"One of the greatest challenges
is to reconstruct the processes which actually formed our
From the Special Topic of
According to our April 2008 Special Topic on
Astrochemistry, the work of Professor Pascale Ehrenfreund
ranks at #5 by total cites, #6 by papers, and #5 by cites
per paper. Her record in this analysis includes 20 papers
cited a total of 445 times. Three of these papers are on
the list of the 20 most-cited papers over the past decade,
and one is on the list of the 20 most-cited papers over the
past two years.
Essential Science IndicatorsSM
Reuters, Prof. Ehrenfreund's record includes two Highly
Cited Papers in the fields of Chemistry and Space Science.
Her most-cited paper is "Organic molecules in the
interstellar medium, comets, and meteorites: a voyage from
dark clouds to the early Earth," (Ehrenfreund P and
Charnley SB, Annu. Rev. Astron. Astrophys. 38:
427-+, 2000), with 211 cites at the time of this feature's
Prof. Ehrenfreund is currently a Research Professor at the Space Policy
Institute at George Washington University's Elliot School of International
Affairs. Over the past decade, she has been Professor at Nijmegen, Leiden,
and Amsterdam Universities in the Netherlands. Since 2001, she led the
Astrobiology Laboratory at the Leiden Institute of Chemistry, and
investigated organic matter in the interstellar medium and in solar system
bodies, including planetary surfaces, comets, and meteorites. She served as
Principal Investigator and Co-Investigator on many different NASA/ESA space
missions, including satellites, planetary probes, and experiments on the
International Space Station.
In the interview below, she talks with
ScienceWatch.com correspondent Gary Taubes about
her highly cited work.
How did you first get involved with research
on organic molecules in the interstellar medium?
I studied molecular biology and astronomy at the University of Vienna and
completed my Master's thesis in the field of protein chemistry at the
Austrian Academy of Sciences in Salzburg. I decided to continue with an
interdisciplinary Ph.D. thesis combining biology, chemistry, and astronomy.
In 1985, my advisors in graduate school at the University of Paris VII, Dr.
Alain Leger and Dr. Louis d'Hendecourt, published the discovery of aromatic
polycyclic hydrocarbons (PAHs) in the interstellar medium. When I arrived
in Paris for my thesis in 1988, I decided to work on this exciting new
topic, looking at the largest organic molecules identified in interstellar
space—that's how I entered the field of astrochemistry.
After my Ph.D., I moved to the University of Leiden and worked
predominantly on ice chemistry in molecular clouds with the late Prof. Mayo
Greenberg and Prof. Ewine van Dishoeck. I was privileged to contribute to
the solid state database for the Infrared Space Observatory (ISO) and to be
involved in the ISO satellite data interpretation for many years. Another
topic I have pursued since the early '90s is the identification of the
carriers of the diffuse interstellar bands (DIBs) and the chemistry of
diffuse interstellar clouds. We reported in 1994 evidence for
C60+ in the interstellar medium. Over the last 15
years, I have been observing DIBs in many galactic and extragalactic
environments with Dr. Bernard Foing, my students, and collaborators all
over the world. Those observations provided important constraints on the
organic nature of the DIB carriers that are ubiquitously observed in the
"...stable isotope measurements
that probe the early solar nebula will help us to
better understand the fundamental questions about
forming planets, solar system chemistry, and the
emergence of life on Earth."
After my habilitation in 1999, I started my own Astrobiology group and
moved more into solar system research, still working with the same
molecules. We investigated the carbon pathways between interstellar and
circumstellar regions and the forming solar system by targeting the
Astronomical observations of interstellar clouds
Laboratory studies on the photostability of organics in space
Mars simulations (survival of organics and microorganisms,
Analyses of the organic composition of carbonaceous meteorites
Space hardware development and ground validation
Extraterrestrial delivery via comets and meteorites has deposited organic
molecules originally formed in the interstellar medium and solar nebula to
the early Earth. The assumption is that some of these molecules may have
been used to build up life here on Earth and maybe on Mars.
Due to our participation in the European ExoMars mission, we also focused
our research in recent years on aspects relevant for life detection on
What prompted you to write your highly cited 2000
review paper in the Annual Review of Astronomy and
Astrophysics? What was your goal in writing that review?
I was invited by the editors. The final document benefited from the
theoretical expertise of my co-author, Steven Charnley, and my experimental
knowledge in astrochemistry. We wanted to collect all the material
available and decide what is truly important concerning organic chemicals
in space, in the interstellar medium, and on comets and meteorites. We
tried to make the connection between these different regimes.
How has the state of the science changed in the
eight years since publication? In other words, what would you write
today that you couldn’t say then?
What has become evident since then is that during solar system formation,
the material that enters the inner region of the forming solar system is
strongly modified. We can see remnant material from the original
interstellar cloud but it is obvious that the solar nebula had its own
active chemistry. And that gives a strong signature to the material, which
is then forming planets.
At the time we wrote our original paper, we did not have enough evidence
for this scenario. Our research field really benefited strongly from recent
cometary missions—in particular, Deep Impact and Stardust. More
meteorites have been analyzed in recent years with advanced laboratory
instrumentation and consequently our knowledge on the early solar nebula
has significantly improved. Recent observations of proto-planetary disks,
using new instruments, can probe deeper and deeper into the interior of
disks; astronomers can look closer to the star, and really begin to
understand what happened in the early stages of planetary development.
Every year, it seems, we get a closer and closer look at this process.
