Meteorites Could Have Supplied the Earth with Phosphorus

Image credit: University of Arizona
University of Arizona scientists have discovered that meteorites, particularly iron meteorites, may have been critical to the evolution of life on Earth.

Their research shows that meteorites easily could have provided more phosphorus than naturally occurs on Earth — enough phosphorus to give rise to biomolecules which eventually assembled into living, replicating organisms.

Phosphorus is central to life. It forms the backbone of DNA and RNA because it connects these molecules’ genetic bases into long chains. It is vital to metabolism because it is linked with life’s fundamental fuel, adenosine triphosphate (ATP), the energy that powers growth and movement. And phosphorus is part of living architecture ? it is in the phospholipids that make up cell walls and in the bones of vertebrates.

“In terms of mass, phosphorus is the fifth most important biologic element, after carbon, hydrogen, oxygen, and nitrogen,” said Matthew A. Pasek, a doctoral candidate in UA’s planetary sciences department and Lunar and Planetary Laboratory.

But where terrestrial life got its phosphorus has been a mystery, he added.

Phosphorus is much rarer in nature than are hydrogen, oxygen, carbon, and nitrogen.

Pasek cites recent studies that show there’s approximately one phosphorus atom for every 2.8 million hydrogen atoms in the cosmos, every 49 million hydrogen atoms in the oceans, and every 203 hydrogen atoms in bacteria. Similarly, there’s a single phosphorus atom for every 1,400 oxygen atoms in the cosmos, every 25 million oxygen atoms in the oceans, and 72 oxygen atoms in bacteria. The numbers for carbon atoms and nitrogen atoms, respectively, per single phosphorus atom are 680 and 230 in the cosmos, 974 and 633 in the oceans, and 116 and 15 in bacteria.

“Because phosphorus is much rarer in the environment than in life, understanding the behavior of phosphorus on the early Earth gives clues to life’s orgin,” Pasek said.

The most common terrestrial form of the element is a mineral called apatite. When mixed with water, apatite releases only very small amounts of phosphate. Scientists have tried heating apatite to high temperatures, combining it with various strange, super-energetic compounds, even experimenting with phosphorous compounds unknown on Earth. This research hasn’t explained where life’s phosphorus comes from, Pasek noted.

Pasek began working with Dante Lauretta, UA assistant professor of planetary sciences, on the idea that meteorites are the source of living Earth’s phosphorus. The work was inspired by Lauretta’s earlier experiments that showed that phosphorus became concentrated at metal surfaces that corroded in the early solar system.

“This natural mechanism of phosphorus concentration in the presence of a known organic catalyst (such as iron-based metal) made me think that aqueous corrosion of meteoritic minerals could lead to the formation of important phosphorus-bearing biomolecules,” Lauretta said.

“Meteorites have several different minerals that contain phosphorus,” Pasek said. “The most important one, which we’ve worked with most recently, is iron-nickel phosphide, known as schreibersite.”

Schreibersite is a metallic compound that is extremely rare on Earth. But it is ubiquitous in meteorites, especially iron meteorites, which are peppered with schreibersite grains or slivered with pinkish-colored schreibersite veins.

Last April, Pasek, UA undergraduate Virginia Smith, and Lauretta mixed schriebersite with room-temperature, fresh, de-ionized water. They then analyzed the liquid mixture using NMR, nuclear magnetic resonance.

“We saw a whole slew of different phosphorus compounds being formed,” Pasek said. “One of the most interesting ones we found was P2-O7 (two phorphorus atoms with seven oxygen atoms), one of the more biochemically useful forms of phosphate, similar to what’s found in ATP.”

Previous experiments have formed P2-07, but at high temperature or under other extreme conditions, not by simply dissolving a mineral in room-temperature water, Pasek said.

“This allows us to somewhat constrain where the origins of life may have occurred,” he said. “If you are going to have phosphate-based life, it likely would have had to occur near a freshwater region where a meteorite had recently fallen. We can go so far, maybe, as to say it was an iron meteorite. Iron meteorites have from about 10 to 100 times as much schreibersite as do other meteorites.

“I think meteorites were critical for the evolution of life because of some of the minerals, especially the P2-07 compound, which is used in ATP, in photosynthesis, in forming new phosphate bonds with organics (carbon-containing compounds), and in a variety of other biochemical processes,” Pasek said.

