OF ALL the planets in our solar system, Mercury is an enigma. The chimeric planet has a face like the moon, yet conceals a metal heart larger than that of Mars; while all of the major planets go around the sun in more or less the same plane, Mercury opts for a jaunty angle; while Earth鈥檚 orbit is essentially round, Mercury prefers an ellipse; and let鈥檚 not forget the magnetic field that it shouldn鈥檛 have. Clearly, the closest planet to the sun is trying to tell us something.
It even had a famous fan: Albert Einstein. Mercury鈥檚 odd motion around the sun was impossible to explain with Newton鈥檚 theory of gravitation alone. The puzzle remained until Einstein used it as the first convincing evidence for his general theory of relativity.
Now astronomers think it holds another secret: how the solar system itself was formed. Ralph McNutt, a planetary scientist at in Baltimore, Maryland, is in no doubt about the planet鈥檚 importance. 鈥淢ercury is the key to the solar system,鈥� he says. If you can explain how such an oddball planet came together, it would go a long way to explaining how all the others formed.
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For more than 30 years we have virtually ignored Mercury. Yet that鈥檚 all about to change thanks to NASA鈥檚 spacecraft. Launched on 3 August 2004, the probe is about to begin a series of three fly-bys which will manoeuvre it into position to enter orbit around Mercury on 18 March 2011.
The inaugural fly-by on 14 January will provide the first opportunity to explore Mercury since 1975, when NASA鈥檚 spacecraft completed its third and final fly-by. Planetologists are getting excited. McNutt, the mission鈥檚 project scientist, and his colleagues have a big list of Mercurial mysteries to solve, and believe that Messenger could crack some of them on its first pass.
Was Mercury once twice the size?
Mercury is peculiarly dense, suggesting it hides a huge iron core, which would account for more than 40 per cent of the planet鈥檚 volume. This is a gigantic proportion compared to Earth鈥檚 core, which fills just 17 per cent of its interior, and its origin is one of the planet鈥檚 biggest mysteries.
One possibility is that the large core may simply reflect the fact that Mercury formed from the hot gas cloud surrounding the sun, where only metals with high melting points could have solidified. Rockier materials would not have condensed so close to the sun, leaving a metal-rich embryonic planet. Or perhaps proto-Mercury formed before the sun鈥檚 fierce heat began and had rocky outer layers which then evaporated as the young sun heated up.
New ideas have emerged as computing power has increased. Planet-formation models suggest that enormous asteroid-like objects were hurtling around the early solar system, colliding and coalescing. Perhaps one of these, from as far away as Mars or beyond, smashed into Mercury so violently that it blasted most of Mercury鈥檚 outer layers into space, leaving the planet just half its original size.
The clues to Mercury鈥檚 formation should lie in its surface composition, but even there the planet shrouds itself in mystery.
If Mercury formed close to the sun, there shouldn鈥檛 be much iron oxide on its surface, since this otherwise common molecule forms more easily at low temperatures. If, however, Mercury formed when building blocks from across the inner solar system coalesced, then its crust should have about the same iron oxide content as Earth鈥檚, regardless of whether a giant impact once blasted Mercury鈥檚 surface layers into space.
The trouble is that Mercury sits between these two extremes. Direct observations from Earth indicate that it is 3 per cent iron oxide by mass, compared to Earth鈥檚 8 per cent. Messenger should be able to clarify this situation. The probe will also measure other key elements and identify the minerals they combine to create across the surface of the planet. For example, if Mercury鈥檚 outer layer evaporated long ago, planetary scientists would expect very low quantities of silicon dioxide and large amounts of magnesium oxide, which has a higher melting point. Other formation scenarios predict different combinations and quantities.
But what if Messenger sees something that no one has predicted? 鈥淧eople can always come up with explanations,鈥� Jeffrey Taylor of the in Honolulu.
Why does Mercury have a magnetic field?
Mercury鈥檚 large, dense core generates more than just confusion. It also gives rise to a magnetic field, as Mariner 10 discovered. The field itself is small 鈥� just one-thousandth of the strength of Earth鈥檚 鈥� but its mere presence was perhaps the biggest surprise of the discoveries made in the 1970s. Put simply, it should not be there.
A magnetic field is usually generated in the core of the planet from a circulating region of electrically conducting, molten material. As large as Mercury鈥檚 iron core is in relation to the planet, it is still only half the diameter of Earth鈥檚 core. This, combined with the thinner layer of insulating rocks around it, means that Mercury鈥檚 core should have long since radiated away its heat and solidified, putting an end to any magnetic field.
