Making nucleosides in interstellar ice analogs
One of the mysteries of the origins of life is how genetic information storing molecules ribonucleic acid (RNA) and deoxyribonucleic acid (DNA) are originated from abiogenesis. The most widely accepted theory "RNA world" hypothesis postulates that the building block of RNA ribonucleosides (consisting of nucleobase and ribose) were formed under early Earth conditions and then aggregated into informational polymers capable of self-replication. This is a key to the transition from the prebiotic world to biology. Many laboratory work have investigated the abiogenesis of ribonucleosides and ribonucleotides (consists of nucleoside and phosphate) under well-controlled laboratory conditions, starting from feedstock prebiotic molecules that are supposedly available on early Earth, including nucleobases, sugar, and phosphate. However, it is unclear whether such genetic information storing molecules can be synthesized under deep space conditions before the planetary system was born. Formation of them under space condition is strong indication that life might be present elsewhere in the universe. This proposal experimental study aims to explore the possibility of forming nucleosides, which is a component of DNA and RNA, under prestellar core conditions. We’ll grow a mixed ice with a similar composition to the ice mantle covering the small dust particles in star-forming regions, and use UV light to trigger the chemistry. Then, we’ll use the second experimental setup in the lab, which combines orbitrap mass spectrometer with laser desorption ionization technique to identify nucleosides in the mixed ices.
This project is supported by a MINT-Innovationen 2023 grant from the Vector Stiftung, awarded to Jiao He and Thomas Henning.
Phase transition of CO ice
Among the over 200 molecular species identified in interstellar clouds, many are organic molecules. It has been proposed that some of these molecules survive the star and planet formation process and are eventually delivered to Earth where they can form the molecular basis of the origin of life. It is now well-established that one of the most important factories of these molecules are ice mantles that cover the dust grains in star-forming molecular clouds. Simple atoms and molecules such as H, O, N, and CO condense from the gas phase onto the grain surface and then react with each other in the ice to form increasingly complex molecules. At the extremely low temperature (10-15 K) in these clouds, the widely accepted mechanism to bring reactive species together --- diffusion--- is severely impeded in the ice, raising the question of the mechanism of their formation. In laboratory experiments we find that the top layers of the ice mantle, which are made primarily of CO, transforms from a disordered phase to a polycrystalline phase at such a low temperature. During the phase transition, reactive species buried inside migrate and react without the need to overcome activation energy for diffusion. We predict that CO ice in interstellar clouds is mostly in the polycrystalline form. The reorganization of CO ice, which occurs below 10 K, may promote mobility of reactive species, and therefore can be a driving force of molecular complexity in molecular clouds.
Jiao He, Francis E. Toriello, Shahnewaz M. Emtiaz, Thomas Henning, and Gianfranco Vidali. Phase Transition of Interstellar CO Ice. ApJL, 915(1):L23, July 2021. Fulltext Link
Jiao He, Sándor Góbi, Gopi Ragupathy, György Tarczay, and Thomas Henning. Radical Recombination during the Phase Transition of Interstellar CO Ice. ApJL, 931(1):L1, May 2022. Fulltext Link
Explaining the non-detection of the 2152 cm-1 CO band
CO is one of the most abundant ice components on interstellar dust grains. When it is mixed with amorphous solid water (ASW) or located on its surface, an absorption band of CO at 2152 cm-1 is always present in laboratory measurements. This spectral feature is attributed to the interaction of CO with dangling-OH bonds (dOH) in ASW. However, this band is absent in observational spectra of interstellar ices. This raises the question of whether CO forms a relatively pure layer on top of ASW or is in close contact with ASW, but not via dangling bonds. We performed laboratory experiments to find out whether the incorporation of NH3 or CO2 into ASW blocks the dOH and therefore reduces the 2152 cm-1 band. We found that annealing the ice reduces the 2152 cm-1 band and that NH3 blocks the dOH on ASW surface and therefore reduces the 2152 cm-1 band more effectively than CO2.
