The solar system has four solid, Earthlike planets (five if you count the Moon). Earth, Mars and Venus are all reasonably familiar, but the fourth terrestrial planet—the innermost planet Mercury—is still largely unknown. If we want to better understand these planets and how they formed, we need more data from Mercury. But soon we'll have some long-awaited answers as the MESSENGER spacecraft settles in for a long visit in Mercury orbit, starting on 18 March 2011.
As an astronomical object, Mercury has been known for as long as we've studied the sky. As a solid planet, though, Mercury was poorly known until 1974 when the Mariner 10 spacecraft passed it at close range. With the spacecraft data and Earth-based observations, we have some good ideas to test with MESSENGER.
Planet Mercury: Composition
Telescopic observations tell us that Mercury is small but heavy. It is the densest planet by a large margin, if we correct for the effects of compression. We deduce that it has a relatively large iron core, three-fourths the diameter of the whole planet, and a thin rocky mantle only about 600 kilometers thick. (Explore this topic at Kevin Healy's planetary density simulator.) Whereas Earth is basically a silicate planet with an iron core, Mercury amounts to an iron planet with a silicate rind.
This fits the overall picture we have of the whole inner solar system: the farther we get from the Sun, the lighter the stuff of the planets. How this results from the solar system's early history is not certain. We have three hypotheses to choose from that MESSENGER can help test. I think of them as the sifting, grilling and bludgeoning scenarios:
- Iron and silicatesmetal and rockwere separated by weight in the original nebula that the planets formed in.
- As the planet formed, the outermost silicate crust was eroded by fierce heat and radiation, then blown away from the Sun.
- As the planet was forming, a giant impact blasted away most of the outer silicate crust (much like what happened to Earth to form the Moon).
Each scenario leaves the Mercurian crust with a different composition. (Specifically, 1 would make a solar/chondritic mantle; 2 would make a refractory-enriched, alkali/FeOx-depleted mantle; 3 would make a Ca/Al/alkali-depleted mantle. Each mantle would subsequently produce its own distinctive crust.) Once MESSENGER can observe the entire planet at close range, we can test Mercury's rocks against these three models. It will take some intricate measurements and reasoning, because space weathering has strongly affected the soil.
Mercurian Volcanism and Tectonics
Every other inner planet displays volcanism, so Mercury ought to too. As MESSENGER approached the planet, using three separate fly-bys to shed its momentum, its cameras confirmed that Mercury has plains of lava, especially in and around the larger impact basins. Unlike the Moon, where the lava plains appear today as dark spots or maria, Mercury doesn't have signs of past lava seas. This fits our picture of Mercury as a very dry planet.
The observations we have so far appear to show that volcanism occurred for at least half of Mercury's history. And there are intriguing hints of Earth-style volcanoes in some places. Mercury is almost certainly volcanically dead today, but MESSENGER mapping will give us a better idea of the role of eruptions during the planet's life story.
The question of tectonics came up in the 1970s with Mariner 10, which imaged long ridges resulting from thrust faults. Mariner 10 covered only half of Mercury, but it documented ridges everywhere. This pointed to a planet that had shrunk as a whole, which one would expect as it cools. (A century ago, planetary shrinkage was the reigning theory for Earth's mountain ranges.) MESSENGER has confirmed that picture on the rest of Mercury, showing that in fact the shrinkage was greater than thought before. The larger impact basins, in contrast, appear to have been soft spots where the planet's overall compression was relieved thanks to the local impact heating.
Mariner 10's instruments detected a magnetic field around Mercury, about a thousand times weaker than Earth's geomagnetic field. Like the geomagnetic field, it's aligned with the north and south geographic poles and is strongly dipolar, just like a single giant magnet. Such a thing could arise from rocks that were magnetized in the past, but Mercury is too hot for its rocks to retain a remanant field that strong.
We already are pretty sure that Mercury has a liquid core, like Earth. A subtle radar technique allowed us to estimate the planet's libration, a tiny wobble caused by the gravity of other objects. In an analogy to the simple household test of telling a raw egg from a cooked one by spinning or shaking it, libration is a clue to the planet's interior. Earth's liquid core was first demonstrated by libration observations in the 1800s. Results announced in March 2007, after making about 20 of these exacting measurements, showed that Mercury's core is indeed likely to be liquid.
That points to a dynamo process, which is the mechanism that maintains Earth's field. There are several different models proposed for a Mercurian dynamo, and MESSENGER data will help in choosing one over the others.
If Mercury has a liquid core, that raises the question of why. A core of pure iron, after 4.5 billion years of cooling, would be frozen solid today. Therefore Mercury's core must have a lighter element in it to keep it liquid (as does Earth's core). It would be nice to have another data point for this question: Mars and the Moon are also tentatively thought to have partially liquid cores, and the structure of Venus is undetermined.
Between the Mariner 10 and MESSENGER missions was a 30-year gap. Because most of us can't wait that long, various scientists with the right equipment turned their hand to this problematic planet. Space-based telescopes like the Hubble cannot be pointed at Mercury because it's too near the Sun. Ground-based telescopes are limited by Mercury's very brief appearances near dawn and dusk. But some sweet techniques were tried out, using ground-based telescopes and radar, that may continue to pay off elsewhere in the solar system.
The latest generation of electronic cameras have snapped useful pictures of Mercury's surface features in visible light. The picture shown at the top of this article is a composite of dozens of images selected from several hundred thousand snapshots taken at Mount Wilson observatory in August 1998. A Swedish experiment yielded more pictures covering the whole planet. Observations in the infrared gave us hints of the composition of Mercury's surface, consistent with feldspar minerals low in iron. That is similar to the ancient highlands of the Moon and parts of the Earth's crust.
I've already mentioned the radar libration measurements that showed a liquid core. In 1992 the Arecibo radar telescope began bouncing signals off Mercury. It yielded crude maps of areas that Mariner 10 did not see, with suggestions of large features that might be volcanoes or lava basins like the lunar maria. It also saw unusually strong reflections from the north polar region, suggestive of ice hiding in places that are never exposed to sunlight. That's a funny thing to imagine on the closest planet to the Sun, where the temperature at noon reaches 450°C. But with no air to blow warmth around, ice can exist easily in shaded places. The same thing appears on the Moon.
Spacecraft Missions to Mercury
By the way, when I say MESSENGER I'm not shoutingit stands for MErcury Surface, Space ENvironment, GEochemistry, and Ranging and is a reference to the Roman god Mercury, associated with the planet, who served as messenger of the gods as well as the patron of commerce and thieves. The MESSENGER mission began in 2004 with the launch of a spacecraft that worked its way into the solar system's hot zone by a series of flybys. It needed a flyby of Earth, two of Venus and three of Mercury itself to shrug off the excess momentum gained by falling so close to the Sun. (Follow progress at the MESSENGER home page.)
The European Space Agency and the Japanese space agency ISAS/JAXA are planning the BepiColombo mission to Mercury in 2014. Two spacecraft will perform flybys and settle into orbit in 2020 for a two-year nominal mission. Given the typical pattern of these things, the mission will surely be extended further if the spacecraft survive.
Sources include Sean Solomon, "A new look at the planet Mercury," Physics Today, January 2011, pp. 50-55.