There is no melting in Antarctica, the rate of snow accumulation is very low, so ice is formed for many years, and its age reaches hundreds of thousands of years. The snow is compressed very slowly and gradually turns into ice. At the final stage of this pressing, the air pores in the packed snow (firn) are closed - and air bubbles are obtained. The atmosphere in its unchanged form has been contained in them for thousands of years.
There are two ways to extract air from ice. The first method is melting: the ice is melted, and all gases come out. But there are gases that dissolve in water, like carbon dioxide CO2. The second method is dry extraction. In dry extraction, a piece of ice is ground in a mill into the smallest particles so that the air is released from the ice.
The air is then collected and transferred to the meter.
Once the atmosphere has been extracted, you can find out its gas composition — directly measure how much oxygen, nitrogen, carbon dioxide, argon, methane, and other gases were in the ancient air. The ratio of O2 and N2 (oxygen and nitrogen) is measured on a spectrometer, the volume of CO2 is measured on laser analyzers, and chromatographs are also used.
It is also possible to establish, for example, the air temperature in the past on the ice. This is studied by using ice isotopic analysis. Water molecules differ in the types of atoms they are made of - isotopes. There are "heavier" isotopes, and there are "lighter" ones, and scientists know that the concentration of heavy molecules in the air is directly related to the condensation temperature at which moisture is formed in the atmosphere. Isotope analysis allows you to determine how many such heavy isotopes are in the water and, therefore, what was the temperature when a particular layer of ice was formed.