carbon-dioxide or methane ice) in Mercury

It’s rare that astronomers declare news with great certainty, given the awesome scope of their work and the level to which they must rely on data gleaned from objects so far away that superlatives quickly run out.

So Thursday’s announcement about the confirmation of water ice (as opposed to, say, carbon-dioxide or methane ice) in Mercury’s poles was, as the scientists put it, an “exclamation point” rather than a plain old period.

Three separate research papers, published in Science Express this week, all arrived at the same conclusion: that the planet closest to the Sun harbors a wealth of water ice, plus dark deposits believed to be organic compounds delivered by comets and asteroids.

The amount of ice is astounding—something on the order of 100 billion to a trillion metric tons. Or, as David Lawrence of the MESSENGER Mercury mission team says, the rough equivalent of layering Washington, D.C. with 2 to 2.5 miles of ice.