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Chiral Ice Crystals in Space

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Akira Kouchi, Takashi Shimonishi, Tomoya Yamazaki, Masashi Tsuge, Naoki Nakatani, Kenji Furuya, Hiromasa Niinomi, Yasuhiro Oba, Tetsuya Hama, Hiroyasu Katsuno, Naoki Watanabe and Yuki Kimura

Submitted: 16 July 2022 Reviewed: 22 July 2022 Published: 22 August 2022

DOI: 10.5772/intechopen.106708

Crystal Growth and Chirality IntechOpen
Crystal Growth and Chirality Technologies and Applications Edited by Riadh Marzouki

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Crystal Growth and Chirality - Technologies and Applications

Edited by Riadh Marzouki and Takashiro Akitsu

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Abstract

We observed the formation of CO, CH3OH, and H2O ices using a cryogenic transmission electron microscope, to determine if chiral ice crystals could form under the conditions of interstellar molecular clouds and young stellar objects (protoplanetary disks) and to clarify the crystalline structure of these ices. Our results suggest that the following ice crystals are chiral: crystalline CO (α-CO) formed on amorphous H2O (a-H2O) grains in a 10-K molecular cloud, crystalline CH3OH formed by the heating of amorphous CH3OH on a-H2O grains at 40–60 K in young stellar objects, and several polymorphs of hydrogen-ordered cubic ice crystals formed by the heating of a-H2O at 80–100 K and direct condensation at 120–140 K in protoplanetary disks. We also investigated candidates for other chiral ices using published data. We found that NH3 I and NH3·H2O I are chiral at low temperature and pressure conditions. If one-handed circularly polarized light is irradiated during the nucleation of these chiral ice crystals, homochiral crystals can be formed. These results have important implications for the origin of interstellar organic molecule homochirality.

Keywords

  • ice crystals
  • chirality
  • CO
  • H2O
  • CH3OH
  • NH3
  • interstellar molecular cloud
  • protoplanetary disk
  • circularly polarized light
  • asymmetric nucleation

1. Introduction

The origin of biomolecular homochirality is one of the most important mysteries of the origin of life. However, asymmetric adsorption and/or asymmetric synthesis on inorganic crystal surfaces is a possible candidate for chiral selection [1, 2, 3]. Quartz and cinnabar are regarded as chiral crystals, and it has been suggested that the surfaces of achiral crystals (e.g., gypsum, calcite, and alkali feldspar) can act as chiral faces [2, 4]. These minerals, as both chiral and achiral crystals, could be formed in evolved bodies, such as meteoritic parent bodies and terrestrial planets; however, it is implicitly considered that there were/are no chiral crystals in interstellar grains. Using transmission electron microscopy (TEM), we demonstrated that chiral crystalline CO (α-CO) would form on icy grains in interstellar molecular clouds [5]; therefore, α-CO in molecular clouds could be regarded as the first chiral crystal in space. To build on this finding, we searched for other chiral ices in space via further laboratory experiments and literature searches. We used the term “ice” to describe a solid at low temperatures (e.g., H2O, CO2, CO, NH3, CH3OH, and their hydrates).

In this chapter, after a brief explanation of icy grains in space in Section 2, we describe the crystal structures of each chiral ice in Section 3. In Section 4, a formation mechanism for homochiral ice crystals in space is discussed. In Section 5, we suggest further areas of study for the determination of the origin of the homochirality of organic molecules on icy grains in space.

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2. Icy grains in space

2.1 The evolution of icy grains during the formation of the solar system

The evolution of icy grains, from an interstellar molecular cloud to a solar system, is schematically illustrated in Figure 1. In 10-K interstellar molecular cloud, icy grains were composed of an amorphous silicate (a-silicate) core, an inner organic mantle, an outer icy mantle of amorphous H2O (a-H2O), and α-CO attached to a-H2O mantle [6]. The composition of ice differs among molecular clouds, as shown in Table 1. The molecular cloud collapsed by gravitational contraction to form protosolar nebulas, during which the icy grains were heated according to their heliocentric distance. In the inner region, the grains were completely sublimated. However, in the outer region, some grains survived. Subsequent cooling led to the formation of crystalline silicates in the inner region and H2O ice crystals in the Jovian region. The aggregation of these grains led to the formation of planets via planetesimals, and remnant planetesimals from this outer region are the comets we observe today.

