Mars bears little resemblance to the primordial planet it must have been. Mars’ atmosphere is a fraction the density of Earth’s — air pressure at the summit of Mount Everest (the highest point on Earth) is 4.89 PSI, while the air pressure at the bottom of Mars’ Hellas Planitia (a 23,465-foot deep crater) is just 0.168 PSI. In other words, the air pressure at the highest point on our planet is 29x higher than the air pressure at the lowest point on Mars.

There have been three main hypotheses for how Mars might have lost its atmosphere: It’s possible that Mars’ atmosphere was eroded by solar wind, that much of the atmosphere was torn away by a cataclysmic impact, or that the low gravity of the planet allowed atmosphere to blow off and dissipate over time. These conditions are not mutually exclusive, and it’s possible that all three of them played a part, but NASA believes it has found sufficient evidence to assign a primary cause. The culprit? Solar wind — particularly the types of energetic blasts emitted by the sun during periods of unrest.