Research

The long-term goal of my research is to produce a comprehensive picture of the processing of volatile elements in the Solar System from the starting materials in the Solar Nebula to the present day. To this end, I have been investigating the origin of organic matter in meteorites (Riebe et al., 2017a), the primordial composition of noble gases in meteorites (Riebe et al., 2017b), the effects of alteration on volatile elements in asteroids and interplanetary dust particles (Riebe et al., 2017c; 2019), regolith processes on asteroids (Riebe et al., 2017d), formation of unusual meteorites (Riebe et al., 2017e; 2018), and the transport of meteoroids from the asteroid belt to Earth (Riebe et al., 2017e, Meier et al., 2017; 2018). You can read more about some of my ongoing research below.

Meier M. M. M., Welten K. C., Riebe M. E. I., Caffee M. W., Gritsevich M., Maden C., and Busemann H. (2017) Park Forest (L5) and the asteroidal source of shocked L Chondrites. Meteoritics & Planetary Science 52,1561-1576.

Meier M. M. M., Bindi L., Heck P. R., Neander A. I., Spring N. H., Riebe M.E.I., Maden C., Baur H., Steinhardt P.J., Wieler R., Busemann H (2018). Cosmic history and a candidate parent asteroid for the quasicrystal-bearing meteorite Khatyrka. Earth and Planetary Science Letters, in revision.

Riebe M. E. I., Stroud R. M., Alexander C. M. O’D., Katz M. B., Nittler L. R, Nuevo M., Sandford S. A., Wang J. (2017a). Preliminary isotopic, chemical, and microstructural investigation of irradiated organic dust analogs. 48th Lunar and Planetary Science Conference #2579.

Riebe M. E. I., Busemann H., Wieler R., Maden C. (2017b). Closed System Step Etching of CI chondrite Ivuna reveals primordial noble gases in the HF-solubles. Geochimica et Cosmochimica Acta 205, 65-83.

Riebe M. E. I., Foustoukos D. I., Alexander C. M. O’D, Steele A., Cody G. D, Nittler L. R. (2017c) Effects of heating on insoluble organic matter in small particles during atmospheric entry. 80th Annual Meteoritical Society Meeting #6252.

Riebe M. E. I., Huber L., Metzler K., Busemann H., Luginbuehl S. M., Meier M. M. M., Maden C., Wieler R. (2017d) Cosmogenic He and Ne in chondrules from clastic matrix and a lithic clast of Murchison: No pre-irradiation by the early sun. Geochimica et Cosmochimica Acta 213, 618-634.

Riebe M. E. I., Welten K. C., Meier M. M. M., Wieler R., Barth M. I. F., Ward D., Laubenstein M., Bischoff A., Caffee M. W., Nishiizumi K., Busemann H. (2017e) Cosmic-ray exposure ages of six chondritic Almahata Sitta fragments. Meteoritics & Planetary Science 52, 2353-2374.

Riebe M. E. I., Busemann H., Goodrich C. A., Maden C., Shaddad M. (2018) Noble gases and cosmic ray exposure ages of the newly discovered Almahata Sitta C1+ureilite breccia sample. 49th Lunar and Planetary Science Conference #2295.

Riebe M. E. I, Busemann H., Alexander C.M.O’D., Nittler L. R., Herd C.D.K., Maden C., Wang J. Wieler R. (2019) Effects of aqueous alteration on primordial noble gases and presolar SiC in the carbonaceous chondrite Tagish Lake. In review

Some of my ongoing research:

Organic matter in meteorites

Primitive meteorites contain up to a few weight percent of organic matter. Organic matter on Earth today is largely formed by biological activity. In meteorites and/or their precursors, organic matter was formed by physical processes and chemical reactions. Organic matter was probably delivered to Earth early in its history by meteorites and comets and could perhaps have been important in the very earliest stages of the evolution of life. Some of the most fundamental questions around organic matter in meteorites that are still debated, including:

How did organic matter in meteorites form?

and

Where did it form, in the Solar System or in the interstellar medium?