There is a slim chance that it is a 鈥渇ossil field鈥�, created by magnetic material deposited in Mercury鈥檚 crustal rocks as the planet solidified. Fossil fields have been detected on both the moon and Mars, but they are relatively small-scale phenomena, dubbed crustal anomalies, which seem unlikely to account for a planet-wide field.
Recent measurements from radio telescopes suggest that there is a molten mantle churning inside Mercury, because of the way the planet wobbles. Such wobbles depend upon the distribution of mass inside a planet 鈥� whether it is moving as a single, solid entity or instead sloshing around because part of it is liquid. Jean-Luc Margot of Cornell University in Ithaca, New York, and his colleagues have recently shown that Mercury鈥檚 wobble is twice that expected from a completely solid object ().
So what鈥檚 keeping the interior molten? A popular theory is that the iron is mixed with sulphur, which would lower the freezing point of the core, allowing it to remain fluid.
There will be no way to probe the composition of the core during Messenger鈥檚 first fly-by, but the path the spacecraft takes as it slingshots around the planet will reveal much about Mercury鈥檚 internal structure. 鈥淲ith a closest approach of 200 kilometres, we will be able to measure the mass distribution of the planet and tell the extent of the molten core,鈥� says McNutt.
At the same time, Messenger鈥檚 magnetometer will be studying the shape of Mercury鈥檚 magnetic field to see whether it resembles that of a classic bar magnet. This would prove the field is generated in the core, as is Earth鈥檚 field.
Yet even if they see this, the job is far from over, says Sean Solomon at the in Washington DC, and the principal investigator for Messenger. He points out that simply scaling down the size and speed of Earth鈥檚 liquid core to Mercury proportions would produce a field far stronger than Mercury鈥檚. 鈥淪omething different must be going on inside Mercury,鈥� he says.
What that might be is still anyone鈥檚 guess, but Messenger鈥檚 fly-bys will help find out. 鈥淲e should get a good measurement of the internal field, and possibly some crustal anomalies also,鈥� says Solomon.
What does the far side of Mercury look like?
Mariner 10鈥檚 carefully planned trajectory around the sun took it repeatedly past the planet, rather than into orbit around it. For every loop the probe made around the sun, the planet completed two orbits. This and Mercury鈥檚 slow rotation rate meant that Mariner 10 always saw the same hemisphere bathed in sunlight, while the other remained hidden in darkness.
As a result, Mariner 10 only managed to image 44 per cent of the planet鈥檚 surface. Being the first craft to orbit the closest planet to the sun, Messenger should finally reveal the rest. One of the structures awaiting us is the whole of Mercury鈥檚 , one of the biggest impact structures in the entire solar system. Mariner 10 photographed only half of it.
Caloris is estimated to stretch for 1350 kilometres and is seemingly ringed by mountains. The basin contains a flat, dark lava plain similar to the lunar maria. The crater may also allow us a glimpse 鈥渋nside鈥� Mercury because the impact will have excavated vast quantities of material from deep within the planet. 鈥淐aloris is a drill hole 鈥� a messy one 鈥� but a drill hole nonetheless,鈥� says Taylor. He suggests that the smash may have thrown lower-crust and even upper-mantle material onto the surface, where Messenger will soon be able to see it.
Messenger will capture images of half of the remaining hemisphere starting this month and fill in the gaps on its second pass, on 6 October. The planet preserves a virtually unblemished record of impacts across its near airless surface. Once the map of Mercury is complete, astronomers will be able to deduce the frequency and ferocity of collisions close to the sun during Mercury鈥檚 lifetime 鈥� crucial for the understanding the how the solar system formed.
Does Mercury have polar ice caps?
As bizarre as it seems for a planet whose sunny side is hot enough to melt lead (see How Mercury measures up), Mercury may have icy deposits. 鈥淩adar echoes from Mercury鈥檚 polar regions are very strong and look like the echoes we get from Mars鈥檚 polar caps, and from the icy satellites of Jupiter,鈥� says John Harmon of the in Puerto Rico.
The specific radar bright spots that the astronomers can see all seem to coincide with known polar craters. This evidence suggests that these craters are 鈥渃old traps鈥� 鈥� permanently shaded regions of Mercury鈥檚 surface where molecules freeze out of the planet鈥檚 ultra-tenuous atmosphere.
Messenger will be unable to peer into the polar craters during its fly-bys, as its closest approach is above Mercury鈥檚 equatorial region. However, mission controllers will turn the probe鈥檚 cameras towards the poles on its first pass to look for telltale signs of icy material boiling off.
鈥淓ven though the crater floor is in shade, the walls can still heat up,鈥� Harmon explains. These walls radiate their heat, warming the ice on the floor sufficiently for some of it to boil back into Mercury鈥檚 pseudo-atmosphere.
