(image by Andy Christie for Scientific American July 99)
astrochem.org the astrochemistry lab at NASA Ames Research Center
The search for life in the Solar System: We may get lucky…
Image of Europa from
ase.jpl.nasa.gov/
…but the search for life in the Solar System is far
more likely to be the search for organic biomarkers
OH
H
Amino acids H
O
HO
HO H
OH H OH
O H OH
Sugars
NH 2
Substituted
N
N aromatics
N N
H
Nucleobases Fatty acids OH O
Juglone (in w alnut & pec an shells)
.
Bull. Soc. Chim 1, 800 (1907)
Nitrogen containing
heterocycles O
Molecules that look like biomarkers
(but may not be convincing indicators of life)
1. Various aromatics (invoked in martian meteorite)
2. Amino acids (components of proteins)
3. Fatty acids (components of membranes)
4. Sweeping generalizations
We make a
little bit of
space in our
laboratory,
and see what
happens.
(image by Andy Christie for Scientific American July 99)
Interstellar ice photochemistry
Drawing a line in the (Martian) sand
OH
O
No more free molecular species, commonly used in
geochemistry i.e., terpenes or PAHs
Unless…you see something way out of equilibrium or
non sensical i.e.,
High ee
Only one (or a very narrow distribution of) chain lengths
or functional groups when many are expected.
Drawing a line in the (Martian) sand
Definitely:
No enzymatic tests that rely on their biochemistry being
the same as ours.
Don’t assume their chromophores will be the same as
ours i.e., Chlorophyll
Keeping an open mind
On the other hand, we don’t want to miss life because
its basic components are different from ours.
Would we get a detection if they used different bases,
different sugars, and had the opposite chirality of
amino acids?
In summary we must be
Knowledgeable & open-minded
without being credulous
In summary we must be
Knowledgeable & open-minded
without being credulous
much like the Astrobiology & Exobiology panels to
whom I am grateful for financial support
Jason Dworkin (GSFC), Scott Sandford, & Louis
Allamandola (Ames)
SETI
A NH2
N
N
HO N N
O
H H
H H
OH H
XA NH2
N
N
HO N N
O
H H
H H
OH H
Liu et al., Science (2003) 302, 868.
T
O
H3C
NH
N O
HO
O
H H
H H
OH H
XT
O
H3C
NH
HO N O
H
O
H H
H H
OH H
A NH2
N
N
HO N N
O
H H
H H
OH H
NH2
N
N
N N
O
HO H
H
OH
OH
H
HO H
OH
Alternative sugars, i.e., allopyranose
Eschenmoser (1999) Science, 284, 2118
T
O
H3C
NH
N O
HO3P-
HO
O
H H
H H
OH H
O
N
NH
N N O
OH
HO
OH
OPO3H
O
N
NH
N N O
FMN, coenzyme in
OH certain oxidation-
reduction reactions in
HO the body
OH
OPO3H
chirality
(non-super imposable mirror images)
(almost)All the chiral amino acids in living things are left handed, and there is also a
bias for left handed amino acids in meteorites. The reason(s) for this is not yet clear.
Cronin, J. R., & Pizzarello, S. (1997) Science 275, 951-955.
OH
Benzo[a]Pyrene
O3
O
OH
CO2H
CO2H
OH OH
On Mars?
from The Mechanisms of Atmospheric Oxidation of Aromatic Hydrocarbons by Calvert, Atkinson, Becker etal., 2002 Oxford U press