In an ongoing project I am investigating the role of irradiation in the formation of complex organic matter. One characteristic of extraterrestrial organic matter is that it has much higher D/H and ¹⁵N/¹⁴N ratios than the Sun. The process(es) that formed organic matter likely contributed to these heavy isotopic compositions, and the capability to fractionate isotopes can be used as one way to evaluate possible formation scenarios. Relatively complex organic molecules could have formed through irradiation of mixtures of simple ices such as water, methanol, carbon monoxide, and ammonia that we know were present in the outer regions of the early Solar System, as well as in the molecular cloud that the Solar System formed from. When such ices are irradiated with UV irradiation the bonds in the molecules break and radicals from. The radicals then recombine to form more complex molecules. Organic dust formed in this way could see even more irradiation, which might alter the matter further and produce heavier isotopic compositions. At NASA Ames, my collaborators Michel Nuevo and Scott Sandford simulate conditions in the outer regions of the Solar System and the molecular cloud and produce organic matter through UV irradiation of ices. The organics produced from ices in the lab are then irradiated further with x-rays and UV. We compare the characteristics of organic matter produced in the experiments with those of the most primitive extraterrestrial organic matter available for laboratory studies to evaluate irradiation as a formation mechanism for extraterrestrial organic matter. My work is focused on the isotopic composition of the organics.

Collaborators:

Larry Nittler (DTM), Conel Alexander (DTM), Scott Sandford (NASA Ames), Michel Nuevo (NASA Ames), Rhonda Stroud (NRL)

Publications:

Riebe M. E. I., Stroud R. M., Alexander C. M. O’D., Katz M. B., Nittler L. R, Nuevo M., Sandford S. A., Wang J. (2017). Preliminary isotopic, chemical, and microstructural investigation of irradiated organic dust analogs. 48th Lunar and Planetary Science Conference #2579.

What happens to small particles when they collide with the atmosphere?

Interplanetary particles (IDPs) are very interesting extraterrestrial samples. They are small (tens of µm) dust grains that travel interplanetary space. IDPs are mainly available to scientists from NASA programs that collect IDPs with airplanes in the stratosphere. Some IDPs come from parent bodies that we do not have meteorites from, perhaps from comets. Such IDPs are primitive in the sense that they have seen less parent body alteration than meteorites and they could work as a window into what Solar System material looked like before parent body alteration. However, IDPs are heated when they collide with the Earth’s atmosphere which might alter the minerals, and perhaps more readily, the organics in the particles.

In this project I am doing experiments to simulate atmospheric entry heating of extraterrestrial organic matter in IDPs. I am then using a variety of analytical techniques (Raman, FTIR, NanoSIMS, IRMS) to investigate the impact of heating on the organic matter. The goals of this study are 1) to better understand the effects of atmospheric entry heating on organics in IDPs, and 2) to find a non-destructive way to identify the least heated, most pristine, particles.

Collaborators:

Larry Nittler (DTM), Conel Alexander (DTM), Dionysis Foustoukos (GL), Andrew Steele (GL), Bjorn Mysen (GL), George Cody (GL),

Publications:

Riebe, M.E.I., Foustoukos, D.I., Alexander, C.M.O., Steele, A., Cody, G.D., Mysen, B.O., Nittler, L.R., (2020) The effects of atmospheric entry heating on organic matter in interplanetary dust particles and micrometeorites. Earth and Planetary Science Letters 540, 116266.

Riebe M. E. I., Foustoukos D. I., Alexander C. M. O’D, Steele A., Cody G. D, Nittler L. R. (2017) Effects of heating on insoluble organic matter in small particles during atmospheric entry. 80th Annual Meteoritical Society Meeting #6252.

The formation of the very special meteorite Almahata Sitta

In 2008, a small asteroid was discovered to be on collision course with Earth. Some 20 h later, the asteroid disintegrated above Sudan and created a large strewn field of debris from which ~700 individual pieces with a total mass of ~10.5 kg have been recovered. The collected pieces of this asteroid are the Almahata Sitta meteorite. Most of these pieces (70-80%) belong to a class or meteorites called ureilites, the rest are a mixture of other types of meteorites. It is very unusual that a meteorite contains that many different kinds of rocks and even more unusual that the percentage of foreign material is that high. There is no consensus on how Almahata Sitta formed. One point of discussion has been weather Almahata Sitta is a regolith breccia or not, i.e., if it came from the top soil layer on the parent body where the material is loose and stirred by impacts, or not. By analyzing noble gases in the samples we can contribute to answer this question in two ways: 1) If Almahata Sitta is a regolith breccia then we expect some Solar wind (SW) noble gases to be present in the matrix portion of the sample. Solar wind is implanted into the top <1 mm of mineral grains when exposed on the surface of an asteroid and should not be present in a sample that originated deeper in the parent body. 2) Some variation in cosmic ray exposure (CRE) ages is possible if the samples originated in a regolith breccia. Cosmic ray exposure ages typically date the time that passed from that the meteoroid was expelled from the parent body until it fell on Earth, but variable cosmic ray exposure ages within the same meteorite might indicate that the meteorite is a regolith breccia. This is because the effects of galactic cosmic rays (GCR) decrease with depth in the asteroid and is negligible at a depth of a few meters. In a regolith breccia, the material gets stirred through impacts and some material spends longer time at shallow depth where there is more exposure to GCR than other material.

