Looking for Carbonates in Dry Places
Liquid water often is considered a requirement for life, on Earth or beyond. And until recently, the presence of extraterrestrial carbonate chemicals – believed to form only in water – was thought to be a reliable indicator of the past or current presence of water.
That belief took a big hit earlier this year when Ciska Kemper, an astronomy doctoral student at the University of Amsterdam, published a study in the journal Nature identifying fine grains of carbonates in the dry dust surrounding two dying stars. Kemper and colleagues say water could not exist there. Not enough time has passed for new water-rich planets to form since the expanding stars vaporized any previously existing planets. And they say the volume of carbonates around the stars is far more than could have been produced by any vaporized planets.
If Kemper’s findings are confirmed, scientists may have to re-examine their assumptions about how soon water was present after our solar system formed. But not everyone agrees that Kemper has, indeed, found carbonates.
Carbonates – A Foolproof Divining Rod?
Carbonates are minerals that form when negatively charged carbonate ions (a carbon atom and three oxygen atoms) combine with a positive ions such as calcium, magnesium or iron. The conventional reaction occurs in solution, and carbonate crystallizes out of the liquid – sometimes with help from marine organisms that incorporate the carbonates into their shells.
The most common carbonates on Earth are calcite – also known as limestone or calcium carbonate – and dolomite, which is made of carbonates of calcium and magnesium. These are the carbonates Kemper claims to have found around the distant stars.
|(Click for larger image.) Hubble Space Telescope image of NGC 6537, also known as the Red Spider Nebula, the other evolved star in which carbonates are found. Again, the carbonates are located in a dusty torus around the central star.|
Kemper based her findings on spectra, patterns of heat and light emissions, from dust around the stars. The spectra, which were collected by the European Space Agency‘s Infrared Space Observatory, an Earth-orbiting telescope that operated during 1995-1998, can be used to identify chemicals in the dust around the stars.
Each of the two dying stars where Kemper says she detected carbonates began like our own Sun, and then evolved into a red giant, which "starts to eject some its material, not like a supernova but a more gradual loss of mass."
The stars and their surrounding dust shells – known as the Butterfly Nebula or Bug Nebula or NGC 6302, and Red Spider Nebula or NGC 6537 – are falling apart, with gas moving away from each star, cooling down and condensing into dust.
The spectra of heat emissions from the dust look like squiggly horizontal lines. Vertical peaks on those lines represent different chemicals that emit heat or infrared light at different wavelengths. Kemper’s group identified one specific peak as indicating calcium carbonate was present.
Around the two nebulae, "there was a part of the spectrum that was still unexplained," says Kemper, who realized "those peaks could be explained as two carbonates. One was dolomite and the other calcite (limestone)."
The discovery was billed as the first detection of carbonates beyond our solar system and the first extrasolar detection of carbonates that could not have formed in the presence of water.
Measurements indicated the amount of carbonates around each star exceeded 30 times Earth’s mass. If water to make the carbonates had come from planets vaporized by the expanding dying stars, 3,000 or more Earth-sized rocky planets would have been required – an unlikely number of planets to orbit one star.
And new planets with water could not yet have formed around the dying stars because only 10,000 years have elapsed since the stars expelled their outer layers, and the dust is not dense enough to form planets, Kemper says.
A Matter of Interpretation
What Kemper’s team detected in the stars’ spectra "is calcium, but it’s not calcium carbonate," and instead probably is the mineral hibonite, or calcium aluminate, Hofmeister says.
The single spectral peak identified by Kemper, argues Hofmeister, is scanty evidence for calcium carbonate. The same peak also is seen in the spectrum of hibonite, and Kemper’s group did not detect other peaks characteristic of carbonates, she says. Hibonite and other minerals with the same calcium peak as calcium carbonate can condense from hot gas ejected by dying stars, Hofmeister says, so there is no need to invoke the idea of carbonates created without water.
