**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

#### **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.*

*Chiral Ice Crystals in Space DOI: http://dx.doi.org/10.5772/intechopen.106708*


#### **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.*

grains led to the formation of planets via planetesimals, and remnant planetesimals from this outer region are the comets we observe today.

#### **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 <sup>+</sup> 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.**

*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 <sup>10</sup><sup>6</sup> Pa (base pressure <sup>&</sup>lt; <sup>2</sup> <sup>10</sup><sup>7</sup> Pa), corresponding to the flux of <sup>2</sup> 1013 molecules cm<sup>2</sup> <sup>s</sup> 1 , and the deposited amount after 60 minutes deposition was estimated to be 1.5 <sup>10</sup><sup>17</sup> molecules cm<sup>2</sup> . 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.*

**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 <sup>10</sup><sup>13</sup> molecules cm<sup>2</sup> <sup>s</sup> 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<sup>1</sup> : 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<sup>1</sup> : 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<sup>1</sup> ) 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

*Chiral Ice Crystals in Space DOI: http://dx.doi.org/10.5772/intechopen.106708*


*a Hydrogen-ordered hexagonal ices called H12–H15 by Raza et al. [18]. <sup>b</sup>*

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

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

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

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

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

*g High-pressure ice, measured at 0.28 GPa.*

*h Two space groups have been proposed.*

**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.*

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.

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 nearand 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<sup>1</sup> [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 NH3H2O differ between 700 and 1100 cm<sup>1</sup> [16], and only a crystalline phase has been measured for NH32H2O 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.
