Powerful appeal of Mars' 'missing' field.

Powerful appeal of Mars' missing' field

Planetary scientists disagree about whether Mars generates its own "intrinsic" magnetic field, as do Earth and most of the solar system's other planets. Some researchers believe Mars once had such a magnetic field, and a far stronger one than any that may exist there today. Now Jayanta Kar of India's National Physical Laboratory in New Delhi proposes in the newly released February GEOPHYSICAL RESEARCH LETTERS that the weakening of a strong magnetic field could have had significant influences on the planet, ranging from the evolution of its atmosphere to the possible development of life.

An intrinsic field is one produced in a planet's molten core by circulation patterns caused by rising heat (convection), which create essentially a dynamo like an electric motor. U.S. and Soviet spacecraft sent to Mars in the 1960s and 1970s with instruments to measures such a field failed to find unambiguous evidence of intrinsic magnetism.

The U.S. Viking spacecraft in the late 1970s did detect an "induced" magnetic field above the planet, formed where the sun-spawned ions called the solar wind strike the top of the Martian atmosphere. Any intrinsic field other than a very weak one, however, would act as a shield, keeping most of the solar wind from ever getting close to the atmosphere.

Some scientists have calculated that if Mars does have an intrinsic field, it is no more than four ten-thousandths as strong as Earth's. On the other hand, Steven A. Curtis of NASA's Goddard Space Flight Center in Greenbelt, Md., and Norman St. Ness of the University of Delaware in Newark suggested in 1988 that the field present early in Mars' history was about 16,000 times stronger than any magnetic field Mars may have now. The field's weakening seems to have happened at least 1.3 billion years ago, they noted, when the dynamo was "switched from its on to its off state." They based this estimate on studies of certain meterorites found on Earth whose rock apparently crystallized on Mars about that time, whose magnetic characteristics indicate the planet's interior may be then have cooled enough to weaken or shut off the dynamo.

Earth is a larger planet than Mars and so it more thickly blankets its core and retains its interior heat. The giant outer planets -- Jupiter, Saturn, Uranus and Neptune -- have kept their heat sources going by compression due to their own gravity. Mars and the sun, however, may have engaged in a sort of shoving match, in which the planet's magnetic field once held the solar wind at bay.

There is no direct evidence for the strength of the solar wind during the period when the Martian dynamo was active, Kar says. However, he adds, observations of stars similar to the sun but at different stages in their evolution can give astronomers an idea.

Studies addressing the sun's early evolution suggest it was then about 25 percent dimmer than now, says Kar, and that the sun's continual loss of its mass was about 25 percent slower than its present rate. Furthermore, the scientist says, investigations of Apollo moon rocks reveal that the average solar wind velocity may have been only three times greater about 3 to 4 billion years ago than it is now, with a pressure at any given distance from the sun of about 2.25 times its present amount. "Clearly, therefore, the pressure of the intrinsic magnetic field in early Mars would exceed the solar wind dynamic pressure," according to Kar, leaving Mars wrapped in a magnetosphere of ions that could not penetrate that magnetic barrier.

The shoving match may not always have gone that way, however. Some astrophysicists believe that the sun spent the first 50 million years of its life in an energetic period called the "T-tauri" phase. At that time, says Kar, mass could have been leaving the sun and heading toward Mars at as much as a million times the present rate, so that even the initially strong Martian magnetic field would not have been able to balance it.

The shoving match after that, however, could have played a role if life were trying to evolve on the Martian surface. Kar notes that some species that became extinct on Earth in the last 2.5 million years did so near reversals in its magnetic field. These reversals may have been accompanied by periods when the whole field was weaker, possibly allowing more ultraviolet tradition from the sun to reach Earth's surface. "It appears," reports Kar, "that the shielding effect of the Martian magnetosphere would have helped sustain biological species until about 1.3 billion years ago," if any ever developed.