University of California NEWSWIRE
March 19, 1997
"We have come up with three possible scenarios, and none of the three looks especially conducive to life," said Laurie Leshin, a UCLA geochemist in the department of earth and space sciences, who will discuss her research at the international Lunar and Planetary Science Conference in Houston on Wednesday, March 19. "If you stretch the imagination, you may be able to argue that one of the three scenarios may be consistent with life, but even under the most charitable scenario, you have to stretch the imagination pretty far."
UCLA's ion microprobe enables scientists to learn the exact composition of samples. The microprobe shoots a beam of ions -- charged atoms -- at a sample, releasing from the sample its own ions that are analyzed in a mass spectrometer. Scientists can aim the beam of ions at specific microscopic areas of a sample and analyze them. The microprobe was used in recent months to determine that life on Earth began at least 3.85 billion years ago and that Mount Everest and the Himalayas evolved as the highest mountain peaks in the world some 15 million years later than scientists had believed.
Supporters of ancient life on Mars argue that evidence of primitive life is associated with crystallized carbonate globules in the meteorite.
Studying bulk samples of the meteorite, advocates concluded the carbonates could have formed at temperatures cool enough to sustain life. Other scientists have argued, based on the mineral chemistry of the carbonates, that a higher temperature could not support life.
"We carefully correlated the chemical composition of the carbonates with their isotope composition, which cannot be done in bulk samples where they are mixed together," said Leshin, a Rubey faculty fellow at UCLA.
Leshin and her colleagues -- Kevin McKeegan, a UCLA research geochemist; and Ralph Harvey, a research scientist at Case Western Reserve University -- are the first scientists to individually pinpoint a wide range of carbonate compositions from the meteorite and analyze their oxygen isotopes.
"What we found," Leshin said, "is that these two seemingly unrelated data sets -- the chemistry of the carbonates and their isotope composition -- are in fact related. Any theory that explains the carbonate formations must also explain the variation in isotopes -- oxygen-18 to oxygen-16, and the calcium content.
"When we placed the samples in the ion microprobe, we found strong evidence that the first formed calcium-rich carbonates contain the lowest ratios of oxygen-18 to oxygen-16," she added.
Leshin said the findings provided hints to the conditions that prevailed when the carbonates formed billions of years ago. The scientists have produced three theories, which will be tested over the next several months, to explain the findings.
The first theory -- which would explain the isotope variations detected by the ion microprobe -- shows that the environment where the rock was located on Mars when the carbonates formed contained a very limited amount of fluid, which consisted largely of carbon monoxide rather than water. If this theory proves to be correct, it virtually rules out the possibility that the meteorite contains any signs of ancient life because water is necessary to support life, Leshin said.
Under a second theory, which also seems to be plausible, the environment from which the carbonates were formed on Mars contained a substantial amount of fluid that interacted with the meteorite. If that is correct, then the variation that the ion microprobe detected in the isotope ratio of oxygen-16 and oxygen-18 would most likely be explained by temperatures that were variable, rising above 200 degrees Celsius -- far higher than could support life, Leshin said.
"Under this scenario, even if the carbonates were at 0 degrees when they neared final crystallization, the temperature was boiling when they started forming," she said.
Under the third theory, the fluid on Mars that interacted with the rock was largely carbon dioxide when the carbonates started forming, and largely water when they were fully crystallized. This theory does not seem conducive to life either, but makes it more difficult to exclude the possibility entirely, said Leshin, adding that under this theory, the argument for ancient life is "conceivable, but not persuasive."
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The findings, announced on March 13, discount the claims of skeptics who believed that the carbonate globules found inside the ancient meteorite had to be formed at extremely high temperatures, far too hot to support life. The findings were published in the current issue of the journal Science.
A group led by University of Wisconsin - Madison geology professor John Valley used an isotopic analysis to find that the carbonate globules must have formed at temperatures no higher than about 100 C (212 F). The study also confirmed that the globules were of Martian, and not terrestrial, origin.
"We have not proven that this represents life on Mars, but we have disproven the high-temperature hypothesis," Valley said.
A separate group, led by Caltech geobiology professor Joseph Kirschvink, examined the magnetic field of the meteorite. Kirschvink and his colleagues concluded the globules must have formed at low temperatures.
"It's very pleasing to know there's other data to support our own," said Everett Gibson, one of the members of the NASA/Stanford team which made the initial announcement of past life on Mars last August. "This is really strong data."
Other scientists had previously claimed the carbonate globules in the meteorite could only have formed at temperatures as high as 650 C (1200 F), far too high for life. The lower temperatures keep a biological origin a possibility, but other, inorganic explanations for the globules are still possible.
The recent results and work by other researchers will be a hot topic at next week's Lunar and Planetary Science Conference in Houston. NASA has scheduled a briefing at the conference on March 19 where several scientists will announce their latest work on the Mars life question.
