Does adenine, a key organic molecule, occur in interstellar dust clouds? If so, those clouds could have delivered the molecule to Earth billions of years ago, a possibility interesting not only in terms of life’s formation on this planet but, of course, on other worlds as well. And as University of Missouri chemist Rainer Glaser notes, the idea of space-borne adenine is not implausible, for adenine is known to occur in meteorites and was identified in 1986 in the organic mantle surrounding comet Halley’s core.
Could adenine have been synthesized on the early Earth? Perhaps, but there are reasons for finding an outside delivery mechanism provocative. Note this from the paper on this work, which appeared in the journal Astrobiology (internal references omitted for brevity). HCN refers to hydrogen cyanide, which can flag the presence of adenine:
The idea of prebiotic adenine synthesis on Earth remains controversial. The HCN-based syntheses rely on the presence of a reducing atmosphere, and there is growing evidence that Earth did start out with a reducing atmosphere. But even with the substrates present, questions remain regarding the kinetic and thermodynamic feasibilities and probabilities of adenine synthesis under various conditions on an early Earth.
Clearly, space-borne adenine would be prime fodder for astrobiologists. The nucleotide adenosine triphosphate (ATP) is formed when phosphates are added to adenosine (a nucleoside formed from adenine and ribose), enabling the transfer of chemical energy in cellular metabolism. If such an essential organic molecule were found in interstellar space, it could actually become a diagnostic tool, helping us identify areas where life is more likely to occur in our galaxy. Here’s Glaser on the possibilities:
“There is a lot of sky with a few areas that have dust clouds. In those dust clouds, a few of them have HCN. A few of those have enough HCN to support the synthesis of the molecules of life. Now, we have to look for the HCN concentrations, and that’s where you want to look for adenine.”
And then this:
“Chemistry in space and ‘normal chemistry’ can be very different because the concentrations and energy-exchange processes are different. These features make the study of chemistry in space very exciting and academically challenging; one really must think without prejudice.”
A University of Missouri news release is here. The paper is Glaser et al., “Adenine Synthesis in Interstellar Space: Mechanisms of Prebiotic Pyrimidine-Ring Formation of Monocyclic HCN-Pentamers,” Astrobiology Vol. 7 No. 3 (2007), pp. 455-470 (available online).
The largest oxygen bearing organic molecule repository
Authors: M. A. Requena-Torres, J. Martin-Pintado, S. Martin, M. R. Morris
(Submitted on 5 Sep 2007)
Abstract: We present the first detection of complex aldehydes and isomers in three typical molecular clouds located within 200 pc of the center of our Galaxy.
We find very large abundances of these complex organic molecules (COMs) in the central molecular zone (CMZ), which we attribute to the ejection of COMs from grain mantles by shocks. The relative abundances of the different COMs with respect to that of CH3OH are strikingly similar for the three sources, located in very different environments in the CMZ. The similar relative abundances point toward a unique grain mantle composition in the CMZ.
Studying the Galactic center clouds and objects in the Galactic disk having large abundances of COMs, we find that more saturated molecules are more abundant than the non-saturated ones. We also find differences between the relative abundance between COMs in the CMZ and the Galactic disk, suggesting different chemical histories of the grain mantles between the two regions in the Galaxy for the complex aldehydes. Different possibilities for the grain chemistry on the icy mantles in the GC clouds are briefly discussed. Cosmic rays can play an important role in the grain chemistry.
With these new detections, the molecular clouds in the Galactic center appear to be one of the best laboratories for studying the formation of COMs in the Galaxy.
Comments: 20 pages, 4 figures, accepted in ApJ
Subjects: Astrophysics (astro-ph)
Cite as: arXiv:0709.0542v1 [astro-ph]
Submission history
From: Miguel Angel Requena-Torres [view email]
[v1] Wed, 5 Sep 2007 13:25:27 GMT (144kb)
http://arxiv.org/abs/0709.0542
There is no doubt that complex molecules can form in space — amino acids, nucleotides, etc. There are two issues to consider, however. One is speed of formation and stability (given the extremes of temperature in space and near stars). More importantly, all molecules involved in life on earth are chiral (non-superimposable on their mirror image), whereas identical molecules, if prebiotic, are racemic. So the crucial question is whether the complex molecules found in space are asymmetric.
The Meaning of Life
We create life, we search for it, we manipulate and revere it. Is it possible that we haven’t yet defined the term?
http://www.seedmagazine.com/news/2007/09/the_meaning_of_life.php
To quote:
“A science in which the most important object has no definition—that’s absolutely unacceptable,” says Popa. “How are we going to discuss it if you believe the definition of life has something to do with DNA and I think it has something to do with dynamic systems? We cannot have a conversation on any level. We cannot make artificial life because we cannot agree on what life is. We cannot find life on Mars because we cannot agree on what life represents.”
Recently, a new voice has entered the debate. Carol Cleland, who teaches philosophy at the University of Colorado and works with NASA’s National Astrobiology Institute—essentially as their philosopher-in-residence—is making a more radical argument: Scientists should simply give up looking for a definition of life. They can’t even begin to understand what life really is, she claims, until they find forms of life profoundly different from those we know here on Earth. Only when we can compare alien life with life on our planet will we understand the true nature of this ubiquitous, ephemeral thing.
Life’s Ingredients May Have ‘Sprinkled’ on Earth
http://bcast1.imaginova.com/t?r=2&ctl=1C98C:4A48D
Study suggests some DNA ingredients may have sprinkled down to young Earth.
