Ch

~ S

Hydrogen

H2

~ 1

Helium

He

A particular form of pyroxene, (Ca, Fe, Mg)2Si2O6. A particular form of olivine, (Mg, Fe)2SiO4.

X can be a molecule of a variety of minerals, and n is greater than or equal to 1.

A particular form of pyroxene, (Ca, Fe, Mg)2Si2O6. A particular form of olivine, (Mg, Fe)2SiO4.

X can be a molecule of a variety of minerals, and n is greater than or equal to 1.

The disc temperatures are generally lower the further we are from the Sun. Therefore as the disc cools a substance condenses rather in the manner of a wave spreading inwards to some minimum distance within which the temperature is always too high.

In the innermost part of the disc the temperatures are probably always too high at the dust condensation stage for anything much less refractory than iron-nickel to condense. At greater distances less refractory dust components appear, including an important range of substances exemplified in Table 2.3 by diopside, forsterite, and alkali feldspars. These are examples of silicates. A silicate is a chemical compound that has a basic unit consisting of atoms of one or more metallic elements and atoms of the abundant elements silicon and oxygen. For example, olivine has the chemical formula (Fe, Mg)2SiO4. Therefore, in the basic unit there is one atom of silicon (Si), four of oxygen (O), and two atoms of iron or magnesium - either two iron atoms or two magnesium atoms, or one of each. A particle of dust consists of very many units, and so the proportion of iron to magnesium in the particle as a whole can be anywhere in the range 0-100%. The particular version of olivine in Table 2.3 (forsterite) has no iron at all. Particular versions are called minerals, naturally occurring substances with a basic unit that has a particular chemical composition and structure. Olivine is thus the name for a range of closely related minerals. The whole family of silicates cover a wide range of compositions.

Silicates are by far the most common refractory substances in the Solar System after iron-nickel, and they are common in rocks, a rock being an assemblage of one or more minerals in solid form. Together with iron-nickel, silicates account for most of the common rocky materials. Note that this is the name of a group of refractory substances and not an implication that they are solid.

In the disc, an extremely important boundary is the distance beyond which water condenses. This is important because there is a considerable mass of water in the disc, and where it condenses it becomes the dominant constituent of the dust grains. Water must have been abundant because oxygen, among the heavy elements, is particularly abundant (Table 1.5), and in a hydrogen-rich gas, at all but very high temperatures, most of the oxygen combines with hydrogen to form water molecules, H2O. Figure 2.7 shows one model of the column mass of the disc versus distance from the proto-Sun at a time well into the dust condensation stage, when the disc probably resembled Figure 2.3(c). The column mass is the total mass in a cylinder of unit cross-sectional area with its axis running perpendicular to the plane of the disc. The increase in column mass at about 5 AU from the proto-Sun is due to the condensation of water beyond this distance. This distance is sometimes called the ice line. Note that the values in Figure 2.7 are illustrative, and not definitive. This applies to the column masses and also to the location of the ice line - in recent models this is around 4 AU.

Though water as H2O condenses beyond 4-5 AU, the dust closer in is not devoid of water: hydrated minerals (Table 2.3) have higher condensation temperatures than water. These are substances that have one or more water molecules attached to their basic unit, or one or more hydroxyl molecules (OH) which are a fragment of the water molecule. Water is one of a group of substances called icy materials. As in the case of rocky materials this is the name of a group of volatile substances with no implication that they are present as solids. The solid form is called an ice. Other important icy materials include ammonia (NH3) and nitrogen (N2), which are shown in Table 2.3 in their hydrated forms, and methane (CH4), shown in hydrated and non-hydrated forms. Carbon monoxide (CO) and carbon dioxide (CO2) are also important icy

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