Survival of organic phases in porous IDPs during atmospheric entry: A pulse‐heating study

Abstract— In this study, we have performed pulse‐heating experiments at different temperatures for three organic molecules (a polycyclic aromatic hydrocarbon [PAH], a ketone, and an amino acid) absorbed into microporous aluminum oxide (Al₂O₃) in order to imitate the heating of the organic molecules in interplanetary dust particles (IDPs) and micrometeorites (MMs) during atmospheric entry and to investigate their survival. We have shown that modest amounts (a few percent) of these organic molecules survive pulse‐heating at temperatures in the 700 to 900 °C range. This suggests that the porosity in IDPs and MMs, combined with a sublimable phase (organic material, water), produces an ablative cooling effect, which permits the survival of organic molecules that would otherwise be lost either by thermal degradation or evaporation during atmospheric entry.     

Amino acid survival in large cometary impacts

Abstract— A significant fraction of the Earth's prebiotic volatile inventory may have been delivered by asteroidal and cometary impacts during the period of heavy bombardment. The realization that comets are particularly rich in organic material seemed to strengthen this suggestion. Previous modeling studies, however, indicated that most organics would be entirely destroyed in large comet and asteroid impacts. The availability of new kinetic parameters for the thermal degradation of amino acids in the solid phase made it possible to readdress this question. We present the results of new high-resolution hydrocode simulations of asteroid and comet impact coupled with recent experimental data for amino acid pyrolysis in the solid phase. Differences due to impact velocity as well as projectile material have been investigated. Effects of angle of impacts were also addressed. The results suggest that some amino acids would survive the shock heating of large (kilometer-radius) cometary impacts. At the time of the origins of life on Earth, the steady-state oceanic concentration of certain amino acids (like aspartic and glutamic acid) delivered by comets could have equaled or substantially exceeded concentrations due to Miller-Urey synthesis in a CO₂-rich atmosphere. Furthermore, in the unlikely case of a grazing impact (impact angle ～5° from the horizontal), an amount of some amino acids comparable to that due to the background steady-state production or delivery would be delivered to the early Earth.     

Concentration and variability of the AIB amino acid in polar micrometeorites: Implications for the exogenous delivery of amino acids to the primitive Earth

Abstract— Micrometeorites (MMs) are extraterrestrial particles ranging in size from 25 μm to 2 mm that survive atmospheric entry and are collected on the Earth's surface. They represent the largest mass flux (MF) of extraterrestrial material (30,000 ± 20,000 t/yr) to the present‐day Earth. Studies of large collections of MMs suggest that about 20% have not been heated to high temperatures and that they contain organic carbon. Since non‐protein amino acids have been found in some carbonaceous meteorites, they might also be found in unmelted MMs. However, previous searches for amino acids in MMs were inconclusive. We combined a new extraction method for amino acids with a highly sensitive analytical method to detect and quantitate amino acids in MMs collected at the South Pole. We found the non‐protein amino acid α‐amino isobutyric acid (AIB) in one of our samples. The non‐detection of this amino acid in the other samples analyzed suggests that there are amino acid‐containing and amino acid‐free MMs, with ?14% of the MMs containing AIB. Since the MF of MMs is much higher than that of carbonaceous chondrites (CMs), amino acids in these small particles would represent an important source of exogenous delivery of organic molecules. Therefore, the results are discussed on the basis of their implications for astrobiology.     

Heating experiments simulating atmospheric entry heating of micrometeorites: Clues to their parent body sources

