Final answer:
Uranus and Neptune appear blue because methane gas in their atmospheres absorbs all colors of sunlight except blue, which is then scattered. This effect is similar to the scattering of sunlight in Earth's atmosphere that makes the sky appear blue.
Step-by-step explanation:
Why Do Uranus and Neptune Appear Blue?
The blue appearance of Uranus and Neptune is due to the way methane gas in their upper atmosphere interacts with sunlight. Unlike the other giant planets in our solar system, which may have clouds composed of ammonia or ammonia ice particles with various colorants, the upper clouds on Neptune are made of methane. The methane gas in the atmosphere of both Uranus and Neptune absorbs all colors of sunlight except blue, which is then scattered, giving these planets their characteristic blue hue.
Neptune, in particular, presents with a pale blue color similar to Earth's atmosphere due to the scattering of sunlight by gas molecules in the upper atmosphere. The effect is akin to the Rayleigh scattering that occurs in Earth's atmosphere, which is why the sky appears blue to us here on the surface. Uranus similarly owes its color to methane, but it has a more featureless appearance because it lacks an internal heat source that would otherwise drive atmospheric convection and create more distinct cloud formations.
Overall, option b) 'The methane gas in their atmospheres absorbs all colors except blue.' is the correct answer to the question of why Uranus and Neptune appear blue.