Space: “Fine meteors are not meteor dust”

Every year, approximately 5,200 tons of fine meteorites reach the soil of our planet. Here is the result of an international program that has been in operation for nearly 20 years, the results of which were recently published in the magazine Earth and Planetary Sciences Letters. Cecil Engrand is a cosmologist, researcher on IJCLab Laboratory From the University of Paris-Saclay (CNRS / IN2P3) and specializes in analyzing these tiny meteorites. She co-authored this study, and told us more about micro-meteorites and the value of studying them. Interview.

Engineering techniques: what is a meteorite accurate? What is the difference with a meteor?

Cecil Engrand: Shooting stars are caused by a millimeter-sized dust. This dust emits light as it enters the atmosphere. Dust ranges between 30 and 200 micrometers in size and is not large enough to emit light. When it enters the atmosphere, some of the dust gets heated up, some evaporates, and for some reason no one has fully understood it yet, others fall to the ground without getting too hot. Micrometeorites are dust that are not destroyed during entry into the atmosphere and reach the ground.

So what is the exact shape of the meteorite?

Micrometeorites are collections of small minerals and organic matter. Most of the tiny meteorites contain about 2% carbon, and the rest are concentrations of small minerals. The most abundant minerals in meteorites are silicates and iron sulfides. Some are found on Earth, such as olivine or pyroxene, but their formations differ from those found in extraterrestrial materials.

Small individual minerals – “grains” between 50 and 100 nanometers in size – formed at the beginning of the formation of the Solar System 4.5 billion years ago. It was formed directly from the gas that was around the sun at that time, or it was inherited from a pre-sun cloud that was used to form the solar system. These grains stick together mechanically to form a slightly larger particle, and they stayed together for 4.5 billion years. This is the order that was used to create planets in staggered accretion. Some dust has escaped from the planetary formation, and this is what we are studying.

Where do subtle meteorites come from?

The tiny meteorites come from the asteroid belt, between Mars, Jupiter and comets. There are several types of comets: those captured by Jupiter and comets that come from very far away, for example from the Oort Cloud.

If we summarize, we find three groups of grains in micro-meteorites. First, those that might have come from nearby comets: they contain mainly silicates and very little carbon. Then there are those that come from comets in the solar system that contain more carbon (tiny “super-carbon” meteors). Finally, there are particles directly related to meteorites that might come from asteroids. Remember, a meteorite is a pebble that comes from space a centimeter or more in size: we have evidence that it came from the asteroid belt between Mars and Jupiter.

But beware: subtle meteors are not meteor dust! If you take a meteorite and powder, you won’t get an accurate meteorite. There are some similarities, but in micro-meteorites the minerals are smaller and the abundance diverges. The Japanese space mission Hayabusa 2 brought back asteroid samples back to Earth in December 2020. Next summer, Paris-Saclay will analyze a portion of it to compare these samples with micro-meteorites. This should allow us to learn more.

What analyzes are we doing on tiny meteorites?

There are many possible analyzes. The prerequisite is to prepare the samples in a different way. Thus every small meteorite will be split into several pieces. For example, a 100 μm particle will be divided into three. In each path, we can perform additional analyzes. The first is through a scanning electron microscope with an x-ray detector to confirm that it is indeed a small meteorite and to determine its type. Then we make very thin sections on another to observe under a transmission electron microscope. This makes it possible to characterize mineralogy at a finer scale and to look at grains of 50 nm in order. Thus, we will be able to distinguish all the individual grains that have stuck together to form tiny meteorites.

It is also possible to perform isotope analyzes to measure and characterize the history of the particle since its formation. We can also perform infrared microscopy to characterize mineral and organic structures. At the University of Paris-Saclay, we are fortunate to have the invention of infrared nanoscale analysis by Alexandre Dazzi about fifteen years ago. This technique allows the mineral and organic composition to be examined at a scale of a few tens of nanometers. Other techniques require particle destruction: this is the case, for example, if one wants to characterize amino acids by liquid chromatography.

What do subtle meteorites tell us about early Earth?

What we’re interested in is to understand how these tiny particles formed, how the grains got stuck together, and how the dust evolved. This gives us information about the mechanisms that were present in the early formation of the solar system.

Another line of research focuses on tiny, super-carbon meteorites, about 1% of the tiny meteorites we detect. Unlike the most common ones, it is dominated by organic matter. They are thought to come from far away, from the regions where comets formed. The input of extraterrestrial organic matter into early Earth about 4 billion years ago is something that is being taken very seriously. What we’re interested in is that in these tiny meteorites we find organic compounds that may have contributed to the supply of prebiotics on primitive Earth.

Amino acids in particular are interesting, found in extraterrestrial matter. This is evidence that the complex chemistry capable of forming these particles can be carried out elsewhere than Earth, including asteroids and comets. The provision of amino acids via the micrometeorites was able to participate in the inventory of the substance that contributed to the formation of life. But beware: we won’t say extraterrestrial matter or meteorites have brought life to Earth! On the other hand, it is said that this extraterrestrial contribution could contain components that could have been used in the complex chemistry of the time that helped the emergence of life.

First Image: Cosmic Sphere © J. Duprat C./Engrand, CNRS