Mars is a frozen desert and Venus a 465-degree pressure cooker, yet both atmospheres are about 96% carbon dioxide—and because neither planet has a global magnetic field, both are still leaking atmospheric particles into space under the solar wind’s assault.

Mars is a frozen desert and Venus a 465-degree pressure cooker, yet both atmospheres are about 96% carbon dioxide—and because neither planet has a global magnetic field, both are still leaking atmospheric particles into space under the solar wind’s assault.

Mars and Venus offer one of planetary science’s cleanest warnings about percentages. Carbon dioxide accounts for about 95.3 percent of the Martian atmosphere and 96.5 percent of Venus’s. Rounded to the nearest whole number, both are 96-percent carbon-dioxide worlds. Stand at either surface, however, and the similarity disappears. Mars is a cold desert beneath air so thin that liquid water is unstable in most present conditions. Venus is hotter t…