Question 1

Why can't Mars have liquid water? Why can't Venus?

Atmospheric pressure, solar winds and higher surface temperature.

Explanation:

Water on Mars exists in the form of ice and in very small quantities as vapor form found in the shallow martin soil. Place where the frozen ice is visible is the north pole of Mars and evidence says that water is present beneath the carbon dioxide ice cap in south pole. With a pressure of 600 pascals, the pure water would freeze or get vaporised. The atmosphere of Venus consists of 0.002% of water vapor as solar winds blow away the upper atmosphere and thus the water is lost. Venus has a high rate of rotation. About 95% of the surface os covered y the carbon dioxide with a strong temperature variation.