How would Mars's atmosphere have evolved differently if the planet had been much closer to the Sun, for example, in Venus's orbit?a. Additional solar heating of Mars's surface would have compensated for its lack of size, so it would have outgassed more water, resulting in atmospheric evolution more similar to Earth's.b. The initial atmosphere of Mars would have been denser and hotter at a location closer to the Sun. Later that atmosphere would mostly have leaked to space because of Mars's small size and higher temperatures. The result would likely be similar to the thin atmosphere of Mars that we know today.c. In the area of Venus's orbit, solar ultraviolet radiation causes a runaway greenhouse effect regardless of the planet's size, so the atmosphere of Mars would have been similar to that of Venus today: thick and hot.d. Solar wind and ultraviolet radiation from the Sun would have completely destroyed Mars's atmosphere and made it similar to the Moon and Mercury at that distance.

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