Even though Mars is one of Earth's two sister planets, it is still in large part a mystery to scientists. Did life ever exist there? Can it exist there now? Is there frozen water? These questions plague scientists, astronomy enthusiasts, and science fiction writers worldwide. For the purposes of the Mars Global Surveyor, however, there is an even more important question: what is the nature of Mars' magnetic field?
To understand this question and therefore the MAG/ER (Magnetometer/Electron Reflectometer) experiment on the Mars Global Surveyor, it is important to first understand magnetic fields and their importance; hence, the focus of this page.
Magnetic fields, simply put, are the areas of influence of a magnet. A magnetic field covers the whole area in which the attraction or repulsion of a magnet can be felt. For an everyday example: most kids know that if someone in their family is making dinner in the kitchen, they will get called in to help if they watch TV nearby in the living room, but not if they are in their bedroom upstairs (if they are lucky). So the chef's "magnetic field," as it were, extends to the living room, but not to the second floor of the house.
That is a magnetic field, but it is important to get the idea of a magnet straight. Everyone has seen and played with magnets; maybe you have sprinkled iron filings near one in a science class or played with the magnetic marbles you can link together as a string. It is not just iron bar magnets that show magnetism, though: the whole idea of magnetism is that in everything (a piece of iron or the graphite in a pencil), a tiny current is caused by electrons whirring around their nuclei. Two currents traveling in the same direction attract; two traveling in opposite directions repel. In a bar magnet like one you might have played with, the tiny atom-magnets are all lined up in the same direction, causing the "magnetic" property due to the attraction of like currents and forming north and south poles due to the lining-up.
Back to magnetic fields -- first of all, they are no haphazard things. Magnetic fields, though they can get pushed around (as we'll see later), keep their currents, and thereby their magnetism, in set places called magnetic field lines, running out of the south pole of the magnet and into the north pole. Since the currents are electrons and charged particles stay along magnetic field lines and do not jump around, this works out pretty well for the lines. This, to extend an analogy, is as if the chef in your house would only be able to say, "Please set the table, dear," if you were standing either two, or five, or fifteen feet away from him or her. The whole idea of magnetic field lines seems rather abstract, but you can prove it to yourself if you put a bar magnet under a sheet of paper and sprinkle iron filings on the paper. The filings will not only be attracted to the magnet, but will form circles of magnetic field lines around it. (See the picture if you do not have a magnet handy.)
Whether you have heard it or not, take it for granted: the Earth is a sort of really big magnet. Just like a bar magnet, the Earth has a magnetic north and south pole and huge connecting magnetic field lines that form what scientists call the magnetosphere. This fact is what makes compasses work -- the north end of a compass repels magnetic north, so they point the same way.
However, do not get the idea that the Earth actually has some huge piece of iron like you might buy at a science store imbedded in it that makes compasses spin around. The cause of Earth's magnetism is actually the Earth's internal dynamo, which is so hot that a typical iron magnet would lose its magnetism, anyway. In this super-hot core, electrically-conducting molten iron flows around through a magnetic field in a closed electric circuit. Because some of the fluid is moving in the magnetic field and some is not, and because a couple of other necessary conditions in the core are satisfied, an electric current starts up as per the laws of physics. As we learned before, the presence of electric currents starts up magnetism. When slow changes in the flow of molten iron in the core occur, the Earth's magnetic field varies.
The magnetosphere is very important to everyone on the planet because it keeps most solar wind that could hurt power and satellites away from the Earth; charged particles coming out of the Sun have to follow the rules and can not jump from one field line to the next to get down to Earth.
As magnetospheres go, though, the Earth is not anything too special. Mercury, Jupiter, Saturn, Uranus, and Neptune all have magnetospheres, and all but Mercury's dwarf ours. Our sister planets, Mars and Venus, are the oddballs: space probes have found no evidence of structured magnetic field lines on either planet, only traces. Since magnets lose their magnetism when heated a lot, it makes sense that Venus, where it is hot enough to melt lead, does not have a magnetosphere. Therefore, it is Mars that is the real mystery: it is pretty cold and is quite like Earth in many ways . . . so why no magnetosphere?
Now, the point of the Mars Global Surveyor's magnetometer comes clear. As you are reading this, MGS is orbiting Mars and mapping out the planet's magnetism (or lack thereof) with its magnetometer. Every once in a while, anomalies are found, where some magnetized substance is buried beneath the surface. These anomalies were thought to have come up from a once-magnetized core and kept their magnetism when the planet lost its overall magnetism. Scientists working with the MGS hope that by mapping these anomalies they can learn about the extinct magnetic core or dynamo within Mars and about Mars' surface evolution.
Want to learn more about magnetism and magnetospheres? Try the Exploration of the Earth's Magnetosphere page.