Units
150 kilo miles?
Two new studies from a NASA Mars probe suggest the red planet's core is surrounded by a layer of molten rock, and the core is smaller than previously thought. Launched in 2016, the InSight lander's Seismic Experiment for Interior Structure (SEIS) project provided data in 2021 that suggested the presence of a large but low- …
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That is an excellent question and it appears that the convective currents in Mars' liquid core reduced and there are a number of theories* about how that happened.
The net result is that the planetary magnetosphere effectively died and that then allowed the strong solar winds to interact directly with the planet’s atmosphere and strip it away. Mars' real problem is that it only has 1/9th of the mass of Earth so that its core was always going to cool down more quickly and the rest is history. You would probably need a planet with at least 1/3rd of the mass of Earth for a planetary magnetosphere to develop and to be self sustaining for multiple billions of years.
*This is just one option - https://www.universetoday.com/154461/we-might-know-why-mars-lost-its-magnetic-field/