Download REQUIRED PERFORMANCES FOR FUTURE LUNAR AND

Annual Meeting of the Lunar Exploration Analysis Group (2016)
5028.pdf
REQUIRED PERFORMANCES FOR FUTURE LUNAR AND ASTEROID NEUTRON SPECTROSCOPY.
K. Ogasawara1, B. Ehresmann1, K. D. Retherford1, K. E. Mandt1, S. A. Livi1, N. Schwadron2, P. Bloser2, J. S. Legere2, M. McConnell2, T. P. McClanahan3, and T. Okada4, 1Southwest Research Institute, 2University of New Hampshire at Durham, 3Goddard Space Flight Center, National Aeronautics and Space Administration, 4Institute of Space
and Astronautical Sciance, Japan Aerospace Exploration Agency.
Instruments using neutron spectrometry have made
significant contributions to planetary science through
the detection of volatiles (including H2O) [1-3], and by
constraining the mechanisms of planetary formation
and surface magmatic processes through the detection
of other neutron-absorbing elements (e.g., iron, titanium, gadolinium, samarium) [4]. However, the spatial
resolution of the neutron measurement technique is
currently quite coarse due to the use of omnidirectional neutron sensors without the usage of bulky
collimators. Omni-directional techniques rely only on
the geometrical cut off of the sensor FOV, restricting
us to the FWHM spatial resolution equal to (or greater
than) the altitude above the target planet/small body
surface. As a consequence, many important science
questions related to the distribution of near-surface
compositions cannot be addressed with currently operated instruments.
Volatiles on the Moon are of great scientific and
exploration interest, particularly the spatial distribution
of hydrogen-bearing minerals, which indicates the potential presence of water. Reduced epithermal neutron
fluxes near the poles have provided compelling evidence for the presence of water [5,6], but the spatial
resolution of the Lunar Prospector Neutron Spectrometer (LPNS) experiment was too coarse to directly determine whether hydrogen enhancements are limited to
PSRs or to the polar regions in general [5, 7, 8]. Updated hydrogen maps by the collimated Lunar Exploration Neutron Detector (LEND) on Lunar Reconnaissance Orbiter (LRO) [6] show that some areas of enhanced hydrogen do not correlate either with permanent shadow or temperature, and the disagreement between the two sets of observations remains hotly debated [9-12].
Neutron instruments can also map volatile content
in the asteroids and small bodies remotely, which is
crucial to identify the type of asteroids. Especially,
Martian moons Phobos and Deimos have gotten a lot
of attention lately, due to their enigmatic origins [13],
and may provide a new aspect of the evolution of the
inner planets in terms of the transportation of water.
The majority of these bodies are irregularly shaped and
small [14, 15]. Thus the irregular mass distribution,
solar radiation pressure, exospheric drag, and gravitational field can perturb the trajectory of the spacecraft
in close proximity to a small body [16]. All of these
factors make orbits less than a few kilometers very
difficult, and consequently, omnidirectional neutron
detectors are unlikely to spatially resolve a small body.
Figure 1 estimates neutron instrument angular resolution as a function of distance from the target body.
The resolution of an omnidirectional neutron detector
is shown as a solid black line. The shaded regions highlight the orbital range for Rosetta and LRO and the
required spatial resolutions for a typical target for neutron spectrometers. For example, in a comet mission
case, the size of the nuclei (~3 km) is a key scale range
to resolve. In the case of Moon orbiting missions, the
crater size (~30 km) is a typical key requirement to
resolve the PSR. Omni-directional measurement cannot
resolve the required scales for these cases.
In this presentation, we will discuss these issues
and possible solutions applicable to neutron sensors.
Figure 1: Omnidirectional neutron resolution as a
function of distance. The yellow area shows the required range to resolve 30 km craters on the Moon,
assuming LRO orbit. The green area is the required
range to resolve the radius of the comet nuclei assuming the Rosetta mission orbital configuration.
References: [1] Feldman et al., 2000; [2] Feldman et
al., 2002; [3] Lawrence et al., 2013 (1997) [4] Lawrence et al., 2010; [5] Feldman et al. 2001; [6] Mitrofanov et al. 2010a; [7] Feldman et al., 1998; [8] Elphic
et al., 2007; [9] Lawrence et al., 2011; [10] Eke et al.,
2012; [11] Mitrofanov et al., 2010b; [12] Sanin et al.,
2012; [13] Murchie et al., 2015; [14] Fujiwara et al.,
2006; [15] Sierks et al., 2015; [16] Scheers 2012