w002 physics astronomy 044 049

8/19/2019 W002 Physics Astronomy 044 049

1/6

A rock that fell from the sky: In April 2002, this meteorite fell to Earth close to the famous castle built by the Bavarian

King Ludwig II – hence its name, “Neuschwanstein”. The photograph shows a 360° view composed of 20 images.


2/6

The Secrets of

Cosmic GrainsThe end of a meteorite’s journey is abrupt by definition,

as the chunk of cosmic material slams into Earth.

A celestial rock like this conceals a great many secrets.

Ulrich Ott from the Max Planck Institute for Chemistry

in Mainz is a scientific detective. He deciphers, for instance,

how long the meteorite traveled through space.

TEXT THORSTEN DAMBECK

The spectacular find was the result of asystematic search undertaken by themeteorite hunter Thomas Grau fromBrandenburg, who was also the first tofind a piece of the “Neuschwanstein”

meteorite in the Alps back in 2002(MAXPLANCKRESEARCH 1/2003, page 16ff.). This time, Grau found his treasure

on the Danish island of Lolland, wherehe netted a small piece of rock aboutthe size of a ping-pong ball; it had bro-ken up and was wedged a few centime-ters deep in a hole in the ground closeto the town of Maribo.

Scientists such as Ulrich Ott fromthe Max Planck Institute for Chemistrybenefit from the instinct of the celestialrock hunter. “This is not the first timewe have obtained our samples in this

way,” he says. The grain of material thepostman delivered this time came fromthe Museum of Geology in Copenha-

On the evening of January17, 2009, a brilliant lightshow caused a great deal ofexcitement. At the sametime as the main evening

news was being broadcast, a meteor litup the sky over northern Germany fora few seconds. Coming from the direc-

tion of Poland, the ball of fire movedacross the Baltic Sea toward Denmarkon a westerly trajectory. Close to 600eye witnesses reported the sight, someeven reporting noises like gun shotsand rolling thunder. In Sweden, a sur-veillance camera documented the spec-tacle, and people in the Netherlandsalso photographed the fireball. Al-though the images turned out to be un-suitable for pinpointing a potential im-pact site with any degree of precision,fragments of the meteorite were alreadybeing discovered in early March.

PHYSICS & ASTRONOMY_Meteorites

P h o t o s : T o s t / D L R

2 | 10 MaxPlanckResearch 45


3/6

It is difficult to investigate the noble

gases in meteorites because the con-

centrations involved are often very

low. The noble gas mass spectrometer

in Mainz operates essentially like this:

The gaseous components of the sam-

ple are first heated gradually to vapor-ize them. Chemical methods remove

the gases that are not noble gases from

the gas mixture, as they are not impor-

tant in the extended analysis. Helium

and neon enter the measuring appara-

tus first, while the other noble gases

are trapped on the surface of liquid-ni-

trogen-cooled (minus 196 degrees Cel-

sius) activated carbon. They are re-

leased from this “interim storage” by

heating, each atomic species at a spe-

cific temperature. Argon is freed first,

at minus 123 degrees Celsius, followed

by krypton. An even higher tempera-

ture (about plus 150 degrees Celsius) is

applied to release xenon.

The analysis operates on the fol-

lowing principle: A heating filamentemits electrons, which collide with the

noble gas atoms, causing them to ion-

ize. The high voltage that is then ap-

plied ensures that the ions of the indi-

vidual noble gas isotopes are def lected

according to their mass. In contrast to

conventional mass spectroscopy, the

pumps are closed off during the meas-

urement. These sensitive measure-

ments depend on not a single gas atom

being lost.

A GOOD NOSE FOR NOBLE GASES

And this is how it is done: After therock was knocked out of its parentbody, it became exposed to the omni-present cosmic radiation. Nuclear reac-tions began to take place in its interior

and the isotope ratios started tochange. As Ott explains, “The interest-ing effects can best be seen for the rareisotopes, such as neon-21.” The nucleiof this special form of the noble gascontain a total of 21 protons and neu-trons. Neon normally contains only0.3 percent of this isotope. “We can usethe Ne-21 content of a meteorite to ac-curately determine its irradiation age,”says Ott. Cosmic ray exposure age isthe technical term for the duration ofthe cosmic journey.

