The close encounter between comet Siding Spring and Mars in 2014 flooded the planet with an invisible tide of charged particles from the comet's coma. The dense inner coma reached the surface of the planet, or nearly so. The comet's powerful magnetic field temporarily merged with, and overwhelmed, the planet's weak magnetic field, as shown in this artist's depiction.
NASA's Mars Atmosphere and Volatile Evolution (MAVEN) mission used the orbiter's magnetometer to monitor how the passing comet affected the magnetic field around Mars.
NASA Goddard Space Flight Center in Greenbelt, Maryland, manages the MAVEN project for NASA's Science Mission Directorate, Washington, and built the mission's magnetometer. MAVEN's principal investigator is based at the University of Colorado's Laboratory for Atmospheric and Space Physics in Boulder. Lockheed Martin Space Systems, Denver, built and operates the spacecraft.
For more information about NASA's Mars Exploration Program, see http://marsprogram.jpl.nasa.gov.