Taking the Temperature of a Martian Meteorite
Inside martian meteorite ALH84001 may lie the fossilized remains of ancient bacteria. Some scientists have suggested that other martian meteorites could have seeded the early Earth with primitive forms of life. Others argue that any asteroid or comet that impacted Mars with sufficient force to eject material into interplanetary space would have superheated the ejecta, and that bacteria could not have survived.
New research by Benjamin Weiss and his colleagues at the California Institute of Technology indicates that at least one such meteorite ALH84001 didn't get all that hot. Weiss's team believes that the interior of ALH84001 was never heated above 40 degrees Celsius (104 degrees Fahrenheit). "The rock had probably never even been above room temperature since before it left Mars," says Weiss.
Because temperatures below 40 C cannot sterilize most bacteria, their study suggests that if meteorites like ALH84001 contained living organisms, they could, in theory, transfer life between planets in the solar system.
"This study has lowered the maximum temperature by a significant amount into the range where biology ought to be able to survive without any trouble," says Jay Melosh, professor of theoretical geophysics at the University of Arizona.
ALH84001 was blasted off of Mars 15 million years ago, most likely by an asteroid or comet impact. The rock tumbled through the solar system, finally reaching Earth approximately 11,000 years ago. The meteorite was discovered at Allan Hills, Antarctica, in 1984.
But the excitement came in 1996 when NASA scientists announced the discovery of unusual structures within the meteorite. These forms are "carbonate globules," flattened spheres measuring up to 250 microns in diameter (or a little less than the width of 3 human hairs). The structure and chemistry of the carbonate globules suggest that they may have been formed with the assistance of primitive, bacteria-like organisms.
Microscopic shapes in the carbonate globules of ALH84001 resemble fossil bacteria on Earth, and the meteorite seems to have the same microscopic mineral grains that are produced by bacteria. In addition, ALH84001 contains organic chemical compounds that resemble the decay products of bacteria.
Weiss and his colleagues undergraduate student Francis Macdonald, geobiology professor Joseph Kirschvink and collaborators at Vanderbilt and McGill Universities were interested in finding out how much ALH84001 had been heated by the impact that sent it hurtling into space. They reasoned that, although no living martian organisms were found in ALH84001, if ALH84001 had made it to Earth without ever getting super-hot, it was conceivable that, billions of years ago, another similar martian meteorite could have transported life from Mars to Earth.
So the scientists heated the rock and looked for changes in the rock's magnetism. Rock magnetization results from coupling between adjacent spinning electrons located inside the rock's minerals. This coupling keeps the spin axes of the electrons in lockstep, creating a strong magnetic field. Heating the rock causes the crystal lattice of its minerals to expand. This breaks the coupling between adjacent electrons and demagnetizes the rock.
"If the magnetization changes during your laboratory heating experiment, then you can say the sample hadn't been heated to that temperature before," says Weiss. "On the other hand, if there is no change in the magnetization, then you can say that it had already been heated to that temperature."
The research team said that because heating the meteorite to 40 degrees Celsius (104 Fahrenheit) did reduce the intensity of some magnetic features, the interior probably had not been above 40 C since it was still a part of the planet Mars. Forty degrees Celsius is about the temperature of a hot summer day in Phoenix. It's possible that throughout most of its life the rock has been even cooler than that.
Weiss's team examined several thin slices of the meteorite using a new device called an Ultra-High Resolution Scanning Superconducting Quantum Interference Device Microscope (UHRSSM). The UHRSSM is designed to detect microscopic differences in the orientation of magnetic lines in rock samples, with a sensitivity up to 10,000 times greater than that of other machines. The scientists heated the samples and then used the microscope to check for resulting changes in magnetization.
"I think [this study] makes it more plausible that if there had been life on Mars, it could've made it to Earth," says Melosh. "And, with a little bit of an extension, if there was early life on Earth it could've made it to Mars. There's really no reason why big impacts on Earth can't eject rocks from the surface of the planet and they get to Mars. It takes a bigger impact than it takes from Mars because our escape velocity is higher and we have a bigger atmosphere. But there's no fundamental reason why that can't happen. The big if,' of course, is whether there was life on Mars, and we don't know if that was the case."
