Though Mars lacks a global protective magnetic shield like that of the Earth, strong localized magnetic fields embedded in the crust appear to be a significant barrier to erosion of the atmosphere by the solar wind. This conclusion by a researcher at the University of California, Berkeley, emerges from a new map of the limits of the planet’s ionosphere obtained by the Mars Global Surveyor spacecraft, which was launched in 1996 and reached the planet 10 months later. The new data show that where localized surface magnetic fields are strong, the ionosphere reaches to a higher altitude, indicating that the solar wind is being kept at bay.