The nearest planet to the sun is the last place one would expect to find water — or anything — frozen. The universe, however, is always full of surprises. Mercury is well known as the most scorched planet in our solar system. At only 36 million miles from the sun and with extremely long daytimes, the surface of Mercury can reach an astounding 800°F. Hardly the environment for ice.

Image credits: MESSENGER / NASA

What a shock it was then, in 1991, when astronomers at the Arecibo Observatory in Puerto Rico discovered circular patches of “extremely reflective” material radiating from Mercury’s surface. The data from the observation suggested the presence not just of water, but of ice on Mercury, an idea previously thought impossible. Since the data just from radar information alone was inconclusive, the matter was greeted with some skepticism for years.

NASA’s more recent Messenger spacecraft has now gathered the best photos and data ever of the possible crater ice, bringing scientists closer to a conclusion; Mercury, despite its scorch, appears to harbor pockets of perpetual water ice.

History of Mercury

The strongest theories of Mercury’s formation state that Mercury originally formed as a much larger planet, but lost approximately half of its mass to the violent fluctuations of the primitive sun, and/or possibly to a collision with a planetesimal (a small planet). The sun theory proposes that Mercury’s original crust may have been vaporized by 8000° F plus surface temperatures imposed by the early sun’s hot and volatile emissions. Mercury may have been originally composed of material with a different chemical composition, but those with a lower vaporization point would have been eliminated.

This could reasonably explain why today Mercury is the only planet in our solar system to contain such a disproportionate amount of metal and silicate, and little else. The composition is roughly two-thirds metal and one-third silicate. Its planetary rotation is extremely slow; about 60 earth days are required to equal one day on Mercury. This causes some portions of the planet to endure prolonged sun exposure and extreme heat while plunging other areas into long, frozen darkness.

Mercury is covered in the multitude of craters that characterize the rocky planets and satellites of our solar system. There is no geologic surface activity, and it lacks a geologically active core, as evidenced by the long-undisturbed craters. Due to its small size and geological constitution, Mercury also lacks any notable atmospheric layer.

How is water ice possible in such a place?

NASA’s MESSENGER found new evidence for water ice at Mercury’s poles

Because of Mercury’s narrow axis, slow rotation, and lack of heat-trapping atmosphere, it is possible to house pockets of frozen water on Mercury’s surface. Mercury, in fact, exhibits the broadest temperature variation of any of the planets in our solar system. The planetary poles are permanently shadowed as are some of its craters. In contrast to the sun-drenched oven on the regular surface, these dark areas often drop to -290°F, more than cold enough to keep water frozen forever. Modern interpretations of Mercury’s undisturbed, ancient craters indicate that there has not been any geological or volcanic activity in a very long time. Without an atmosphere to trap and disperse heat nor any geothermal heat from Mercury’s center, the craters are in permafrost.

The extraterrestrial origins of water ice on Mercury

Where did all the water come from in the first place? Water is actually fairly plentiful in the galaxy. Hydrogen is found everywhere as the chemical basis of most inorganic matter. Oxygen is produced as a byproduct of star activity. When they meet under cooler temperatures, H2O, or water, is the usual result. As a matter of fact, most of the universe’s oxygen is tied up in water and carbon dioxide, so the availability of extraterrestrial water is in no short supply.

The question, of course, is how it was delivered to Mercury. The most popular theory is that ice-filled comets and asteroids pummeled Mercury and the rest of the solar system at a turbulent time early in the solar system’s history, releasing countless tons of water onto each of the planets. Much of the interest is centered around a class of meteorites known as carbonaceous chondrites, which are known to contain substantial amounts of ice in addition to a fascinating mixture of prebiotic organic ingredients, such as amino acids, a discovery that will surely lead to more astonishing revelations as we learn more.

How was water ice discovered on Mercury?

In 1991, Puerto Rico’s Arecibo radio telescope transmitted a circularly polarized, coded radar wave toward Mercury. The wave was reflected off Mercury and back toward Earth, where Arecibo received its images. What they found was that although Mercury’s silicate component is already very reflective, there were also circular areas of an even brighter reflectivity near the poles. At the time, the areas were suspected to be water ice, but there was no other data on which to investigate.

Doubt has now been almost completely eradicated with the data received from the recently completed MESSENGER spacecraft project. An acronym for “Mercury surface, space environment, geochemistry, and ranging”, MESSENGER began orbiting Mercury in 2011 and continued to send the most comprehensive data ever collected until it ran out of fuel and crashed into Mercury’s surface in 2015. The new data left little question as to whether or not there is water ice on Mercury.

MESSENGER used laser pulses, fired at the planet’s surface, to create highly detailed maps. Like before, reflective anomalies at the poles suggest the presence of water, but this time were correlated with up-to-date temperature models that confirmed the reflective areas as frozen water. Columbia University’s principal investigator, Sean Solomon, is quoted as saying: “For more than twenty years the jury has been deliberating on whether the planet closest to the sun hosts abundant water ice in its permanently shadowed polar regions. MESSENGER has now supplied a unanimous affirmative verdict.”

With one question answered, more are raised

With the question of the existence of water ice on Mercury resolved, it leads to more questions about its origins, those carbonaceous chondritic meteorites. Along with the discovery of extraterrestrial water not only on Mercury, but also on Mars and our moon, has come the more astonishing revelation of extraterrestrial organic compounds, such as amino acids. This has the potential to change everything known about the origins of life on Earth, and the possibility of similar organic evolutions elsewhere. Is life extraterrestrial in origin? Did the building blocks of nucleic replication ride in on a meteorite from a distant, dark unknown? The answer to such questions may never be found, but it will certainly compel the fundamental truth-seeking that is the engine of all of our discoveries.