“I think one of the most exciting aspects of this discovery is the fact that iron meteorites form by the process of planetesimal differentiation,” Lauretta said. That is, the building-blocks of planets, called planestesmals, form both a metallic core and a silicate mantle. Iron meteorites represent the metallic core, and other types of meteorites, called achondrites, represent the mantle.

“No one ever realized that such a critical stage in planetary evolution could be coupled to the origin of life,” he added. “This result constrains where, in our solar system and others, life could originate. It requires an asteroid belt where planetesimals can grow to a critical size ? around 500 kilometers in diameter ? and a mechanism to disrupt these bodies and deliver them to the inner solar system.”

Jupiter drives the delivery of planetesimals to our inner solar system, Lauretta said, thereby limiting the chances that outer solar system planets and moons will be supplied with the reactive forms of phosphorus used by biomolecules essential to terrestrial life.

Solar systems that lack a Jupiter-sized object that can perturb mineral-rich asteroids inward toward terrestrial planets also have dim prospects for developing life, Lauretta added.

Pasek is talking about the research today (Aug. 24) at the 228th American Chemical Society national meeting in Philadelphia. The work is funded by the NASA program, Astrobiology: Exobiology and Evolutionary Biology.

Original Source: UA News Release

New Research Doubts Life in Martian Meteorite

Image credit: NASA
The scientific debate over whether a meteorite contains evidence of past life on Mars continues to intensify, with colleagues of the team that announced the possibility in 1996 revealing new findings that may cast doubt on some of that earlier work.

?These new findings illustrate the excellent scientific process that was ignited by the announcement in 1996 of possible meteorite evidence of past life on Mars,? said Dr. Steven Hawley, Associate Director, Office of Astromaterials Research and Exploration Science at the Johnson Space Center. ?As work on this fundamental question continues, it is quite likely the final answer may not be known until Mars samples can be retrieved for study by scientists there or back on Earth.?

In the recent study, a team of scientists based largely at JSC found that a mineral in Mars meteorite ALH84001 that had been asserted to be most likely caused by an ancient microscopic organism may have been caused by a non-biological process. The team, led by D.C. Golden of Hernandez Engineering Inc. in Houston and including many NASA scientists from the Office of Astromaterials Research and Exploration Science, will have its work published in the May/June issue of American Mineralogist. The same office includes Dave McKay, Everett Gibson and several other scientists who contributed to the 1996 findings.

The new paper reports that magnetite, an iron-bearing mineral found in Martian meteorite ALH84001, was likely caused by inorganic processes, and that those same processes can be recreated in the laboratory, forming magnetite identical to that found in the Mars meteorite.

Magnetite crystals in ALH84001 have been a focus of debate about the possibility of life on Mars. The 1996 study led by McKay suggested that some magnetite crystals associated with carbonate globules in ALH84001 are biogenic because they share many characteristics with those found in bacteria on Earth. A study led by Kathie Thomas-Keprta in 2000 showed that some of the magnetite crystals in ALH84001 carbonate globules are characterized by elongation, a ?unique habit? identical to magnetite grains produced by bacteria on Earth.

Golden and his team first investigated whether an inorganic process can produce magnetite crystals identical to those in ALH84001 claimed by Thomas-Keprta?s team to be biogenic. Then, they sought to replicate the tenet of McKay?s 1996 hypothesis that the purported biogenic magnetite grains in ALH84001 are identical to those produced by a bacterium called MV-1.

Golden?s team concluded that the shapes of the MV-1 and ALH84001 elongated crystals differ. Their study concluded that inorganic processes can make the magnetite crystals in ALH84001, so any claim to a biological source is uncertain. Golden?s team found that decomposition of iron-bearing carbonate under high heat produced magnetite crystals identical to those found in ALH84001.

?The strength of the inorganic process provided here is that not only does it produce elongated magnetite crystals identical to those of the ALH84001 meteorite, but also it produces a whole range of features found in the meteorite,? said Golden, a mineralogist at JSC.

McKay, chief scientist for astrobiology at JSC, stands by his 1996 findings. ?We originally proposed a suite of four lines of evidence which, taken together, were consistent as a package with a possible biological origin,? McKay said. ?The Golden group has singled out one very specific feature, the shape of the magnetite crystals, to try to discredit the whole biogenic hypothesis. Their alternative inorganic hypothesis, thermal decomposition of carbonate, will not explain many of the features described by us in ALH84001. A plausible inorganic model must explain simultaneously all of the properties that we and others have suggested as possible biogenic properties of this meteorite.?

