First Measurements of Neutrons at Venus Using the MESSENGER Neutron Spectrometer

The MESSENGER Neutron Spectrometer (NS) is part of the Gamma-Ray and Neutron Spectrometer (GRNS) instrument on board the MESSENGER spacecraft. The measurement goals of the NS are to identify and measure the abundance of hydrogen at high northern latitudes on Mercury, measure composition-sensitive neutron-flux intensities related to iron and titanium abundances on Mercury's surface, and provide neutron flux measurements that will be used for analyzing data from the GRNS Gamma-Ray Spectrometer sensor. The NS uses lithium glass and borated plastic scintillators to measure thermal, epithermal, and fast neutrons. The MESSENGER spacecraft executed a gravity assist flyby of Venus on 5 June 2007 during which time the NS was turned on. Strong cosmic ray-induced neutron signals from the Venus atmosphere in all energy ranges covered by NS were clearly observed. This is the first time that neutrons have been observed from Venus as well as the first planetary neutrons seen with the NS. Since these neutrons originate from the Venus atmosphere, there is no new information about Venus surface composition in these data. However, because these neutrons come from a source with a known composition, they provide ideal calibration data that will be needed to understand future neutron observations at Mercury.