Reference: Jiao He, Giulia Perotti, Shahnewaz M. Emtiaz, Francis E. Toriello, Adwin Boogert, Thomas
Henning, and Gianfranco Vidali. Ammonia, carbon dioxide, and the non-detection of the 2152
cm−1 CO band. A&A, 668:A76, December 2022. Fulltext link
Formation of interstellar molecules
In cold and dense molecular clouds, atoms and simple molecules condense onto submicron size dust grains and forms new molecular species. Many complex organic molecules (defined as carbon-containing molecules with at least 6 atoms) are formed on grain surface instead on in the gas phase. We use the two atomic/molecular beam lines to deposit reactants onto a dust grain analogue and then uses a combination of Fourier Transform Infrared spectrometry and temperature programmed desorption (TPD) techniques to identify and quantify the molecules that are formed on the grain surface.
Quantifying grain surface processes
The setup was originally designed for surface science studies. We utilize the techniques in surface science to study the various physical and chemical processes on interstellar dust grain surface. We mainly study the sticking, diffusion, and desorption on grain surfaces.
Sticking is the first step of grain surface chemistry. Atoms and molecules in the gas phase that collide with the grain surface can either be bounced back to the gas phase or stick to the surface and participate in the chemistry in the solid state. Quantifying the sticking coefficient of atoms and molecules on realistic grain surface is therefore important to the chemistry in the interstellar medium. We use the time-resolved surface scattering technique to accurately measure the sticking rate.
Atoms and molecules on the grain surface diffuse to react with each other and therefore form new chemical bonds. We measure the diffusion rate on the surface of amorphous silicate and amorphous water ice, which are two surfaces relevant to interstellar environment.
CO2 diffusion on non-porous amorphous solid water surface
The diffusion of molecules on interstellar grain surfaces is one of the most important driving forces for the molecular complexity in the interstellar medium. Due to the lack of laboratory measurements, astrochemical modeling of grain surface processes usually assumes a constant ratio between the diffusion energy barrier and the desorption energy. This over-simplification inevitably causes large uncertainty in model predictions. We present a new measurement of the diffusion of CO2 molecules on the surface of non-porous amorphous solid water (np-ASW), an analog of the ice mantle that covers cosmic dust grains. A small coverage of CO2 was deposited onto an np-ASW surface at 40 K, the subsequent warming of the ice activated the diffusion of CO2 molecules, and a transition from isolated CO2 to CO2 clusters was seen in the infrared spectra. To obtain the diffusion energy barrier and pre-exponential factor simultaneously, a set of isothermal experiments were carried out. The values for the diffusion energy barrier and pre-exponential factor were found to be 1300+-110 K and 107.6+-0.8 s-1.
Reference: Jiao He, Paula Caroline Pérez Rickert, Tushar Suhasaria, Orianne Sohier, Tia Bäcker, Dimitra
Demertzi, Gianfranco Vidali, and Thomas K. Henning. New measurement of the diffusion of carbon
dioxide on non-porous amorphous solid water. Molecular Physics, 0(0):e2176181, 2023. Fulltext link
Molecules on grain surface, either condensed from the gas phase or formed on the surface, can return to the gas phase by thermal desorption, reactive desorption (also called chemical desorption), or photo desorption. We focus on thermal desorption and reactive desorption. We quantify the desorption energy (or binding energy) distribution using temperature programmed desorption (TPD) technique. We also use the time-resolved reactive scattering technique to quantify the reactive desorption rate.
Infrared spectra of interstellar ices
Dust grains in dense clouds are typically covered by an ice mantle composed of H2O, CO2, CO, NH3, CH4, CH3OH and other minor components. The composition of the ice mantle is mostly determined by comparing the observed infrared absorption spectra in interstellar clouds with laboratory measurements of ices. We measure the infrared spectra of interstellar relevant ices under various physical conditions. Our measurements are useful in interpreting the observed spectra in space, and in characterization of the physical and chemical environments of the ice mantle.
Structure of ice mantle
The ice mantle covering dust grains provides a medium where chemical reactions occur. Because the rate of chemical reactions is typically much higher on surface than in the bulk ice, it is important to find out the porosity of the ice mantle on dust grains. So far, the structure of the ice mantle is under debate. We measure the infrared absorption spectra of ice mantle analogues to infer the structure of ice mantle.
Formation of complex organic molecules by energetic processing of ices
The ice mantle on dust grains is subject to bombardment of cosmic rays and UV photons. Complex organic molecules can be formed in the “energetic processing” of ices. We are currently in the construction of new UHV setup dedicated to the formation of complex organic molecules, particularly those relevant to the origin of life.