CBSE Class 9 Answered

Mars' atmosphere is both shallow and thin, less than 1% as dense as Earth's. Even if liquid water were present (which it is not), the atmosphere cannot hold any appreciable water vapor. Much of Mars' oxygen is locked into surface rocks, and the remainder is combined as carbon dioxide. While CO₂ makes up 95% of the Martian atmosphere, the low pressure means insufficient concentrations to support Earth vegetation. And while Martian temperatures may reach 20°C (68°F) in the Martian summer, it can also be as low as -140°C (-220° F). The atmosphere is highly inefficient at heat transfer.

Venus is the other extreme of conditions. While Mars' atmosphere is cold and rarified, Venus' is hot and dense. The same net result is the absence of liquid water or vapor transport. Also the same is the predominance of carbon dioxide, with much less nitrogen than Earth. The temperature and pressure of Venus are much greater than Earth : the surface pressure is 92 times as great, and temperatures average 460°C (770° F). The atmospere is highly efficient at heat transfer, so there are few places on the surface more than a few degrees cooler. The only "rain" is droplets of sulfuric acid in high Venusian clouds of sulfur dioxide.