ALH84001 was discovered in 1984 in the Allan Hills region of Antarctica by an annual expedition of the National Science Foundation?s Antarctic Meteorite Program. Its Martian origin was not recognized until 1993. One of about 30 meteorites discovered on Earth thought to be from Mars, it is a softball-sized igneous rock weighing 1.9 kilograms (4.2 pounds). With the exception of ALH84001, all are less than 1.3 billion years old. ALH84001 is 4.5 billion years old.

To view the study on the Internet, visit:

http://www.minsocam.org/MSA/AmMin/AmMineral.html For information about space research on the Internet, visit: http://spaceresearch.nasa.gov/Video to accompany this release will air on the NASA Television Video File at 11 a.m. CDT May 5. NASA TV is on AMC-9, transponder 9C, C-Band, at 85 degrees west longitude, frequency 3880.0 MHz, polarization vertical, audio monaural at 6.80 MHz.

Original Source: NASA News Release

Silicate Found in a Meteorite

Image credit: WUSTL
Ann Nguyen chose a risky project for her graduate studies at Washington University in St. Louis. A university team had already sifted through 100,000 grains from a meteorite to look for a particular type of stardust ? without success.

In 2000, Nguyen decided to try again. About 59,000 grains later, her gutsy decision paid off. In the March 5 issue of Science, Nguyen and her advisor, Ernst K. Zinner, Ph.D., research professor of physics and of earth and planetary sciences, both in Arts & Sciences, describe nine specks of silicate stardust ? presolar silicate grains ? from one of the most primitive meteorites known.

“Finding presolar silicates in a meteorite tells us that the solar system formed from gas and dust, some of which never got very hot, rather than from a hot solar nebula,” Zinner says. “Analyzing such grains provides information about their stellar sources, nuclear processes in stars, and the physical and chemical compositions of stellar atmospheres.”

In 1987, Zinner and colleagues at Washington University and a group of scientists at the University of Chicago found the first stardust in a meteorite. Those presolar grains were specks of diamond and silicon carbide. Although other types have since been discovered in meteorites, none were made of silicate, a compound of silicon, oxygen and other elements such as magnesium and iron.

“This was quite a mystery because we know, from astronomical spectra, that silicate grains appear to be the most abundant type of oxygen-rich grain made in stars,” Nguyen says. “But until now, presolar silicate grains have been isolated only from samples of interplanetary dust particles from comets.”

Our solar system formed from a cloud of gas and dust that were spewed into space by exploding red giants and supernovae. Some of this dust formed asteroids, and meteorites are fragments knocked off asteroids. Most of the particles in meteorites resemble each other because dust from different stars became homogenized in the inferno that shaped the solar system. Pure samples of a few stars became trapped deep inside some meteorites, however. Those grains that are oxygen-rich can be recognized by their unusual ratios of oxygen isotopes.

Nguyen, a graduate student in earth and planetary sciences, analyzed about 59,000 grains from Acfer 094, a meteorite that was found in the Sahara in 1990. She separated the grains in water instead of with harsh chemicals, which can destroy silicates. She also used a new type of ion probe called the NanoSIMS (Secondary Ion Mass Spectrometer), which can resolve objects smaller than a micrometer (one millionth of a meter).

Zinner and Frank Stadermann, Ph.D., senior research scientist in the Laboratory for Space Sciences at the university, helped design and test the NanoSIMS, which is made by CAMECA in Paris. At a cost of $2 million, Washington University acquired the first instrument in the world in 2001.

Ion probes direct a beam of ions onto one spot on a sample. The beam dislodges some of the sample’s own atoms, some of which become ionized. This secondary beam of ions enters a mass spectrometer that is set to detect a particular isotope. Thus, ion probes can identify grains that have an unusually high or low proportion of that isotope.

Unlike other ion probes, however, the NanoSIMS can detect five different isotopes simultaneously. The beam can also travel automatically from spot to spot so that many hundreds or thousands of grains can be analyzed in one experimental setup. “The NanoSIMS was essential for this discovery,” Zinner says. “These presolar silicate grains are very small ? only a fraction of a micrometer. The instrument’s high spatial resolution and high sensitivity made these measurements possible.”

