Interstellar ice

Interstellar ice consists of grains of volatile materials in an icy phase. These grains form within the interstellar medium, which is the space between stars. This ice and dust are the primary materials used to build solar systems. You can find these ice grains in the dense regions of molecular clouds. These clouds are the massive nurseries where new stars are born. Because these regions are so cold, they allow molecules to freeze onto dust grains. This process creates the foundation for planets and other celestial bodies.

The formation of these ice mantles follows a specific step-by-step process. First, molecules collide with tiny dust grains in the cold environment. Once they land, the atoms undergo thermal motion across the surface of the grain. This motion allows the atoms to eventually form chemical bonds with one another. As these bonds form, new substances like water and methanol are created. This layer of frozen material is called an icy mantle. Most of these mantles are amorphous, meaning they lack a structured shape. They only become crystalline when they are near a star.

Interstellar ice is made of many different chemical components. Water is the most common substance found in these grains. In sites observable from Earth, water makes up about 60% to 70% of the ice. Methanol is another major component found in these clouds. Other important molecules include ammonia, carbon monoxide, and carbon dioxide. You might also find frozen formaldehyde and molecular hydrogen. There are also smaller amounts of nitriles, ketones, esters, and carbonyl sulfide. Each of these chemicals contributes to the complex makeup of the interstellar medium.

Scientists determine what these ices are made of by using an infrared spectrum. This is a way of studying light patterns to identify substances. When starlight passes through a molecular cloud, the molecules in the cloud absorb energy. This absorption happens at specific frequencies of vibration for the gas and dust. By looking at these absorption patterns, scientists can see the "fingerprints" of the ice. They then compare these patterns to known ice samples found here on Earth. This comparison allows them to identify the exact composition of the distant ice.

In September 2012, NASA scientists shared important findings about organic chemistry in space. They studied polycyclic aromatic hydrocarbons, which are often called PAHs. They found that under the conditions of the interstellar medium, these PAHs undergo changes. These changes include hydrogenation, oxygenation, and hydroxylation. These processes transform the PAHs into much more complex organic molecules. This is a vital step toward creating amino acids and nucleotides. These are the raw materials needed to build proteins and DNA.

This chemical transformation also explains why some materials are hard to find. As PAHs transform into complex organics, they lose their specific spectroscopic signature. This loss may be why scientists struggle to detect PAHs in certain areas. They are often missing from the outer regions of cold, dense clouds. They are also frequently absent from the upper molecular layers of protoplanetary disks. Understanding these chemical shifts helps scientists map the evolution of matter in space.

Interstellar ice is incredibly old and connects directly to our own world. Research published in the journal Science suggests that much of our water is ancient. About 30% to 50% of the water in our Solar System existed before the Sun was born. This includes the water in Earth's oceans and the rings around Saturn. It is also found in meteorites from other planets. This means the water you drink today has roots in the deep history of the galaxy.

We have even seen how tough this ice can be through spacecraft missions. In November 2014, the Philae spacecraft landed on comet 67P/Churyumov–Gerasimenko. The mission revealed a large amount of water ice beneath a layer of dust. A team using the MUPUS instrument tried to hammer a probe into the surface. They found that the ice was surprisingly hard. Even as they increased the power of the hammer, they could not penetrate deep into the surface. This shows that the ice in our solar system can be a very solid and durable material.