Figure 1.

Evolution of icy grains in space (grain sizes < 1μm). The compositions of the grains are indicated by different colors: Blue: CO, pale blue: H2O, yellow: Organic matter, and brown: Silicate. Note that CH3OH, NH3, and minor components have been omitted for simplicity. Oval and polygonal grain forms are amorphous and crystalline, respectively. The density of the nebular gas, mainly composed of H2, is indicated by the intensity of the green filling. The red and blue arrows indicate heating and cooling, respectively.

MoleculeMCsMYSOsLYSOsComets
H2O100100100100
CO9–673–26(<3)–850.4–30
CO214–4311–2712–504–30
CH3OH(<1)–12(<3)–31(<1)–250.2–7
NH3<7∼73–100.2–1.4
CH4<31–31–110.4–1.6
H2COn.d.2–7∼60.11–1
HCOOH<2(<0.5)–6(<0.5)–40.06–0.14
NH4+4–139–344–25n.d.

Table 1.

Composition of ice in molecular clouds [7], young stellar objects [7], and comets [8] relative to H2O. Abbreviations: MC, molecular cloud; MYSO, massive young stellar object; LYSO, low-mass young stellar object; n.d., no data.

2.2 Infrared observation of ices

Information about the composition and crystallinity of icy grains can be gained from infrared (IR) astronomical observations. Table 1 lists the main components of icy grains observed in molecular clouds and young stellar objects [7], including comets [8]. The most abundant component for all the objects is H2O. The next most abundant components are CO and CO2, although the abundance of CO varies depending on the object. For all the objects, the abundance of CH3OH relative to H2O ranges from lower than the detection limit to ∼30%. Because CH3OH can be formed from CO via the H-atom addition reaction on icy grains [9], it is suggested that the amount of CH3OH reflects the evolutionary stage of objects. Although NH3 is not detected in molecular clouds, it is detected in young stellar objects, while considerable amounts of NH4+ are tentatively assigned to all the objects [7]. It should be noted that the composition of cometary ices is quantitatively consistent with that of interstellar ices, suggesting an interstellar origin for cometary ices [8]. Among the crystals of these abundant molecules, possible chiral crystal candidates are H2O, CO, CH3OH, NH3, and their hydrates, which will be discussed in the following section.

The comparison of astronomically observed and laboratory-measured IR spectra provides us with information on the crystallinity of ices, both amorphous and crystalline. H2O ice is easily identified because of the spectral feature of the OH stretching mode around 3 μm, which differs between amorphous and crystalline H2O ices [10]. The observed features of a molecular cloud (Elias 16) and a circumstellar envelope of an evolved star (OH231.8 + 4.2) could be fitted by a-H2O at 23 K and crystalline H2O ice at 77 K, respectively [11]. For a young stellar object (Orion BN), the observed feature could be fitted by a mixture of a-H2O at 23 and 77 K and crystalline H2O ice at 150 K [11]. These results, consistent with a theoretical study [12], are reflected in the crystallinity of the H2O ice depicted in Figure 1.

Figure 2 shows the IR spectra of a-CO and α-CO measured by us. The sample deposition was done at a very low temperature (6 K) with a slow deposition rate (2 × 1013 molecules cm−2 s−1), ensuring that the produced CO ice sample is amorphous [5]. The IR spectrum measured just after deposition shows an asymmetric feature with a peak near 2136 cm−1: the IR spectrum of a-CO. During warming up to 22 K, the band shape gradually changed. The IR spectrum measured at 22 K (Figure 2B) shows a rather symmetric feature with a peak near 2138 cm−1: the IR spectrum of α-CO. Recently, He et al. [13] reported the IR spectra of solid CO measured with a reflection-absorption IR spectrometry. They observed a very slight change in the peak position (∼1 cm−1) during warm-up and attributed this change to the phase transition from a-CO to α-CO. However, it should be noted that determination of crystallinity based on a reflection-absorption IR spectrometry measurement tends to be difficult and it is probable that their ice sample after deposition could be a mixture of a-CO and α-CO. Thus, we consider that the spectra shown in Figure 2 are the first IR spectra of “pure” a-CO and α-CO measured in a laboratory. It is expected that a comparison of these laboratory spectra with astronomical observations will be made in the near future, which will further the discussion of the crystallinity of solid CO in molecular clouds.

