MERCURY

Key Concepts

Mercury has an unusual orbit, with a tidally-created resonance between its orbital period and its rotation period.

Careful measurement reveals that the rotation period is equal to EXACTLY 2/3 the orbital period. Every time Mercury goes two times around the Sun, it turns around three times on its axis. This coincidence is actually the result of tidal braking. An observer located on the surface of Mercury would discover that the time between one noon and the next is 176 days, twice the orbital period. Thus, if you lived on Mercury, you would experience a ``solar day'' that was twice as long as a ``year''. The Sun would be above the horizon for 88 days, and would be, on average, more than six times brighter than it appears from Earth. Thus, on Mercury, it becomes very hot during the daytime.

(1) Mercury has no permanent atmosphere because it is hot and has a low escape velocity.

Remember, temperature is a measure of the average random speed of atoms or molecules in a substance. By raising the temperature of a planet's atmosphere, for instance, we are increasing the speed with which the molecules in the atmosphere travel. If the average random speed of the molecules is greater than the escape velocity, then the air molecules will escape into outer space.

At a fixed temperature, the average speed of a molecule depends on its mass. Low-mass molecules travel faster than the sluggish high-mass molecules. Thus, as an atmosphere is heated up, the low-mass atoms & molecules (like hydrogen and helium) are lost first; more massive molecules (like carbon dioxide) don't escape until higher temperatures are attained. Examples:

Mercury is now a planet without an atmosphere. Because Mercury doesn't have a thick insulating blanket of air, temperatures at the surface plummet at night.

(2) Like the Moon, Mercury has cratered highlands and lava-covered plains.

Only one spacecraft has visited Mercury: Mariner 10, which was launched into an eccentric orbit that would cause it to swing past Mercury once every 176 days (two mercurian orbital periods). Pictures were taken of the surface of Mercury during the first three fly-bys, in March 1974, Sept. 1974, and March 1975. (After that, Mariner 10 exhausted the supply of fuel for its maneuvering rockets; it's still orbiting the Sun, but it's now an inert hunk of space junk.) Thus, most of what we know about Mercury comes from data over a quarter-century old.

The pictures sent back by Mariner 10 reveal a surface that looks much like the Moon's; a mosaic image of Mercury, as seen by Mariner 10, is given below; note the heavy cratering over most of the surface.


[Image credit: Mariner 10, NASA]

The highlands of Mercury, like the highlands of the Moon, are heavily cratered. The only noteworthy difference between mercurian craters and lunar craters is that craters on Mercury have lower rims, thanks to the greater gravitational acceleration on Mercury than on the Moon. (If you weigh 150 pounds on Earth, you would weigh 57 pounds on Mercury, and only 25 pounds on the Moon.)

The lowlands of Mercury, like the maria of the Moon, have been covered with lava flows, and are less heavily cratered than the highlands. The lava-covered plains of Mercury are less conspicuous than the maria of the Moon, largely because they are the same color as the highlands of Mercury. (The maria of the Moon, by contrast, are much darker than the highlands of the Moon.)

Given the current similarities of Mercury and the Moon, it is probable that their geological histories are similar: a period of heavy bombardment, followed by a period when lava welled up into the lowlands, followed by a long boring period when not much happens. Mercury appears to be geologically dead now.

One of the most prominent features on Mercury is the Caloris Basin; like Mare Imbrium on the Moon, it is a large impact basin (it's about 1300 km across) which has been flooded with lava. Directly opposite the Caloris Basin, at the antipodes, is a region of jumbled terrain. The jumbled terrain consists of a chaotic mix of oddly-shaped hills. The jumbled terrain was created when the impact that created the Caloris Basin sent extremely powerful seismic waves racing through Mercury's interior. The waves converged on the point directly opposite the point of impact, and tossed huge chunks of the crust about, creating the chaotic, weird, jumbled terrain we see today.

Mercury also has a number of high cliffs, called ``scarps''. The scarp pictured below is about 3 kilometers high and 500 kilometers long. It was created, along with the other scarps on Mercury, when the uneven cooling of the planet made the crust wrinkle and crack.


[Image credit: Mariner 10, NASA]

(3) Mercury has an extremely large iron core.

Not much is known about the innards of Mercury, given the lack of seismic data. However, one unusual fact stands out: Mercury is extraordinarily dense. The average density of the planet is 5440 kg/m3, nearly as dense as the Earth's value of 5500 kg/m3. The density of Mercury is even more remarkable when you consider that, being a small planet, it is not as strongly compressed by its own gravity. If Mercury and the Earth were not gravitationally compressed, then the Earth's density would be merely 4400 kg/m3, while Mercury's density would still be 5300 kg/m3.

To be so extremely dense without the benefit of gravitational compression, Mercury must be made of material that is intrinsically very dense. If we assume that, like the Earth, Mercury consists of a rocky mantle over an iron-nickel core, we find the following result:

Thus, Mercury apparently consists of an iron-nickel sphere the size of our Moon covered by a relatively thin mantle of rock.

Why is the iron core of Mercury so large? One hypothesis says that Mercury, like the Earth, once had a relatively thick mantle, but lost it in a giant impact with a planetesimal. According to this hypothesis, about 4.6 billion years ago, when the planets were being formed by the collision of planetesimals, the young Mercury suffered a head-on impact with a giant planetesimal (maybe 2000 kilometers across). The collision released a vast amount of energy, which vaporized the outer, lower-density rocky mantle of Mercury, and sent it flying off in all directions. Most of the rocky mantle's material achieved escape speed, and was permanently lost; only a small amount remained to settle back down onto the central iron core, making the thin rocky mantle that Mercury has today.

(An alternate hypothesis states that the skimpy rock mantle of Mercury simply results from the fact that Mercury formed close to the Sun, where the temperature of the early solar nebula was high. Also iron was able to condense into planetesimals, it was too hot for a variety of silicate rocks to condense. Lacking those varieties of rock with a low condensation temperature, the mantle was thinner than it otherwise would have been.) More will be learned about Mercury when the next mission to Mercury is launched; the MESSENGER spacecraft is scheduled for launch in March 2004, and will enter Mercury orbit on April 6, 2009.

updated: