11-3 Mercury is cratered like the Moon but has a surprising magnetic field

Spacecraft observations at close range were necessary to reveal Mercury’s unusual properties

Imagine an alien astronomer observing Earth from Mercury, with the same telescope technology available to our own astronomers. Such an astronomer would be unable to resolve any features on Earth less than a few hundred kilometers across. It would be impossible for her to learn about Earth’s mountain ranges or volcanoes. Earth-based astronomers face the same limitations in studying Mercury. To truly understand Mercury, it is essential to study this world at close range using spacecraft.

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Mercury Spacecraft: Mariner 10 and MESSENGER

The first spacecraft to provide some knowledge about Mercury was Mariner 10, which flew close to Mercury on three occasions in 1974 and 1975. Two brief flybys in 2008 produced the first images of Mercury from the spacecraft MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and Ranging) before it settled into orbit in 2011. MESSENGER can distinguish features as small as 18 m—a vast improvement over Mariner 10’s 1-km resolution. Figure 11-5 shows a real-color image of Mercury from MESSENGER.

Figure 11-5: R I V U X G
MESSENGER Image of Mercury This real-color image shows the natural gray color of Mercury’s surface. Much like the Moon, Mercury’s surface contains many craters, but none of the Moon’s dark maria. The lack of darker maria is consistent with a low iron content measured on Mercury’s surface.
(NASA)

Mercury’s Cratered Surface

As Mariner 10 closed in on Mercury, scientists were struck by the Moonlike pictures appearing on their television screens. It was obvious that Mercury, like the Moon, is a barren, heavily cratered world, with no evidence for plate tectonics. These surface features are what we would expect based on Mercury’s small size (see Table 7-1 and the figure that opens this chapter). As we saw in Section 7-6, a small world is expected to have little internal heat, and, hence, little or no geologic activity to erase an ancient, cratered surface (see Figure 7-11). But craters are not the only features of Mercury’s surface; there are also gently rolling plains, long, meandering cliffs, and an unusual sort of jumbled terrain. As we will see, Mercury’s distinctive surface shows that this planet is not merely a somewhat larger version of the Moon.

Figure 11-6 shows a typical close-up view of Mercury’s surface. The consensus among astronomers is that most of the craters on both Mercury and the Moon were produced during the first 700 million years after the planets formed. Debris remaining after planet formation rained down on these young worlds, gouging out most of the craters we see today. We saw in Section 10-4 that the strongest evidence for this chronology comes from analysis and dating of Moon rocks. No spacecraft has landed on Mercury, so we are not able to make the same kind of direct analysis of rocks from the planet’s surface.

Figure 11-6: R I V U X G
Mercurian Craters and Plains MESSENGER took this true-color image of a region near Mercury’s equator at a range of 2800 km (1700 mi). Lava flows formed the plains inside and to the west of the crater on the right.
(NASA/Johns Hopkins U. Applied Physics Laboratory/Carnegie Institution of Washington)

Scarps: Evidence of a Shrunken Planet

Images also reveal numerous long cliffs, called scarps, meandering across Mercury’s surface (Figure 11-7). Some scarps rise as much as 3 km (2 mi) above the surrounding plains and are 20 to 500 km long. These cliffs probably formed as Mercury cooled and contracted a few kilometers in size. Scarps are analogous to wrinkles on fruit—as the fruit dries out and shrinks in size, its surface is deformed. Just as we saw on the Moon (Figure 10-8), when a scarp cuts through a crater, it means that the scarp formed after the crater. Eventually, using age estimates related to craters, astronomers hope to date the scarps of Mercury. Some scarps on the Moon indicate very recent geologic activity: will Mercury reveal a similar surprise? Even with the scarps, there are no features on Mercury that resemble the boundaries of tectonic plates. Thus, we can regard Mercury’s crust, like that of the Moon, as a single plate.

Figure 11-7: R I V U X G
A Scarp A long cliff, or scarp, called Beagle Rupes runs across this MESSENGER image. This scrap is almost a kilometer high and extends for more than 600 km (370 mi) across a region near Mercury’s equator.
(NASA/Johns Hopkins U. Applied Physics Laboratory/Carnegie Institution of Washington)

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Fire and Ice: The Caloris Basin and Mercury’s Poles

The most impressive feature discovered by Mariner 10 was a huge impact scar called the Caloris Basin (calor is the Latin word for “heat”). The Sun is directly over the Caloris Basin during alternating perihelion passages, making it the hottest place on the planet once every 176 days. In Figure 11-8, enhanced-colors help to identify different chemical compositions present on the surface of the Caloris Basin. The human eye would see mostly gray, as seen in the other Mercury images. With enhanced coloring, bright orange spots near the rim of the basin are visible. These spots are thought to indicate volcanic material, while the blue areas suggest an above-average titanium oxide content. Material that appears blue is thought to have originated a few kilometers below the surface and was exposed through impact events.

Figure 11-8: R I V U X G
The Caloris Basin About 4 billion years ago an impact on Mercury created an immense basin, shown in orange in this enhanced-color MESSENGER image. The crater is about 1500 km (960 mi) in diameter. The impact fractured the surface extensively, forming several concentric chains of mountains. The mountains in the outermost ring are up to 2 km (6500 ft) high. Craters inside the basin (shown in blue) were formed later by smaller impacts.
(NASA/Johns Hopkins U. Applied Physics Laboratory/Arizona State U./Carnegie Institution of Washington)

The Caloris Basin, which measures 1500 km (960 mi) in diameter, is both filled with and surrounded by smooth lava plains. The basin was probably gouged out by the impact of a large meteoroid that penetrated the planet’s crust, allowing lava to flood out onto the surface and fill the basin. Because fewer craters pockmark these lava flows than elsewhere on the surface, the Caloris impact must be relatively young. It must have occurred toward the end of the crater-making period—the Late Heavy Bombardment—that dominated the first 700 million years of our solar system’s history.

The impact that created the huge Caloris Basin must have been so violent that it shook the entire planet. On the side of Mercury opposite the Caloris Basin, Mariner 10 discovered a possible consequence of this impact—a jumbled, hilly region covering about 500,000 square kilometers, about twice the size of the state of Wyoming. Geologists theorize that seismic waves from the Caloris impact became focused as they passed through Mercury. Perhaps as this concentrated seismic energy reached the opposite side of the planet, called the antipode, it deformed the surface and created jumbled terrain. This same process may also have taken place on the Moon: There is a region of chaotic hills on the side of the Moon directly opposite from the large impact basin called Mare Orientale. What about antipodal effects on Earth? Detailed calculations of Earth’s dinosaur-killing asteroid impact (Chapter 15) indicate that the antipodal region, on the opposite side of Earth, would have been raised 3 to 5 meters!

Even though temperatures at Mercury’s equator are high enough to melt lead, there is the possibility of water-ice at the poles. In 1991, radar waves that bounced from the surface showed an unexpectedly high reflectivity near the poles, and ice is highly reflective. Further analysis showed that the radar reflections came precisely from permanent shadows within craters near the pole, which are well below the freezing point of water. This idea gained even more credibility in 2010 with the discovery of water in a permanently shadowed crater on the Moon (Section 10-2).

Finally, in 2012, the MESSENGER spacecraft confirmed water-ice by detecting excess hydrogen in these shaded craters, where the hydrogen is part of water molecules. Analysis of hydrogen concentrations indicates a layer of almost pure water-ice more than 30 cm thick beneath a layer about 15 cm thick containing little water. Some astronomers speculate that the water was delivered by comets, but the source is unknown.

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Mercury’s Interior and Iron Core

By measuring how much Mariner 10 was deflected by Mercury’s gravity, scientists were able to make a precise determination of Mercury’s mass. Given the mass and the diameter of Mercury, they could then calculate that the planet has an average density of 5430 kg/m3. This value is quite similar to Earth’s average density of 5515 kg/m3. However, as shown in Figure 11-9, a much larger fraction of Mercury is composed of iron.

Figure 11-9: The Internal Structures of Mercury and Earth Compared to Earth’s core, Mercury’s iron-rich core takes up a much larger fraction of the planet’s volume. Indeed, Mercury is the most iron-rich planet in the solar system. Mercury’s iron core covers 83% of its radius and 68% of its volume, whereas Earth’s core is only 55% of its radius and 17% of its volume.
(NASA)

CAUTION!

You might wonder how Mercury and Earth can have similar densities while Mercury’s iron fraction is twice as large as Earth’s. In Earth, which is 18 times more massive than Mercury, lighter material is compressed by gravity to create an overall high density. Mercury gets its own high density from its abundance of intrinsically heavy iron.

Several theories have been proposed to account for Mercury’s high iron content. According to one theory, the inner regions of the primordial solar nebula were so warm that only those substances with high condensation temperatures—like iron-rich minerals—could have condensed into solids. Another theory suggests that a brief episode of very powerful solar winds could have stripped Mercury of its low-density mantle shortly after the Sun formed. A third possibility is that during the final stages of planet formation, Mercury was struck by a large planetesimal. Supercomputer simulations show that this cataclysmic collision would have ejected much of the lighter mantle, leaving a disproportionate amount of iron to reaccumulate to form the planet we see today (Figure 11-10).

Figure 11-10: Stripping Mercury’s Mantle by a Collision To account for Mercury’s high iron content, one theory proposes that a collision with a planet-sized object stripped Mercury of most of its rocky mantle. These four images show a supercomputer simulation of a head-on collision between proto-Mercury and a planetesimal one-sixth its mass.
(Courtesy of W. Benz, A. G. W. Cameron, and W. Slattery)

Clues About the Core: Mercury’s Magnetic Field

An important clue to the structure of Mercury’s iron core came from the Mariner 10 magnetometers, which discovered that Mercury has a magnetic field similar to that of Earth but only about 1% as strong. MESSENGER has confirmed the presence of a global magnetic field, which indicates that Mercury contains moving liquid material in its interior that conducts electricity. As in Earth, this moving material would act as a dynamo to generate the magnetic field. Hence, we conclude that at least part of Mercury’s core must be in a liquid state.

Mercury’s magnetic field comes as something of a surprise. The ancient, cratered surface of Mercury is evidence for a lack of geologic activity (that erases craters), which in turn shows that Mercury must have lost the internal heat that powers such activity. Again, we would expect that such a small planet would lose much of its internal heat (see Section 7-6). But the existence of a global magnetic field shows that Mercury is still partially molten inside, which is evidence that the planet has some internal heat. Thus, Mercury’s small size and dense cratering are in contradiction to its global magnetic field.

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Another mystery is that a planet must rotate fairly rapidly in order to stir its molten interior into the kind of motion that generates a magnetic field. But as we saw in Section 11-2, Mercury rotates very slowly. It takes 58.646 days to spin on its axis. We still have much to learn about the interior of Mercury.

Mercury’s Unsolved Mysteries

A very unusual and unexplained property of Mercury’s magnetic field is that it does not originate at the planet’s center. Instead, MESSENGER found that Mercury’s magnetic field originates about 488 km north of the planet’s center—an offset equal to a whopping 20% of the planet’s radius! No other planet or moon has shown such a large offset and this offset is currently one of Mercury’s greatest mysteries. Yet, another related mystery is perhaps the most perplexing of all.

Mercury presumably formed close to the Sun under high temperatures. (The early migration expected in our solar system was mostly by the Jovian planets.) It is generally found that under higher temperatures, certain atomic elements—referred to as volatile elements—“boil off” out of planetary material. We saw in Section 10-5 that the Moon has fewer volatile elements than Earth, just as would be expected if the Moon was created from a fiery impact with Earth. However, Mercury has an unexpectedly high abundance of volatile elements, as if it formed at lower temperatures.

Not only is Mercury’s high volatile content a problem in trying to explain its formation so close to the hot Sun, but it also poses a challenge to subsequent large impacts with Mercury. For example, we saw that one explanation for Mercury’s large iron core is that a large impact blasted away some of the lighter outer material (Figure 11-10), but this hot impact event is inconsistent with a high volatile content; volatile elements would be expected to cook out of the remaining surface material, and then evaporate into space. It is also tempting to propose that a large impact created Mercury’s off-center magnetic field, but this too runs against the observed high abundance of volatile elements on Mercury’s surface.

Simply put, Mercury holds unsolved mysteries that are certain to keep astronomers busy. While additional data from MESSENGER will help to answer some questions, it will certainly open up new questions as well.

CONCEPT CHECK 11-2

Why is it thought that Caloris Basin would have formed near the end of the Late Heavy Bombardment?

CONCEPT CHECK 11-3

In Figure 11-10, rocky mantle is vaporized about 6 minutes after Mercury is hit by a large planetesimal. Would this be more consistent with a low or high abundance of volatile elements on Mercury?