Office of News and Public Affairs
University of Wisconsin-Madison
March 13, 1997
The study, published March 14 in the journal Science by a team led by University of Wisconsin-Madison geochemist John W. Valley, lends powerful new support to the notion that the carbonate globules found within the meteorite, dubbed ALH84001, were formed on the Red Planet under conditions consistent with life.
The isotopic procedures employed by Valley and his colleagues were developed specifically for the Mars rock. Results contradict claims that the carbonate globules found in the rock were formed at blistering temperatures too hot to support life, or were formed on Earth, two primary arguments advanced against the meteorite as evidence of past life on Mars. "Everything we see is consistent with biological activity, but I still wouldn't rule out low-temperature inorganic processes as an alternative explanation" said Valley. "We have not proven that this represents life on Mars, but we have disproven the high-temperature hypothesis."
Valley said the high-temperature origin hypothesis relies on a set of thermodynamic assumptions that don't measure up on Earth, and therefore don't apply to an ancient Mars that may have had conditions more conducive to life.
"If the same assumptions are applied to the carbonates found in the Earth's oceans, one would erroneously conclude that the water temperatures are over 1,000 degrees Fahrenheit and the surface pressures are several thousand atmospheres," Valley said.
"These carbonates in the meteorite are easily explained by low-temperature processes similar to those commonly found on Earth," he said.
The meteorite at the center of the scientific controversy was blasted off the surface of Mars about 15 million years ago and fell to Earth about 13,000 years ago.
There is also widespread agreement that the rock is very old, probably 4.5 billion years, and that it formed in the Martian crust. The age of the rock sparked interest, because it formed at a time when the Red Planet was warmer, wetter and potentially more hospitable to life.
The new study was conducted by a team that includes Valley, John M. Eiler and Edward M. Stolper of the California Institute of Technology, Colin M. Graham of the University of Edinburgh, Everett K. Gibson of NASA's Johnson Space Center, and Christopher S. Romanek of the University of Georgia.
The analysis was made with a device designed to analyze minute samples of material gleaned from spots less than one-quarter of the diameter of a human hair. Known as an ion microprobe, it uses a beam of high-energy plasma to burn tiny craters on the surface of a sample, in this case a polished sample no bigger than a grain of rice. The vaporized material is held in a vacuum and drawn into a mass spectrometer for isotopic analysis.
The advantage of the ion microprobe, said Valley, is that it allows for minuscule amounts of material to be sampled, one million times less than would typically be necessary. Employing the microprobe, Valley and his colleagues were able to look deep within the carbonates themselves and make the first in situ measurements of the controversial globules.
"Making these analyses in situ has never been done before," he said. "For the first time, we can actually see what we analyze."
He described the carbonates as "pancakes within pancakes" having a distinct chemistry in each. "We can go in and look for differences or similarities within the carbonates themselves."
"Without the ion microprobe, one doesn't really know what's being analyzed. We found that the globules are different. There is a very intricate concentric mineral, chemical and isotopic zonation (within the globules)."
Valley's team measured the ratios of two different isotopic species of oxygen and two of carbon. They found that the carbon ratios in the meteorite are high, higher than in Earthbound rocks.
"This rules out the idea that these features formed while the meteorite was lodged in the Antarctic ice," said Valley. "Such ratios have never been measured in a terrestrial sample."
Oxygen isotope ratios are also high, Valley said, but he noted that the significant discovery is that the oxygen isotopes are not evenly distributed within the sample. "The ion microprobe allows us to determine which parts of the meteorite have more of a particular oxygen isotope."
The life on Mars hypothesis has been challenged on the grounds that the carbonates formed in chemical equilibrium above 1200 degrees Fahrenheit. The new data prove that the meteorite is not in isotopic or chemical equilibrium.
"There is no self-consistent evidence to suggest such a high-temperature genesis," said Valley. "All of the chemical, mineralogical and isotopic evidence that we present is consistent with a low-temperature origin."
The upshot of the analysis is that the carbonates most likely precipitated at temperatures below 200 degrees Fahrenheit, under conditions hospitable to some forms of microscopic life.
California Institute of Technology
March 13, 1997
Moreover, the conclusions of California Institute of Technology researchers Joseph L. Kirschvink and Altair T. Maine, and McGill University's Hojatollah Vali, also suggest that Mars had a substantial magnetic field early in its history.
Finally, the new results suggest that any life on the rock existing when it was ejected from Mars could have survived the trip to Earth.
In an article appearing in the March 13 issue of the journal Science, the researchers report that their findings have effectively resolved a controversy about the meteorite that has raged since evidence for Martian life was first presented in 1996. Even before this report, other scientists suggested that the carbonate globules containing the possible Martian fossils had formed at temperatures far too hot for life to survive. All objects found on the meteorite, then, would have to be inorganic.
