Final answer:
The key factor for the presence of methane clouds on Uranus and Neptune and their absence on Jupiter and Saturn is temperature. Uranus and Neptune are cold enough to allow methane to condense into clouds, unlike the warmer gas giants.
Step-by-step explanation:
The presence of methane clouds on Uranus and Neptune, but not on Jupiter and Saturn, can be attributed to the atmospheric composition and temperatures of these planets. Jupiter and Saturn have higher concentrations of hydrogen and helium and possess extensive mantles of liquid hydrogen. The uppermost clouds on these two planets are composed of ammonia crystals because of the relatively higher temperatures near their cloud tops, around 140 K.
In contrast, Uranus and Neptune have higher densities and are depleted in hydrogen and helium, which classifies them as ice giants. They contain more icy materials such as water, methane, and ammonia, as well as having a core of ice and rock. While both have similar atmospheric temperatures to Jupiter and Saturn, their colder temperatures favor the formation of methane clouds. Specifically, Neptune's upper clouds are made of methane, forming at a cooler temperature of 70 K and a pressure of 1.5 bars, which is cold enough to allow methane to condense into cloud form.
Therefore, the temperature is the key factor in determining why Uranus and Neptune have methane clouds while Jupiter and Saturn do not. The colder temperatures of the ice giants allow methane to condense into clouds, which is not possible at the warmer temperatures of the gas giants' cloud tops.