Mercury A to Z: Amorphous Ice

Hello, friends!  Welcome to my very first post for this year’s A to Z Challenge.  You don’t know what the A to Z Challenge is?  That’s okay.  You can click here if you want to learn more.  My theme for this year’s challenge is the planet Mercury, and in today’s post the letter A is for:

AMORPHOUS ICE

It gets really hot on Mercury.  You probably knew that already.  Mercury is, after all, the planet closest to the Sun.  But it may surprise you to learn that it also gets really cold on Mercury.  Extremely cold.  Like, we’re talking spit-goes-clink levels of cold.

Much like the Moon, Mercury has virtually no atmosphere.  That means there’s no atmospheric convection to transfer heat from the dayside of Mercury to the nightside.  Atmospheres can also act as a sort of blanket to keep a planet’s surface warm during the night.  But again, Mercury has virtually no atmosphere.  No blanket effect.  All the heat Mercury’s surface soaks up during the long Mercurian day is lost during the equally long Mercurian night.  As a result, the nightside of Mercury is one of the absolute coldest places in the entire Solar System.

Now, imagine if there were a place on Mercury where it is always night and never day.  Places like that exist at the bottoms of deep, dark craters clustered around Mercury’s north and south poles.  Shielded by crater rims and tall crater walls, the bottoms of those polar craters are cloaked in eternal darkness, and they are eternally cold.  Anything that happens to fall into one of those craters would freeze solid and could stay frozen for millions or even billions of years.

Back in the 1990’s, scientists began to suspect that those deep, dark craters around Mercury’s poles might be full of water (frozen water, obviously, but still… water).  And then in the 2010’s, NASA’s MESSENGER space probe took a closer look and confirmed it.  There is, in fact, water (in ice form) on Mercury.  Water on Mercury, of all places!

But I remind you again, the bottoms of Mercury’s polar craters are obscenely and stupidly cold.  Too cold for water to freeze the way it freezes on Earth.  On a molecular scale, the ice we find here on Earth has a neat and orderly crystalline structure.  Scientists call our Earthly kind of ice “ice Ih” or “hexagonal ice,” because the water molecules fit together in a hexagon pattern.  But the ice on Mercury is more likely to be what scientists call “amorphous ice.”

Amorphous ice is what happens when water freezes so fast that the water molecules don’t have time to arrange themselves in any sort of crystalline structure.  On a molecular scale, the water molecules are scattered haphazardly about.  No hexagons.  No patterns or shapes.  The ice is structurally shapeless—a.k.a., amorphous.  This doesn’t occur often here on Earth, except in certain astrophysics laboratories, but amorphous ice is extremely common out in space.

Comets and asteroids?  Whatever water they have is, partially or wholly, in the form of amorphous ice.  The surfaces of Europa, Ganymede, and the other icy moons of the outer Solar System?  They may be partially composed of amorphous ice.  And the ice inside those polar craters on Mercury (and similar polar craters on the Moon)?  You can bet on that being amorphous ice, too.

WANT TO LEARN MORE?

Water can freeze in so many different ways, with so many different crystalline and non-crystalline structures.  Here’s a brief video from Sci-Show about all the different kinds of ice scientists currently know about.

I also want to recommend this article from ZME Science, briefly summarizing the history of how water ended up on Mercury, how scientists on Earth first detected it, and how the MESSENGER mission later confirmed it.

Lastly, this is a far more technical source than the other two, but this paper on amorphous ice in the Solar System is the best source I could find stating, explicitly, that the ice on Mercury is probably amorphous ice.