The laboratory is filled with the

whirring of pumps – the measurementsrequire an ultrahigh vacuum. Atop the

gen, where Grau had handed in hisfind. The laboratory scales recorded amere 115 milligrams – but still morethan enough for the analysis to be per-formed. Prior to this, colleagues in Den-

mark and Münster had already carriedout a classification: Maribo – the offi-cial name of the Baltic meteorite – is acarbonaceous chondrite, a specimenrich in carbon. Less than five percent ofmeteorite falls belong to this rare class.

NOBLE GASES HOLD THE KEY

TO THE JOURNEY TIME

The Max Planck scientists in Mainzhave been researching meteorites fordecades. Since 1969, they have alsobeen analyzing samples brought back

by lunar travelers – no scientific insti-tution outside the USA received asmany moon rocks back then as theCosmochemistry Department in Mainz.Ott’s cosmic research objects havealways been meteorites, however: “Ourwork has an interdisciplinary ap-proach,” explains the researcher, whosescientific home is at the interfacebetween astrophysics, chemistry andthe geosciences.

“Here we are running a temperatureseries,” explains Ulrich Ott on the way

to the laboratory where the Maribosample is already being strongly heat-ed. Temperatures of between 400 and1,800 degrees Celsius are applied overa period of several days to graduallydrive the gaseous components out ofthe celestial rock. Ott and his fellow sci-entists are particularly interested in thenoble gases, or to be more precise, inthe ratios of the noble gas isotopes, be-cause they can be used to calculate thetime taken to travel from the realm ofthe planetoids to the point of impact inthe Danish countryside. P h

o t o s : M P I f o r C h e m i s t r y ( b o t t o m

2 ) A x e l G r i e s c h ( t o p )


pipes of the apparatus, which are heat-insulated with aluminum foil, sits aglass vessel from which a dozen finger-like protuberances point downward.These contain the samples wrapped up

in silvery nickel foil. The samples comefrom a wide variety of different mete-orites. In addition to the sample fromMaribo and a number of others, thereis also a very special sample: “A col-league from Vienna sent us this one,”says Ott. “The mineralogists were not

sure whether it was a meteorite at all.They suspected that if it really was fromspace, it was possibly from Mars.”

The noble gas analysis of the mys-terious object is already complete, andOtt is certain that it is not from Earth.On the contrary, it has completed a

long cosmic odyssey. “The high con-centrations of Ne-21 are evidence of a

» The Max Planck scientists in Mainz have been working with meteorites for decades.The work at the institute is interdisciplinary. The scientific home of the experts is at

the interface between astrophysics, chemistry and the geosciences.

46 MaxPlanckResearch 2 | 10


4/6

1 Ulrich Ott examines the printout of a mass spectrum. This is how the spectra were recorded 20 years ago; now it is done electronically.

But the conventional technology is always used in parallel just to be on the safe side.2 Weighed – and found to be a lightweight. If you expect to find a large piece of rock in the laboratory, you will be disappointed.

A tiny amount of material is sufficient for the analysis. The tiny grain of material from the Maribo meteorite weighed just 115 milligrams.

3 The samples from the meteorites are stored in Plexiglas bottles like these until they are analyzed in Ott’s laboratory.

1

2 3


5/6

1 The meteorites carefully wrapped in nickel foil in the glass sample vessel, where they are

freed from adsorbed gases by moderate heating.

2 It looks a little like modern art but is really a cryogenic separation trap. This is a kind of

interim storage for the noble gases driven out of the meteorite samples by heating.

1

2


6/6

()

high irradiation age, around 20 millionyears.” But that would indicate that itis not from Mars, because meteoritesfrom Mars manage the space transfermore rapidly, typically within a fewmillion years.

NANODIAMONDS MADE OFA MERE 1,000 CARBON ATOMS

The gas mass spectrometer that helpsUlrich Ott and his fellow scientists de-cipher the celestial rock is a very sensi-tive piece of equipment. It has to be,

because noble gases are often presentin meteorites in low concentrations(see box on page 46). These measure-ments have nevertheless been routineat the Max Planck Institute for Chem-istry for some time, sort of a servicethat forms part of a standard series of

analyses in meteorite research. As thenumbers for the noble gases from heli-um to xenon appear on the screen, Otttalks about his real research field:presolar grains. These phases are em-bedded in some meteorites and can beinvestigated separately.

