Modern refineries can break up the heavier molecules in crude oil, using heat and pressure in a "cracking" reactor.

They can also combine molecules of the lightest hydrocarbons into heavier molecules.

In addition, the heavy molecules can stick to the cylinder walls, leaving deposits that can retain heat.

Here it is possible to lose heavier molecules that would not normally be lost.

Even the natural level of 13 C abundance (1%) may allow weak bands due to'heavy' molecules to be observed.

The heavy molecules of the thick atmosphere were too fast-moving to be contained by the Moon's gravity.

This diffusion results in the heavier and lighter molecules separating.

The heavier molecules are pulled harder by gravity.

This means that light molecules will move a lot faster, on average, than heavy molecules at the same temperature.

Thus, heavy molecules are pumped much more efficiently than light molecules.