Figure 2.

Infrared spectra of solid CO. The CO gas was vapor deposited onto a Si(111) substrate at 6 K. The pressure of the chamber during deposition was 7.1 × 10−6 Pa (base pressure < 2 × 10−7 Pa), corresponding to the flux of 2 × 1013 molecules cm−2 s−1, and the deposited amount after 60 minutes deposition was estimated to be 1.5 × 1017 molecules cm−2. The deposited sample was warmed up stepwise with an increment of 2 K. The IR spectra were measured with a transmission configuration. The spectra measured at 6 K (a-CO) and 22 K (α-CO) are shown in (A) and (B), respectively.

The laboratory-measured spectra of amorphous and crystalline CH3OH phases differ [14]; however, because the astronomically observed spectra of the OH stretching modes of CH3OH overlap with those of H2O, it is difficult to obtain information about the crystallinity of CH3OH. Zanchet et al. [15] measured the near- and mid-IR spectra of amorphous and crystalline NH3 at 15 and 85 K, respectively, and found that both spectra were similar, except for a band around 1100 cm−1 [15], which demonstrates the difficulty of obtaining information on the crystallinity of NH3. At 83 K, the measured IR spectra of the amorphous and crystalline phases of NH3·H2O differ between 700 and 1100 cm−1 [16], and only a crystalline phase has been measured for NH3·2H2O at 100 K [17]. However, it is expected that a comparison of these laboratory spectra with astronomical observations will be made in the near future.

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3. The crystal structures of ices at low temperatures and pressures

Table 2 lists chiral ice crystal candidates in molecular clouds and protoplanetary disks, and the crystal structures of the respective species are described based on our observations and published data.

SpeciesPhaseSpace groupH-orderT (K)Method [Ref]
H2OXIhaP21OT [18]
XIhbP212121OT [18]
XIccP21OT [18]
XIcdP21212OT [18]
XIceP41212OT [18]
XIcfP41OT [19]
IIIgP41212D250N [20]
COαP213n/a14–30N [21], E [5]
CH3OHαP212121O15, 110N [22], E [23]
NH3IP213O77X [24], N [25]
NH3·H2OIP212121O∼110X [24], N [26]
NH3·2H2OIhP212121O100X [17]
P213PO4–174N [26, 27]
CH4IIhP213O12, 25I [28]
P213OT [29]
Fm3¯cPO24.5N [30]

Table 2.

Candidate chiral ice crystals in molecular clouds and protoplanetary disks. Abbreviations: O, order; D, disorder; PO, partial order; T, theory; N, neutron diffraction; X, X-ray diffraction; E, electron diffraction; I, far-infrared spectroscopy.

Hydrogen-ordered hexagonal ices called H12–H15 by Raza et al. [18].


Hydrogen-ordered hexagonal ices called H6 and H7 by Raza et al. [18].


Hydrogen-ordered cubic ices called C10 and C11 by Raza et al. [18].


Hydrogen-ordered cubic ice called C7 by Raza et al. [18].


Hydrogen-ordered cubic ice called C2 by Raza et al. [18].


Hydrogen-ordered cubic ices called d by Geiger et al. [19].


High-pressure ice, measured at 0.28 GPa.


Two space groups have been proposed.


3.1 CO

α-CO is a thermodynamically stable phase of solid CO at low temperatures [31]. We observed the morphology of CO deposited on a-H2O by TEM, as shown in Figure 3 [5]. The CO formed three-dimensional polyhedral crystals, and the diffraction pattern confirmed that the CO crystals were α-CO. It has long been debated whether the crystal structure of α-CO is an orientationally ordered phase (space group: P213) or a disordered one (space group: Pa3¯). Wang et al. [21] confirmed by neutron diffraction that α-CO belongs to P213, as proposed by Vegard [32]. Therefore, we concluded that α-CO in molecular clouds would be chiral.

Figure 3.

Transmission electron microscopy image of (A) crystalline CO (α-CO) deposited on amorphous H2O at 19 K, and (B) the electron diffraction pattern of α-CO on amorphous Si at 19 K. White scale bar = 500 nm.

