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Water activity (aw) is a measure of the partial vapor pressure of water within a substance. This vapor pressure controls how much water would be absorbed by a gas in contact with the substance, and more generally, to what degree the water might be extracted, e.g., for a living organism's functions, and the water activity of a world's surface material is an indicator of its habitability. The measure compares the substance's vapor pressure with that of liquid water, quantified as a fraction, "1" meaning they are the same, and fractions near one indicating a high water activity, conducive to life.
The concept is used in food science for the purposes of preserving food (e.g., preventing mold from occurring), and has been used in analysis of whether life is possible on Mars, and is similarly applicable to other astronomical bodies. A high water activity is consistent with harboring life. Water activity is affected by chemicals dissolved in the water (e.g., salts) and by temperature, i.e., changing the temperature changes the water activity, independent of other changes.
The vapor pressure within a solid as compared with the vapor pressure of a surrounding gas (humidity) determines whether the substance is absorbing water from the gas or water is drawn from the solid (or there is an exact balance). This is demonstrated in kitchen freezers by "freezer burn", frost forming on items due to the dryness of the cold air surrounding frozen food drawing water out of the food, much of which then freezes on the surface.