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Astrochemistry - May 2008

Lodders Dr. Katharina Lodders
From the Special Topic of Astrochemistry

According to our April 2008 Special Topic on astrochemistry, the scientist whose work ranks at #6 is Dr. Katharina Lodders, with 4 papers cited 351 times. Dr. Lodders also authored the paper at #1 on our 10-year list with 323 citations: "Solar system abundances and condensation temperatures of the elements," (Astrophys. J. 591[2]: 1220-47, part 1, 10 July 2003). This paper is also a Highly Cited Paper in the field of Space Science in Essential Science IndicatorsSM from Thomson Reuters. Dr. Lodders is an Associate Research Professor in the Department of Earth & Planetary Sciences at Washington University in St. Louis, Missouri.

In the interview below, she talks with about her highly cited work.

  Please tell us a little about your research and educational background.

I am a chemist by education. I received my Doctorate in 1991 from the Johannes-Gutenberg University in Mainz, Germany, and did my thesis work at the Department for Cosmochemistry at the Max-Planck Institute for Chemistry in Mainz. This work was about chemistry during core formation in the Earth and in other planets and also related to meteorites.

" is interesting to follow how the elements produced in different stars eventually end up in new stars and planets."

I wanted to continue to work on cosmochemistry, which is closely related to planetary science and astronomy. In 1992, I started working at Washington University in St. Louis as a postdoctoral researcher, and currently I have a position as Research Associate Professor here.

  What do you consider the main focus of your research, and what drew your interest to this particular area?

My research focuses on gas and dust chemistry in stellar environments and planetary atmospheres. I got interested in dust formation in stellar environments because dust grains that condensed in the ejecta of evolved stars, such as supernovae, are found in some meteorites. Obviously, star dust was one ingredient when our solar system formed. Other than hydrogen and helium, all other chemical elements that we have in our solar system were made in previous generations of stars.

The stardust grains in meteorites formed from the ejected matter of dying stars. Therefore, grains can contain larger proportions of the elements that the star produced during its lifetime, but grains may not capture all elements completely, and some elements stay in the gas. We model these chemical fractionations of the elements during the grain formation processes with chemical thermodynamics and kinetics. Overall, it is interesting to follow how the elements produced in different stars eventually end up in new stars and planets.

The second major theme of my work is the atmospheric chemistry of gas giant planets, both inside and outside of the solar system, and of brown dwarfs, which are cool objects with insufficient mass to become real stars. The exoplanets and brown dwarfs were only first discovered in 1995. The different compositions of our gas giant planets—Jupiter, Saturn, Uranus, and Neptune—tell us something about the chemistry during the formation of our solar system. It is a natural extension to apply the methods developed for the planets in our solar system to study the newly discovered planets. Differences and similarities in the chemistry of our gas giant planets and gas giant exoplanets can help us understand how planetary systems form.

  Your most-cited paper in our astrochemical Special Topic is the 2003 Astrophysical Journal article, "Solar system abundances and condensation temperatures of the elements." Would you sum up the major points of this review for our readers?

The composition of the Sun is commonly used as the baseline composition for comparing and modeling elemental abundance variations in stars as well as planets and meteorites. The solar elemental composition is derived from spectroscopy of the Sun's photosphere and from geochemical analyses of certain meteorites that contain the elements in similar proportions as the Sun. In principle, the solar, or solar-system, composition can be well determined from the analysis of the Sun's photospheric spectrum and meteorite analysis, but refinements of the data are still necessary.

The last major evaluation of solar and meteoritic abundances of all stable elements was done in 1989. Since then, re-analyses led to significant revisions for important elements like C and O in the Sun. There were also updates for several other elements and their isotopes that had been re-measured in the Sun or in meteorites.

The composition of our Sun is not only important for comparison to astronomical observations of other stars or for input to models of stellar evolution. It is also important because it is representative of the material from which everything in the solar system—the Sun, the planets, their moons, asteroids, and comets—formed through chemical and physical processes.

When the solar system formed, the elements were chemically fractionated through condensation and evaporation processes. At a given temperature, some elements prefer to be in a condensed phase whereas other elements prefer to be in the gas. The condensation temperatures of the elements are a quantitative measure of the temperatures that are needed to vaporize an element out of solids or condense it out of the gas.

The changes in solar abundances, especially in C and O, influence what gas molecules and solids are present at a given temperature and total pressure. I found it therefore practical to summarize the best available abundance determinations for the Sun and meteorites, derive the solar system elemental composition, and then to use these data and chemical thermodynamics to calculate a self-consistent set of condensation temperatures.

  Many of your original papers deal with the chemical composition of stellar atmospheres. Would you talk a little about this aspect of your work?

I am interested in the chemical speciation of the elements in planetary and cool stellar atmospheres. The temperatures of the outer atmospheres of cool stars, brown dwarfs, and giant planets are low enough that gas molecules become abundant. Several molecular gases give characteristic absorption features—molecular fingerprints so to speak—in the spectra of cool stars and brown dwarfs, and the investigation of the underlying chemistry is part of my work.

"Differences and similarities in the chemistry of our gas giant planets and gas giant exoplanets can help us understand how planetary systems form."

What makes these atmospheres quite interesting is that clouds can form out of different materials. On Earth we have water clouds, but chemical modeling predicts that brown dwarfs and gas giant planets have cloud layers of refractory ceramics, iron metal, salts, water, and ammonia ice, depending on how cool the atmosphere is on the outside. Jupiter has all these materials as clouds with increasing depth, but only the ammonia clouds on the top can be seen. There are already hints of silicate clouds in brown dwarfs and very hot exoplanets, and it is probably just a matter of time until the spectral signatures of different cloud materials become clearly detectable for exoplanets.

  Where do you see your research going in five to ten years?

I expect that other types of star dust minerals will be discovered in meteorites, and that we will learn more about the compounds that are present when stars and planetary systems form. We have made several predictions as to which star dust minerals should be there; some have been found, but others that we expect are elusive so far. The minerals may indeed form at the stars, but a lot can happen to dust after leaving its birthplace near a star, and the chemical modifications need to be better understood.

I also expect that exoplanets will remain an exciting research area. Currently, new exoplanet detections and more physical and chemical information about them are almost regular items in the weekly news. It will be interesting to see how different or similar planetary systems with Earth-size planets will be from our own, and how unique the chemical pathways are for making planets like our Earth.

  What should the "take-away lesson" about your work be for the general public?

A lot of gas and dust chemistry happens when stars and planetary systems are born, and when stars die. On an astronomical scale, our Earth is a "dust ball". Chemistry tells us what the dust is made of, and how it can be formed.

Dr. Katharina Lodders
Department of Earth & Planetary Sciences
Washington University
St. Louis, MO, USA

Dr. Katharina Lodders's most-cited paper with 323 cites to date:
Lodders K, "Solar system abundances and condensation temperatures of the elements," Astrophys. J. 591(2): 1220-47, part 1, 10 July 2003. Source: Essential Science Indicators from Clarivate Analytics.

Keywords: astrochemistry, solar nebula, stellar environments, planetary atmospheres, dust formation, gas giant planets, brown dwarfs.


Special Topics : Astrochemistry : Katharina Lodders - Special Topic of Astrochemistry