Early Mars was warmer and wet, which means that its atmosphere was very likely denser and richer in the greenhouse gas CO2 than it is today. So where did the carbon go?
On Earth, a dense early CO2-rich atmosphere was largely converted into thick deposits of carbonate minerals such as calcium carbonate (CaCO3) by the action of reef-building organisms. But surveys of
Mars from orbit have turned up only small areas of carbonate deposits, apparently insufficient to account for the missing carbon. (Though some have argued that more might be buried out of view from orbit.)
NASA’s Mars MAVEN (Mars Atmosphere and Volatile Evolution) orbiter has been studying the interaction of the Martian atmosphere with the solar wind since its arrival at Mars in 2014. The data has allowed a picture to be developed which show that the solar wind, particularly during strong solar storms, accounts for the loss of Martian CO2 over the past few billion years: the time since Mars last had significant liquid water on its surface. It appears that early Mars had an atmosphere composed predominately of CO2 which was slightly less dense than Earth’s atmosphere is today, but which has been sputtered away by solar storms. This sputtering process, which is still ongoing and has been observed by Mars MAVEN, correctly accounts for the measured relative abundances of carbon isotopes. At the same time Mars began this process of losing carbon, cooling and drying, on Earth bacteria and archaea began modifying the atmosphere, lithosphere and climate in profound and lasting ways.
Of course, these models do not rule out the presence of bacteria on early or even contemporary Mars, but if Mars ever had life it evidently never “took charge” of the planet’s atmosphere to anything like the degree Earth life has done here.