Mars was well protected from the solar wind

Today Mars is a cold, icy and dry desert with a rarefied atmosphere. However, 3-4 billion years ago, there was a lot of liquid water on the planet and hydrological cycles operated, which indicate that the atmosphere was also denser, retaining moisture and providing heating due to the greenhouse effect. It is believed that the whole problem of Mars is in small dimensions and the lack of a molten iron-nickel core.
A greater attraction would better keep the particles of the atmosphere, and the global magnetic field created by the currents in the core would protect them from erosion under the action of the solar wind. Deprived of their Mars quickly lost the gas shell, and soon – and water. However, the new work of Swedish astronomers suggests a slightly different picture: it seems that the Red Planet is not so defenseless under the solar wind.
It is worth saying that solar radiation partially ionizes particles of the upper layers of the atmosphere of Mars. Ion currents, in turn, induce magnetic fields. It was assumed that these fields do not create any significant protection from the solar wind, but Robin Ramstad and his colleagues from the Swedish Institute of Space Physics have shown that this is not so.
Scientists used the data of the particle sensor ASPERA-3, which works on board the European probe Mars Express. The instrument recorded the flow of ions leaving the Martian atmosphere, and these observations were correlated with solar activity. It turned out that the level of ionizing ultraviolet radiation strongly influences the intensity of this flux, but the solar wind is practically nonexistent.
According to astronomers, this requires reviewing the possible rate of loss of Mars atmosphere in the direction of a sharp decline. Given the new data, it was unlikely to lose more than 0.01 bar of pressure, even for 3.9 billion years. However, today it totals 0.01 bar, and to create a greenhouse effect 3.9 billion years ago should have been at least 1 bar. So now I’ll have to explain how everything else has vanished.