Mercury is the sun’s smallest and innermost planet. Like Venus, it is a lesser planet.

A result of Mercury’s closeness to the sun, it can only be seen from Earth in the eastern and western horizons.

Mariner 10 and MESSENGER have both visited Mercury recently, during their swing around the solar system.

The surface is similar to the Moon’s, with massive craters suggesting lengthy periods of geological inactivity.

After being studied by both spacecraft, this planet is presently the least known. This will change with new probes.


Mercury’s name comes from the Roman era.

The five brightest planets, the Sun, the Moon, and the Earth were all they knew at the time. They called these objects after their most beloved deities out of superstition. They named the planet Mercury, their messenger deity and traveling god. Mercury is often depicted in a winged cap and winged shoes.

Mercury in winged cap and winged shoes
What’s so hot about Mercury?

Mercury is one of the five visible planets.

As a result, the planet’s exact discovery date is uncertain. Thomas Harriot and Galileo Galilei were the first to use a telescope to study it.

What are little planets made of?

You could call Mercury a heavy metal planet, and you wouldn’t be far off the mark. Scientists have established that the planet is composed of seventy percent metallic material and thirty percent silica.

Most of that metal is iron, one of the heavier elements. In fact, after our satellites passed by the planet they discovered that the core of Mercury is pretty much all iron – and the same size as the iron/nickel core of the Earth! Since the planet is so much smaller than Earth, that means the core on Mercury is over fifty percent of its total volume. Here on Earth, our core is just a measly seventeen percent. All that heavy metal makes Mercury very dense, and it takes its time to rotate. If you are a dedicated procrastinator you should think about living on Mercury, where one day equals fifty-eight Earth days.

Such a large molten core gives Mercury top honors among planets in the solar system for volcanoes. Although in the case of Mercury the active volcanoes are not the big fiery mountains we have here on Earth. They are gaping fissures on Mercury’s surface, constantly belching out pyroclastic material from the core, spreading out for hundreds of miles and actually adding to the thickness of the planet’s crust. Of course, with daytime temperatures more than 800 degrees Fahrenheit, the molten lava doesn’t have much chance to cool down and solidify. But wait – there’s more! The reason that Mercury is not a big puddle of pyroclastic mush is because at night (and those are long nights, remember – 58 Earth days) temperatures plunge to below -189 degrees Fahrenheit. That’s cold enough to keep even a Popsicle happy!

If you doubt the solidity of Mercury’s surface (as many astronomers and astrophysicists did right up until the 1960s) you have just to look at the findings from the Goldstone Deep Space Communications Complex in Barstow, California. Huge impact craters, like those on our Moon, have been detected on Mercury, with clear radial streaks indicating a large meteor impact.

Since the planet has a hard crust it might be possible for space craft to one day land on the planet to explore it in more detail. Or at least fly around it in a low enough orbit to get a better view of the place.

And, in fact, some enterprising international agencies are doing just that. Several years ago the Japan Aerospace Exploration Agency (abbreviated to initials JAXA) and the European Space Agency (known to its fans as ESA) launched a probe meant to orbit Mercury starting in 2025. This probe will send back information on indications of ice at Mercury’s poles and water vapor inside its many crepuscular volcanoes. Water is the holy grail to exobiologists – those men and women dedicated to the proposition that all life starts with water – because if there is even trace amounts of H2O on Mercury it could mean the chance for life to form there.

The Mercury probe is named BepiColumbo.

Professor Giuseppe (Bepi) Colombo (1920-1984), a mathematician and engineer from Padua, Italy, is the inspiration for the name BepiColombo.

An unknown resonance causes Mercury’s habit of revolving three times every two orbital cycles around the Sun, which he discovered.

Another reason Mercury is of great interest to scientists here on Earth is because the molten core of the planet is so similar to Earth’s. This being the case, scientists are anxious to find out more about the core there, which is much closer to the surface of the planet, in order to discover more about our own core. To date, it remains technologically impossible to penetrate the Earth’s mantle far enough to study the core in any detail. In other words, we don’t really know what that molten ball of metals is up to down there. Scientists have discovered that it rotates at a faster rate than the outer mantle and crust. And that that has something to do with why the Earth’s magnetic poles move around every million years or so. Reverse polarity is a real phenomena, not just a sci-fi term, and it could have a very severe effect on communications and utilities if it happens anytime soon. So scientists would like to know how to predict it in advance. Which may happen if BepiColombo is able to gather enough detailed information from Mercury.

How it all started

Scientists generally believe that the core accretion model helped form Mercury. Meaning that billions of years ago a clump of dust and gas meandering around the Sun decided to gravitate together. Eventually it formed into Mercury. Mercury is not a stray moon, nor a chunk broken off from somewhere else. It is completely unique.