THE X-RAY SPECTROMETER for MERCURY MESSENGER. R. D. Starr1, G

Mercury: Space Environment, Surface, and Interior (2001) 8055.pdf

THE X-RAY SPECTROMETER FOR MERCURY MESSENGER. R. D. Starr1, G. C. Ho2, C. Schlemm2, R. E. Gold2, J. O. Goldsten2, W. V. Boynton3, and J. I. Trombka4, 1Department of Physics, The Catholic University of America, Washington, DC 20064 (richard.starr@gsfc.nasa.gov), 2Johns Hopkins University, Applied Physics Labo- ratory, Laurel, MD 20723 ([email protected], [email protected], [email protected], [email protected]), 3 Department of Planetary Science, Space Sciences Building, University of Arizona, Tucson, AZ 85721 ( [email protected]) , 4Goddard Space Flight Center, Code 691, Greenbelt, MD 20771 ([email protected]).

Introduction: Mercury is the closest planet to the keV), Si (1.740 keV), S (2.308 keV), Ca (3.691 keV), Sun and because it is so close, it is difficult to study Ti (4.508 keV), and Fe (6.403 keV). The strength of from Earth-based observatories. Its proximity to the these emissions from planetary surfaces is strongly Sun has also limited the number of spacecraft to visit dependent on the chemical composition of the surface this tiny planet to just one, Mariner 10, which flew by as well as on the incident solar spectrum, but is of suf- Mercury twice in 1974 and once in 1975. Mariner 10 ficient intensity to allow orbital measurement by de- provided a wealth of new information about Mercury, tectors like those on the MESSENGER spacecraft. yet much still remains unknown about Mercury’s geo- The coherently and incoherently scattered solar X- logic history and the processes that led to its formation. rays are one source of background signal. Astronomi- The origin of Mercury’s metal-rich composition is just cal X-ray sky sources, which could also be sources of one area of investigation awaiting more and improved background, are eliminated at Mercury, because the data to sort between competing hypotheses. Mercury XRS is collimated to a 12° field of view, and the planet plays an important role in comparative planetology, completely fills the field of view even when the space- and many of the processes that were important during craft is at apoapsis. its formation are relevant to the Earth’s early history. Because incident solar X-rays are the excitation MESSENGER (MErcury Surface, Space ENvi- source for X-rays generated from a planetary surface, ronment, GEochemistry, and Ranging) is a Discovery knowledge of the solar spectrum is necessary for mission that has been designed to fly by and orbit quantitative analyses. The solar flux from 1 to 10 keV Mercury [1], [2], [3]. It will launch in March 2004, fly- is composed of a continuum and discrete lines, both of by Mercury in 2007 and 2008 and enter an elliptical which vary with solar activity. This process is well orbit in April 2009. During the one-year orbital phase, understood and theoretical models accurately predict a suite of instruments on board the MESSENGER the solar spectrum. The solar intensity decreases by spacecraft will study the exosphere, magnetosphere, three to four orders of magnitude from 1 to 10 keV. surface, and interior of Mercury. One of these instru- Fluorescent lines as well as the scatter-induced back- ments will be an X-Ray Spectrometer (XRS) that will ground, therefore, have greater intensity at lower ener- measure surface elemental abundances. Remote X-ray gies. As the level of solar activity increases, relatively spectroscopy has been accomplished before on the more output occurs at higher energies, the slope of the Apollo 15 and 16 missions, and more recently on spectrum becomes less steep, and the overall magni- NEAR Shoemaker [4]. tude of the X-ray flux increases. This process is called The MESSENGER XRS will measure characteris- hardening. tic X-ray emissions induced in the surface of Mercury Instrument Design: The XRS is an improved ver- by the incident solar flux. The Ka lines for the ele- sion of the NEAR Shoemaker X-Ray Spectrometer ments Mg, Al, Si, S, Ca, Ti, and Fe will be detected design [5]. Three gas proportional counters view the with spatial resolution on the order of 40 km when planet, and a state-of-the-art Si-PIN detector mounted counting statistics are not a limiting factor. These on the spacecraft sunshade views the Sun. (See Table measurements can be used to obtain quantitative in- 1.) The energy resolution of the gas proportional formation on elemental composition. counters (~850 eV at 5.9 keV) is sufficient to resolve X-ray remote sensing: The X-ray spectrum of a the X-ray lines above 2 keV, but Al and Mg filters on planetary surface measured from orbit is dominated by two of the planet-facing detectors are required to dif- a combination of the fluorescence excited by incident ferentially separate the lower energy X-ray lines from solar X-rays and coherently and incoherently scattered Al, Mg, and Si. This balanced filter technique has solar X-rays. The sampling depth is dependent on en- worked well on NEAR Shoemaker [4]. A Be-Cu hon- ergy, but is always less than 100mm. The most promi- eycomb collimator provides a 12° FOV, which is nent fluorescent lines are the Ka lines (1–10 keV) smaller than the planet at apoapsis and eliminates the from the major elements Mg (1.254 keV), Al (1.487 X-ray sky background. Mercury: Space Environment, Surface, and Interior (2001) 8055.pdf

XRS FOR MERCURY MESSENGER: R. D. Starr et al.

