A geochemist from MSU has assessed the oxidative environment inside asteroids

IMAGE: This is a schematic of the oxygen concentration EMF cell.

Image credit: 
Valentin Osadchii

A postgraduate of the Faculty of Geology at Moscow State University working as a part of an international team has assessed the oxidative environment and its changes inside asteroids from the core to the surface. This gives the authors of the study a better understanding of how the planets were formed. The paper was published in Meteoritics and Planetary Science.

Asteroids were formed by accretion in the early stages of the Solar System development. Accretion is growth of a celestial object by the attachment of the material attracted to it by gravity. When sufficient mass was reached, the temperature and pressure in the center of the asteroids increased, which led to the formation of a protoplanet, an "embryo" of a planet. Further accretion of such celestial bodies led to planet formation. But this process was not always completed: there are more than 700,000 asteroids known in the Solar System at the moment. Some of them were called minor planets until recently.

The processes that were taking place in our planet at the formation stage can be studied through meteorites - the asteroids whose orbits have crossed ours - that ceased to develop as planets. The trace of these transformations on Earth has become obliterated long ago. About 87% of all meteorites found are ordinary chondrites mainly consisting of spherical formations, chondrules, made of silicon (Si), iron (Fe), aluminum (Al), magnesium (Mg) and manganese (Mn) silicates. Geologists have studied the meteorite samples of this subgroup provided by the Museum of Extraterrestrial Material of the Laboratory of Meteoritics of Vernadsky Institute of Geochemistry and Analytical Chemistry, RAS.

The substance exists in an oxidized state in the center of ordinary chondrites. This would not be surprising on Earth because of the high concentration of oxygen in the atmosphere, but in space, where there is no oxygen, this raises questions. According to a common model, the oxidized state of the substance appears due to the water contained in the meteorites, which is exuded when the temperature in the center of the body increases. The temperature is lower at the surface, so the substance is oxidized to a lesser extent. In order to build this model, other scientists studied the chemical composition of the main minerals in meteorites and conducted a thermodynamic calculation of the oxygen pressure based on these data. The disadvantage of this method is that such calculations are indirect and cannot take into account all the factors affecting the oxidation process.

"We conducted a direct measurement of the partial pressure (the pressure of an individual component of the gas mixture) of oxygen in a series of meteorites. It turned out that the variations in the initial composition of the asteroid, that is, its inhomogeneity, as well as its complex structure caused by the cracks due to collisions with other bodies and their accretion, have a large effect on the oxygen pressure," said one of the authors of the paper Valentin Osadchii, postgraduate at the Department of Geology at Moscow State University.

The studied chondrites belonged to different chemical groups and were metamorphosed (varied by temperature and pressure) to different degrees.

Scientists were able to make direct measurements of the partial pressure of oxygen in ordinary chondrites using the electromotive force (EMF) method with solid electrolyte. In this method two systems are taken in one of which the pressure of the test substance, oxygen in this case, is known.

The Earth atmosphere was used as a comparison system in this study, since the partial pressure of oxygen in it is known. Regarding this pressure, the authors measured the pressure in the second vessel - in an ampoule with an electrolyte containing a sample of a meteorite substance, whose main minerals are native iron, olivine, and pyroxene. Olivine and pyroxene contain magnesium in variable amounts - they are so-called solid solutions with a variable chemical composition. Depending on the amount of magnesium the partial pressure of oxygen changes.

The measurement is as follows: two vessels containing oxygen are separated by a substance called an electrolyte. Its works as a membrane: if the oxygen pressure on one side is greater than on the other, the oxygen tries to pass through the membrane. It can happen only if the O2 molecule turns into two O2- ions. Only these ions can pass through the membrane. At the same time, an electron will pass through a wire connecting the two vessels. If we connect a voltmeter to this wire, we will know the EMF of the process, or how much the electrons "want" to run from one vessel to another. This value is directly related to the difference in pressure in both vessels.

"Before this study, all researchers looked at the composition of the mineral, made a lot of measurements, averaged them, and then wrote down the chemical reaction according to the composition to get the oxygen pressure in the system. This was somewhat problematic, because there are other impurities in these minerals besides magnesium. In order to perform all the calculations correctly, one must have a lot of thermodynamic data which is not available yet. The key difference of our study is that we directly measure the partial pressure of oxygen, with no need to know the real composition inside the meteorite, and we know what the oxygen pressure was at the moment when all processes in the meteorite stopped," the scientist explained.

The scientists found out that the composition of the substance was almost homogeneous, which did not correspond to many theories. By the spread of the oxygen partial pressure they accessed the homogeneousness of the substance at the accretion stage.

"We concluded that even if there was oxidation, it was insignificant. That is, we can judge the amount of water in these bodies. Of course, we cannot draw some global conclusion about the origin of the Earth, because it was a rather narrow study, but nevertheless, it allows us to understand the conditions under which the planets were formed. By studying the spread of the oxygen partial pressure, we can understand how homogeneous the substance was at the accretion stage. According to our data, the composition of the substance of asteroids and protoplanets was fairly homogeneous, although there were theories that the substance was quite inhomogeneous," Valentin Osadchii concluded.

Lomonosov Moscow State University