Kemper replies that disagreement over whether the peak in the stardust’s chemical signature really reveals carbonate boils down to differing laboratory measurements of carbonate spectra. The German spectroscopists who co-authored Kemper’s study compared their laboratory carbonate spectra to the patterns from dust around the dying stars. Hofmeister is using different laboratory measurements of carbonate spectra, which look different than those used by Kemper’s colleagues. Hofmeister says those differences are minor, and the main problem is that the spectra from the dying stars do not display the other peaks typical of carbonate.
Kemper says calcium carbonate was detected based on the single peak because other spectral peaks characteristic of the mineral are not detectable due to the temperature of the dust around the two dying stars. For now, she says, she stands by her interpretation. "Until I’ve seen and used [Hofmeister’s spectra], I will stick with carbonates."
Carbonates Closer to Home
Why all the fuss about carbonates around a couple of dying stars light-years away? Because if Kemper truly did detect carbonates around these stars – if carbonates can form in the absence of water – astronomers may have to rethink their assumptions about the presence of water during the formative stage of our own solar system.
Extremely old carbonates have been found in meteorites known as chondrites, which are believed to be primitive leftovers from the solar system’s early stages, when huge amounts of swirling gas and dust gradually clumped into progressively larger objects and eventually formed asteroids and planets.
Because carbonates were thought to form only in the presence of water, their occurrence in chondrites – which were dated by the decay of radioactive components – was interpreted to suggest that planet- or asteroid-sized bodies had formed by the time the solar system was a mere 20 million years old, some 4.54 billion years ago.
"That scenario may now be due for a revision," the European Space Agency said after publication of Kemper’s study.
The discovery of carbonates not formed by water, Kemper and her colleagues wrote in their paper in Nature, "suggests that some of the carbonates found in solar system bodies no longer provide direct evidence that liquid water was present on large bodies early in the history of the solar system."
But cosmochemist Laurie Leshin isn’t so sure. "I have not seen this particular work causing a massive questioning of the [water-based] origin of carbonates in meteorites," she says.
Other minerals in the chondrites indicate that water was present in the young solar system, and those minerals formed in association with carbonates, says Leshin, an associate professor of geological sciences and associate director of the Center for Meteorite Studies at Arizona State University. She notes that there also is chemical evidence the carbonates formed from a fluid.
The Martian Connection
When Nature published Kemper’s discovery of carbonates purportedly produced without water, some news accounts incorrectly said the findings raised doubts that water was involved in the formation of carbonates in Martian meteorite ALH84001 – a rock that some NASA scientists argue shows evidence of past life on Mars.
But Kemper’s study said no such thing. She and other experts believe the Martian meteorite’s carbonates did form in the presence of water.
"It was silly [for some news media] to bring the meteorite into it," says geologist David McKay, chief scientist for astrobiology at NASA’s Johnson Space Center and leader of the team that advocates the hotly disputed notion that ALH84001 harbors evidence of past life on Mars. "There’s no connection at all with the carbonates in the meteorite, which are clearly precipitates from water."
Kemper suggests three ways carbonates may form in space without liquid water. Thin ice layers on dust grains in space may move just enough to react with silicates and form carbonates. Or carbonates may condense directly from carbon dioxide and gaseous calcium oxides. Or silicate dust grains react with water vapor to form hydrated silicates, which then react with carbon dioxide to form carbonates.
Leshin says conditions in space would not be conducive to those mechanisms, yet "if I was going to make up a list, those are the three I would come up with."
Kemper says these three possibilities "should all be investigated in the lab."
So far, she is aware of no such experiments, and "it’s disappointing. It’s clear much more work is needed on this subject, not only in the laboratory. We have to look for astronomical evidence for the presence of carbonates."
Leshin agrees: "Understanding the cycle of biogenic elements is a fundamental thing, and carbon is the most biogenic element. It’s really important to follow the carbon in all its forms and all its places. So in that sense, it’s really important to understand what happens to carbon around old stars, what happens to carbon around young stars. We need to understand how carbon gets incorporated into planets and pre-planets. This is at the forefront of astrobiology."