ack no edit.
that should say hydrogen is too light to exist in our atmosphere (in any appreciable amount)
and Venus if you tossed it out around Mars' location, I'd expect a vastly different evolutionary path it would have taken. The surface gravity is still 90% of Earth's after all.
It would...the water that once where in Venus atmosphere has been stripped away by the solarwind...put it out in Mars orbit and things look differently...the "double distance -> square thingy" matters