The following text is for questions 67 to 71.

The Moon is the only natural satellite of the Earth. The Moon is the second brightest object in the Earth's sky after the Sun. The natural satellites of the other planets in the solar system are also sometimes referred to as moons.

    The Earth's Moon is now known to be a slightly egg-shaped ball composed mostly of rock and metal. It has no liquid water, virtually no atmosphere and is lifeless. The Moon shines by reflecting the light of the Sun. Although the Moon appears bright to the eye, it reflects on average only 12 percent of the light that falls on it. This reflectivity of 0.12 is similar to that of coal dust. This reflectivity is called albedo.

    The temperatures on most of the Moon's surface are too extreme for water or ice to exist, ranging from a maximum of 127°C (261°F) at lunar noon to a minimum of -173°C (-279°F) just before lunar dawn. Temperatures in permanently shadowed areas near the lunar poles, however, may consistently be as low as -220°C (-364°F). Comets and micrometeoroids that strike the Moon release gases that contain water. The gases would form an extremely thin atmosphere that would then migrate to the coldest regions of the poles and condense, forming ice that combines with the lunar soil. 

". . . then migrate to the coldest regions of the poles and condense . . . . " (Paragraph 3) 

What does the underlined word mean?