Using a primary beam of cesium ions, Nguyen painstakingly measured the amounts of three oxygen isotopes ? 16O, 17O and 18O ? in each of the many grains she studied. Nine grains, with diameters from 0.1 to 0.5 micrometers, had unusual oxygen isotope ratios and were highly enriched in silicon. These presolar silicate grains fell into four groups. Five grains were enriched in 17O and slightly depleted in 18O, suggesting that deep mixing in red giant or asymptotic giant branch stars was responsible for their oxygen isotopic compositions.

One grain was very depleted in 18O and therefore was likely produced in a low-mass star when surface material descended into areas hot enough to support nuclear reactions. Another was enriched in 16O, which is typical of grains from stars that contain fewer elements heavier than helium than does our sun. The final two grains were enriched in both 17O and 18O and so could have come from supernovae or stars that are more enriched in elements heavier than helium compared with our sun.

By obtaining energy dispersive x-ray spectra, Nguyen determined the likely chemical composition of six of the presolar grains. There appear to be two olivines and two pyroxenes, which contain mostly oxygen, magnesium, iron and silicon but in differing ratios. The fifth is an aluminum-rich silicate, and the sixth is enriched in oxygen and iron and could be glass with embedded metal and sulfides.

The preponderance of iron-rich grains is surprising, Nguyen says, because astronomical spectra have detected more magnesium-rich grains than iron-rich grains in the atmospheres around stars. “It could be that iron was incorporated into these grains when the solar system was being formed,” she explains.

This detailed information about stardust proves that space science can be done in the laboratory, Zinner says. “Analyzing these small specks can give us information, such as detailed isotopic ratios, that cannot be obtained by the traditional techniques of astronomy,” he adds.

Nguyen now plans to look at the ratios of silicon and magnesium isotopes in the nine grains. She also wants to analyze other types of meteorites. “Acfer 094 is one of the most primitive meteorites that has been found,” she says. “So we would expect it to have the greatest abundance of presolar grains. By looking at meteorites that have undergone more processing, we can learn more about the events that can destroy those grains.”

Original Source: WUSTL News Release

More Support for Life in Martian Meteorite

Image credit: NASA
University of Queensland researchers have confirmed the theory life once existed on Mars.

Dr John Barry, from UQ?s Centre for Microscopy and Microanalysis, together with former UQ researcher Dr Tony Taylor, found their proof in the water trap at the ninth hole of the Howestern golf course at Birkdale.

Mud samples from the golf course contained magnetic crystals which matched those found in a meteorite discovered in Antartica in 1984.

In 1996 NASA announced it had found primitive bacteria in that meteorite and since then debate has raged in the scientific community whether the organism were from Mars.

Dr Taylor, together with his PhD co-supervisor Dr Barry, examined the mud samples using a world-first breakthrough in electron microscopy and found the fossil bacteria and the new samples were identical.

?Tony developed a new technique to capture specimens for the electron microscope which allowed us to see through the bacteria and into the gel surrounding the magnetic crystals inside the bacterium,? Dr Barry said.

?This gave us a lot more information about the structure than what we would have seen before.?

Dr Taylor, who now works for the Australian Nuclear Science and Technology Organisation in Sydney, said this research seriously challenges doubts of sceptical scientists by discovering that many bacteria match the features found in the Martian meteorite.

?Our research shows that the structures found in the NASA meteorite were more than likely made by bacteria present on Mars four billion years ago, before life even started on Earth,? said Dr Taylor.

Dr Taylor said the discovery was the product of painstaking research conducted with other scientists in the 1990s that vastly improved imaging techniques to study bacterial structures. Ultraviolet light was the key and resulted in the detailed analysis of 82 different bacterial types – a major improvement on the 25 identified at that time.

?We became very excited when we discovered that many of the bacteria found had the same biosignature, which resembles a tiny backbone surrounded by cartilage, as that of the Martian fossils,? Dr Taylor said.

Emeritus Professor Imre Friedmann, one of the original NASA scientists to make the life on Mars claim said he was thrilled by the news.

?The Study of Taylor and Barry now presents evidence that the same features occur in a wide range of bacteria that live on Earth today. The tiny structures, chains of crystals of the mineral magnetite, are comparable to animal skeletons on a microscopic scale, ? Professor Friedmann said.

Dr Barry and Dr Taylor?s research was published recently in the Journal of Microscopy.

Original Source: University of Queensland News Release