Strictly speaking, all planets have tails -- they're just extremely diffuse (as Mars' is). Even the Moon has a tail. It's material being blown away by the solar wind.<br /><br />The Earth's ionosphere is not spherical at all -- it is blown into a long tail (called the magnetotail) stretching out beyond the orbit of the Moon! But it is much shallower on the sunward side of Earth. The Sun blows material away, basically. If we didn't have a magnetosphere to redirect the solar wind, there would probably be a lot less air on Earth, and consequently no liquid surface water. <div class="Discussion_UserSignature"> <p> </p><p><font color="#666699"><em>"People assume that time is a strict progression of cause to effect, but actually from a non-linear, non-subjective viewpoint it's more like a big ball of wibbly wobbly . . . timey wimey . . . stuff."</em> -- The Tenth Doctor, "Blink"</font></p> </div>