That's because we think Mars used to have a wetter past, and when water had enough energy, it broke up into hydrogen and oxygen, and hydrogen being so light, it escaped into space, and the oxygen that was left oxidized or rusted the ground, making that familiar rusty red color that we see.
Perquè pensem que Mart va tenir un passat molt més humit, i que quan l'aigua va tenir prou energia, es va separar en hidrogen i oxigen, i com que l'hidrogen era tan lleuger, es va fugar cap a l'espai, i l'oxigen que va quedar va oxidar o rovellar el sòl, provocant aquest familiar color vermell rovellat que veiem.