COMPARISON OF TERRESTRIAL PLANETS

- ON EARTH DIFFERENTIATION CONTINUES TODAY AS

PLANET COOLS BY THERMAL CONVECTION - PLATE TECTONICS, SO PLANET IS "GEOLOGICALLY ALIVE"

WITH MOUNTAIN BUILDING, EARTHQUAKES,

VOLCANOES, ETC.

- ON MERCURY AND MARS, - SMALLER PLANETS THAT LOSE HEAT FASTER - COOLING HAS GONE SO FAR THAT THEY HAVE THICK LITHOSPHERE AND ARE "GEOLOGICALLY DEAD" (small animals have to eat a lot more times their weight to keep alive in cold weather)

- NOT CLEAR ABOUT VENUS

- EARTH IS "GEOLOGICALLY YOUNG" - MERCURY AND MARS

ARE "GEOLOGICALLY OLD" - THOUGH THEY FORMED AT THE SAME TIME (dog years versus human years)

- MAYBE: END OF PLATE TECTONICS ON MARS STOPPED

VOLCANISM, WHICH PRODUCES "GREENHOUSE" GASES

IN ATMOSPHERE THAT KEEP PLANET WARM, SO MARS'

SURFACE GOT TOO COLD FOR LIQUID WATER AND

MOST LIFE (if there ever was any)