Though Mars lacks a global protective magnetic shield like that of the Earth, strong localized magnetic fields embedded in the crust appear to be a significant barrier to erosion of the atmosphere by the solar wind.
This conclusion by a researcher at the University of California, Berkeley, emerges from a new map of the limits of the planet's ionosphere obtained by the Mars Global Surveyor spacecraft, which was launched in 1996 and reached the planet 10 months later. The new data show that where localized surface magnetic fields are strong, the ionosphere reaches to a higher altitude, indicating that the solar wind is being kept at bay.
The findings suggest that these crustal fields could have played an important role in the past evolution of Mars' atmosphere. If, as some Mars experts think, much of the planet's atmosphere was stripped away by the solar wind, these maps show where the solar wind did, and continues to do, the most damage.
"The ionosphere is what shields the densest part of Mars' atmosphere from being swept away by the solar wind," said David Mitchell, a research physicist at UC Berkeley's Space Sciences Laboratory who compiled the map from spacecraft data. "Our data show for the first time that the crustal magnetic fields are a major factor limiting erosion of the atmosphere in some regions. These fields are like umbrellas scattered over the surface protecting the atmosphere."
The map of the ionosphere will be presented by Mitchell and his colleagues on Saturday (Dec. 16) during a morning poster session at the San Francisco meeting of the American Geophysical Union.
Mars at one time presumably had an interior dynamo like that of the Earth, which would have generated a global magnetic field to shield the atmosphere from the solar wind.
Data reported in 1998 from the same spacecraft indicated that Mars probably lost its magnetosphere about four billion years ago, at which time the atmosphere would have felt the full force of the ionized particle sirocco from the sun. While at one time the planet apparently had an atmosphere dense enough to allow liquid water to flow on the surface, most of that has since disappeared.
A crucial finding was that ancient asteroid or comet impacts wiped out part of the crustal magnetism, and those regions were not subsequently remagnetized. Because these demagnetized craters are some four billion years old, the dynamo must have wound down at least that long ago.
"Finding these demagnetized and very ancient crater sites helped us date when the dynamo turned off, which was a big help, because now we know when, in our models, to turn on erosion by the solar wind," said Mitchell. "What I'm doing is trying to determine where you can apply the erosion. So now we have the when and the where, and we can estimate the how much."
Though he hopes soon to use this information to estimate how quickly the Martian atmosphere dissipated over time, "We're not at the stage yet where we can apply this new information to modeling the atmospheric loss," he said.
Mitchell's colleagues include Robert P. Lin, professor of physics at UC Berkeley and director of the Space Sciences Laboratory; Henri Reme of the Centre d'Etude Spatiale des Rayonnements (CESR) in Toulouse, France; Paul A. Cloutier of the Department of Physics and Astronomy at Rice University; and J. E. P. Connerney and Mario Acuna of NASA's Goddard Space Flight Center.
The data were obtained by an electron reflectometer aboard Mars Global Surveyor, an instrument built at the Space Sciences Laboratory and CESR to map surface magnetic fields. Mitchell used the instrument to determine when the spacecraft, orbiting about 400 kilometers above the surface, was inside the planet's ionosphere or outside in the gale of the solar wind. This is possible because the energy spectrum of ionospheric electrons is distinctly different from that of solar wind electrons.
Between February 1999 and April 2000, the spacecraft mapped the position of the ionopause - the boundary between the ionosphere and the solar wind. The final map was an average over this time period and over thousands of orbits, representing the probability at any given point that the spacecraft was within or outside the ionosphere.
When he compared this map with a map of the surface magnetic fields obtained by a magnetometer also aboard the spacecraft, he found that the ionosphere extended to the highest altitudes over the strong crustal magnetic fields. Over areas of weak magnetic field, the ionopause rarely reached as high as the spacecraft orbit, whereas over strong magnetic areas it nearly always reached the spacecraft at 400 kilometers altitude, and probably extended hundreds of kilometers higher.
"The correlation is striking," Mitchell said. "When the spacecraft is flying over the magnetic anomalies, it is almost always in the ionosphere, whereas when it's over magnetically weak regions, 90 percent of the time it is in the solar wind."
Mars' crustal magnetic fields themselves are a mystery, because they are nearly as strong at the surface as the Earth's magnetic field - a few tenths of a Gauss, compared to a third of a Gauss on Earth. Plus they are arrayed in east-west bands of alternating polarity, extending for over 1,000 kilometers north to south like a bar code across the planet's surface. Scientists still do not know what materials produce this strong field, or why it occurs in alternating bands.
Mitchell said the crustal fields have been there for four billion years, fending off the solar wind. Despite this protection over part of the planet, however, the solar wind is still considered the most likely cause of the loss of Mars' atmosphere.
The atmosphere today extends hundreds of kilometers into space, where the solar wind can ionize the atoms and sweep them away. At such high altitudes, however, the density is very low. The ionosphere, though it does not extend to such high altitudes, nevertheless protects the densest part of the atmosphere from this type of erosion.
The magnetic field lines can be pictured as half cylinders lined up side by side on the surface, oriented east and west. The place where the half-cylinders touch are areas of strong vertical magnetic field, where ionized hydrogen and helium of the solar wind are able to funnel down to low altitudes. The tops of the cylinders are areas of strong horizontal magnetic field, which acts as an umbrella to shield the underlying atmosphere from the solar wind.
"These crustal magnetic anomalies form cylindrical magnetic objects that shield the atmosphere much like the Earth's dipole field does the entire Earth, except on Mars it is local," he said. "You can see a very interesting pattern of shielded regions and cusps where the solar wind funnels down in between."
Space Sciences Lab at Berkeley
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