MESSENGER Mercury Thermal Neutron Map (MEAP-4)

MEAP4 – Thermal Neutron Map Derived Analysis Product

Instrument: Gamma Ray Spectrometer

PDS Bundle: urn:nasa:pds:izenberg_pdart14_meapDOI: 10.17189/1518648

PDS Collection: urn:nasa:pds:izenberg_pdart14_meap:data_tnmap

For more information about MEAP4 products, see Peplowski et al., 2015, “Geochemical terranes of Mercury’s northern hemisphere as revealed by MESSENGER neutron measurements”, doi:10.1016/j.icarus.2015.02.002.

For more information about MEAP products, see the Messenger Advanced Product Specification.

The Gamma Ray Spectrometer (GRS) Thermal Neutron Map Derived Analysis Product (MEAP4) is the first map of variable abundances of thermal neutron absorbing elements on Mercury’s surface. This map uses GRS Anti-Coincidence Shield (ACS) data collected from 1 March 2013 to 28 February 2014 (CDR_CSC).

The GRS consisted of a high-purity Ge (HPGe) sensor surround by a BC454 anti-coincidence shield (ACS) for background reduction. The HPGe offered exceptional energy resolution, which was necessary to characterize many elements of interest on Mercury (e.g. Na, Cl). HPGe had to be operated at cryogenic temperatures, and as a result the GRS included a cryocooler to keep the sensor at <90 K. The ACS measured incident neutrons and charged particles, the latter providing a veto signal to reduce backgrounds in the HPGe resulting from galactic cosmic rays.

The GRS operated at peak performance from shortly after Messenger’s orbit insertion about Mercury (18 March 2011) until 11 October 2011. Data acquired after 11 October 2011 was subject to period degradation in HPGe energy resolution due to anomalous low-energy counts. The HPGe detector was operated intermittently in 2012 until the failure of the cryocooler on 15 June. The failure occurred after ~9,500 hours of operation, well past the estimated cooler lifetime of 8,000. The ACS was operated near continuously, and on 25 February 2013, a flight software update was uploaded to optimize ACS neutron measurements. ACS neutron measurements are complementary to those of the Messenger Neutron Spectrometer (NS).

Production of the thermal neutron absorption map was as follows:

  1. Raw ACS energy deposition spectra were fit and the neutron capture peak area was extracted.

  2. Peak areas were temperature corrected.

  3. Peak areas were normalized to a reference altitude (1000 km) altitude and vertical Doppler velocity (0) via comparison to modeled count rates. Peak areas were normalized to a reference galactic comic ray flux value, sampled via the NS triple coincidence count, to a value of 26.5 triples/sec.

  4. Peak areas were empirically corrected to a constant (nadir) viewing geometry.

The map is stored in two formats, IMG and JPEG2000 (JP2). The image file has 259200 elements, which corresponds to the original 720 x 360 degree map (0.5 degree wide pixels in east longitude, latitude). The elements are arranged rom the top (90 lat, -180 lon) across, then down (last entry is -90,180). Each element is a byte. Pixel values range from 0 to 255, converting them to physical units (10-4 cm2/g) requires multiplying the pixel value by 0.222860. Pixel values of zero are unmapped, and this corresponds to all latitudes less than (south) of 20 deg N.

The MEAP4 product is named THERMAL_NEUTRON_MAP.xxx, where xxx is either IMG or JP2.

MEAP4 PDS4 Logical Identifier:

urn:nasa:pds:izenberg_pdart14_meap:data_tnmap:thermal_neutron_map