A UHRSSM magnetic microscope. Credit: Neil Brake, Vanderbilt University.
In a study conducted in 1997, the Caltech research team found that heating the meteorite to 110 C (230 F) caused changes in the rock's magnetism. Before this experiment was performed, most scientists believed the entire rock had been super-heated upon being blasted off the martian surface. The discovery that the interior of the meteorite had never been above 110 C was completely unexpected.
"During that study, Kirschvink heated the sample to 110 C," says Weiss. "He never expected that the rock had not been above that temperature, so when he found that the rock had only gotten to 110 C, he realized that it was he who made it get that hot!"
One hundred ten degrees Celsius was a whole lot cooler than the temperature to which scientists thought the rock had been heated. Previous estimates ranged between 200 C (392 F) to over 500 C (932 F). This finding in 1997 led the team to conduct their most recent study on the meteorite samples, to see if the rock might have remained even cooler.
"Because 110 Celsius is still too hot for most life, the first study could not tell us whether the rock was heat-sterilized or not during its ejection," says Weiss. "This prompted us to do the second study in 2000 in which we were careful only to heat the rock to 40 C before we measured its magnetic properties. But, as it turned out, even 40 C was a higher temperature than the rock had ever seen."
So how is it possible for a rock to be hit so hard that it flies up into space, escaping the gravity of its own planet and yet still remain relatively cool?
ALH84001 and other martian meteorites were once rocks on the Red Planet. An asteroid impact hurled ALH84001 into space, where it orbited the Sun for 15 million years. The meteorite landed on Earth 11,000 years ago. Click here to enlarge.Credit: Don Davis.
"The amount of heating depends more on impact velocity than the size of the impact," says Melosh. "The non-heating effect the fact that you can get material ejected at very high speed without much shock heating and pressure depends upon that material coming from the free surface. If you take a watermelon seed between your fingers and squeeze it, if there's an open surface upward where it isn't confined, it'll shoot out at high speed.
"So for rocks near the free surface," Melosh continues, "you can eject material at very high speed without seeing very much in the way of shock damage. And that's where these rocks have to come from. In fact, all of the martian meteorites seem to have been near-surface rocks, maybe a meter or two down. Just how much material gets away without high shock just simply depends on the size of the impact."
"In the 2000 paper we provided the first demonstration that a rock could be transferred from one planet to another without being heat-sterilized," says Weiss. "This provides critical support for the panspermia hypothesis that life could have been transferred between planets, and perhaps have originated on a planet other than Earth."
Weiss and his colleagues are now working on a variety of paleomagnetic problems related to the evolution of life and planetary bodies such as Mars, Earth, and the moon.
"We had to demonstrate that ALH84001 still retains magnetization that it acquired on Mars in order to complete our 2000 and 1997 studies," says Weiss. "This is very interesting, since this magnetization might then tell us something about the strength and duration of Mars's magnetic field, its core, and its thermal history. It turns out that the magnetization in ALH84001 is extremely old, and that is telling us a lot about what happened to the planet during its earliest history.
"Magnetic fields are important to the evolution of life because they protect the atmosphere from being depleted of light gases by the impact of the solar wind," Weiss continues. "Mars currently has no global magnetic field, but if it did in the past, it may have been able to sustain a thicker atmosphere that was more amenable to life."
Related Web Pages
More on this story
Benjamin Weiss and Joseph Kirschvink, "Life From Space? Testing Panspermia With Martian Meteorite ALH84001" (Planetary Society)
Determining the temperature history of ALH84001 (Planetary Science Research Discoveries)
Micromagnets in ALH84001 (NASA Astrobiology Institute)
Chains of micromagnets in ALH84001 (NASA Astrobiology Institute)
Fossil Life in ALH84001? (Lunar and Planetary Institute)
Ancient Martian Life in Allan Hills 84001? (New Mars)