Monday, March 7, 2011

Our Solar System Part 5: Mercury

Mercury, named after Mercurius, the Roman messenger and god of trade, is a small mysterious planet. It is relatively bright because of the reflection off of its rocky surface, like the moon, but it orbits so close to the Sun that it is usually lost in the Sun's glare. You might get a chance to see it at twilight along the horizon close to where the Sun comes up or sets.

Try just after sunset in late winter and in early spring and just before sunrise in late summer and early autumn, with clear skies.

This is a photo sequence of Mercury (the tiny white dots) exactly 33 minutes after sunset on 12 consecutive evenings taken in April 2004 in the United Kingdom by Tony Cook.

Gaps in the sequence were because of cloudy evenings. This progression in Mercury's position across the sky hints at a very eccentric orbit. In fact, Mercury has the most eccentric orbit of all the planets.

This is what Mercury looks like close up. 

This is a false colour image of the planet taken by MESSENGER during its flyby in 2008. NASA's MESSENGER probe just moved into orbit around Mercury a few days ago, March 17. We will explore this current NASA mission in a moment.

As first glance, Mercury seems to be nothing more than a small broiled and cratered dead planet, nothing much to see. Upon closer investigation, however, it reveals some fascinating mysteries about how the solar system came to be and about its own history as well.

Let's Start With Mercury's Orbit

Mercury's distance from the Sun varies between 46 and 70 million kilometers. Its orbit looks like a cross section of an egg with Mercury orbiting where the shell is and the Sun located in the yolk. It actually orbits faster when it is near perihelion, which means closest to the Sun, thanks to greater gravitational pull. Mercury is in a 3:2 spin orbit resonance, which means it rotates on its axis 3 times for every 2 orbits around the Sun. Because of the planet's slow rotation and fast orbit, a day on Mercury lasts twice as long as a year. A day, from one sunrise to the next, lasts 176 Earth days, while it only takes 88 Earth days to make one orbit around the Sun, one Mercury year. This arrangement is stable only because Mercury's orbit is so eccentric (non-circular) and, to make matters more complicated, the eccentricity of Mercury's orbit gradually varies between zero (a perfect circular orbit) and 0.45 (a nearly perfect elliptical orbit) over a period of millions of years. Computer simulations suggest that the gravitational influence of Jupiter causes Mercury's eccentricity, and that it's eccentricity may gradually increase to the point where Mercury may someday collide with Venus, sometime within the next 5 billion years.

What is perhaps even more intriguing is Mercury's advancing perihelion. What this means is that Mercury's orbit around the Sun is not really a stationary ellipse but a flower petal shape. The orbital path itself rotates gradually over time, as shown in this diagram.

Mercury is the small brown globe and the Sun is the fiery red large globe. This diagram is obviously not drawn to scale and the advancement is greatly enhanced.

This kind of planetary movement can usually be explained by changing gravitational forces exerted by other planets, motions governed by Newton's laws of classical mechanics. In Mercury's case, however, the advancing perihelion can only be satisfactorily explained by Einstein's theory of general relativity. This is how it works: 

Einstein Gravity Versus Newton Gravity

General relativity explains a gravitational field as a curvature in space-time. Gravity is not a force per se but rather it's a geometry where the source of the curvature is the stress-energy tensor (representing mass). The stress-energy tensor describes the density and flux of energy and momentum in space-time. It can be an attribute of matter, radiation or non-gravitational force fields. In other words, the stress-energy tensor is the source of the gravitational field in general relativity, just like mass is the source of the gravitational field in Newtonian physics. The stress-energy tensor, or stress-energy-momentum tensor as it's sometimes called, generalizes the Newtonian stress tensor. It covers mass as well as conditions other than mass, territory where Newtonian gravity can't go. An example of this territory is the observed gravitational pull on massless photons (units of electromagnetic radiation such as visible light), a phenomenon called gravitational lensing.

This 10-minute video clipping from the PBS series, "The Elegant Universe," is helpful with visualizing space-time gravity:

