Mars
- Low atmosphere
- low pressure
- very cold
"The reason it is so hard to land on Mars is that the atmospheric pressure is low, less than 1% of Earth’s surface pressure. This means that any probe will descend very rapidly to the surface, and must be slowed."
Venus
- extremely hot
- high pressure
- high atmosphere
"In the planet’s dense atmosphere, carbon dioxide, a greenhouse gas, somehow built up to toxic levels, now accounting for 96.5 percent of Venus’ air, as compared to 0.04 percent of Earth’s."