3.2 H2O

3.2.1 Ice Ih, Ic, and XI

Hydrogen-disordered hexagonal ice (ice Ih) is a thermodynamically stable phase of solid H2O under low pressures at temperatures >72 K [33]. At temperatures <72 K, hydrogen-ordered ice XI becomes the thermodynamically stable phase [34, 35]. It is widely accepted that doping (e.g., KOH) is essential for the formation of ice XI at low temperatures [36]. A thermodynamically metastable phase of hydrogen-disordered cubic ice (ice Ic) also exists at temperatures between 100 and 200 K [33]. The crystal structures of ice Ih and ice Ic are very similar except for the stacking sequences of their layers: ice Ih is ABABAB and ice Ic is ABCABC. The space groups of ice Ih, ice XI, and ice Ic are P63/mmc, Cmc21, and Fd3¯m, respectively, and all these crystals are not chiral.

3.2.2 Hydrogen-ordered cubic ice

Although the existence of hydrogen-ordered cubic ice (ice XIc) has been discussed theoretically [18, 19, 37, 38, 39], there has been no experimental evidence for this crystal structure. Raza et al. [18] and Geiger et al. [19] suggested 11 and 4 different structures, respectively, for ice XIc. We observed the annealing of ice Ic deposited on an a-SiN thin film by TEM and found that several polymorphs of ice were formed at temperatures between 100 and 130 K without doping [40]. Figure 4 represents the TEM images and corresponding electron diffraction patterns of the ice XIc formed by the annealing of a-H2O and ice Ic, showing the formation of ice XIc. However, we could not determine which structures were formed in terms of the different structures proposed by Raza et al. [18] and Geiger et al. [19]. However, because five-twelfths of the proposed structures were chiral (space group: P41212, P21212, P21, and P41), some chiral crystals might be included not only in our samples but also in H2O ice crystals in space. Furthermore, the calculation of the infrared spectra of ice XIc, as demonstrated by Geiger et al. [19], will help identify ice XIc in space.

Figure 4.

Transmission electron microscopy images and corresponding electron diffraction patterns of hydrogen-ordered cubic ice (ice XIc) formed by (A) the annealing of amorphous H2O at 130 K and (B) the annealing of ice Ic at 130 K. The blue and yellow arrowheads indicate the diffraction spots of d = 4.50 Å and d = 6.41 Å, respectively, originating from ice XIc. The white and yellow scale bars are 500 nm and 2 nm−1, respectively.

3.2.3 Hydrogen-ordered hexagonal ice

As mentioned in Section 3.2.1, the thermodynamically stable phase of hydrogen-ordered hexagonal ice is ice XI. However, Raza et al. [18] proposed 15 different structures of hydrogen-ordered hexagonal ice (XIh) as metastable phases, and seven-fifteenths of the proposed structures are chiral (space group: P212121, P21, and P1). Although there has been no experimental investigation of this, we should consider the occurrence of these polymorphs, as demonstrated in the Section 3.2.2.

3.2.4 Ice III

Here, it is worthwhile commenting on the structure of ice III, although ice III is stable only at higher pressures between 210 and 344 MPa and higher temperatures between 238 and 256 K [33]. The space group of ice III is P41212, meaning that the arrangement of the oxygen atoms is ordered and chiral. Conversely, the arrangement of the hydrogen atoms is disordered [20]. Therefore, we concluded that the surface of ice III does not behave as an asymmetric catalyst on which asymmetric adsorption and/or asymmetric synthesis can proceed.

3.3 CH3OH

α-CH3OH is a thermodynamically stable phase of solid CH3OH at temperatures < 157 K under low pressure [41]. Torrie et al. [22] showed by neutron diffraction that α-CH3OH is chiral, including the positions of the hydrogen atoms (space group: P212121). Furuya et al. [23] observed the deposition of CH3OH on a-H2O using TEM between 90 and 120 K and found that α-CH3OH was formed at temperatures >100 K; however, they did not observe the crystallization of amorphous CH3OH (a-CH3OH). Using TEM, we observed the formation of α-CH3OH crystals during the warming of a-CH3OH deposited on a-H2O (Figure 5). Therefore, we concluded that if α-CH3OH crystals were formed by the heating of a-CH3OH on a-H2O grains in young stellar objects (i.e., protoplanetary disks), α-CH3OH were/are chiral.

Figure 5.