Table 1. XRS Characteristics improved version was subsequently flown on the Mars Pathfinder rover. The XRS solar flux monitor is a Measured Elements Mg, Al, Si, S, Ca, Ti, Fe third-generation design that uses discrete resets rather Solar Monitor Si-PIN, 300 mm thick, 0.12 than resistive feedback to compensate for diode leak- mm2 age currents. This design gives it better energy resolu- Detectors 3 gas proportional counters, tion (~300 eV at 5.9 keV) and greater radiation immu- 10 cm2 each nity. Field-of-view 12°, Be-Cu honeycomb col- XRS Sensitivities: The MESSENGER X-Ray limator Spectrometer will detect several elements of geologic Window Beryllium 25mm significance on the surface of Mercury. The spatial Balanced Filters 8.5 mm Mg; 8.5 mm Al resolution will depend upon proximity to the planet Energy Range 0.7 to 10 keV and the intensity and shape of the exciting solar spec- Energy Resolution 850 eV fwhm @ 5.9 keV trum. Table 2 lists the required integration times for Maximum input rate 20 kHz identifying the listed elements at the 10% uncertainty Integration period 50 s @ periherm; 2000 s @ level for different solar conditions for MESSENGER. apoherm Integration times vary due to elemental abundance, Improvement of the signal to background ratio solar spectrum and background rejection efficiency. compared to the NEAR XRS was of primary impor- Because of the very steep slope of the incident solar tance in the design for MESSENGER. Background spectrum, the fluorescence from the heavier elements reduction in the planet-facing gas tubes is enhanced by (Fe, Ti, and Ca) will only be detected during solar a set of anti-coincidence wires, located near the pe- flares. Fluorescence from Mg, Al, and Si will be de- riphery of the tubes, which detect penetrating cosmic- tected even during quiescent solar conditions. ray and gamma-ray events. An internal Be liner blocks The XRS will attain its finest spatial resolution X-rays produced by cosmic-ray interactions in the de- (~40 km) at periherm, which occurs over Mercury’s tector Ti tube walls. Northern Hemisphere. However, due to the extreme It is also important to note that the XRS observing elliptical orbit of the MESSENGER spacecraft, this geometry on MESSENGER will be significantly im- will amount to only 15 minutes out of every 12-hour proved over that on NEAR. Measurements where the orbit or 180 hours during the one-year orbital phase of solar incidence angle and detector-viewing angle are the mission. In the Southern Hemisphere spatial reso- less than 60° and the instrument field of view is filled lution will be ~3000 km. by the planet are best. On MESSENGER these condi- References: [1] Solomon, S. C. et al. (2001) tions will occur about 50% of the time. On NEAR this Planet. Space Sci. accepted for publication. [2] Gold R. number was only about 5%. E. et al. (2001) Planet. Space Sci. accepted for publi- The solar monitor that tracks the solar X-ray input cation. [3] Santo A. G. et al. (2001) Planet. Space Sci. to the planet, is a small (~0.1 mm2) Si-PIN detector accepted for publication. [4] Trombka, J. I. et al. with a thin Be window that provides thermal protec- (2000) Science 289, 2101-2104. [5] Trombka, J. I. et tion. The NEAR Shoemaker mission was the first to al. (1997) JGR 102, 23,729-23,750. [6] Brückner J. fly the high-resolution (~600 eV at 5.9 keV) Si-PIN X- and Masarik J. (1997) Planet. Space Sci. 45, 39-48. ray detector technology (as a solar flux monitor). An

Table 2. XRS Observation Times Proportional counter with 95% Proportional counter with 75% Assumed background rejection background rejection Element abndances Normal Flare Normal Flare Fe 2.3% — 80 s — 2 min Ti 1.0% — 3 min — 4 min Ca 4.0% — 10 s — 30 s Si 21.5% 7 min 2 min 18 min 3 min Al 3.0% 2 hr 22 min 8 hr 50 min Mg 22.5% 5 min 2 min 11 min 3 min Counting times are those required to reproduce the assumed composition at the 10% uncertainty level. Assumed abundances are from Brückner and Masarik [6]. Count rates are scaled from those made by NEAR in July 2000 (1.78 AU) to the assumed composition at Mercury (at 0.387 AU).