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Mars is about half the linear size of Earth. It therefore has about 1/4 the surface area and 1/8 the volume. The surface to volume ratio of Mars is twice what it is for Earth. Since internal heat is stored throughout the planet but radiated only through its surface, Mars must have cooled from its initial molten state more quickly than Earth. Thus, while Earth still has a molten component in its core, Mars is believed to have a solid core. This difference may be significant as regards the Martian magnetic field. Today, the Martian field is negligible. But, when Mars had a molten core, it may also have had a more substantial magnetic field - as Mars Global Surveyor results now seem to imply! A magnetic field is essential to life: the field acts as a shield, protecting the surface from lethal particle radiation from the sun and the galaxy. Mars has a day only slightly longer than a day on Earth, and a year twice as long. The axial tilt of Mars (not shown) is almost identical to that of Earth so that Mars has alternate seasons in its northern and southern hemispheres. Other comparative data are given for reference.
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