A meteorite that fell to Earth from Mars purported to offer evidence of past Martian biology has fallen from grace. That's the assertion from two scientists who say the rock's strongest link to life -- as claimed by other researchers -- has broken down.

In December 1984, ALH 84001 -- often called the "Mars rock" -- was picked up in Antarctica by a National Science Foundation-sponsored meteorite-hunting expedition. Tossed into space by an asteroid or comet that hit Mars billions of years past, the tiny piece of rock eventually found its way to Earth. It is believed to have landed in Antarctica some 13,000 years ago.

But in the May 14 issue of the Proceedings of the National Academy of Sciences, researchers David Barber of the University of Greenwich, London and Ed Scott of the University of Hawaii argue against those tiny crystals of iron oxide in ALH 84001 being formed by organisms on Mars. They believe their findings offer definitive evidence that should put to rest years of dispute over the origin of the iron oxide grains.

The work of Barber and Scott show that the planes of oxygen atoms in the magnetite crystals are aligned with those in the surrounding carbonate crystal. This proves, they contend, that the magnetite crystals could not have formed elsewhere and then been deposited in the carbonate. They must have formed exactly where they are observed today in the carbonate. This observation is counter to what the NASA-led team has argued.

Barber and Scott also discovered oriented crystals of magnesium oxide in the Martian carbonate. They conclude that the iron and magnesium oxides both formed when the carbonate was hot and had partly decomposed to form carbon dioxide gas.

Since the meteorite was heated by an impact on Mars four billion years old that melted and vaporized many minerals in the rock, Barber and Scott infer that the iron and magnesium oxides formed then. Martian organisms cannot be responsible for the size and shape of any magnetite crystal in ALH 84001.

Although the researchers argue the new findings derail a biological connection to ALH 84001, the meteorite surely contains valuable clues as to early solar system formation.

"Meteorites are cosmic gifts that keep on giving, and giving," said Scott of the Hawaii Institute of Geophysics and Planetology, University of Hawaii at Manoa, in Honolulu.

For example, if Mars and the Moon were both bombarded by asteroids four billion years ago, why should we find an ancient meteorite from Mars but no equally ancient lunar meteorites or returned samples from the Moon of comparable size?

In addition, can the magnetite crystals in ALH 84001 tell us the strength of the magnetic field on Mars four billion years ago?

An ancient Martian rock, even one that has apparently failed to bring evidence of extra-terrestrial life, is a precious gift, Scott said.

After years of argument, Scott told SPACE.com, the scientific thrash out over the meteorite underscores a number of points.

"ALH 84001 certainly shows how difficult it may be to assess future samples returned from Mars. That has been quite a surprise! After six years of intense laboratory study of many tens of grams of rock, we think we know how the magnetite grains in ALH formed. We have yet to agree on how the carbonate formed, and we have yet to find any convincing evidence for life," Scott said.

Scott said that by returning samples from Mars, those specimens are less likely to have been severely damaged by shock, like ALH 84001. However, it is possible that scientists will face many of the same difficulties when samples are brought back to Earth, he said.

Has the debate over ALH 84001 been helpful or a waste of scientific time?

"The studies and the debates have been an essential and invaluable stage in our learning process. ALH 84001 has provided a great technical challengeto study a rock at diverse scales, to relate what we see at the centimeter scale to what is going on at the nanometer scale. Studies of ALH have helped to expand the field of the geology, biology, and planetary sciences into the nanometer world," Scott said.

ALH and the discovery of extrasolar planets have also stimulated the expanding new field of astrobiology, Scott added.

"You could argue that it has been a major stimulus in helping to integrate geology and biology in the planetary sciences. Its no longer okay to focus only on the living or the non-living," Scott said. "We have discovered exciting new connections on the Earth and on Mars. It is not enough to address the simple question 'are we alone?' We have to understand more complex questions that lend a new perspective to planetary science. Why is Earth so different from Mars and Venus? How special is the Earth? How common are Earth-like planets?"