Abstract— Depending on their velocity, entry angle and mass, extraterrestrial dust particles suffer certain degrees of heating during entry into Earth's atmosphere, and the mineralogy and chemical composition of these dust particles are significantly changed. In the present study, pulse-heating experiments simulating the atmospheric entry heating of micrometeoroids were carried out in order to understand the mineralogical and chemical changes quantitatively as well as to estimate the peak temperature experienced by the particles during entry heating. Fragments of the CI chondrites Orgueil and Alais as well as pyrrhotites from Orgueil were used as analogue material. The experiments show that the volatile elements S, Zn, Ga, Ge, and Se can be lost from 50 to 100 μm sized CI meteorite fragments at temperatures and heating times applicable to the entry heating of similar sized cosmic dust particles. It is concluded that depletions of these elements relative to CI as observed in micrometeorites are mainly caused by atmospheric entry heating. Besides explaining the element abundances in micrometeorites, the experimentally obtained release patterns can also be used as indicators to estimate the peak heating of dust particles during entry. Using the abundances of Zn and Ge and assuming their original concentrations close to CI, a maximum heating of 1100–1200 °C is obtained for previously analyzed Antarctic micrometeroites. Thermal alteration also strongly influenced the mineralogy of the meteorite fragments. While the unheated samples mainly consisted of phyllosilicates, these phases almost completely transformed into olivine and pyroxene in the fragments heated to ≥800 °C. Therefore, dust particles that still contain hydrous minerals were probably never heated to temperatures ≥800 °C in the atmosphere. During continued heating, the grain size of the newly formed silicates increased and the composition of the olivines equilibrated. Applying these results quantitatively to Antarctic micrometeorites, typical peak temperatures in the range of 1100–1200 °C during atmospheric entry heating are deduced. This temperature range corresponds to the one obtained from the volatile element concentrations measured in these micrometeorites and points to an asteroidal origin of the particles.     

Carbon isotopic composition of acetic acid generated by hydrous pyrolysis of macromolecular organic matter from the Murchison meteorite

Abstract— Low molecular weight monocarboxylic acids, including acetic acid, are some of the most abundant organic compounds in carbonaceous chondrites. So far, the ¹³C‐ and D‐enriched signature of water‐extractable carboxylic acids has implied an interstellar contribution to their origin. However, it also has been proposed that monocarboxylic acids could be formed by aqueous reaction on the meteorite parent body. In this study, we conducted hydrous pyrolysis of macromolecular organic matter purified from the Murchison meteorite (CM2) to examine the generation of monocarboxylic acids with their stable carbon isotope measurement. During hydrous pyrolysis of macromolecular organic matter at 270–330 °C, monocarboxylic acids with carbon numbers ranging from 2 (C₂) to 5 (C₅) were detected, acetic acid (CH₃COOH; C₂) being the most abundant. The concentration of the generated acetic acid increased with increasing reaction temperature; up to 0.48 mmol acetic acid/g macromolecular organic matter at 330 °C. This result indicates that the Murchison macromolecule has a potential to generate at least ?0.4 mg acetic acid/g meteorite, which is about four times higher than the amount of water‐extractable acetic acid reported from Murchison. The carbon isotopic composition of acetic acid generated by hydrous pyrolysis of macromolecular organic matter is ?‐27‰ (versus PDB), which is much more depleted in ¹³C than the water‐extractable acetic acid reported from Murchison. Intramolecular carbon isotope distribution shows that methyl (CH₃‐)‐C is more enriched in ¹³C relative to carboxyl (‐COOH)‐C, indicating a kinetic process for this formation. Although the experimental condition of this study (i.e., 270–330 °C for 72 h) may not simulate a reaction condition on parent bodies of carbonaceous chondrite, it may be possible to generate monocarboxylic acids at lower temperatures for a longer period of time.     

An experimental study on impact-induced alterations of planetary organic simulants

The present study systematically investigates shock-induced alteration of organic simulants of planetary bodies (OSPBs) as a function of peak shock pressure and temperature by impact experiments. Our results show that the composition and structure of OSPBs are unchanged upon impacts at peak pressures ≤~5 GPa and temperatures ≤~350 °C. On the other hand, these are dramatically changed upon impacts at >7–8 GPa and > ~400 °C, through loss of hydrogen-related bonds and concurrent carbonization, regardless of the initial compositions of OSPBs. Compared with previous results on static heating of organic matter, we suggest that shock-induced alteration cannot be distinguished from static heating only by Raman and infrared spectroscopy. Our experimental results would provide a proxy indicator for assessing degree of shock-induced alteration of organic matter contained in carbonaceous chondrites. We suggest that a remote-sensing signature of the 3.3–3.6 μm absorption due to hydrogen-related bonds on the surface of small bodies would be a promising indicator for the presence of less-thermally-altered (i.e., <350 °C) organic matter there, which will be a target for landing to collect primordial samples in sample-return spacecraft missions, such as Hayabusa2 and OSIRIS-REx.     