Mercury's Orbit Defies Newtonian Physics

The orbit of Mercury is an example of a two-body (or Kepler) problem in general relativity, where we are trying to describe the motion and gravitational fields of two bodies interacting with each other. The body in question can range from an individual photon to a galaxy. This is the kind of problem that is solved when calculating how light bends around a massive star or black hole, or the amount of energy that is gradually lost through the orbiting of two binary stars with each other. At first thought, you might think these should be simple calculations but they are not. The solutions to these equations are nonlinear and that means they can only yield a close approximation in terms of predicting these kinds of motion (most physical "real-life" situations are nonlinear in nature - the weather is an example, where simple changes in one part of the system produce complex and often chaotic effects throughout). In our case with Mercury, one solution, called the Schwarzschild solution, almost exactly solves the Kepler problem because the Sun is so much more massive (and its spin is relatively slow) than the planet orbiting it, that the Sun's gravity can be considered for these equations to be the sole contributor to the gravitational field (obviously this is not really true because Jupiter - and other planets to some extent - also impact Mercury as mentioned and Mercury itself has a gravitational impact, but here Mercury is relatively so small we can ignore all of that) around Mercury. When you solve the equations you get a geodesic motion for Mercury that is in agreement with careful observations of its orbit. The advancing perihelion of Mercury, along with the gravitational bending of light, are often used as supporting evidence for the validity of general relativity. The Schwarzschild solution means that Mercury takes the shortest path between two points in curved space-time (space-time is four-dimensional so it's pretty well impossible to visualize). This is what is meant by geodesic, and it means that Mercury's orbit "inches forward" very gradually, at a rate of 5600 arc seconds per century. To give you an idea of how miniscule this rate of movement is, there are 60 arc seconds per 1 arc minute and 60 arc minutes in 1 degree and 360 degrees in a full circle. It takes over 12 million orbits to make one whole "flower," or about 3 million years.

Its orbit is not the only unusual thing about Mercury.

Mercury's Unusual Composition

Mercury looks much like the moon does. But looks can be deceiving. Although Mercury is small, smaller than any other planet and even smaller than the moons Ganymede and Titan, it is the second densest planet in the solar system, with Earth being the densest of all planets. If the effect of gravitational compression is factored out (Earth is so much more massive than Mercury that its interior is squished more by gravity), it is actually far denser than Earth as well. The source of this density is a relatively very large metallic core that makes up almost half of Mercury's volume. Earth's core, by comparison, makes up about 17% of its volume. Mercury has a metal to silicate ratio much higher than that of the meteorites from which it formed. The other rocky planets have ratios that are very close to the ratio of meteorites. Why it would have such a different composition than other rocky planets is a source of debate.

When the inner solar system (inside 4 astronomical units or AU) was very young, it was too hot for volatile molecules like water and methane to condense, so the planets that formed in this region, Mercury, Venus, Earth and Mars, could only form from chunks of rock composed of high melting-point compounds like iron, nickel and aluminum. These rocks, or asteroids (or planetisimals if you want to call them that at this early point in solar system's evolution), were made almost entirely of metals and silicates. Silicate minerals are what make up most rocks. 90% of Earth's crust is silicate mineral. These minerals contribute most of the mass of the other rocky planets too, as well as the moons and asteroids. Metals are quite rare in the universe. They made up only about 0.6% of elements in the gas cloud from which the solar system formed, so these rocky planets are therefore quite small, compared to the much larger gas and ice giants (Saturn, Jupiter, Neptune and Uranus) that formed farther out. The rocky protoplanets that first formed were very small, about 1/25 of Earth's mass, but this region was littered with asteroids so the protoplanets grew as asteriods bombarded them and the pieces stuck together.

There are currently three theories about Mercury's strange composition:

Theory #1

Proto-Mercury might have been quite a bit larger than it is today. It may have been struck by a large asteroid, several hundred kilometers across. According to computer models, this impact would have stripped away much of the original crust and mantle, while leaving most of the core behind.

Theory #2

Mercury may have formed before the Sun's energy output had stabilized. In this scenario, Mercury was originally about twice its current size. As the newly formed Sun contracted, it could have emitted so much energy that the space around it which enveloped Mercury could have reached temperatures as high as 10,000 K (Kelvins). This heat would have vapourized much of Mercury's surface rock and carried it away on the powerful solar wind.

Theory #3

The rotating gas cloud from which the Sun and the planets were forming could have created enough drag on all the particles and debris, which were sticking together and forming Mercury, that lighter elements simply blew away before they could stick to the planet, leaving a disproportionate amount of heavy elements behind.

Each of these hypotheses predicts a different surface composition of Mercury. Astrophysicists hope that MESSENGER, the probe currently in orbit around Mercury, will be able to test these theories.


In 2004, NASA launched the Mercury Surface, Space Environment, Geochemistry and Ranging (MESSENGER) probe to study the chemical composition, geology and magnetic field of Mercury. This is an artist's rendering of what it looks like orbiting Mercury.