Transmission electron microscopy image and corresponding electron diffraction pattern of (A) amorphous CH3OH (a-CH3OH) deposited on amorphous H2O (a-H2O) at 82 K and (B) those of crystalline CH3OH (α-CH3OH) at 110 K formed during the heating of a-CH3OH.

Sugisaki et al. [42] observed by calorimetry that the glass transition and crystallization of glassy CH3OH occurred at about 103 and 105 K, respectively. Luna et al. [14] also observed by IR spectroscopy that the crystallization temperature of a-CH3OH is between 100 and 110 K.

3.4 NH3 and its hydrates

3.4.1 NH3

NH3 I is a thermodynamically stable phase of solid NH3 at temperatures <200 K under low pressure [43]. Olovsson and Templeton [24] and Reed and Harris [25] showed by X-ray diffraction and neutron diffraction, respectively, that NH3 I at 77 K is chiral, including the positions of the hydrogen atoms (space group: P213). Hewat and Riekel [44] also confirmed this by high-accuracy neutron diffraction at temperatures between 2 and 180 K. It has been observed by IR spectroscopy that the crystallization temperature of a-NH3 is ∼80 K [44, 45].

3.4.2 NH3·H2O

NH3·H2O I is a thermodynamically stable phase of solid NH3·H2O at temperatures <194 K under low pressure [46]. Olovsson and Templeton [24] showed by X-ray diffraction that NH3·H2O I at 113 K is chiral, including the positions of the hydrogen atoms (space group: P212121). Loveday and Nelmes [26] confirmed the space group by neutron diffraction at 110 K.

3.4.3 NH3·2H2O

NH3·2H2O I is a thermodynamically stable phase of solid NH3·2H2O at temperatures <176 K under low pressure [47]. Bertie and Shehata [17] showed by X-ray diffraction and IR spectroscopy that NH3·2H2O I at 100 K is chiral, including the positions of the hydrogen atoms (space group: P212121). However, neutron diffraction studies showed that the space group is P213 and the hydrogen is partially ordered at temperatures between 4 and 174 K [26, 27]. Among the four hydrogen sites, the hydrogen occupancy of two of the sites was unity (order), while those of the other two sites were one-third and two-thirds. They considered that the transition to the ordered phase is frustrated by kinetics, as in the transition of pure ice Ih to ice XI. Fortes et al. [27] suggested the occurrence of a hydrogen-ordered phase (space group: P212121) at temperatures <140 K because the ordered phase must be thermodynamically more stable than the disordered phase at low temperatures. We therefore suggested the occurrence of a hydrogen-ordered phase (space group: P212121) at lower temperatures in space, although the equilibrium structure at low temperatures still remains unclear. Further studies on the formation of the ordered phase using a dopant should be undertaken.

3.5 CH4

CH4 II is a thermodynamically stable phase of solid CH4 at temperatures <20.4 K under low pressure [48]. Savoie and Fourier [28] suggested that the CH4 II space group is P213 based on the measurement of far-IR spectra at 12 and 25 K. Hashimoto et al. [29] also suggested the same space group based on the calculation of pair-interaction potentials. Press [30] showed by neutron diffraction at 24.5 K that six of the eight molecules were ordered while the remaining two were orientationally disordered, with a space group of Fm3¯c. Kobashi et al. [49], based on IR and Raman spectra, suggested that, theoretically, the space group of CH4 II is Fm3¯c. Greiger et al. [50] analyzed the total neutron cross section assuming the structure proposed by Press [30]. Although recent studies have only referred to Fm3¯c, these results do not rule out the existence of the space group P213. Therefore, we cannot eliminate the possibility that both space groups exist.

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4. The formation of homochiral ice crystals

4.1 Sources of circularly polarized light

One-handed circularly polarized light (CPL) from an astronomical source could play an essential role in the homochirality of ice crystals. Neutron stars have been suggested as possible sources of CPL [51]; however, CPL at visible and UV wavelengths has not been observed [52], and it is unlikely that a neutron star could encounter a molecular cloud where our solar system was born [53]. In contrast, CPL produced in star-forming regions is considered to be more important because CPL has been observed [54], and the possibility of a star-forming region and a molecular cloud occurring together is very large. Therefore, our discussion of the homochirality of ice crystals assumes that the CPL originated in a star-forming region.

