How do we know where meteorites come from?
Article | Updated 11 months ago
Meteorites are solid pieces of natural space debris that do not completely disintegrate during their descent through the atmosphere. Available evidence and research suggest most meteorites appear to be fragments of asteroids in solar orbits between Mars and Jupiter, but some meteorites also originate from Mars and the Moon. Today, seventy meteorites are recognised to have come from the planet Mars. In its collections, the Western Australian Museum contains samples of three Martian meteorites and two samples of lunar meteorites.
But, how do scientists know where a meteorite comes from?
Here is Nakhla, or rather a fragment of Nakhla, a Martian meteorite that fell in Egypt in 1911. This is the story of its study, and that of all rocks suspected to be from Mars.
To determine a meteorite’s origin, researchers begin their study by identifying the rock type and then dating the meteorite. Nakhla revealed it was formed 1.3 billion years ago, much later than most other meteorites that date from the birth of the Solar system, 4.56 billion years ago. Then scientists go through a process of elimination as to where the meteorite could not have come from.
Is this rock from Earth?
During the meteorite’s descent through the Earth’s atmosphere, the molten surface layer solidifies into a thin black crust, smooth and sometimes brilliant, termed “fusion crust”. This crust is black if the meteorite is collected shortly after its fall, but may turn brown or even disappear due to weathering with time. Some depressions on the meteorite’s surface, named regmaglypts, can also be noted due to its passage through our atmosphere. 85% of meteorites falling on Earth are chondrites, containing small, round particles named chondrules, which are composed mostly of silicate minerals. Finding these small silicate balls in a rock suggests it is a meteorite. Moreover, most meteorites contain a substantial quantity of iron-nickel metal and have a density above the average density of terrestrial rocks. Although some terrestrial rocks might look like meteorites, careful research can establish an extra-terrestrial origin and confirm that a rock is a meteorite.
Is this meteorite from the Moon?
Nakhla is an igneous rock, which means it was formed through the cooling and solidification of magma or lava. Therefore this meteorite is a fragment of a large parent body which experienced active volcanic activity. The meteorite is 1.3 billion years old, whereas volcanism has not been present for 3 billion years on the Moon. Nakhla is too young to have come from the Moon. Furthermore since 1969 we have gained much detailed information about the Moon from rocks returned to the Earth by manned Apollo space missions launched by the United States’ NASA Space program and unmanned missions by the former Soviet Union. So far, one hundred and three meteorites from the Moon are known thus scientists are able to more easily recognise lunar meteorites.
Is this meteorite from an asteroid?
We have extensive knowledge about meteorites from asteroids, as they are quite common, thus these types can be easily identified. Furthermore, volcanism has not been present in asteroids for over 4.4 billion years. Nakhla’s parent body is not an asteroid.
Is this meteorite from Venus?
Recent volcanism has occurred on Venus, but its high gravity might prevent ejection of fragments intersecting the Earth’s orbit.
Mars seems to be the only possible candidate
Mars has no active volcanism at the present time but evidences suggest that the Olympus Mons may still be volcanically active 115 million years ago, a very recent age in geological terms.
To confirm or refute the hypothesis of a Martian origin, scientists perform analysis on the chemical composition of the meteorite and use in particular a method based on the oxygen composition.
A single chemical element may be found under several variants depending on its number of neutrons. These variants are called ‘isotopes’. Thus natural occurring oxygen is composed of three stable isotopes which are 18O (with eighteen neutrons), 17O (seventeen neutrons) and 16O (sixteen neutrons). Scientists established the diagram below showing the ratio of amounts of 18O and 17O contained in terrestrial rocks, lunar samples and several meteorites. In all the rocks that belong to the same parent body the oxygen isotopes 17O and 18O is ratios lie on the same line. This results in the fact that the rocks which have the same origin are aligned on the same line. For example the terrestrial rocks are all grouped on the same line. The Moon is also placed on this line, suggesting that it was formed from the Earth. The diagram also shows that the SNC meteorites (for Shergottite, Nakhlite and Chassignite, three meteorite groups then recognised as from Mars) are aligned together, suggesting that these meteorites come from the same parent body. Our meteorite Nakhla gave its name to the group Nakhlite and, thus, belongs to the SNC meteorite group.