Atmospheric entry heating of micrometeorites at Earth and Mars: Implications for the survival of organics

The atmospheric entry heating of micrometeorites (MMs) can significantly alter their pre‐existing mineralogy, texture, and organic material. The degree of heating depends predominantly on the gravity and atmospheric density of the planet on which they fall. For particles falling on Earth, the alteration can be significant, leading to the destruction of much of the pre‐entry organics; however, the weaker gravity and thinner atmosphere of Mars enhance the survival of MMs and increase the fraction of particles that preserve organic material. This paper investigates the entry heating of MMs on the Earth and Mars in order to examine the MM population on each planet and give insights into the survival of extraterrestrial organic material. The results show that particles reaching the surface of Mars experience a lower peak temperature compared to Earth and, therefore, experience less evaporative mass loss. Of the particles which reach the surface, 68.2% remain unmelted on Mars compared to only 22.8% on Earth. Due to evaporative mass loss, unmelted particles that reach the surface of Earth are restricted to sizes <70 μm whereas particles >475 μm survive unmelted on Mars. Approximately 10% of particles experience temperatures below ~800 K, that is, the sublimation temperature of refractory organics found in MMs. On Earth, this fraction is significantly lower with less than 1% expected to remain below this temperature. Lower peak temperatures coupled with the larger sizes of particles surviving without significant heating on Mars suggest a much higher fraction of organic material surviving to the Martian surface.     

Temperature structure in the lower atmosphere of Venus: New results derived from pioneer Venus entry probe measurements

The interpretation of unexpected characteristics of Pioneer Venus temperature measurements, and of the large difference between these and the Venera results, is aided by new Venus temperature profiles derived from engineering measurements of the Pioneer Venus Small-Probe Net Flux Radiometer (SNFR) instruments. To facilitate correction of a temperature-dependent radiometric response, these instruments monitored the temperatures of their deployed radiation detectors. The accurate calibration of the temperature sensors, and their strong thermal coupling to the atmosphere, make it possible to deduce atmospheric temperatures within 2°K (at most altitudes) using a simple two-component thermal model to account for lag effects. These independent temperature profiles generally confirm to high accuracy, the small-probe results of A. Seiff, D. B. Kirk, R. E. Young, R. C. Blanchard, J. T. Findlay, G. M. Kelly, and S. C. Sommer (1980a, J. Geophys. Res.85, pp. 7903–7933) concerning vertical structure and horizontal contrast in the lower atmosphere, although the stable layer below 25 km is found to be slightly more stable (by about 0.4°K/km) and absolute temperatures are an average of 2°K higher. The measured Day-Night thermal contrast is compatible with predicted responses to the diurnal variation in solar heating, except near the cloud base, where 3–5°K differences may be due to thermal radiative heating differences associated with different cloud opacities. Temperature contrasts between latitudes 30 and 60° are roughly consistent with cyclostrophic balance. But pressure and temperature measurements by the Pioneer Venus Sounder probe at 4° latitude, when compared to Small-probe results, imply unreasonably large equatorward accelerations of 100 (m/sec)/day. Poleward accelerations compatible with cyclostrophic balance can be obtained if Sounder-probe temperatures are increased by a scale-factor correction reaching 6–7°K at 13 km.     

Structure of the Venus mesosphere and lower thermosphere from measurements during entry of the pioneer Venus probes

Data on the thermal structure of the nightside middle atmosphere of Venus, from 84 to 137 km altitude, have been obtained from analysis of deceleration measurements from the third Pioneer Venus small probe, the night probe, which entered the atmosphere near the midnight meridian at 27°S latitude. Comparison of the midnight sounding with the morning sounding at 31°S latitude indicates that the temperature structure is essentially diurnally invariant up to 100 km, above which the nightside structure diverges sharply from the dayside toward lower temperatures. Very large diurnal pressure differences develop above 100 km with dayside pressure ten times that on the nightside at 126 km altitude. This has major implications for upper atmospheric dynamics. The data are compared with the measurements of G. M. Keating, J. Y. Nicholson, and L. R. Lake (1980, J. Geophys. Res., 85, 7941–7956) above 140 km with theoretical thermal structure models of Dickinson, and with data obtained by Russian Venera spacecraft below 100 km. Midnight temperatures are ～ 130°K, somewhat warmer than those reported by Keating et al.     

