About 10 parts per billion
I believe the answer is Mars.
The planet Uranus is known to change color. Its cyan color is due to the presence of methane gas in its atmosphere. As the planet orbits the sun, different concentrations of methane cause it to appear to change color from greenish-blue to a more turquoise hue.
Yes, methane has been detected in the atmosphere of Mars by several instruments on spacecraft and telescopes. The presence of methane is significant because it could potentially be a sign of microbial life or geological activity on the planet. However, the exact source of the methane on Mars is still under investigation.
There is actually Methane in Mars' atmosphere. The finding of methane is a huge leap forward in the understanding of Mars. Even though it is only around 10 ppb. Having methane tells Scientists that there is something going on within Mars. If it is some kind of life-form or vulcanicactivity is not known. But either would be a great leap in understanding the red planet, and the origin of life.
"When H.G. Wells wrote about invaders from Mars, not much was known about the planet."
Lots
The only outer planet without a thick atmosphere is Uranus. It has a thin atmosphere composed mostly of hydrogen and helium with traces of methane.
Not much gas at all, as Mars is a rocky planet with not much of an atmosphere.
Neptune is much colder than Mars.
Uranus is much larger than Mars. Uranus is the seventh planet from the Sun and is classified as an ice giant, while Mars is the fourth planet from the Sun and is known as the "Red Planet".
Mars is an inner planet.
Mars. Not Mars. Jupiter is a lot bigger than Earth, but the Sun is much bigger than a planet.