Mars.
If Mars used to have a thick atmosphere made of CO2, where is all the CO2 now? Unlike Hyrodgen or Helium, CO2 does not readily escape from an atmosphere.