"Just a moment," Caxton protested. "Captain, even so - Mars attack us?  Mars?  You know more about these things than I do, but wouldn't that be about like us attacking Jupiter?  I mean to say, we have about two and a half times the surface gravity that Mars has, just as Jupiter has about two and a half times our surface gravity.  Somewhat analogous differences, each way, on pressure, temperature, atmosphere, and so forth.  We couldn't stay alive on Jupiter... and I don't see how Martians could stand our conditions.  Isn't that true?"