Comparing Venus, Earth, and MarS
VENUS
Venus is the second planet from the sun, between the orbits of Mercury and Earth. Because it is closer to the Sun
than Earth, the sunlight that strikes it is almost twice as intense as that which strikes Earth. The Venusian
atmosphere is 96% CO2. This high concentration of carbon dioxide results in a runaway greenhouse effect such that
the surface temperature is 477 OC. The atmosphere is much “thicker” than Earth’s. So much that the air pressure at
the surface of the planet is 90 times that on Earth. It is so thick that only about 25% of the sunlight that strikes it
passes to the surface; the remaining 75% is reflected to space. Consequently there is a constant eerie, reddish light at
the surface of the planet during the day.
The Venusian atmosphere has no oxygen to speak of. It is filled with clouds made of sulfuric acid, a very
corrosive substance. The planet rotates slowly compared to Earth. One day on Venus is as long as 117 days on
Earth. The surface of Venus is similar to Earth’s in composition and density, and Venus is only 5% smaller than
Earth. Venus is considered more similar to Earth than any other planet in the Solar System.
MARS
Mars, the next planet beyond Earth, is similar to Earth in several ways as well. The tilt of its rotational axis and its
rotational speed are almost identical to Earth’s. So the Martian day is about the same length as Earth’s, and Mars
has seasons like Earth. The Martian atmosphere is very thin. The air pressure at the surface is 166 times less than
that on Earth. Carbon dioxide constitutes 95% of this atmosphere. But unlike Venus there is no substantial
greenhouse effect. So the planet is subject to a wide range of temperatures. The temp can be as low as -128O C or as
high as 37O C during a Martian summer. At the north and south poles, the temperatures remain cold enough yearround to maintain permanent ice caps made of frozen carbon dioxide (dry ice). The atmospheric pressure on Mars is
far too low to allow the existence of liquid water. It is believed that what water does exist on Mars is frozen either
beneath the polar ice caps or in clouds. The surface of Mars is extensively covered with iron oxide (rust) giving the
planet its characteristic red color.
On the back: Create a Venn diagram comparing Venus, Mars, and Earth. Use the information to explain
why Earth is best suited for life to exist.