Magnetic Mars

April 15, 2013

Magnets, with their magical action-at-a-distance property, have inspired many a child's interest in science. Permanent magnets, and a variety of electromagnets I built when I was about nine years old, were the probable catalyst for my physics career.

Many movies of the era portrayed laboratories with bubbling flasks of chemicals, and I played with chemistry sets for a time. Electricity, however, was a more fascinating subject, especially when sparks were involved.

One method for generating sparks is to connect an inductor to a hefty battery and momentarily touch the wire ends to complete and break the circuit (see figure). If you want to create a lot of sparks, as I did as a child, you would connect one wire to a metal file and scrape the other wire across the file.[1]

To paraphrase the expression about mirrors, it's all done with magnetism. It also generates radio frequency interference, so don't use too large a battery. Of course, the usual disclaimers about parental supervision, safety glasses, etc., apply; and, there's also an electrical shock hazard.[1]

Creating sparks with an inductor. The stored energy in a magnetic field is discharged as a spark when the electrical circuit is broken.[1] (Illustration by the author using Inkscape.)

Our knowledge of magnetism goes back at least to the sixth century B.C. to the Greek philosopher, Thales. Thales wrote about the attraction of lodestones, which are magnetized specimens of the mineral, magnetite (Fe₃O₄), to themselves, and to iron. The Earth's magnetic field was made apparent with the first magnetic device, the compass, also known from antiquity.

The first modern study of magnetism was done by the English scientist, William Gilbert, and published in 1600 as De Magnete.[2] Gilbert provided evidence that the Earth was a giant magnet, and he actually created a sphere of lodestone, which he called a terrella, as a model to prove his theory.

William Gilbert (1544-1603) The gilbert, a unit of magnetomotive force, was named in his honor. The gilbert is not an SI unit, but rather a cgs unit. The SI unit for magnetomotive force is one ampere flowing in a loop of wire, and there's a conversion factor between the SI unit and a gilbert of (10/4π) which makes the gilbert slightly smaller. (Gilbert portrait by Charles Henry Granger (1812-1893), via Wikimedia Commons.)

Gilbert's De Magnete was not just concerned with the magnetic moment of the Earth. It included information on practical magnetism, such as the method shown in the following woodcut for magnetizing a piece of iron.

Magnetizing an iron piece. Hot iron is worked on an anvil with its axis aligned north-south. The Latin word for North is septentrio, and the Latin word for south is avster/auster).[2] (Via Wikimedia Commons.)

The intensity of Earth's magnetic field, 0.25 to 0.65 gauss when measured at its surface, is small compared with that of permanent magnets. The barium ferrite (BaFe₁₂O₁₉) component of composite refrigerator magnets has a field strength of up to several thousand gauss. Small though it may be, we're fortunate that Earth has a magnetic field, since it directs cosmic rays around the Earth.

Cosmic rays are charged particles, so they experience a force when moving in a magnetic field, much like a current-carrying wire will be deflected when a magnet is brought nearby. An interesting example of this effect is the dancing filament of an incandescent light bulb incorporating a permanent magnet.[3] The Earth's magnetic field lines deflect most cosmic rays to the poles, where they are manifest in auroral displays.

Mars, however, doesn't have a global magnetic field. That, and its lack of a dense atmosphere, will make conditions difficult for astronauts. Its lack of a global magnetic field may be the reason why Mars has so little atmosphere, since the solar wind would have stripped it away over the course of billions of years.

NASA has planned a space probe, the Mars Atmosphere and Volatile Evolution (MAVEN) mission, scheduled for launch this year. One purpose of this probe is to do a careful magnetic survey of Mars, which still retains patches of magnetic field in its crust (see figure). MAVEN will reach Mars orbit in September 2014.[4]

Magnetic fields on Mars. Unlike Earth, Mars doesn't have a global magnetic field. There are just remnants of a past field locked into crustal minerals. (Still image from a NASA video.)

Small though they be, these magnetic patches might create pockets of atmosphere that are protected from the solar wind and cosmic rays.[4] The magnetometers of MAVEN are sensitive flux-gate devices based on the magnetic saturation of a material. When exposed to a magnetic field, such materials will saturate magnetically more quickly in one direction than the other. The electronics to sense such a change is easy to design, but the power demand of the saturating coils is often a problem.

The biggest problem NASA faces is creating a "magnetically clean" spacecraft; namely, one in which its materials of construction and electrical currents for operation won't generate false signals.[4] When probing Mars for very small magnetic fields, every stray bit of magnetic field in the spacecraft would generate measurement errors.

References:

1. Please read our Site Disclaimer. "The scientific procedures and experiments described on these pages assume an appropriate level of adult supervision, safety training, personal protective equipment (such as safety glasses, etc.), and other safety facilities. You must follow all the customary, prudent, and necessary procedures of the appropriate scientific discipline for the following: 1) Safe storage, preparation, handling, and disposal of any chemicals or other potentially dangerous materials mentioned in these pages. 2) Safe use of machines, experimental apparatus, and other equipment."
2. William Gilbert, "De Magnete," 1600 (original Latin); English translation. The illustration is from Book III, Chapter XII.
3. Robert J. Kyp, "Electric Light Bulb with Oscillating Filament," US Patent No. 3,237,053, February 22, 1966.
4. Claire De Saravia, "Measuring Mars: The MAVEN Magnetometer, NASA Goddard Space Flight Center Press Release, March 26, 2013.
5. Magnetic putty absorbing a rare-earth magnet, YouTube video, Mar 25, 2013.