Mercury: The Key to Terrestrial Planet Evolution
Mercury, Venus, Earth, and Mars are terrestrial (rocky) planets. Among these, Mercury is an extreme: the smallest, the densest (after correcting for self-compression), the one with the oldest surface, the one with the largest daily variations in surface temperature - and the least explored. Understanding this "end member" among the terrestrial planets is crucial to developing a better understanding of how our own Earth formed, how it evolved, and how it interacts with the Sun. To develop this understanding, the MESSENGER mission, spacecraft, and science instruments are focused on answering six of the key outstanding questions that will allow us to understand Mercury as a planet:
- Why is Mercury so dense?
- What is the geologic history of Mercury?
- What is the structure of Mercury's core?
- What is the nature of Mercury's magnetic field?
- What are the unusual materials at Mercury's poles?
- What volatiles are important at Mercury?
- Question 1: Why is Mercury so dense?
Each of the terrestrial (class of planets that are like Earth) planets is composed of a dense iron-rich core surrounded by a mantle of magnesium and iron silicates (rock). The topmost layer of rock, the crust, formed from minerals lower in melting point than those in the underlying mantle, either during differentiation of a large volume of molten silicate early in the planet's history or by the later ascent and accumulation of melts generated within the mantle.
The density of each planet reflects the balance of iron-rich core and silicate-rich mantle and crust. Mercury's uncompressed density (what its density would be without compaction of its interior by the planet's own gravity) is 5.3 grams per cubic centimeter, by far the highest of all the terrestrial planets. Mercury's density implies that 65% of the planet is metal-rich core - twice as much as Earth!
There are three major theories to explain why Mercury is so much denser and more metal-rich than Venus, Earth, and Mars. Each theory predicts a different composition for the rocks on Mercury's surface. According to one idea, before the planet formed from the solar nebula, drag by thin nebular (cloud) gas favored accretion of dense particles so that Mercury became enriched in metal, but this process didn't change the composition of the silicates (minerals). In this case surface rock composition would be similar to that of the other terrestrial planets. According to another idea, tremendous heat from the early Sun vaporized part of the outer rock layer of proto-Mercury and left the planet a metal-rich cinder. This idea predicts a rock composition poor in easily evaporated elements like sodium and potassium. The third idea is that giant impacts, soon after Mercury formed, stripped off the primordial crust and upper mantle. This idea predicts that the present-day surface is made of rocks highly depleted in those elements (silicon, aluminum, and oxygen) that would have been concentrated in the primordial (first-developed) crust.
MESSENGER will determine which of these ideas is correct by measuring the composition of the surface. X-ray and gamma-ray spectrometers will measure the elements present in the surface rock and determine if volatile elements are depleted or if elements that tend to be concentrated in planetary crusts are deficient. A visible-infrared spectrometer will determine which minerals are present.
Question 2: What is the geologic (rock) history of Mercury?
As amazing as it seems, 25 years after Mariner 10 visited Mercury, only 45% of the planet has been photographed by spacecraft. The part that has been seen is cratered and ancient like the surface of Earth's moon. Slightly younger, but still very old, plains sit between the largest old craters. Many scientists think that the plains are volcanic. This interpretation is reinforced by a slightly different color of the plains from the ancient craters, which may indicate a different rock
composition. But because landforms diagnostic of volcanism are too small to be visible in Mariner 10 pictures, the origin of the plains remains uncertain. Unlike Earth and Venus, very few features are seen on Mercury that are clearly due to tectonic forces having reshaped the surface. Hummocky terrain, on the opposite side of the planet from the largest crater, may have formed when the planet's shape focused seismic (vibrational) energy from the impact into concentrated regions. More remarkable are huge, lobate (rounded projection) scarps (cliffs) nearly a mile in height and hundreds of miles long; they're common on Mercury but rare on Mars, for example. These features are thought to have formed by compression, that is, squeezing of Mercury's brittle outer layer.
MESSENGER will bring a variety of investigations to bear on Mercury's geology, in order to determine the sequence of processes that have shaped the surface. The X-ray, gamma-ray, and visible-infrared spectrometers will determine the elemental and mineral makeup of rock units composing the surface. The camera will image the previously unseen portion of the planet, and nearly all of the surface will be imaged in stereo to determine the planet's global topographic variations and landforms. The laser altimeter will measure the topography of surface features even more precisely in the northern hemisphere. Comparing the topography with the planet's gravity field, measured by tracking the MESSENGER spacecraft, will allow determinations of local variations in the thickness of Mercury's crust.
Question 3: What is the structure of Mercury's core?
