It is the last stop before Pluto and 4.5bn kilometres from the Sun, and yet Neptune has some of the wildest weather in the solar system. Winds of over 2,000km per hour (nine times faster than Earth’s fastest winds) whip up extreme storms, and exotic clouds (made of ammonia, methane and hydrogen sulphide) come and go. The methane in Neptune’s atmosphere absorbs red light and makes the planet appear blue to us. Meanwhile the high-level cirrus-like clouds, made of frozen methane, give the planet its ever changing pattern of bright white dots and dashes.
But with around 900 times less sunlight than Earth, and an average temperature of -170°C, how does Neptune manage to have such changeable weather? The question is a long-standing conundrum in planetary science, with some scientists saying it must be sunlight variation and others believing galactic cosmic rays (high-energy particles from outer space) have greater influence. Now a new study reveals that it is the combination of the two that influences Neptune’s weather patterns.
Karen Aplin from the University of Oxford and Giles Harrison from the University of Reading studied changes in Neptune’s brightness since 1972 and modelled the effect of sunlight and cosmic rays. “We saw a clear cosmic-ray effect acting in combination with ultraviolet from the Sun,” says Aplin, whose findings were published in Nature Communications.
It seems that the cosmic rays swipe electrons off molecules in Neptune’s atmosphere and, combined with small changes in sunlight, help to modulate the formation of clouds. Which begs the question are cosmic rays influencing the weather on other planets too?