Three billion years ago, liquid water existed on Mars as well as on Earth. Today, this is only the case on our home planet. Why is that? Mars no longer has such a strong magnetic field as the Earth. Therefore the solar wind can take the atmosphere better there than here. The magnetic field is generated in the outer core of the Earth, where liquid iron rotates (this is called a "geodynamo"). About 565 million years ago, however, the strength of the magnetic field decreased to 10 percent of its present strength. Then the field mysteriously recovered and regained strength…