The MESSENGER spacecraft has been in orbit around Mercury since March 2011.
Magnetic field measurements taken by MESSENGER have yielded definitive evidence
for a weak dipolar internal field, generated by dynamo action in Mercury’s core.  The
weak field, together with Mercury’s proximity to the sun, mean that Mercury’s magnetosphere
is small and highly dynamic, responding to changes in solar wind conditions on time
scales of seconds to a Mercury year.   Why is Mercury’s field so weak?  What is the history
of the planet’s magnetic field?  How and when can solar wind plasma enter the magnetosphere
and even reach Mercury’s surface and what are the consequences?  In this talk I will
review results from MESSENGER that shed light on the planet’s magnetic field
and indicate that Mercury’s deep interior, surface and space environment are intimately coupled.