4.2 CPL flux in a molecular cloud

We estimated the CPL photon flux in a molecular cloud based on a simplified model. We assumed two cases: i) a molecular cloud illuminated by the interstellar radiation field and ii) a molecular cloud illuminated by radiation from a nearby massive star. Case i) assumed an isolated star formation, while case ii) assumed a clustered star formation in a massive star-forming region. In both cases, we assumed a 0.1 pc diameter molecular cloud with a hydrogen density of 2 x 105 cm−3. We used Weingartner and Draine’s [55] standard dust extinction curve with an RV parameter of 5.5 to mimic dust in dense clouds [56].

For i), a standard interstellar radiation field model [57] was assumed for the incident radiation spectrum. For (ii), the incident radiation field was simulated by blackbody radiation from a B3-type star (mass = 8 solar mass, luminosity = 2.8 x 103 solar luminosity, and surface effective temperature = 2.3 x 104 K), which was located 0.1 pc away from the molecular cloud. IR observations have indicated that circularly polarized IR emissions with a degree of circular polarization of up to 20% extend in a 0.1–0.7 pc area in high−/intermediate-mass star-forming regions [58, 59].

We assumed that the CPL was generated within the molecular cloud by the dichroic extinction of incident radiation [60]. A theoretical study predicted that dichroic extinction can produce a degree of circular polarization of up ∼10% in star-forming clouds [61]. Here, we assumed that the radiation penetrating the molecular cloud resulted in a 10% degree of circular polarization.

The estimated flux of the CPL in the molecular cloud is summarized in Figure 6. On the surface of the molecular cloud, the photon flux reflects sources of radiation and does not change with wavelength. At the middle points (r = 0.025 pc), however, the photon flux decreases with decreasing wavelength. The intensities of the photon fluxes at 200 nm in the cases of i) and ii) were ∼ 10−1 and ∼ 103 photons cm−2 s−1, respectively, suggesting that the photon flux of case i) was too weak for a photochemical reaction but that of case ii) was effective.

Figure 6.

The wavelength dependence of the flux of circularly polarized light in the molecular cloud. For incident radiation sources, (A) and (B) assume an interstellar radiation field and blackbody radiation from a massive star, respectively. The respective colored marks represent the photon fluxes integrated over the following wavelength regions: Cyan, 90–150 nm; blue, 150–250 nm; green, 400–700 nm; magenta, 800–1200 nm; and red, 1000–2600 nm.

We noted that cosmic-ray-induced UV (CRUV) is a dominant source of UV photons in well-shielded regions [62, 63]. The total photon flux of CRUV is estimated to be 104 photons cm−2 s−1 [64], which is orders of magnitude higher than the estimated photon fluxes at the middle and core points in cases i) and ii). However, because CRUV photons are produced in dense regions, they would be irradiated to icy grains before experiencing dichroic extinction. Thus, we did not consider CRUV to be a source of CPL. If circularly polarized UV light plays an important role in the production of enantiomeric excess, then relevant photo processing would occur on the shallow molecular cloud surface, where the external UV overwhelms the CRUV. Because the volume fraction of the middle part of the molecular cloud is ∼0.88, we discuss the asymmetric nucleation of ice crystals using a curve at the middle points (r = 0.025 pc) in the following section.

4.3 Asymmetric nucleation by one-handed CPL

Solid CO is formed via CO deposition from the vapor phase in molecular clouds. The crystallinity of solid CO, either amorphous or crystalline, can be determined by the CO flux in a molecular cloud [5]. Because the CO flux in a molecular cloud is much smaller than the critical flux in which amorphous CO (a-CO) is formed, α-CO should be formed. When one-handed CPL is irradiated during the nucleation of α-CO, the formed crystals might have an enantiomeric excess (Figure 7). When there are no metal or high-index nanoparticles on icy grains, α-CO can absorb UV-CPL, which may result in excess enantiomeric crystals. In this case, the formation of one-handed α-CO would only occur in the shallow part of the molecular cloud because the UV-ray penetration depth is not so large (see Figure 6). However, when there are metal or high-index nanoparticles on the icy grains, the peak absorption wavelength could be transferred to the visible wavelength region, and the peak could be enhanced compared to that of the UV region [65, 66, 67], resulting in excess enantiomeric crystals, possibly up to several tens of percent. This could be supported by laboratory experiments on chiral crystallization [68, 69] and theoretical work [70, 71, 72]. In this case, the formation of one-handed α-CO would occur not only in shallow parts of a molecular cloud but also in deeper parts because the penetration depth of visible rays is considerably deep (see Figure 6).

