Читать книгу Introduction to Nanoscience and Nanotechnology - Chris Binns - Страница 38

Nanoparticles themselves do not stop at the top of the Earth's atmosphere and cosmic particles (referred to as “dust” by astronomers) are spread throughout space from a number of sources. Supernovae (Figure 2.1e) have already been mentioned but others include outflowing material from carbon‐rich stars, which is rich in silicon carbide and titanium carbide particles [21] as well as various forms of pure carbon particles including fullerenes (see Chapter 3). As with the particle populations measured in the Earth's atmosphere, when measured as the number density or surface area, it is nanoparticles (<100 nm) that dominate the distribution (Figure 2.2). Thus nanoparticles provide a significant proportion of the solid surface area in space on which chemical reactions can take place.

Dust particles accelerate the process of condensation of gas clouds by gravity to form stars and planets thus nanoparticles were an important ingredient in the initial formation of our own sun and its planets, including the Earth. It is interesting to note that the special behavior of nanoparticles compared to the bulk matter discussed in Chapter 1 is also important in this context. For example, a significant fraction of particles produced by supernova explosions contain iron (from the core of the exploding star) and are magnetic. The magnetic interaction between the particles in space, which is orders of magnitude stronger than their gravitational attraction, can significantly accelerate the process of condensation and for this to work the particles must be single‐domains, that is, permanently magnetized. As discussed in the previous chapter this requires that they are smaller than a critical size of about 100 nm. Once stars and planets are formed they produce interplanetary particles by various processes. For example, in our own solar system the Jovian satellite Io, which it has a very high volcanic activity sprays vast quantities of particles into the rest of the solar system [22].

Comets are thought to preserve the pristine dust that was present during the formation of the solar system and this dust along with water is shed in the comet's tail as it is warmed during its closest encounter with the sun. In 2004, the European Space Agency's Rosetta spacecraft was launched and 12 years later was installed in orbit around the comet 67P/Churyumov‐Gerasimenko, from where spectacular images of the 4 km wide nucleus like the one in Figure 2.14a were obtained. The orbiter collected pieces of dust from the plumes emerging from the comet on sticky targets and it was the first space mission that had an atomic force microscope (see Chapter 5, Section 5.4.4) onboard that could image the grains with nanometre resolution [23]. Typical dust grains had sizes of a few μm but closer inspection showed that these particles were compact agglomerates of smaller grains as shown in Figure 2.14b. The image displays a single dust grain about 1 μm across but it is composed of at least seven smaller particles smaller than 500 nm across and there is evidence that these are composed of yet smaller particles. This is consistent with current models of protoplanetary growth where the dust is composed of a hierarchy of aggregates with the smallest units being nanoparticles.

Figure 2.14 Rosetta mission to comet 67P/Churyumov‐Gerasimenko. (a) Photo of the 4 km wide comet nucleus taken by the Navcam aboard the ESA Rosetta mission to comet 67P/Churyumov‐Gerasimenko from a distance of 28.6 km.

Source: ESA/Rosetta/NAVCAM/CC BY‐SA IGO 3.0. Reproduced under Creative Commons CC BY‐SA 3.0 license.

(b) Atomic force microscope (AFM) image of a single ~1 μm dust particle from the comet showing that it is composed of smaller particles.

Source: Reproduced with the permission of the Nature Publishing Group from [23].