Mercury – Iron Planet

Mercury Data

Mass: 3.30 _ 10²³ kg or 0.055 of Earth’s

Diameter: 4878 km or 0.38 of Earth’s

Surface gravity: 0.38 gees

Axial tilt: 0.1°

Mean surface temperature: 427 Celsius

Rotation period: 58.65 days

Orbital period: 87.98 days or 0.24 years

Inclination of orbit to ecliptic: 7.0°

Orbital eccentricity: 0.206

Distance from the Sun: 0.31–0.47 AU

Sunlight strength: 4.5–10.4 times Earth’s

Satellites: 0

The first planet from the Sun is Mercury, only 40 percent larger than our Moon. It is a heavily cratered ball of rock and iron, without a satellite of its own, huddled

in to the searing Sun more than two-thirds as close as the Earth is. On average, Mercury orbits the Sun at 0.39 AU, once in just 87.97 days. It’s a proximity that makes for an exceedingly high surface temperature. Daytime temperatures peak at 430 Celsius at the subsolar point, then drop by more than 600 Celsius at night. No other planet exhibits a more extreme range of temperature. And compared with the orbits of all other planets except Pluto, which are nearly circular, none is more eccentric than Mercury’s. The planet’s significant iron content also gives it a very high density, comparable to the Earth’s. And it is because of its odd orbit and high density that astronomers suspect Mercury’s present appearance might be an accident – the outcome of a cosmic collision deep in the planet’s past.

Physical Overview

Even a cursory glance at Mercury reveals a battered world, not unlike the Moon. The colour is slightly different, though: more coppery than the grey that characterises our satellite. There are no volcanoes, and Mercury is devoid of an atmosphere. It does occasionally capture some gas – helium and hydrogen – from the solar wind, but the density is far too low for it to be considered ‘air’ in any real sense. Also, liquid water has never existed on Mercury – nor any water at all aside from that brought in small amounts by cometary impacts. This is because the planet formed so close to the Sun that such volatile compounds could not condense. And so, lacking volcanism, air and water – which on Earth are powerful forces of erosion – Mercury’s surface is a fossil, geologically dead. It has not changed significantly in several billion years.

For this reason Mercury’s main surface features are its impact craters. These are the scars that betray the heavy bombardment the planet endured after it had formed. Craters appear over almost the entire surface and at a wide variety of sizes, forming a mountainous terrain known as the highlands. However, the surface relief in the highlands is not as high as might be expected. Many of the craters are quite shallow. It is as if they have been flooded by ancient lava flows early in the planet’s history. Some older craters may even have been buried altogether. Because of this the rolling spaces between individual craters, called the intercrater plains, tend to be somewhat smooth. Meanwhile, there are other, much more extensive smooth areas called lowlands or smooth plains. These are found near the planet’s north pole or around and within giant impact features called basins. The largest of these, the Caloris basin, is a vast circular patch about 1300 kilometres across – big enough to contain the British Isles. It was formed when a large planetesimal, 100–150 kilometres in diameter, ploughed into the young planet with the force of a trillion megaton-nuclear bombs. The blast melted the surface locally, forming as it solidified the smooth, round impact scar that we see today.

Aside from craters and basins, Mercury boasts a number of fractures called scarps. Some of these are up to 500 kilometres long and form cliffs that in places jut nearly 4 kilometres into the black sky. They often cut right across craters and basins and are thus more recently formed. Astronomers suspect that the scarps are faults caused by horizontal compression in the crust – evidence, they say, that the planet has gradually contracted and cracked upon cooling. Very likely, therefore, much of Mercury’s interior has frozen solid; the body is too small to retain heat for

as long as the Earth. But, at the same time, the planet has a measurable magnetic field, and this implies that the interior must at least be partially molten, perhaps in the very centre. Without a fluid interior the planet would lack the convection needed to generate its magnetic field.

Cosmic Casualty

The interior of Mercury must also be very rich in metals such as iron, for the planet has the second highest average density in the Solar System, after Earth. Of course, because it accreted so close to the Sun where only the densest substances could condense, its high metal content is to be expected. But the planet has such a high mass for its size that its iron core must be phenomenal. It extends out to 75 per cent of the planet’s radius. This has led some astronomers to the conclusion that Mercury suffered a cataclysmic collision with another large planetoid while it was still accreting. It is quite conceivable that a large enough impact – with a body perhaps half the size of the young Mercury itself – could have melted the planet’s original rocky mantle, jettisoning it into space where it later fell into the nearby Sun. Only the iron-rich core would have escaped annihilation. If this happened late enough, Mercury would not have been able to recover its original mass owing to the rapidly diminishing number of planetesimals in its vicinity. Instead, its growth was stunted, and the result is the planet we see today: dwarfish, dense, and with a relatively odd orbit.

At closest approach to the Sun Mercury is only 0.306 AU from it. But the other end of its orbit lies at 0.467 AU, more than 1.5 times further out. This means that from the surface of Mercury the Sun varies its diameter from two times to more than three times that seen from Earth. The orbit is also tilted with respect to the so-called ecliptic, the centre plane of the Solar System, by 7 degrees. These characteristics may have been imparted to the planet in the very same impact that vaporised its original surface and blasted it into space.

Evolution of Mercury

From the general physical characteristics of Mercury, researchers now think they have some idea how the planet has evolved. Its oldest terrain, accounting for 70 per cent of the surface, is the highlands. The craters there were formed during the early heavy bombardment phase and are thus some 4200 million years old. They date back to a mere 400 million years after the end of accretion in the Solar Nebula. The highland intercrater plains, where some craters have been buried partially or totally, are obviously somewhat younger. These plains are gigantic lava flows that oozed out of the planet’s crust on a global scale about 4 billion years ago. Meanwhile, the last major episode of activity on the planet was that which followed the Caloris basin impact. This devastating blow brought more lava to the surface and formed the localised smooth plains or lowlands. These areas have few impact craters and so must have been laid down after most of the heavy bombardment had finished. Thus the Caloris basin is about 3800 million years old – the youngest terrain on the planet. Aside from the cracks brought about as the planet gradually cooled and contracted, Mercury’s airless surface hasn’t really changed since the Caloris event. The rugged landscape we see now dates to just 800 million years after the formation of the planet itself. It has been frozen solid ever since, for the last 80 per cent of the planet’s existence.

There is one other way in which Mercury has changed, though, and that has to do with its rotation period. Currently the planet takes 58.65 days to spin once on its axis. But when the planet first emerged from the busy rubble of the protoplanetary disc it would almost certainly have been going much faster. The cause of Mercury’s gradual spin down was the tidal force exerted by the nearby Sun. Because of the way in which gravity diminishes with distance, the Sun’s pull on Mercury’s surface is much greater than its influence at the planet’s centre. Thus in a sense the Sun tries to hold on to Mercury’s surface layers as the planet rotates. Yet at the same time, Mercury naturally tries to take its surface layers with it as it spins. This tug-of-war – known as tidal friction – caused Mercury to gradually slow down. As a result it now has the second-slowest rotation period in the entire Solar System. Only Venus spins more slowly than Mercury. And that is the next planet we encounter as we move outward through the Solar System, away from the Sun. 

Source :

Mark A. Garlick. The Story Of The Solar System. University Press: Cambridge. 2002.