Hot Water

12 October 2014

A group of sunspots. NASA Goddard Space Flight Center
A group of sunspots.

Yesterday I talked about how water can form on the Moon. It might seem a bit surprising that water exists on the Moon, but it doesn’t sound like a crazy idea. What does sound crazy is the idea that there is water on the Sun’s surface, and yet we know that there is.

The surface of the Sun (specifically the photosphere) has a temperature of about 6000 K. It is so hot that hydrogen atoms are ionized, and molecules can be ripped apart. It’s pretty inhospitable for a molecule such as water. Despite this, the potential for water is there. Oxygen is produced in stars through the CNO fusion cycle, and we have observed quantities of oxygen in the Sun’s spectra. Hydrogen is the most abundant element in the universe, and most of the Sun’s mass is hydrogen. All that’s needed is a cool enough temperature for the hydrogen and oxygen to come together to form water.

It turns out there is just such a place in sunspots. We normally think of sunspots as dark regions on the Sun. They aren’t actually dark, but they are cooler and dimmer than the rest of the Sun, which is why they appear dark in solar images. Within a large sunspot, the temperature can be as cool as 3500 K, which is cool enough for water to form. Naturally it only exists as water vapor, but there really is water on the Sun’s surface.

The red line is the theoretical spectra for water at 3000K. The blue line is the observed curve in sunspots. Polyansky, et al
The red line is the theoretical spectra for water at 3000K. The blue line is the observed curve in sunspots.

The presence of water on the Sun has long been suspected, but proving it has been a real challenge. That’s because water has a complex spectra with millions of absorption lines. These lines also vary with temperature, making it even more challenging. Experimentally measuring the line spectra of water vapor at 3500 K isn’t feasible, so you need to calculate the expected spectra using computer simulations.

In 1997, a team did just that.1 They were able to calculate more than 6 million absorption lines for very hot water, and then compared the results to observed spectra deep within sunspots. They found a clear match, showing that water does indeed form within “cool” sunspots.

So yes, there really is water on the Sun’s surface. It’s a fact worth remembering if you ever want to win a bar bet.

  1. Polyansky, Oleg L., et al. “Water on the sun: line assignments based on variational calculations.” Science 277.5324 (1997): 346-348. ↩︎