all three began with volcanically-outgassed CO 2 - and H 2 O- rich atmospheres the mass of Mars is the essential reason its atmosphere is so much thinner today than those of Venus and Earth if Venus had the mass and size of, say, earth's moon, it would probably have a nitrogen-rich rather than a CO 2 -rich atmosphere, but the atmospheric density would be about the same in either case as long as there is a source of atmospheric gases, the ability of any 
(the atmospheric density; would be; about the same in either case; T:as long as there is a source of atmospheric gases, the ability of any) (the mass of Mars; is; the essential reason its atmosphere is so much thinner today) (its atmosphere; is; so much thinner; T:today) (Venus; had; the mass and size of, say, earth's moon) (it; would probably have; a nitrogen-rich rather than a CO 2 -rich atmosphere) Context(it would probably have,List([244, 266))):(all three; began; with volcanically-outgassed CO 2 - and H 2 O- rich atmospheres the mass of Mars is the essential reason) (there; is a source of; atmospheric gases)
it would probably have,List([244, 266))
the atmospheric density the mass of Mars its atmosphere Venus it all three there
would be is is had would probably have began is a source of
about the same in either case T:as long as there is a source of atmospheric gases, the ability of any
the essential reason its atmosphere is so much thinner today
so much thinner T:today
the mass and size of, say, earth's moon
a nitrogen-rich rather than a CO 2 -rich atmosphere
with volcanically-outgassed CO 2 - and H 2 O- rich atmospheres the mass of Mars is the essential reason
atmospheric gases