Once and for All, Scientists Have Revealed What's at the Center of Mars

so it's mostly liquid iron, which begs the question of why it doesn't have a magnetic field. from the article:

Although Mars does not currently have a magnetic field, scientists hypothesize that there was once a magnetic shielding similar to Earth’s core-generated field due to traces of magnetism lingering in Mars’ crust. Lekic and Schmerr noted that this might mean that Mars gradually evolved to its current conditions, changing from a planet with a potentially habitable environment into an incredibly hostile one. Conditions in the interior play a key role in this evolution, as might violent impacts, according to the researchers.