This is the second probe to reach Mercury. The first one was Mariner 10, which reached the planet in 1974. MESSENGER made three flybys of the planet before entering into orbit around it on March 18 of this year. Its flight path from Earth was complicated because a probe can't be sent straight from Earth to Mercury. It would be so strongly accelerated by the Sun's immense gravity on the way to Mercury that it would fly past Mercury far too fast to enter into orbit, even if an enormous amount of fuel could be spent. In addition to this, other probes can break in a planet's atmosphere using friction (aerocapture) to slow down without using too much fuel but Mercury has essentially no atmosphere. MESSENGER flew by Earth once, by Venus twice, and by Mercury three times, so that it could be positioned with the right velocity to enter a stable orbit. It had to travel 8 billion km to visit a planet only 92 million km away. This is what its complicated trajectory looks like.

It captured some beautiful images of Earth as it made its gravity-assist swingby in 2005, shown here.

MESSENGER started filming about 66,000 km above South America and the last image was taken 436,000 km away the next day (farther away from Earth than the Moon).

MESSENGER is the first probe to study Mercury in detail. It has two cameras (MDIS) that are mapping the planet's landforms. A laser altimeter (MLA) will create a map of the 3-dimensional topography of the planet surface. A radio science experiment will use the Doppler effect to assess the distribution of mass and the thickness of the crust. An X-ray spectrometer (XRS) and a gamma ray/neutron spectrometer (GRNS) will identify elements in the crust and look for signs of any water ice in permanently shadowed craters in the planet's far north and south poles. An atmospheric and surface spectrometer (MASCS) will search for any gases in Mercury's thin atmosphere and look for surface minerals. Finally, an energetic particle and plasma spectrometer (EPPS) will study the magnetosphere around Mercury and a magnetometer (MAG) perched on a 3.5 metre boom will map the magnetic field. The mission will end sometime in 2012 when the probe runs out of the fuel it needs to maintain its orbit.

It will take several months of data analysis to get a global picture of Mercury. Meanwhile, click here to view the latest images from MESSENGER as they come into NASA.

What We Already Know About Mercury

Mercury's surface is very similar to the Moon. There are extensive plains and heavy cratering suggesting that there haven't been any volcanoes or other geological activity for billions of years. All this cratering records a period of heavy bombardment by comets and asteroids, about 4.5 billion years ago, during Mercury's formation and probably again around 3.8 billion years ago during the late heavy bombardment. It was during this period that Mercury was volcanically active, forming the smooth plains much like the maria visible on the Moon today.

The temperature on Mercury's surface ranges from 100 K to 700 K, with a steep temperature gradient from the equator to the poles. The sunlight striking its surface is between 5 and 10 times more intense than what strikes the top of Earth's atmosphere. In spite of this, we know there is a lot of radar reflection at the poles and that could be a sign of water ice. It would be covered by a layer of regolith, preventing its sublimation and loss into space. This water could have come from either outgassing from the planet's interior or from comet impacts (these are also the two probable sources of Earth's water).

Mercury doesn't have enough mass, and therefore gravity, to hold onto any atmosphere for long. But it does have a thin unstable exosphere containing hydrogen, helium, and other gases. These atoms are continuously lost and replenished. Hydrogen and helium probably come from the solar wind and are deposited on the surface. Radioactive decay might supply more helium as well as sodium and potassium. MESSENGER has already found relatively plentiful calcium, magnesium, oxygen and other elements as well. It also found water vapour and water-related ions like O+, OH- and  H3O+. The heavier elements that were found may come from the vapourization of the surface crust by micrometeorite impacts.

Mercury's magnetic field is strong enough to deflect solar wind, thus creating a magnetosphere, a protective envelope. Its magnetic field is a dipole, like Earth's magnetic field, and scientists think it arises from a dynamo effect caused by the circulation of its liquid iron-rich outer core. Mercury's eccentric orbit would cause strong enough tidal effects to keep its outer core liquid, even though, based on its small size, its core should have cooled and solidified long ago. MESSENGER discovered on its 2008 flyby that Mercury's magnetic field is also very "leaky." When magnetic fields carried by the solar wind connect with Mercury's magnetic field, they twist up into tornado-like vortices and form holes in the magnetosphere through which solar wind can enter and directly strike the planet's surface. This is called magnetic reconnection and it actually happens on Earth too. Our planet also has a leaky magnetosphere although, luckily for us, these leaks are very small and happen only rarely, during the most powerful solar storms.

Finally, enjoy this 8-minute video. It sums up what we know about this small but fascinating planet:

Next we will explore Neptune.


  1. You're linked (again) on Educators' News.
    Thanks for a great blog.

    Steve Wood
    Educators' News
    Senior Gardening

  2. Gravity is a little big bigger than in Newton’s law; it increases with speed where the maximum is the double gravity in the case of light.
    Global Physics also predicts the anomalous precession of Mercury’s orbit as Paul Gerber did 20 years before Einstein.

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