* Astronomy

A new study of old rocks on Earth could force a revision of theories about Mars. The results suggest ancient Mars might have been more magnetic than thought, challenging basic assumptions about the evolution of the red planet.
Data from the Mars Global Surveyor spacecraft have revealed a pattern of magnetic stripes on Mars similar to fields permeating the sea floors of Earth. It appears that Mars may have once had a rather dynamic geology, similar to activity on our planet today, with important consequences for ancient Martian life.
Unlike modern Earth, Mars has almost no magnetic field today. Evidence has suggested Mars didn't have a very strong magnetic field early on, either.
Our planet's magnetism is created by the rubbing of a solid inner core against a liquid outer core, which rotate at different rates and act as a dynamo. The magnetic field helps deflect cosmic radiation and solar particles, making Earth comparatively more habitable.



Magnetism is recorded in the structure of rocks. Superheated material, when it cools, takes on a structure parallel to the prevailing magnetic field at the time.
On Earth a magnetic banded pattern is created when molten lava wells up from the planet's interior. Along the divide where material cools to form new crust, iron-rich minerals preserve a record of the the magnetic field that prevails on the planet at the time. Earth's magnetic field reverses its polarity every million or so years, causing the alternating magnetic signature. The discovery of banded magnetic patterns on Mars suggests a similar process at work.
A planet's magnetic activity changes over the eons, in part because a young planet cools and solidifies as it ages, so ancient bedrock can serve as a time capsule for magnetism, a sort of fossil compass.



Magnetic bands on Mars in the southern highlands near the Terra Cimmeria and Terra Sirenum regions, centred around 180 degrees longitude from the equator to the pole. It is where magnetic stripes possibly resulting from crustal movement are most prominent. The bands are oriented approximately east - west and are about 100 miles wide and 600 miles long, although the longest band stretches more than 1200 miles. The false blue and red colours represent invisible magnetic fields in the Martian crust that point in opposite directions. The magnetic fields appear to be organized in bands, with adjacent bands pointing in opposite directions, giving these stripes a striking similarity to patterns seen in the Earth's crust at the mid-oceanic ridges.
A study in 2003 found the core of Mars, at least the outer part, is liquid.
Surveys in the 1990s of magnetic fields on Mars, by the orbiting Mars Global Surveyor, detected the signatures of relatively intense magnetism in some of the planet's more modern surfaces. But the fields were found to be very weak in two large and old impact basis, called Hellas and Argyre. Each basin, carved out by a colossal space rock, is more than 3 billion years old. The data implied that Mars had a weak magnetic field back then.
It is thought that without a substantial magnetosphere to protect it, much of Mars's atmosphere is exposed directly to fast-moving particles from the Sun.


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"In 1989 the Soviet Phobos probe made direct measurements of the atmospheric erosion. When the spacecraft passed through the solar wind wake behind Mars, onboard instruments detected ions that had been stripped from Mars's atmosphere and were flowing downstream with the solar wind. If we extrapolate those Phobos measurements 4 billion years backwards in time, solar wind erosion can account for most of the planet's lost atmosphere." - Dave Mitchell, space scientist at the University of California at Berkeley.

That analysis has influenced theories of how Mars cooled after its formation and when its inner layers developed distinct boundaries.
The new research calls into question the validity of measuring magnetism from an orbital perch.
A team led by Stuart Gilder of the Paris Earth Physics Institute found that rocks in the 2-billion-year-old Vredefort impact crater in South Africa -- the oldest such structure on Earth -- are highly magnetized, yet from above the magnetism appears weak. Two other ancient craters reveal similar differences.


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The basic reason is simple: While magnetism is strong in individual rocks, the direction varies from rock to rock in these impact craters, so when examined from a distance, they cancel each other out.
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"Meteorite craters can then seem to be magnetic or non-magnetic, depending on how close the magnetometer is to the source. Viewed from satellite altitudes of 100–400 kilometres, Martian impact basins would appear magnetically featureless if the magnetic vectors of their source rocks vary in direction over distances of a few kilometres or less." - David Dunlop, a University of Toronto researcher.



Exactly why the rocks are magnetized randomly is more complicated.
Based on differing mineral structures in the rocks, Gilder and his colleagues hypothesize that when a space rock hits, the shock of the event would briefly create intense localized magnetic fields. Rocks that cool during this initial period would be magnetized with orientation related to these temporary field. Other rocks would cool more slowly, and would take on the planet's magnetic orientation.