However, based on magnetic evidence, Kirschvink and his colleagues say that the rock has certainly not been hotter than 350 degrees Celsius in the past four billion years -- and probably has not been above the boiling point of water. At these low temperatures, bacterial organisms could conceivably survive.
"Our research doesn't directly address the presence of life," says Kirschvink. "But if our results had gone the other way, the high-temperature scenario would have been supported."
Kirschvink's team began their research on the meteorite by sawing a tiny sample in two and then determining the direction of the magnetic field held by each. This work required the use of an ultrasensitive superconducting magnetometer system, housed in a unique, nonmagnetic clean lab facility. The team's results showed that the sample in which the carbonate material was found had two magnetic directions -- one on each side of the fractures.
The distinct magnetic directions are critical to the findings, because any weakly magnetized rock will reorient its magnetism to be aligned with the local field direction after it has been heated to high temperatures and cooled. If two such rock fragments are attached so that their magnetic directions are separate, but are then heated to a certain critical temperature, they will have a uniform direction.
The igneous rock (called pyroxenite) that makes up the bulk of the meteorite contains small inclusions of magnetic iron sulfide minerals that will entirely realign their field directions at about 350 degrees C, and will partially align the field directions at much lower temperatures. Thus, the researchers have concluded that the rock has never been heated substantially since it last cooled some four billion years ago.
"We should have been able to detect even a brief heating event over 100 degrees Celsius," Kirschvink says. "And we didn't."
These results also imply that Mars must have had a magnetic field similar in strength to that of the present Earth when the rock last cooled. This is very important for the evolution of life, as the magnetic field will protect the early atmosphere of a planet from being sputtered away into space by the solar wind. Mars has since lost its strong magnetic field, and its atmosphere is nearly gone.
The fracture surfaces on the meteorite formed after it cooled, during an impact event on Mars that crushed the interior portion. The carbonate globules that contain putative evidence for life formed later on these fracture surfaces, and thus were never exposed to high temperatures, even during their ejection from the Martian surface nearly 15 million years ago, presumably from another large asteroid or comet impact.
A further conclusion one can reach from Kirschvink's work is that the inside of the meteorite never reached high temperatures when it entered Earth's atmosphere. This means, in effect, that any remaining life on the Martian meteorite could have survived the trip from Mars to Earth (which can take as little as a year, according to some dynamic studies), and could have ridden the meteorite down through the atmosphere by residing in the interior cracks of the rock and been deposited safely on Earth.
"An implication of our study is that you could get life from Mars to Earth periodically," Kirschvink says. "In fact, every major impact could do it."
Kirschvink's suggested history of the rock is as follows:
The rock crystallized from an igneous melt some 4.5 billion years ago and spent about half a billion years on the primordial planet, being subjected to a series of impact-related metamorphic events, which included formation of the iron sulfide minerals.
After final cooling in the ancient Martian magnetic field about four billion years ago, the rock would have had a single magnetic field direction. Following this, another impact crushed parts of the meteorite without heating it, and caused some of the grains in the interior to rotate relative to each other. This led to a separation of their magnetic directions and produced a set of fracture cracks. Aqueous fluids later percolated through these cracks, perhaps providing a substrate for the growth of Martian bacteria.
The rock then sat more or less undisturbed until a huge asteroid or comet smacked into Mars 15 million years ago. The rock wandered in space until about 13,000 years ago, when it fell on the Antarctic ice sheet.
NASA Headquarters, Washington, DC
Johnson Space Center, Houston TX
March 14, 1997
Douglas P. Blanchard, Ph.D., Chief of JSC's Earth Science and Solar System Exploration Division, will moderate a six-person panel of co-authors of each of the six abstracts on the Mars issue that are scheduled for presentation later that day during a special plenary session of the Lunar and Planetary Science Conference in the adjacent Teague auditorium at JSC.
The briefing will be carried live on NASA Television with two-way question and answer capability from participating NASA centers. As of March 15, NASA Television will be broadcast at a new satellite location. The new satellite is GE-2, Transponder 9C at 85 degrees West longitude, frequency 3880.0 MHz, audio 6.8 MHz.
The text of complete abstracts to be presented may be obtained in advance of the event by contacting the web site for the 28th Lunar and Planetary Science Conference.
The conference will be held March 17-21, 1997 in Houston. Sessions will be held at JSC and the Lunar and Planetary Institute (LPI), 3600 Bay Area Boulevard, Houston, TX.
Already featured on ASTRONET:
Michael Carr and Thomas Gold: mounting evidence for past life on Mars
More evidence for Martians?
Possible source craters for Martian meteorite found
Discovery of possible early Martian life
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