The whirling chunks of dry ice skimming across the water surface in “Icy Bodies” bring to mind some aspects of the behavior of comets.  Like the dry ice in “Icy Bodies,” comets are also made of ices — mostly water ice, methane ice, and ammonia ice (with some carbon monoxide ice).  In contrast to most large bodies of matter, which contributed their mass to the formation of the planets, comets were ejected deep into space as a result of their encounters with those planets.

In “Icy  Bodies” the dry ice shards spew vapor because they are ejected into a warmer liquid.  The reaction of the dry ice and warm water causes sublimation (the process in which a solid turns into a gas without first becoming a liquid).  When comets emit vapor it is because the Sun is warming them.  Most comets spend almost all their time in a very deep freeze, too far from the Sun to undergo the process of sublimation.  Only a tiny fraction of them plunge into the inner Solar System on orbits that allow us to see them as they are warmed by the Sun. 

Photo Credit: Conrad Jung

The vapors being released by the chunks of dry ice in the sculpture are actually propelling these “Icy Bodies” and always trail behind them.  The tails of comets act more like windsocks, always pointing away from the Sun, as they blow in the “solar wind,” a stream of ionized hydrogen and helium that radiates outward from the sun. Thus, a comet's tail follows the comet when it approaches the Sun and precedes it when it is traveling away from the Sun.  The “Icy Bodies” here are easily visible, but real comets are so small (a few miles across) and so far away (usually hundreds of millions of miles) that generally all we can see are the vapors.  

Right around a comet is a cloud called a “coma,” which can be seen as a bright pinprick of light.  Comets have two tails.  One is the yellowish “dust tail” which is composed of rocky dust and heavier molecules and tends to be curved due to the comet’s orbital motion.  The other is the bluish  “ion tail,” which is made of charged particles and always points outward because of the velocity of the solar wind.  Although these tails look impressive, they are actually far emptier than the best laboratory vacuum we are able to produce.

Although comets, unlike the sculpture’s “Icy Bodies,” are not actually propelled by their vapor tails, something similar can happen to them, to a lesser extent.  As the ice begins to violently boil off the surface of a comet, it can have a “rocket effect” and actually change the comet’s orbit.  Thus, even the periodic comets (those that return on a predictable orbit) may have slightly different paths each time they appear.  The dry ice shards in “Icy Bodies” evaporate completely after a time, and that will happen to comets too if they end up on orbits that bring them repeatedly into the warm inner Solar System. When comets evaporate, they leave behind a stream of rocks, which are responsible for the various meteor showers the Earth passes through each year.  When the “Icy Bodies” in the sculpture evaporate, nothing perceptible is left.

Some of the effects visible in ”Icy Bodies” do not occur with comets. Because the dry ice shards in “Icy Bodies” are very light bodies moving on top of a liquid, certain effects of surface tension (a force that causes molecules near the surface of a liquid to experience unequal attraction) are at play. Sometimes the dry ice shards repel each other and sometimes they collide and stick, depending on the ways surface tension push and pull them.  If the “Icy Bodies” in the sculpture were much heavier you would not see them careen across the water.  An apt analogy is that water striders (spider-like insects) can skate across the top of a stream but frogs cannot. 

Comets, unlike the ice shards in “Icy Bodies,” are not skimming across water.  There are trillions of them in the Sun’s retinue, but they are moving through such a vast volume that they almost never come near each other or another planetary body.  Comets either move through space indefinitely or get pulled close enough to the Sun to undergo sublimation.