Letter: Letters : Water shortage

From Craig Lindsay

Aberdeen

You state that: “Roughly a million molecules of water fall into the upper
atmospheres of Jupiter, Saturn, Uranus and Neptune each second”
(In Brief, 13 September, p 27).

At first glance that figure sounds impressive. However, a quick calculation
reveals it to be totally unimpressive. I calculate that to accumulate sufficient
water to fill a normal cup (300 millilitres) at this rate would take some 320
billion years—roughly 20 times the age of the Universe.

Has something been lost in the translation from Nature to New Scientist? If
not, it seems that any inhabitants of those planets would hardly need to prepare
for a deluge.

A million molecules spread over the whole planet is indeed a minuscule
amount. What we meant to say was “a million molecules per square centimetre at
the top of the atmosphere”, which yields a rather larger amount of water.
However, that is still far less than the amount of water supposedly falling to
Earth in the form of mini-comets—Ed.