How Kryptonite Works

Just before the explosion that destroyed Krypton, Kryptonians Jor-El and Lara outfitted their son Kal-El's birthing matrix for space travel. They sent Kal-El to Earth, where Kansas farmers Martha and Jonathan Kent found and adopted him. The Kents named the baby Clark, and he grew up to be Superman.

As Kal-El's birthing matrix traveled through space, it pulled fragments of the destroyed planet, made radioactive in the explosion, along in its wake. This radioactive debris became known as Green Kryptonite, or simply kryptonite, and it is deadly to super-powered Kryptonians. It does not react with oxygen, so it does not combust when it enters Earth's atmosphere. However, kryptonite is not invulnerable -- you can cut it, chip it, crush it and melt it with acid.

When exposed to Green Kryptonite, super-powered Kryptonians instantly become weak. With prolonged exposure, they die. Green Kryptonite has this effect because the interaction between two substances: its radiation and the Kryptonian's cells.

On Earth, a variety of naturally-occurring and manufactured substances emit radiation. They do this through one of three processes:

Atoms that undergo any of these processes often have lots of extra energy. They emit this energy as gamma rays, which are electromagnetic pulses -- they're made of energy, not matter. Each of these forms of radioactive decay creates ionizing radiation, which can knock electrons off of atoms. X-rays, another form of electromagnetic energy, are also a form of ionizing radiation. Check out How Nuclear Radiation Works for more information.

Relatively speaking, alpha and beta particles cannot penetrate very far into matter. Gamma rays and X-rays, on the other hand, can penetrate matter, including human bodies. Their ability to displace electrons from atoms can cause cells to mutate, sometimes causing cancerous tumors. Fortunately, lead blocks both gamma rays and X-rays. It's able to do so because of its high electron density. The rays are unable to penetrate the dense web of electrons found in a piece of lead.

Like radioactive Earth elements, kryptonite emits radiation, although exactly how kryptonite atoms decay is unknown. However, kryptonite radiation seems to behave like gamma or X-ray radiation -- it can penetrate objects and living bodies but cannot penetrate lead. This suggests that kryptonite radiation is a form of electromagnetic energy, like gamma or X-rays, rather than particles of matter.

If Superman comes into contact with Green Kryptonite, he instantly becomes very weak. With enough exposure, he could die. Kryptonite has this effect because of the way it interacts with Superman's cells.

Much of Superman's power comes from the Earth's yellow sun. His cells are like living photovoltaic, or solar, cells -- they can store the energy from sunlight. Inside a photovoltaic cell, light comes into contact with a semiconductor, like silicon. The light's energy releases electrons from the silicon, and an electric field forces them to flow in one direction. In this way, a solar cell produces electricity.

You could also compare Superman to a plant that uses photosynthesis to make its own food. Through photosynthesis, plants use energy from the sun to turn water and carbon dioxide into oxygen and sugar.

It's unclear exactly which method best describes the way Superman's cells use sunlight, if either of them describe it at all. It's also unclear exactly why light from a yellow sun affects Superman in a way that light from a red sun does not. If Rao were a red dwarf, the explanation could be simple -- yellow stars are bigger, brighter and hotter than red dwarfs. However, Rao is a red giant, meaning it is bigger, brighter, and a little cooler than the Earth's sun. Although we do not know precisely why yellow light is so important, we do know that Superman needs it in order to have super powers.

There are a few possible explanations for how Green Kryptonite keeps Superman from getting power from the sun:

As a result, Superman can no longer take advantage of the powers Earth's yellow sun gives him -- he loses his powers, and he may die.

Green Kryptonite is the most abundant and most frequently used form of kryptonite. But it wasn't always been part of the Superman universe, and it wasn't always green. An unpublished Superman story from the 1940s featured a precursor to kryptonite, a substance called K-metal. The deadly element itself made its debut in the "Superman" radio series -- not the comic book -- in 1943. Its original purpose was to give voice actor Bud Collyer, who played the role of Superman, a vacation. With Superman incapacitated by kryptonite, another voice actor could supply his incomprehensible moans, filling the role until Collyer returned.

Kryptonite made another radio appearance in 1945 and appeared in a movie serial in 1948. But it didn't find a place in the comics until "Superman" #61 in 1949, more than 10 years after Superman's debut in "Action Comics" #1. Radio scripts and comic book art portrayed the substance as red, grey, green and metallic, but eventually the writers settled on green as the color of kryptonite.

In addition to "plain" Green Kryptonite, multiple varieties have appeared on the scene through the years. These varieties are different isotopes of the same element, and they can come in different grades, or strengths. The different isotopes have distinctly different effects on Superman and other life forms, but these effects most likely all stem from the disruption of cells. Some varieties of kryptonite appeared in only one comic book issue or story arc. For example:

In addition, red, white, blue and gold varieties of kryptonite, created through various means, appeared in the comic books until 1985.

By the 1970s, Kryptonite was everywhere -- common criminals had pieces stashed away as protection from Superman. In "Superman" #233, an experiment gone wrong transformed all of the Green Kryptonite on Earth into iron. "Superman" #255 eliminated it from the rest of the universe. But soon, the radioactive mineral was back.

The year 1985 marked a turning point for Superman, Kryptonite and the entire D.C. Comics universe, which had become a multiverse full of alternate worlds. A miniseries called "Crisis on Infinite Earths" made major changes to the comic books' reality. The miniseries is an example of retroactive continuity, or retcon. Retcon is an attempt to clean up years of comic book history and get rid of unnecessary characters, plot holes or, in this case, whole universes. These changes are retroactive -- the post-Crisis universe is not simply the way the D.C. Comics world works now; it's the way it has always worked.

In the post-Crisis reality, fewer versions of kryptonite exist, and the substance is relatively rare.

In addition to "Crisis on Infinite Earths," another recent retcon attempt is a miniseries called "Infinite Crisis," but it did not substantially affect the use or presence of kryptonite.