The c2d Spitzer spectroscopic survey of ices around low-mass young stellar objects II: CO2
Authors: Klaus M. Pontoppidan, A. C. A. Boogert, Helen J. Fraser, Ewine F. van Dishoeck, Geoffrey A. Blake, Fred Lahuis, Karin I. Oberg, Neal J. Evans II, Colette Salyk
(Submitted on 28 Nov 2007)
Abstract: This paper presents Spitzer-IRS spectroscopy of the CO2 15.2 micron bending mode toward a sample of 50 embedded low-mass stars in nearby star-forming clouds, taken mostly from the “Cores to Disks (c2d)” Legacy program. The average abundance of solid CO2 relative to water in low-mass protostellar envelopes is 0.32 +/- 0.02, significantly higher than that found in quiescent molecular clouds and in massive star forming regions.
It is found that a decomposition of all the observed CO2 bending mode profiles requires a minimum of five unique components. Roughly 2/3 of the CO2 ice is found in a water-rich environment, while most of the remaining 1/3 is found in a CO environment. Ground-based observations of solid CO toward a large subset of the c2d sample are used to further constrain the CO2:CO component and suggest a model in which low-density clouds form the CO2:H2O component and higher density clouds form the CO2:CO ice during and after the freeze-out of gas-phase CO. It is suggested that the subsequent evolution of the CO2 and CO profiles toward low-mass protostars, in particular the appearance of the splitting of the CO2 bending mode due to pure, crystalline CO2, is first caused by distillation of the CO2:CO component through evaporation of CO due to thermal processing to ~20-30 K in the inner regions of infalling envelopes.
The formation of pure CO2 via segregation from the H2O rich mantle may contribute to the band splitting at higher levels of thermal processing (>50 K), but is harder to reconcile with the physical structure of protostellar envelopes around low-luminosity objects.
Comments: Accepted for ApJ
Subjects: Astrophysics (astro-ph)
Cite as: arXiv:0711.4616v1 [astro-ph]
Submission history
From: Klaus Martin Pontoppidan [view email]
[v1] Wed, 28 Nov 2007 22:43:30 GMT (1577kb)
http://arxiv.org/abs/0711.4616
Complex Organic Materials in the Circumstellar Disk of HR 4796A
Authors: J.H. Debes, A.J. Weinberger, G. Schneider
(Submitted on 19 Dec 2007)
Abstract: We combine HST/NICMOS imaging photometry of the HR 4796A disk at previously unobserved wavelengths between 1.71-2.22\micron with reprocessed archival observations to produce a measure of the dust’s scattering efficiency as a function of wavelength. The spectrum of the dust, synthesized from the seven photometric measures, is characterized by a steep red slope increasing from 0.5 \micron to 1.6 \micron followed by a flattening of the spectrum at wavelengths $ less than $ 1.6 \micron.
We fit the spectrum with a model population of dust grains made of tholins, materials comprised of complex organic materials seen throughout the outer parts of our Solar System. The presence of organic material around a star that may be in the later stages of giant planet formation implies that the basic building blocks for life may be common in planetary systems.
Comments: 5 pages, emulateapj, Accepted to ApJL
Subjects: Astrophysics (astro-ph)
Cite as: arXiv:0712.3283v1 [astro-ph]
Submission history
From: John H. Debes [view email]
[v1] Wed, 19 Dec 2007 21:06:58 GMT (177kb)
http://arxiv.org/abs/0712.3283
Arecibo telescope finds critical ingredients for the soup of life in a galaxy far, far away
http://www.news.cornell.edu/stories/Jan08/Arecibo.methanimine.html
Jan. 14, 2008
Astronomers from Arecibo Observatory radio telescope in Arecibo, Puerto Rico, have detected for the first time the molecules methanimine and hydrogen cyanide — two ingredients that build life-forming amino acids — in a galaxy some 250 million light years away.
When combined with water, the molecules form glycene, the simplest amino acid and a building block of life on Earth.
The astronomy team, led by Arecibo astronomer Christopher Salter, announced the discovery Jan. 11 in a poster presented at the American Astronomical Society meeting in Austin, Texas.
The Arecibo astronomers focused on the distant galaxy Arp 220, an ultra-luminous starburst galaxy, because it forms new stars at a very high rate. They used the 305-meter, or 1,000-foot diameter, Arecibo radio telescope, the world’s largest and most sensitive, to observe the galaxy at different frequencies. The observations, made in April 2007, were the first use of the 800 megahertz wide-band mode of the telescope’s main spectrometer.
The molecules were found by searching for radio emission at specific frequencies. Each chemical substance has its own unique radio frequency, much like people have unique fingerprints.
“We weren’t targeting any particular molecule, so we didn’t know what we were going to find — we just started searching, and what we found was incredibly exciting,” said Tapasi Ghosh, an Arecibo astronomer.
“The fact that we can observe these substances at such a vast distance means that there are huge amounts of them in Arp 220,” said Emmanuel Momjian, a former Arecibo astronomer, now at the National Radio Astronomy Observatory in Socorro, N.M. “It is indeed very intriguing to find that the ingredients of life appear in large quantities where new stars and planets are born.”
In addition to Salter, Momjian and Ghosh, the other researchers included Arecibo astronomers Robert Minchin and Mikael Lerner; Barbara Catinella, a former Arecibo astronomer now at the Max Plank Institute for Astrophysics in Germany; and Mayra Lebron, a former Arecibo astronomer now at the University of Puerto Rico.
The Arecibo Observatory is part of the National Astronomy and Ionosphere Center, a national research center operated by Cornell for the National Science Foundation.