=== Magnetosphere ===
Mars has a very weak magnetic field. Data from various missions has established an upper limit of 2.1 * 10<sup>18</sup> N-m/T on the [[magnetic dipole moment]] of Mars. According to [[Russell Humphreys]]'s model of the creation of magnetic fields, Mars probably had a magnetic dipole moment ''at creation'' of 1.51 * 10<sup>23</sup> N-m/T. Thus Mars's magnetic field has been decaying very rapidly. This tremendous decay, and the presence on [[Mercury]] of a magnetic field of significant strength, baffles [[astronomy|astronomers]] who have assumed that magnetic fields form on rapidly moving planets that have conductive and liquid cores that can act as dynamos. In fact, Humphreys asserts that the characteristic that determines which of the [[Terrestrial Planet|terrestrial planets ]] will have a persistent magnetic field is the core radius, and that the cores of all four of these planets have similar conductivities. Mercury and Earth have larger cores than do Mars and Venus. The lengths of the sidereal days on those worlds do not matter.<ref name=Humphreys/>
=== Geology ===