What’s the most challenging aspect of your
One of the greatest challenges is to reconstruct the processes which
actually formed our solar system. The number of well-studied meteorites and
comets is still scarce.
This is why some of the ultimate questions—the questions relevant to
the origin of life on Earth—are still wide open. What material came
to the young planets via comets and meteorites? Where did the water come
from on planet Earth? What were the conditions on the early Earth that
allowed life to form and proliferate? These are major questions, and
difficult ones to answer.
Are you satisfied with the pace of research in the
field? What you’ve learned since you entered it in the
I’m happy for every piece of the puzzle we find, but the progress has
been rather predictable. The Stardust mission confirmed ideas that some
scientists had already, namely that the solar nebula was very active. It
was a natural development in my career to move into solar system research
and astrobiology. In 1999 I learned to analyze extraterrestrial samples in
Prof. Jeff Bada's group at the University of California, San Diego, on my
sabbatical leave, and I also got involved in life-detection strategies and
instrumentation. That was a great experience that paved the way for my
future research. I am convinced that astrochemistry will greatly benefit
from a strong exchange between astronomers and meteoriticists.
How do you see the current state of affairs in
your field and its prospects for the future?
I have to say that I think the future looks bright. In the next decade, we
will have a European space mission, Rosetta, that will land on Comet
67P/Churyumov-Gerasimenko (in 2014). This will allow us, for the first
time, to study the composition of a cometary nucleus in situ. This
space mission promises exciting results about the formation of our solar
system and the material that has been deposited on the young Earth through
extraterrestrial delivery. The US Mars mission Phoenix touched down on May
25, 2008 in polar latitudes and look for ice and organics. And there are
two additional robotic mars missions in preparation that will investigate
organic material and life: the US Mars Science Laboratory in 2009 and the
European ExoMars mission in 2013. We may even have an asteroid
sample-return mission in the not-too-distant future.
"Extraterrestrial delivery via
comets and meteorites has deposited organic molecules
originally formed in the interstellar medium and solar
nebula to the early Earth. The assumption is that some
of these molecules may have been used to build up life
here on Earth and maybe on Mars."
In astronomy, the instrumentation keeps getting better and better.
Herschel, a European mission, will be launched at the end of this year to
do sub-millimeter astronomy. There are large-scale telescopes, such as
ALMA—the Atacama Large Millimeter/submillimeter Array—that will
be coming online soon. Then the James Webb Telescope, the replacement for
the Hubble Space Telescope, will be going up in the next decade. So there
are a lot of opportunities in astronomy to continue to look at the
structure of interstellar clouds, proto-stellar disks, proto-planetary
The improvement of laboratory instrumentation will lead to more accurate
data of meteorites. In particular, stable isotope measurements that probe
the early solar nebula will help us to better understand the fundamental
questions about forming planets, solar system chemistry, and the emergence
of life on Earth.
Do you have any new research projects you would
like to discuss?
I am currently strongly involved in the preparation of the future European
Mars mission ExoMars, scheduled for launch in 2013. ExoMars will be the
first robotic mission to Mars that is dedicated to the search for life. I
work on instrumentation development of the key instruments UREY and MOMA,
and I also try to understand where organic molecules could actually survive
on Mars. Organic material and life are believed to be destroyed by the
combination of radiation and oxidizing agents and the absence of liquid
water on the surface.
I am part of the Wisconsin Astrobiology Research Consortium funded by the
NASA Astrobiology Institute and perform research to study the stability of
organic material in Martian soil analogs with particular emphasis on the
interplay between the mineral matrix and organics. We also monitor the
effect of a simulated Martian environment on microorganisms and organics in
order to understand how and where biomolecules may survive in the
subsurface of Mars. These ground-based data are used to calibrate our
instruments that will search for life on Mars in 2013.
What unexpected or serendipitous events arose in
the course of your research?
In this kind of research, progress can sometimes be achieved by just
grasping chances. Last month, we had a press release from my former student
Dr. Zita Martins about her thesis work. She was analyzing carbonaceous
meteorites from the Antarctic and found unprecedented amino acid levels
that were 20-30 times higher than in any meteorite ever measured before. So
we looked at the data and said, "Okay, that can't be right, so let's do it
again." But we measured it again and it turned out to be true.
There are lucky surprises. I remember once we had a week scheduled on a
telescope, and we had terrible, dreadful weather throughout the run. One
night, Dr. Emmanuel Dartois and I opened the dome for two hours and
observed at the zenith—we had no choice, because this was the only
position without clouds. The observed distant star BD+63 1964 showed the
strongest diffuse interstellar bands ever measured and we submitted a
letter to Astronomy & Astrophysics after the observing run.
Surprises happen and make science so very exciting.
Leiden Institute of Chemistry
Leiden, The Netherlands
Space Policy Institute
Elliot School of International Affairs
Washington, DC, USA
Ehrenfreund's most-cited paper with 211
cites to date:
Ehrenfreund P, Charnley SB, "Organic molecules in the
interstellar medium, comets, and meteorites: a voyage from
dark clouds to the early Earth," Annu. Rev. Astron.
Astrophys. 38: 427+, 2007. 211 cites. Source:
Essential Science Indicators
Keywords: pascale ehrenfreund,
interstellar medium, comets, meteorites, astrochemistry, astrobiology,
ice chemistry, molecular clouds, diffuse interstellar bands,
proto-planetary discs, planet formation, alma, herschel mission, james
webb telescope, rosetta cometary mission, exomars.