“We are mainly concerned with tinydiamonds that contain a mere 1,000carbon atoms,” explains the Max Planckscientist. These nanodiamonds are old-er than the solar system. It was noblegas analyses that identified them as rel-

ics from interstellar space, and that firstshowed that meteorites contain suchancient components.

In addition to the miniature dia-monds, the scientists also analyze larg-er grains, although even these are stillon the micrometer scale. They use com-plex methods to extract the grains fromthe matrix of the meteoritic rock. Someof the isotopes in these grains exhibitdramatic, distinctive features. Investi-gations by Peter Hoppe, also from theMax Planck Institute for Chemistry inMainz, show that, while the isotope ra-

tio of carbon with atomic weights 12and 13 is around 90 almost everywherein the solar system, these grains arecompletely out of the ordinary: somehave values two orders of magnitudelower, while others exhibit an isotoperatio almost 100 times higher.

Why is this? It is assumed that starseject dust at the end of their life. An-cient stellar matter from red giants orexploding supernovae thus reachedthe primeval solar cloud from whichthe Sun, the meteorites and the plan-ets formed. The isotope anomaliesthus provide insight into the interiorof these stars and the synthesis of thechemical elements that occur there – asort of genetic fingerprint of our stel-lar ancestors.

NEW MEASURING EQUIPMENT

FOR INTERSTELLAR DUST

The Maribo meteorite may also containthe primeval grains; Ott’s measure-ments indicate a nanodiamond contentin the parts per thousand range. An im-portant condition for this is that Mari-bo was subjected to only relatively mildheating, no more than 200 degrees Cel-sius, in the four and a half billion yearsof its existence. There will probably beno detailed search for presolar materi-al, however, because only a small

amount – just 30 grams – of Maribo ma-terial has been found in Denmark so far.The institute in Mainz will see a leap

in the sensitivity of its noble gas analy-ses this year, as a new piece of measur-ing equipment for the analysis of mi-crometeorites, interstellar dust andindividual presolar grains goes into op-eration. In the meantime, all noble gas-es from the Maribo sample have beendetermined in the laboratory and themeasurement data evaluated. Ott is notsurprised by the result: “Maribo’s radia-tion age is around one million years,” he

explains. This is a very short journeytime compared to the ordinary stonymeteorites, but not unusual for this classof meteorite. The crumbly materialcould probably not resist the harsh con-ditions in space for much longer. And inthe end, the heating process in Mainz

also proved too much for the grain ofcosmic dust: “It vaporized,” says Ott un-moved. “After all, our measurementsaren’t a non-destructive technique.”

GLOSSARY

Isotope

Term for various types of atoms of a given

chemical element whose nuclei have the

same atomic number (number of pro-

tons) but different numbers of neutrons

and thus also different mass numbers.

Cosmic radiation

High-energy particle radiation fromspace that consists mainly of protons,

electrons and completely ionized atoms.

The radiation originates from the Sun,

but also from supernovae and neutron

stars, as well as extragalactic sources

such as active galaxies and quasars.

Meteoroid, meteor, meteorite

These terms are often confused. A mete-

oroid is a small or large chunk of material

that orbits in space. If such a meteoroid

enters Earth’s atmosphere, it produces a

bright phenomenon called a mete or.

A particularly bright meteor is also called

a fireball or bolide. Most of the grains

are only the size of a speck of dust and

burn up, but the larger chunks fall toEarth as meteorites.

Supernova

The sudden bright flare-up of a star,

increasing its brightness several million

to several billion times. There are

several types of supernovae, and two

basic mechanisms: the explosion of an

individual massive star that has used

up all of its nuclear fuel at the end of its

life; and the detonation of one of the

stars of a binary star system, caused by

the transfer of material between two

stars (white dwarves).

2 | 10 MaxPlanckResearch 49


w002 physics astronomy 044 049

Documents