Transcript STARDUST Presentation

Science (1999) Vol. 283 p 1135-1138
1
Introduction
• A Brief Introduction - Max
• How we know polycyclic aromatic hydrocarbons
are ubiquitous and abundant in space - Lou
• Interstellar conditions and how we simulate
them - Scott
• How we analyzed the samples (L2MS) - Dick
• Our results and their astrobiological significance
- Max
• Conclusions - Max
2
A little context…

Space was considered chemically barren
for most of the 20th Century

The spell was broken in the 1960’s and
1970’s with these discoveries:
OH
(early 60’s)
NH3 (1968)
H2CO (1969)*
CO
(1970)
11.3 µm emission (1973)
* …polyatomic molecules containing at least 2 atoms other than H can
form in the interstellar medium.” Snyder, Buhl, Zuckerman, Palmer
3
Center of the Orion Nebula
4
EMISSION FROM ORION
5
6
Soot Particles are Mainly PAHS
PAH MOLECULE
SOOT PARTICLE
7
Darken room and
cover projector lens
for fluorescence
demonstration
8
UV Pumped Infrared Fluorescence
UV
9
PAH EMISSION FROM NEARBY SPIRAL
GALAXY
MESSIER 81.
10
PAH EMISSION FROM THE SOMBRERO
GALAXY MESSIER 104.
11
What happens to PAHS in Cold, Dark
Interstellar Clouds ???
TOP OF THE HORSEHEAD NEBULA
12
Astrochemistry - A middling difficult enterprise
“Physicists love the early universe -- because it
is EASY. You’ve got protons, electrons, light, and
that’s it. Once atoms come together, you get
chemistry, then biology, then economics… it
pretty much goes to hell.”
-Andrew Lange (5/3/2000)
13
How do we simulate chemistry in the
interstellar medium?
• Much of the material in galaxies exists in ‘Dense Molecular
Clouds’ that consist of a mixture of dust, gas, and ices
14
How do we simulate the interstellar medium?
• These ‘dense’ clouds are the site of star formation
• Material from these clouds can find its way into/onto
newly formed planets
15
How do we simulate the interstellar medium?
• The dust in these dense clouds blocks out starlight and
their interiors can get very cold (T < 50 K).
• The pressures are very low
16
How do we simulate the interstellar medium?
• The radiation field can be high (UV and particle radiation)
•This radiation clearly illuminates PAHs associated with the clouds
Visible Light
PAH Emission
17
Interstellar Dust: ice mantle evolution
Thus, at the low temperatures found in these clouds,
most molecules are expected to freeze out onto the dust
grains where they may be exposed to ionizing radiation
Bernstein, Sandford, Allamandola , Sci. Am. 7,1999, p26 18
We can get an idea of what the ices are made of by
measuring the absorption spectra of the cloud material
The main ice ingredient is always H2O.
19
So, to simulate dense cloud conditions we need to
recreate low T, low P, high radiation conditions with
PAHs in H2O-rich ices exposed to radiation
Cryo-vacuum Sample Head
20
Lots of “plumbing”…
Cryo-vacuum System
(w/o spectrometer)
H2 Lamp On
21
Brown Organic Residue Produced by
Low Temperature UV Ice Irradiation
22
Analysis of the Samples
23
Laser-Desorption Laser-Ionization Mass Spectrometer
24
Two-Step Laser Mass Spectrometry
I. Laser desorption of neutral
molecules
pulsed IR
laser
A
A B
BA
B
plume of
neutral
molecules
II. Laser ionization of selected
species
selective
ionization of
aromatics
pulsed
UV laser
A
A+
A+ B
A
B A+
B
to detector
sample
25
Principles of
Time-of-Flight Mass Spectrometry
Kinetic Energy = zV = 1/2mv2
Arrival Time = t = d/v
= d/[(2z V/m)]1/2
= d[m/(2zV)]1/2
26
Two-Step Laser Mass Spectrometry
pulsed
IR beam
Reflectron
Acceleration
grids
Mass
Deflectors
Einzel
lens
MCP
detector
27
QuickTime™ and a
Motion JPEG OpenDML decompressor
are needed to see this picture.
28
The peaks at 316, 332, and 348 amu correspond to the addition
of one to three O atoms, respectively, likely in the form of
ketones or hydroxyl side groups (or both).
29
The peak at 290 amu corresponds to the addition of an O atom
with loss of two H atoms, consistent with an ether bridging the
molecule’s bay region.
30
Summary
31
Astrobiological Implications: The Search for Life
and see a whale breaching in the oceans of Europa32
Astrobiological Implications: The Search for Life
Alkylated PAHs were invoked as
biomarkers in the Martian
meteorite ALH84001
McKay et al., (1996) Science, Vol. 273, p. 924-930.
"Search for past life on Mars: Possible relic biogenic
activity in martian meteorite ALH84001"
33
Astrobiological Implications: The Search for Life
OH
O
Juglone (in walnut & pecan shells)
Bull. Soc. Chim . 1, 800 (1907)
O
OH
O
OH
Aloe -Emodin
Arch. Pharm. 247, 81 (1909)
R
O
Rhein (extract of chinese rhubarb)
Ann. 50, 196 (1844)
34
Astrobiological Implications: The Origin of Life
We see this class of compounds facilitating the most basic
chemical reactions in "primitive" organisms thus we believe
that these molecules are ancient
O
R
H2
or + S8
R-H
O
T. tenax
R'
H2S
Thermoproteus tenax (a "primitive" organism) use menaquinones as their primary quinone, and in most Bacteria and
Archaea, MK and related naphthoquinones seem to be very fundamental = ancient: are manufactured via Shikimate, couple
important biochemical reactions (i.e. Fumarate to Succinate), are involved in active transport of amino acids, and replace or
augment ubiquinone or plastoquinone as electon transport and oxidative phosphorylation co-enzymes
35
Conclusions
• The results explain many molecules seen in meteorites.
• These species resemble biomarkers, and thus are relevant
to the search for life.
• They are members of a class of compounds that is
ubiquitous in space.
• Quinones play fundamental roles in life's chemistry now and
probably did so from the beginning.
36
Thanks
Advice, edits, and patience of our friends here at
NASA-Ames and Stanford,
Technical support from dedicated lab technicians,
Support from our local management and,
Financial support from NASA's Astrophysics and
Planetary Science Divisions at NASA HQ
Our thanks also to our coauthor colleagues who were
unable to attend this presentation. It wouldn’t have
happened without them.
37
Prof. Zare receiving H. Julian Allen Award
from Simon P. Worden, Ph.D., BGen. (USAF, Ret.),
who is the Director of the NASA Ames Research Center
38
Photo of all presenters: Simon P. Worden, Scott A. Sanford, Richard N. Zare,
Max P. Berstein, and Louis J. Allamandola.
Unfortunately, two other authors could not be present: J. Seb Gillette
and Simon J. Clemett.
(Photo by Dr. Jennifer Heldmann)
39
40