Pyrolisis of phosphorylated molecules and survivability limits during the atmospheric passage in earth-like planets

There is evidence that space energy sources could give place to the appearance of phosphorylated nucleosides outside of Earth. These compounds may have been delivered mainly by interplanetary dust particles due to the lower temperatures experienced during atmospheric deceleration and impacts to the terrestrial surface. In this report, we communicate the results of pyrolytic studies to simulate atmospheric survivability of adenosine-5′-diphosphates (ADP) (and adenosine-5′-monophosphate, adenosine and adenine as degradation products) at temperatures <500°C and at various time intervals. Our results revealed that phosphorylated and non-phosphorylated nucleosides transported by IDPs having sizes of 10⁻⁶- could resist temperatures up to 500°C generated during atmospheric entry. However, atmospheric passage should not exceed a time due to the thermal lability of these molecules. Because of the high half-life showed by ADP in the presence of meteoritic powder, it is thought that extraterrestrial delivery of very complex biomolecules is more suitable under such protected conditions. These data indicate that the formation of a Fe²⁺- and/or Ca²⁺-complex could increase the stability of the molecules in the presence of meteoritic matter. Therefore, if the synthesis of nucleosides, nucleotides or oligonucleotides could take place in icy bodies, then micron-sized dust could have contributed significantly to the availability of phosphorylated nucleosides in the early Earth or in extrasolar early Earth-like planets, and thereby could have allowed the arising of an early biological activity.     

Bulk mineralogical changes of hydrous micrometeorites during heating in the upper atmosphere at temperatures below 1000 °C

Abstract— Small particles 200 μm in diameter from the hydrous carbonaceous chondrites Orgueil CI, Murchison CM2, and Tagish Lake were experimentally heated for short durations at subsolidus temperatures under controlled ambient pressures in order to examine the bulk mineralogical changes of hydrous micrometeorites during atmospheric entry. The three primitive meteorites consist mainly of various phyllosilicates and carbonates that are subject to decomposition at low temperatures, and thus the brief heating up to 1000 °C drastically changed the mineralogy. Changes included shrinkage of interlayer spacing of saponite due to loss of molecular water at 400–600 °C, serpentine and saponite decomposition to amorphous phases at 600 and 700 °C, respectively, decomposition of Mg‐Fe carbonate at 600 °C, recrystallization of secondary olivine and Fe oxide or metal at 700–800 °C, and recrystallization of secondary low‐Ca pyroxene at 800 °C. The ambient atmospheric pressures controlled species of secondary Fe phase: taenite at pressures lower than 10^?2 torr, magnesiowüstite from 10^?3 to 10^?1 torr, and magnetite from 10^?2 to 1 torr. The abundance of secondary low‐Ca pyroxene increases in the order of Murchison, Orgueil, and Tagish Lake, and the order corresponds to saponite abundance in samples prior to heating. Mineralogy of the three unmelted micrometeorites F96CI024, kw740052, and kw740054 were investigated in detail in order to estimate heating conditions. The results showed that they might have come from different parental objects, carbonaterich Tagish Lake type, carbonate‐poor Tagish Lake or CI type, and CM type, respectively, and experienced different peak temperatures, 600, 700, and 800?900 °C, respectively, at 60–80 km altitude upon atmospheric entry.     

Prebiotic synthesis of adenine and amino acids under Europa-like conditions.

In order to simulate prebiotic synthetic processes on Europa and other ice-covered planets and satellites, we have investigated the prebiotic synthesis of organic compounds from dilute solutions of NH4CN frozen for 25 years at -20 and -78 degrees C. In addition, the aqueous products of spark discharge reactions from a reducing atmosphere were frozen for 5 years at -20 degrees C. We find that both adenine and guanine, as well as a simple set of amino acids dominated by glycine, are produced in substantial yields under these conditions. These results indicate that some of the key components necessary for the origin of life may have been available on Europa throughout its history and suggest that the circumstellar zone where life might arise may be wider than previously thought.     

The ultrafine mineralogy of a molten interplanetary dust particle as an example of the quench regime of atmospheric entry heating

Abstract— Melting and degassing of interplanetary dust particle L2005B22 at ～1200 °C was due to flash heating during atmospheric entry. Preservation of the porous particle texture supports rapid quenching from the peak heating temperature whereby olivine and pyroxene nanocrystals (3 nm-26 nm) show partial devitrification of the quenched melt at T ? 450 °C–740 °C. The implied ultrahigh cooling rates are calculated at ～105 °C/h–106 °C/h, which is consistent with quench rates inferred from the temperature-time profiles based on atmospheric entry heating models. A vesicular rim on a nonstoichiometric relic forsterite grain in this particle represents either evaporative magnesium loss during flash heating or thermally annealed ion implantation texture.     

