A quiet controversy has surfaced regarding the usefulness of the Russian-built and managed LEND instrument on-board the Lunar Reconnaissance Orbiter, now approaching its 12,300th orbit of the Moon.
Intended as a sharper-resolution follow-up to neutron flux detection performed by Lunar Prospector (1998-1999) a team of investigators representing four prestigious institutions are expressing doubts about the actual resolution of the LEND Collimated Sensors for EpiThermal Neutrons (CSETN).
|LRO instrument payload schematic. The LEND package|
is at bottom center [NASA].
“Serious questions have been raised concerning the effectiveness of the LEND CSETN for actually returning a sharper map of the lunar neutron flux,” the team writes in “What is LEND collimated detector really measuring?” a presentation prepared for the 43rd Lunar and Planetary Science Conference (2012), in March.
Cosmic rays of a wide range of energies rain in on the inner Solar System from beyond the Sun’s interplanetary magnetic field, at a more or less constant rate that varies by roughly 100 percent to a peak infall from all directions at solar cycle minima and dropping by half inversely to solar cycle max. As these energetic nuclei bombard the lunar surface the result is a predictable flux of freed neutrons scattered into space. As a detector in low lunar orbit passes overhead these newly space borne neutrons can be related to their origin on the lunar surface.
The presence of hydrogen, in the form of hydroxyl molecules, hydrogen gas or was water ice, in the upper half meter of the lunar surface will absorb or refract some of these neutrons. It was the confirmation of such an apparent detection by Lunar Prospector that gradually convinced many scientists of the otherwise thought unlikely presence of cold- trapped volatiles, perhaps in the form of water ice, near the Moon’s highest latitudes.
But the neutron detection array on-board the highly budget-restricted Lunar Prospector was of very low resolution, and though the detection of hydrogen by that instrument was mostly confined to the lunar poles its detection was not confined directly to permanently shadowed regions, the only place where water ice was believed possible.
The two complementary detectors of the Lunar Exploration Neutron Detector (LEND), flying on LRO, carried high hopes of a great improvement on the Lunar Prospector measurements. The work by Eke, Teodoro and Lawrence, et al., first appearing in Science last fall, “uses the Lunar Prospector results in combination with data from the LEND CSETN to demonstrate that less than 5 percent of the LEND CSETN counts come from the (detector’s actual) field of view.”
The abstract “What is the LEND collimated detector really measuring,” (LPSC 2012, #2211) can be reviewed HERE.
A preliminary letter responding to similar critical comments from Lawrence, et al., written by LEND team members I.G. Mitrofanov, et al., was published in Science, February 14, 2011. It can be read HERE.