Bacteria living on the surface of the skin split these molecules into amino acids, which the bacteria consume, and a volatile compound known as thioalcohol.

Using this technique, deep learning systems can be trained to detect small amounts of volatile compounds with potentially wide applications in medicine, forensics, environmental analysis and others.

Chilling them significantly reduced the level of volatile compounds - chemicals that easily become gases and strongly affect smell.

The researchers found that the old-book smells had more than a dozen volatile compounds, ranging from the vinegary punch of acetic acid to the burntsugar and almond whiff of benzaldehyde.

Outgassing of volatile compounds through the first volcanoes would have contributed to the formation of the planets' atmospheres.