Thermoremanent magnetization (TRM) in hematite is larger than TRM in magnetite for grain sizes≥10 &mgr;m. We show that hematite's weak spontaneous magnetization Ms causes its strong TRM, since the self-demagnetizing field Hd opposing large domain wall displacements is proportional to Ms. In hematite, Hd is comparable to the Earth's magnetic field but in magnetite. Hd is 1000 times larger. As a result, Earth's field TRM of MD hematite (≈0.3 Am2/kg) outweighs TRM and induced magnetization of MD magnetite (≈0.01--0.02 Am2/kg) and rivals TRM of single-domain and PSD magnetite as a source of magnetic anomalies on Earth and perhaps on Mars. ¿ 2001 American Geophysical Union