MARS - TOO COLD
IN PAST MARS SEEMS TO HAVE HAD LIQUID WATER, AS SHOWN BY SEDIMENT DEPOSITS
NOW, HOWEVER, IT DOESN’T
SOMEHOW MARS CLIMATE CHANGED — PERHAPS SEVERAL TIMES — BETWEEN HOT/WET PERIODS WHEN
"GREENHOUSE" GASES IN ATMOSPHERE KEPT PLANET WARM — TO COLD/DRY TIMES WITHOUT A GREENHOUSE WHEN THE SURFACE GOT TOO COLD FOR LIQUID WATER AND MOST LIFE (if there ever was any)