The greenhouse of Titan

Both non-gray radiative equilibrium and gray convective equilibrium calculations for Titan indicate that the discrepancy between the equilibrium temperature of an atmosphereless Titan and the observed infrared temperatures can be explained by a massive molecular hydrogen greenhouse effect. The convective calculations indicate a probable minimum optical depth of 14, corresponding to many tens of km-atm of H₂, and total pressures of ～0.1 bar. The tropopause is several hundred km above the Titanian surface and at a temperature of about 90°K. Methane condensation is likely at this level. Such an atmosphere is unstable against atmospheric blow-off unless typical mesosphere scale heights are < 25km, an unlikely situation. Blow-off can also be circumvented by exospheric temperatures near the freezing point of hydrogen. It is considered more plausible that the present atmosphere is in equilibrium between outgassing and blow-off of the one hand and accretion from protons trapped in a hypothetical Saturnian magnetic field on the other; or exhibits uncompensated blow-off of outgassing products. To maintain the present blow-off rate without compensation for all of geological time requires an outgassing equivalent to the volatilization of a few km of subsurface ices. Photo-dissociation of these volatilized ices produces the observed high abundance of H₂ as well as large quantities of complex organic chromophores which may explain the reddish coloration of the Titanian cloud deck. An extensive circum-Titanian hydrogen corona is postulated. Surface temperatures as high as 200°K are not excluded. Because of its high temperatures and pressures and the probable large abundance of organic compounds, Titan is a prime target for spacecraft exploration in the outer solar system.     

Sublimation extraction coupled with gas chromatography-mass spectrometry: A new technique for future in situ analyses of purines and pyrimidines on Mars

We have developed a sublimation technique coupled with chemical derivatization and gas chromatography mass spectrometry (GC-MS) to detect nucleobases and other volatile organic compounds derived from bacteria in Mars analog materials. To demonstrate this technique, a sample of serpentine inoculated with Escherichia coli (E. coli) cells was heated to 500 °C for several seconds under Martian ambient pressure. The sublimate was collected on a cold finger, then derivatized and analyzed by GC-MS. We found that adenine, cytosine, thymine and uracil were the most abundant molecules detected in the sublimed E. coli extract by GC-MS. In addition, nucleobases were also detected in sublimed extracts of a deep-sea sediment sample, seawater, and soil collected from the Atacama Desert in Chile after heating the samples under the same conditions. Our results indicate that nucleobases can be easily isolated directly from natural samples using sublimation and then detected by GC-MS after chemical derivatization. The sublimation-based extraction technique is one approach that should be considered for use by future in situ instruments designed to detect organic compounds relevant to life in the Martian regolith.     

Interstellar Grains as Amino Acid Factories and the Origin of Life

Some two decades ago, Hoyle and Wickramasinghe (1976) proposed that the physical conditions inside dense molecular clouds favour the formation of amino acids and complex organic polymers. There now exists both astronomical and laboratory evidence supporting this idea. Recent millimeter array observations have discovered the amino acid glycine (NH₂CH₂COOH) in the gas phase of the dense star-forming cloud Sagittarius B2. These observations would pose serious problems for present-day theories of molecule formation in space because it is unlikely that glycline can form by the gas-phase reaction schemes normally considered for dense cloud chemistry. Several laboratory experiments suggest a new paradigm in which amino acids and other large organic molecules are chemically manufactured inside the bulk interior of icy grain mantles photoprocessed by direct and scattered ultraviolet starlight. Frequent chemical explosions of the processed mantles would eject large fragments of organic dust into the ambient cloud. Large dust fragments break up into smaller ones by sputtering and ultimately by photodissociation of individual molecules. Hence, a sizeable column density (N≈ 10¹⁰−10¹⁵ cm^-2) of amino acids would be present in the gaseous medium as a consequence of balancing the rate of supply from exploding mantles with the rate of molecule destruction. Exploding mantles can therefore solve the longstanding molecule desorption problem for interstellar dense cloud chemistry. A sizeable fraction of the organic dust population can survive destruction and seed primitive planetary systems throughout our galaxy with prebiological organic molecules needed for proteins and nucleic acids in living organisms. This possibility provides fresh grounds for a new version of the old panspermia hypothesis first introduced by Anaxagoras. It is shown that panspermia is more important than asteroid and cometary organic depositions onto primitive Earth. Furthermore, no appeal to Miller-Urey synthesis in a nonoxidizing atmosphere of primitive Earth is then needed to seed terrestrial life. This revised version was published online in July 2006 with corrections to the Cover Date.     

