Which of the following statements best reflects our current understanding of the past and present of Mars' atmosphere?

W) It has always been about as thick Earth's atmosphere today

X) It has always been much thinner than Earth's atmosphere today

Y) It was thick shortly after formation, but has since thinned due to solar wind

Z) It was thin shortly after formation, but has since thickened due to volcanic outgassing
Y) IT WAS THICK SHORTLY AFTER FORMATION, BUT HAS SINCE THINNED DUE TO SOLAR WIND