Molecular Monday: Water on Neptune

Welcome to Molecular Mondays! Every other Monday, we examine the atoms and molecules that serve as the building blocks of our universe, both in reality and in science fiction. Today we continue our investigation of:


Meet Neptune, the other blue planet.

Nv01 Neptune Blues

Neptune may be named after the ancient god of the sea, but the planet’s striking blue color is caused by a high concentration of atmospheric methane, not water. Methane absorbs red light and reflects blue light back into space. So Neptune has nothing to do with that most precious commodity in space: water.


Okay, this sort of statement should no longer come as a surprise to me or anyone else, but the planet Neptune does in fact have water. In fact, water is sort of everywhere in space, which makes sense because the water molecule is made of two of the most common elements in the universe: hydrogen and oxygen.

Furthermore, water is the simplest possible combination of hydrogen and oxygen. So of course we’re going to find it everywhere. Even on Neptune. Why would anyone think water is somehow special to Earth? (Why did I think that for so long?)

But water on other planets rarely behaves the way it does here on Earth. As we’ve seen in two previous Molecular Monday posts (click here and here), the freezing and boiling temperatures of water change depending on pressure and salinity. At a certain point, known as the triple point, water’s freezing and boiling temperatures are the same, so water skips over its liquid phase, transitioning directly from gas to solid and vice versa.

Nv01 Water Water Everywhere

So the planet Neptune might actually live up to its sea god name. Not only does it have water, but there is a slim possibility that deep in the planet’s interior, the pressure and temperature are just right to support an ocean of liquid water. If Neptune were just a bit colder, the odds of such an ocean forming would improve dramatically.

Discovering liquid water on Neptune would be pretty cool, but there’s something even cooler (or perhaps hotter) about Neptune’s water. In the next edition of Molecular Monday (two weeks from today), we’ll take a closer look at Netune’s “icy” interior.

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Today’s post is the beginning of Neptune month for the 2015 Mission to the Solar System. Click here to learn more about this series.

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