One of the more surprising discoveries by Mariner 10 is that Mercury has a global magnetic field. That makes Mercury the only terrestrial planet besides Earth to have a global field. Earth's magnetic field is thought to be generated by swirling motions in the molten liquid outer portions of the core. (Only the
inner part of Earth's core is actually solid.) But Mercury is so much smaller than Earth (4,878 kilometers [3,030 miles] vs. Earth's 12,756 kilometers [7,926 miles]) that the core should long ago have cooled and solidified. In fact, cooling and contraction of the core may have been the driving force behind the global wrinkling that formed Mercury's lobate scarps. How could a cooled, solid core generate a magnetic field? One possible answer is that the core hasn't yet frozen completely, due to dissolved low melting-point elements like sulfur. Another is that the present-day magnetic field is a frozen-in remnant of Mercury's primordial magnetic field. Understanding the state of Mercury's core would explain a great deal about how terrestrial planets like Earth can generate a magnetic field.
Using the laser altimeter, MESSENGER will determine the presence or absence of a liquid outer core in Mercury by measuring the planet's libration. Libration is the slow wobble of the planet around its rotational axis. The libration of the rocky outer part of the planet will be twice as large if it is floating on a liquid outer core than if it is frozen to a solid core. This libration experiment, when combined with measurement of the gravity field by radio tracking of the spacecraft, will also help to determine the size of the core and how much of it is solid.
Question 4: What is the nature of Mercury's magnetic field?
Earth's magnetic field is very dynamic, and constantly changing in response to activity of the Sun including the solar wind and solar flares. We see the effects of these dynamics on the ground as they affect power grids and electronics, causing
blackouts and interference with radios and telephones. Mercury's magnetic field was shown by Mariner 10 to experience similar dynamics; understanding them will help us to understand the interaction of the Sun with Earth's field. Although Mercury's magnetic field is thought to be a miniature version of Earth's, Mariner 10 didn't measure Mercury's field for long enough to characterize it well. We're not even sure just how strong Mercury's field really is. Earth's field is a "dipole" field, which means that on a global scale Earth acts like there is a giant bar magnet at its core. On average, Mercury's field is also a dipole field. In contrast, the Moon and Mars lack a global dipole magnetic field, but they have local magnetic fields centered on different rock deposits. It's not certain how such local fields were formed, and it's also not clear how much of Mercury's field comes from smaller local fields (as on the Moon and Mars).
MESSENGER's magnetometer will characterize Mercury's magnetic field in detail from orbit over four Mercurial years (each Mercurial year equals 88 Earth days), to determine its exact strength and how its strength varies with position and altitude. The effects of the Sun on magnetic field dynamics will be measured by the magnetometer and by an energetic particle and plasma spectrometer.
Question 5: What are the unusual materials at Mercury's poles?
Mercury's axis of rotation is oriented nearly perpendicular to the planet's orbit, so that in the polar regions sunlight strikes the surface at a constant grazing angle. The interiors of large craters at the poles are permanently shadowed and remain perpetually cold, below -212ēC (-350° F). Radar images of the polar regions, first obtained in 1991, show that the large craters' interiors are highly reflective at radar wavelengths. The most common material that could explain this behavior is -- ice! On the planet closest to the Sun! The tiny flow of ice from infalling comets and meteorites could be cold-trapped in these Mercurial polar deposits over billions of years, or water vapor might outgas from the planet's interior and be frozen out at the poles. Alternatively, it has been suggested that the polar deposits consist of a different material, perhaps sulfur sublimated over the eons from minerals in the surface rocks.
MESSENGER's gamma-ray and neutron spectrometer will detect hydrogen in any polar deposits. Alternatively, sulfur would be detected by the ultraviolet spectrometer and energetic particle spectrometer from tenuous vapor over the deposits. Understanding the composition of Mercury's polar deposits will clarify the inventory of volatile materials in the inner solar system.
Question 6: What volatiles are important at Mercury?
Mercury is surrounded by an extremely thin envelope of gas. It is so thin that, unlike the atmospheres of Venus, Earth, and Mars, the molecules surrounding Mercury don't collide with each other -- instead they bounce from place to place on the surface like so many rubber balls. This is called an "exosphere." Six elements are known to exist in Mercury's exosphere: (1) hydrogen, (2) helium, (3) oxygen, (4) sodium, (5) potassium, and (6) calcium. Hydrogen and helium come, at least partly, from the stream of hot, ionized gas emitted by the Sun -- the "solar wind." Some of the hydrogen and oxygen may also come from ices that fall into Mercury in comets or meteorites. The sodium, potassium, and some of the oxygen most likely come from rocks on the surface. Several different processes may have put these elements into the exosphere, and each yields a different mix: vaporization of rocks by impacts, slow evaporation of elements from the rocks due to sunlight, sputtering by solar wind ions, or emission of gases from the planet's interior. MESSENGER will determine the composition of Mercury's exosphere using its ultraviolet spectrometer and energetic particle spectrometer. The exosphere composition measured by these instruments will be compared with the composition of surface rocks measured by the X-ray and gamma-ray spectrometers, to determine which processes have contributed molecules to the tenuous atmosphere.
the above text from Messenger’s web site