I started studying Almahata Sitta during my PhD and found that the CRE ages are more variable in the Almahata Sitta samples than previously thought. However, the results in the Riebe et al. (2017) study were not sufficient to conclude with certainty that Almahata Sitta is a regolith breccia. I am currently involved in two ongoing Almahata Sitta projects. Amy Plant, PhD student at ETH Zurich, is analyzing a large number of Almahata Sitta samples to investigate if the variation in CRE ages detected in Riebe et al. (2017) can be confirmed. In another study, I am looking at the noble gases in a newly characterized Almahata Sitta sample that might be coming from a matrix surrounding the more rigid clasts in Almahata Sitta that we analyzed previously. This sample is very interesting as there is a larger chance of detecting Solar wind gases in this kind of sample than in the more rigid samples.

Collaborators:

Henner Busemann (ETH), Matthias Meier (ETH), Amy Plant (ETH), Rainer Wieler (ETH), Kees Welten (Berkley), Cyrena Goodrich (LPI), Addi Bischoff (Munster), Marc Caffee (Purdue) and more.

Publications:

Riebe M. E. I., Busemann H., Goodrich C. A., C. Maden (2020) Noble Gases in an Almahata Sitta Sample Rich in C1 Like Material. Goldschmidt abstract. https://doi.org/10.46427/gold2020.2204

Riebe M. E. I., Welten K. C., Meier M. M. M., Wieler R., Barth M. I. F., Ward D., Laubenstein M., Bischoff A., Caffee M. W., Nishiizumi K., Busemann H. (2017) Cosmic-ray exposure ages of six chondritic Almahata Sitta fragments. Meteoritics & Planetary Science 52, 2353-2374.

Riebe M. E. I., Busemann H., Goodrich C. A., Maden C., Shaddad M. (2018) Noble gases and cosmic ray exposure ages of the newly discovered Almahata Sitta C1+ureilite breccia sample. 49th Lunar and Planetary Science Conference #2295.

Alteration, presolar grains, and primordial noble gases

Aqueous alteration is low temperature alteration in the presence of water that occurred on many asteroids in the early Solar System. In this project, the goal is to better understand the effects of this alteration on constituents of meteorites, in particular on the noble gases and presolar grains.

I am working with the unique meteorites Tagish Lake which contains clasts that experienced different degrees of aqueous alteration. I’ve done detailed investigations of primordial noble gases in four samples that experienced different degrees of aqueous alteration. One unexpected result was that the samples have large variations in the estimated concentrations of presolar grains based on the concentrations of noble gases that they carry. This does not correlate with degree of alteration. By identifying presolar grains in the same samples using NanoSIMS isotopic mapping I found the same variations in concentrations of presolar grains from NanoSIMS images as from primordial noble gases. This indicates that aqueous alteration might have redistributed presolar grains in the Tagish Lake meteorite. If so, then this has implications for our understanding of aqueous alteration in asteroids as most petrological studies have concluded that aqueous alteration was isochemical and without significant mass transfer. It also complicates using presolar grains as an indicator of degree of alteration as people have tried to do.

Collaborators:

Henner Busemann (ETH), Rainer Wieler (ETH), Larry Nittler (DTM), Conel Alexander (DTM), Chris Herd, Colin Maden

Publications:

Riebe, M.E.I., Busemann, H., Alexander, C.M.O.D., Nittler, L.R., Herd, C.D.K., Maden, C., Wang, J., Wieler, R., (2020) Effects of aqueous alteration on primordial noble gases and presolar SiC in the carbonaceous chondrite Tagish Lake. Meteoritics & Planetary Science. 55, 1257-1280.

postprint available at the ETH Research collection: https://doi.org/10.3929/ethz-b-000366471

dataset available at the ETH Research collection: https://doi.org/10.3929/ethz-b-000469217