Earth, Mars, and Venus are volcanically active today or
have been volcanically active in their geological past, and
all must have shared the intense cometary showers of the
distant past. The similarity in the geological histories of
these three planets suggests that their early secondary atmospheres
might also have been quite similar. Venus and
Earth are similar in both mass and composition. Mars also
has a similar composition, but its mass is only about a tenth
that of Earth or Venus. Venus is 0.3 astronomical unit (AU)
closer to the Sun than Earth is, and Mars is 0.5 AU farther
from the Sun. Table 9.1 shows that the atmospheres of
Venus and Mars today are nearly identical in composition.
They are both composed mostly of carbon dioxide, with
much smaller amounts of nitrogen. Earth’s atmosphere, in
contrast, is made up of mostly nitrogen and oxygen, with
only a trace of carbon dioxide