Soluble organic compounds in the Tagish Lake meteorite

The C2 ungrouped Tagish Lake meteorite preserves a range of lithologies, reflecting variable degrees of parent‐body aqueous alteration. Here, we report on soluble organic compounds, including aliphatic and aromatic hydrocarbons, monocarboxylic acids, and amino acids, found within specimens representative of the range of aqueous alteration. We find that differences in soluble organic compounds among the lithologies may be explained by oxidative, fluid‐assisted alteration, primarily involving the derivation of soluble organic compounds from macromolecular material. In contrast, amino acids probably evolved from precursor molecules, albeit in parallel with other soluble organic compounds. Our results demonstrate the role of parent‐body alteration in the modification of organic matter and generation of prebiotic compounds in the early solar system, and have implications for interpretation of the complement of soluble organic compounds in carbonaceous chondrites.     

In situ experimental formation and growth of Fe nanoparticles and vesicles in lunar soil

We report the results of the first dynamic, in situ heating of lunar soils to simulate micrometeorite impacts on the lunar surface. We performed slow‐ and rapid‐heating experiments inside the transmission electron microscope to understand the chemical and microstructural changes in surface soils resulting from space‐weathering processes. Our slow‐heating experiments show that the formation of Fe nanoparticles begins at ~575 °C. These nanoparticles also form as a result of rapid‐heating experiments, and electron energy‐loss spectroscopy measurements indicate the Fe nanoparticles are composed entirely of Fe⁰, suggesting this simulation accurately mimics micrometeorite space‐weathering processes occurring on airless body surfaces. In addition to Fe nanoparticles, rapid‐heating experiments also formed vesiculated textures in the samples. Several grains were subjected to repeated thermal shocks, and the measured size distribution and number of Fe nanoparticles evolved with each subsequent heating event. These results provide insight into the formation and growth mechanisms for Fe nanoparticles in space‐weathered soils and could provide a new methodology for relative age dating of individual soil grains from within a sample population.     

Numerical Simulation of Ionospheric Disturbances Generated by the Chelyabinsk and Tunguska Space Body Impacts

Numerical simulation of atmospheric disturbances during the first hours after the Chelyabinsk and Tunguska space body impacts has been carried out. The results of detailed calculations, including the stages of destruction, evaporation and deceleration of the cosmic body, the generation of atmospheric disturbances and their propagation over distances of thousands of kilometers, have been compared with the results of spherical explosions with energy equal to the kinetic energy of meteoroids. It has been shown that in the case of the Chelyabinsk meteorite, an explosive analogy provides acceptable dimensions of the perturbed region and the perturbation amplitude. With a more powerful Tunguska fall, the resulting atmospheric flow is very different from the explosive one; an atmospheric plume emerges that releases matter from the meteoric trace to an altitude of the order of a thousand kilometers.     

Amino acids derived from Titan tholins

An organic heteropolymer (Titan tholin) was produced by continuous dc discharge through a 0.9 N2/0.1 CH4 gas mixture at 0.2 mbar pressure, roughly simulating the cloudtop atmosphere of Titan. Treatment of this tholin with 6N HCl yielded 16 amino acids by gas chromatography after derivatization of N-trifluroacetyl isopropyl esters on two different capillary columns. Identifications were confirmed by GC/MS. Glycine, aspartic acid, and alpha- and beta-alanine were produced in greatest abundance; the total yield of amino acids was approximately 10(-2), approximately equal to the yield of urea. The presence of "nonbiological" amino acids, the absence of serine, and the fact that the amino acids are racemic within experimental error together indicate that these molecules are not due to microbial or other contamination, but are derived from the tholin. In addition to the HCN, HC2CN, and (CN)2 found by Voyager, nitriles and aminonitriles should be sought in the Titanian atmosphere and, eventually, amino acids on the surface. These results suggest that episodes of liquid water in the past or future of Titan might lead to major further steps in prebiological organic chemistry on that body.