The oxygen isotopes ratio is substantially different in terrestrial rocks, SNC meteorites and most meteorites groups. This chemical signature cannot affirm a Martian origin by itself but allowed scientists to separate meteorites which belonged to the same parent bodies. Further analyses had to be undertaken to determine what the SCN meteorites’ common parent body was and, thus, where Nakhla came from.
Bubbles of Martian atmosphere
Let us leave aside Nakhla for a little while and take us a look at EETA 79001, the first meteorite that provided conclusive proof of a Martian origin.
When an asteroid or comet collides with another body, such as a planet, fractured fragments of the impacted body can be ejected under favorable conditions. Sometimes these fragments were heated so that the rocks were partly melted and trapped gases from the body’s atmosphere.
Mars’ atmosphere is known since analysis in situ performed by the NASA’s Viking probes in 1976. Scientists found out that the abundance of rare isotopes (neon, argon, xenon, krypton) trapped in the meteorite EETA 79001 was exactly the same as the composition of Mars’ atmosphere. This substantial correlation suggests the meteorites have a large probability to be from Mars.
Scientists thus have the evidence of EETA 79001’s Martian origin. Moreover, as EETA 79001 shows the same oxygen isotopes composition as SNC and is on the same line of the diagram, we can conclude that EETA 79001 and the SNC, including Naklha, come from one single parent body: the planet Mars.
What about meteorites from the rest of our Solar System?
Most of the meteorites found on the Earth appear to be fragments from the asteroid belt between Mars and Jupiter and other ones originate from the Moon and Mars. However, another origin is possible.
In early 2012, the extra-terrestrial rock NWA 7325 was found in the Moroccan desert. The owner of the 35 fragments of the meteorite immediately noticed the intense green-coloured heart and the very shiny crust of the rock and sent a sample to the Washington University for further analysis.
After a scientific investigation, scientists discovered a substantial quantity of magnesium, aluminium and calcium silicates but a very low concentration of iron. This chemical composition closely resembles observations on Mercury’s crust ran by the probe Messenger, in orbit around the planet. Researchers also undertook the study of NWA 7325’s oxygen isotope composition to understand its parent body signature. The results of the study support the evidence that NWA 7325 belongs to a differentiated body. However its oxygen isotope composition does not match the Earth or Mars. Scientific investigations have not resolved the mystery of NWA 7325’s origin yet, but showed some evidence of a possible Mercurian origin.
If this hypothesis was confirmed, NWA 7325 would be the first meteorite to come from Mercury ever identified on Earth. To confirm or refute a Mercurian origin, the study requires a comparison with Mercury’s rocks, either with return samples from the planet, such as for the Moon, or with analyses in situ, such as for Mars.
Scientists have found evidence of supervolcanoes on Mars. Find out more in this Nature Video, 3 October 2013.
To find out more about the potentially new Mercurian meteorite we invite you to read the following papers about research performed on NWA 7325:
Y. Amelin, P. Koefoed, T. Iizuka, and A. J. Irving, U-Pb Age of ungrouped achondrite NWA 7325, 76th Annual Meteoritical Society Meeting (2013), 5165.pdf
A. J. Irving, S. M. Kuehner, T. E. Bunch, K. Ziegler, G. Chen, C. D. K. Herd, R. M. Conrey, and S. Ralew, Ungrouped mafic achondrite Northwest Africa 7325: a reduced, iron-poor cumulate olivine gabbro from a differentiated planetary parent body, 44th Lunar and Planetary Science Conference (2013), 2164.pdf
I. Jabeen, A. Ali, N. R. Banerjee, G. R. Osinski, S. Ralew, and S. DeBoer, Oxygen isotope compositions of mineral separates from NWA 7325 suggest a planetary (Mercury?) origin, 45th Lunar and Planetary Science Conference (2014), 2215.pdf