Profile of Mercury Help

The Year And The Day

Mercury’s mean orbital radius is 38.7 percent that of Earth, or about 58 million kilometers (36 million miles). The planet’s distance from the Sun varies considerably; the orbit is far from a perfect circle. At perihelion, Mercury is 46 million kilometers (29 million miles) from the Sun; at aphelion, it is 70 million kilometers (44 million miles) distant. This, together with the fact that Mercury’s year is 88 Earth days long while its day (measured relative to the distant stars) is 59 Earth days long, gives the planet “seasons” of a sort.

Even though the equatorial plane of Mercury is tilted by only 2 degrees relative to the plane of its orbit around the Sun, the planet’s changing distance from the Sun produces a maximum-to-minimum solar brilliance ratio of more than 2:1. That is, at perihelion, Mercury gets more than twice as much energy from the Sun as it does at aphelion. This does not matter in any practical sense; from a human point of view, the surface broils in daylight and freezes at night. However, if Earth’s orbit were as eccentric as that of Mercury, life on this planet would have evolved along a much different course. The weather would be more violent, the tides would be greater, and things in general would be rougher than they are. Earth’s 23.5-degree axial tilt causes seasonal variability, but nothing of the sort that would take place if the irradiation from the Sun were twice as great at some times compared with other times.

Composition

Mercury is a lone planet; it has no moons, except perhaps for a few tiny, captive asteroids or degenerate comet nuclei. Mercury is more dense than any other planet in the solar system except Earth and is considerably smaller, only about 4,880 kilometers (3,030 miles) in diameter. Figure 5-2 shows the relative sizes of Mercury and Earth.

Figure 5-2. Mercury has only about 3/8 the diameter of Earth; it is only a little larger than Earth’s moon.

Mercury has a surface pockmarked with craters. The planet is even more rugged than our own Moon. If Mercury were suddenly put in place of our Moon, we would notice only a slight difference in size and brightness, but the “facial features” would be markedly different. Moreover, the tides on our planet would be much more variable, for Mercury has a core of iron that is unusually large in proportion to the planet’s overall size. The dense core is overlaid with a crust of silicate rock.

There is no evidence of geologic activity on Mercury. Because it is so small, it has cooled off to the point that future volcanism will not occur. There is plenty of evidence that volcanoes spilled lava out onto the landscape in the distant past, but today the planet’s surface heat is attributable entirely to the Sun. This keeps the daytime temperature hot enough to melt lead. In one region, known as the Caloris Basin because the Sun is near the zenith there when Mercury is at perihelion, the midday temperature rises above 400°C (750°F).

The surface of Mercury is crisscrossed by a network of cliffs 2 to 4 kilometers (3 to 6 miles) high. This suggests that as the interior of the planet cooled off, it shrank enough to cause the crust to fall inward in places. Imagine the violence of such an event if Earth were to endure it! When a piece of our own terra firma collapses or slides a few meters along a fault line, it makes news headlines all around the world. Suppose that the San Andreas fault were rocked by a quake that sent the coast of California plunging 3 kilometers in a couple of minutes? Imagine the same thing happening along all the major fault lines of the world at once!

Atmosphere

Surprisingly, Mercury has a thin atmosphere. It is only a tiny fraction as dense as that of Earth at the surface; in fact, the “air” on Mercury would constitute an excellent laboratory vacuum. Argon and helium gases have been detected. The Sun continually blows Mercury’s atmosphere off into space, but at the same time, the heat of the Sun drives new atmospheric atoms out of the surface.

There is no weather, as such, on Mercury. However, there is evidence that water ice might exist in some of the craters near the pole. The Mariner 10 mission in the mid-1970s analyzed the surface of Mercury extensively, photographing most of it, and there is apparently some sort of frost that has settled in the depths of polar craters where the Sun never shines. The minimal tilt of the planet’s axis keeps certain small regions of the polar surface in shadows at all times. If humans ever attempt to land on Mercury, these would be the logical places to touch down.

Practice problems of this concept can be found at: Mercury and Venus Practice Problems