Figure 7.

Schematic illustration of the formation of homochiral CO crystals on the icy grains in molecular clouds.

A similar process might occur during the crystallization of a-H2O to form chiral ice crystals and hydrogen-ordered cubic and hexagonal ices (see Table 2) in protosolar nebula, as shown in Figure 1. The crystallization temperature of a-H2O under a 105-years’ timescale is ∼90 K [12]. The penetration depth of UV to visible CPL in protoplanetary disks is smaller than that in molecular clouds. However, icy grains could be moved to the surface of the disk by turbulent motion [73] and irradiated with CPL, resulting in the formation of one-handed, hydrogen-ordered H2O crystals. When ice crystals were recondensed during the cooling of the solar nebula (Figure 1), one-handed, hydrogen-ordered H2O crystals might be formed by the same mechanism.

The crystallization of a-CH3OH and a-NH3 also occurred in the protoplanetary disk. Crystallization temperatures of a-CH3OH and a-NH3 under a 105-years’ timescale can be estimated from those in the laboratory (a-CH3OH: ∼100 K [14, 42] and a-NH3: ∼80 K [15, 45]) and from the assumption that the slopes of a-CH3OH and a-NH3 in the plot of the timescale of crystallization vs. the inverse of the temperature lie between those of H2O [12] and CO2 [6]. We found that the crystallization temperatures of a-CH3OH and a-NH3 under the 105-years’ timescale were 40–60 K and 20–40 K, respectively. The formation of one-handed α-CH3OH, NH3 I, and NH3 hydrates might also occur, as in the crystallization of a-H2O. In this way, various kinds of homochiral ice crystals could be formed in protoplanetary disks.

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5. Conclusion and outlook

The results of this study indicate the possibility that there were/are many chiral ice crystals in space and that homochiral ice crystals might form by the irradiation of CPL in the star-forming region. These findings have important implications for the origin of the homochirality of organic molecules in space, and the pursuit of the following three suggested areas of study would further our understanding of this.

The crystallinity of CH3OH and NH3 in space and the formation mechanism of α-CH3OH, NH3 I, and their hydrates in protoplanetary disks are still unclear. Therefore, astronomical observations of the crystallinity of these ices are highly desirable.

For chemical reactions on icy grains, only a-H2O ice has been considered as a substrate. The adsorption and subsequent surface diffusion of atoms (H, C, N, and O), small molecules (e.g., CO, CO2, and H2CO), and radicals (e.g., OH, HCO, and NH), followed by surface two-body reactions to form larger molecules on a-H2O at low temperatures have been calculated using astrochemical reaction network models [74]. However, this study indicated the possibility of the growth of single ice crystals on grains. On the surface of α-CO, the adsorption behavior of atoms differs greatly from that on a-H2O [6]. Therefore, it is expected that atoms, except for C and small molecules/radicals, are not adsorbed on the surface of singe-crystalline H2O ice I. Instead, larger molecules/radicals diffuse easily on the surface of singe-crystalline H2O ice I, which leads to the formation of more complex organic molecules. Furthermore, the asymmetric adsorption/synthesis of organic molecules on homochiral ice crystals might also proceed.

The search for enantiomeric surfaces on achiral ice crystals, as investigated in minerals [2, 4], is another important subject that should be explored.

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Acknowledgments

Part of this work was supported by a Grant-in-Aid for Scientific Research from the Japanese Ministry of Education, Culture, Sports, Science, and Technology and from the Japan Society for the Promotion of Science.

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Conflict of interest

The authors declare no competing financial interests.

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Written By

Akira Kouchi, Takashi Shimonishi, Tomoya Yamazaki, Masashi Tsuge, Naoki Nakatani, Kenji Furuya, Hiromasa Niinomi, Yasuhiro Oba, Tetsuya Hama, Hiroyasu Katsuno, Naoki Watanabe and Yuki Kimura

Submitted: 16 July 2022 Reviewed: 22 July 2022 Published: 22 August 2022

© 2022 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution 3.0 License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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