Mars is an attractive planet because many of the processes that happened on Earth, happened there as well. Moreover, every time one goes to a more detailed scale, Mars seems to be a new place. With NASA's MGS spacecraft, we have had a revolution: we now know that Mars has a magnetic field, and that Mars shows no sign of a core-source magnetic field. When the magnetic spectra of Earth and Mars are compared, it illustrates the dominance of the internal static magnetic field of lithospheric origin at Mars, in contrast to the dominant role played by the core field on the Earth.
The Martian magnetic field of lithospheric origin is one order of magnitude larger than the terrestrial lithospheric field, when directly measured at comparable altitudes above the planet (400 km). The most intense sources are observed in the Cimmeria region, where fields in excess of 1500 nT were observed near Mars periapsis at 100 km altitude. A deep analysis of the magnetic signature of the impact craters on Mars, Earth and Moon is an exciting topic for the planetary evolution comparison, topic in which I have been involved and interested.The global Martian field lasted for only the first few hundred million years of Mars' life, when temperature and heat flow conditions in the planet's molten core were favorable to churn it into a magnetic dynamo. Why did the Martian magnetic field disappear? What is the spatial coherence of the magnetization as a function of location or age? What factors explain the order of magnitude, or larger difference in the Terrestrial and Martian magnetic fields of lithospheric origin? Basic questions concerning the interaction of the solar wind with Mars' magnetic field can also be addressed, such as: What is the magnetic structure of the regional "magnetospheres"? And how is it affected by parameters such as solar zenith angle and solar wind conditions? Can any signatures of magnetic reconnection between the piled-up interplanetary magnetic field and the crustal field be detected? A way to find a more exhaustive answer to these questions is to get a better mapping of the Mars' magnetic field.
Although the Hermean magnetic field is thought to be a miniature version of Earth's, it is far from being accurately characterized. Its origin is likely intern, but there is still some debate about more exotic sources, such as remanent rocks in an inhomogeneous shell or induced magnetic field on a planetary scale or thermoelectric currents. On average, Mercury's field seems to be a dipole field, in contrast to the Moon and Mars which lack such a field field, but have local magnetic fields centred on different rock deposits.
MESSENGER and Bepi Colombo missions, carrying magnetometers, will characterize Mercury's magnetic field in detail from orbit, in order to determine its exact strength and how its strength varies with position and altitude.
Bepi Colombo will set off in 2016 on a journey lasting approximately 6 years. After arriving around Mercury it will also map the magnetic field, and then we should be able to give unambiguous proofs of the origin of the Hermean magnetic field. This would be the first time we will be able to demonstrate beyond the shadow of a doubt the existence of a liquid core on another terrestrial planet. If for the Mercury magnetic field a core origin is found, this will have other consequences, the first one being to estimate the size of the core. As Co-I in MERMAG payload, I have been involved in preparing this mission in order to achieve the proposed goals.