Mercury's magnetosphere is known to be affected by enhanced ram pressures and
magnetic fields inside interplanetary coronal mass ejections (ICMEs). Here we
report detailed observations of an ICME compressing Mercury's dayside
magnetosphere to the surface. A fast CME launched from the Sun on November 29
2013 impacted first MESSENGER, which was orbiting Mercury, on November 30 and
later STEREO-A near 1 AU on December 1. Following the ICME impact, MESSENGER
remained in the solar wind as the spacecraft traveled inwards and northwards
towards Mercury's surface until it reached and passed its closest approach to
the planet (at 371 km altitude) without crossing into the magnetosphere. The
magnetospheric crossing finally occurred 1 minute before reaching the planet's
nightside at 400 km altitude and 84$^\circ$N latitude, indicating the lack of
dayside magnetosphere on this orbit. In addition, the peak magnetic field
measured by MESSENGER at this time was 40% above the values measured in the
orbits just prior to and after the ICME, a consequence of the magnetospheric
compression. Using both a proxy method at Mercury and measurements at STEREO-A,
we show that the extremely high ram pressure associated with this ICME was more
than high enough to collapse Mercury's weak magnetosphere. As a consequence,
the ICME plasma likely interacted with Mercury's surface, evidenced by enhanced
sodium ions in the exosphere. The collapse of Mercury's dayside magnetosphere
has important implications for the habitability of close-in exoplanets around M
dwarf stars, as such events may significantly contribute to planetary
atmospheric loss in these systems.