What the ice is made of is another question. While it could be water ice, it could also be anything that reflects rather than absorbs the radar signals, such as sulphur. If the polar deposits turn out to be water ice, they must be the remains of comets that have collided with Mercury. If they are sulphur, they will have originated in the planet鈥檚 interior and seeped out as a result of volcanic activity. Once Messenger settles into orbit in 2011, it will investigate the polar deposits in greater detail.
Why is Mercury鈥檚 orbit so tilted?
Of all the major planets, Mercury has the weirdest orbit. It is elliptical, swinging 46 million kilometres from the sun right out to 70 million kilometres and back again. The 88-day orbit is tilted too, inclined at about 7 degrees to the orbital plane of Earth.
At first glance, Mercury鈥檚 odd orbit seems to be compelling evidence that it was walloped by another large body 鈥� perhaps the same impact that may have stripped its outer layers. As ever, though, things are not clear-cut.
鈥淵ou don鈥檛 need a giant impact to do this,鈥� says Solomon. 鈥淕ravitational interactions can pump up oddities in planetary orbit.鈥� Such interactions occur when planetary objects continually pass close to one another, which could have happened to Mercury during the formation of the solar system. Repeated gravitational nudges can force bodies into increasingly elongated orbits. 鈥淎ll you need is for an orbit to be stable, rather than circular,鈥� says Solomon, citing the various 鈥渆xoplanets鈥� now being found around other stars, many of which also have elliptical orbits.
There is probably no single observation that Messenger can make to determine the origin of Mercury鈥檚 orbit. Instead, when results from the probe鈥檚 many investigations are collated to provide the big picture of the planet鈥檚 formation history, we will know whether a large impact was likely sometime in its past. If it was not, the focus will turn to modelling Mercury鈥檚 orbit using the softly, softly approach of gravitational interactions.
Is there physics beyond Einstein?
Orbiting so close to the sun, Mercury feels its gravitational pull most keenly, making it the perfect place to test general relativity. As Einstein showed, the effects of general relativity constantly alter the planet鈥檚 path. On its elliptical orbit, Mercury dips in and out of the dent in space caused by the sun鈥檚 mass, and this turns the planet鈥檚 orbit. Any slight inconsistencies in this motion might reveal new physics in action.
However, Messenger was not designed to test fundamental physics. 鈥淲e鈥檒l feel the effects of relativity 鈥� and have to correct for them 鈥� but we won鈥檛 be able to test relativity to more stringent limits than before,鈥� says McNutt.
Fortunately, Messenger is not the only Mercury mission we have to look forward to. The European and Japanese space agencies plan to launch a joint mission to Mercury in 2013, and this one does plan to probe fundamental physics. Called , it is larger than Messenger and will consist of two orbiting spacecraft. One will scrutinise the surface of Mercury while the other will investigate the details of its magnetic field. Researchers are already referring to Messenger as the appetiser to BepiColombo鈥檚 main course.
For general relativity, BepiColombo will carry radio equipment that allows mission controllers to track the position of the spacecraft to an accuracy of 10 centimetres. This accuracy will enable them to deduce the motion of the planet to within 10 metres. At present, the planetary position is only known to an accuracy of several kilometres.
Gravitational physicists and cosmologists are becoming increasingly convinced that general relativity must break down beyond a certain level of accuracy, as a result of the new energy fields they postulate to account for the accelerating expansion of space. Each new field that theorists introduce produces a subtle deviation from the behaviour that relativity predicts for gravity. If BepiColombo detects such violations of general relativity, they will discover a strong clue as to the nature of these mysterious energy fields.
Once in orbit around Mercury, the craft will test general relativity in two ways. First, it might simply detect a subtle deviation in the position of the planet that relativity cannot account for. Second, and more decisively, mission scientists will time the delay in radio signals the spacecraft sends back to Earth as Mercury begins to go behind the sun. This delay will be caused by the signal dropping into the sun鈥檚 gravitational well before 鈥渃limbing鈥� out the other side.
This phenomenon is predicted by relativity to be the equivalent of the radio signals travelling an extra 70 kilometres through space. With a tracking accuracy of 10 centimetres, BepiColombo will measure this distance more accurately than before and spot any anomalies.
Luciano Iess at the University of Rome La Sapienza, Italy, conducted a with the spacecraft at Saturn in 2003. It just reached the accuracy at which violations of relativity are expected to show up, but none was seen. Iess is now principal investigator of BepiColombo鈥檚 radio science experiment. 鈥淲e will improve on the accuracy of the Cassini experiment by a factor of 10,鈥� he says, making it the sharpest test of general relativity yet.
