Abstract:
Gamma-ray spectroscopy techniques have proven to be useful in mapping the surface composition of bodies in the Solar system. This paper reviews three space missions that made use of gamma-ray spectroscopy and their contributions to space exploration and planetary science endeavours.
Keywords: Gamma-ray spectroscopy, 2001 Mars Odyssey, MESSENGER, Dawn, germanium semiconductors, neutron spectroscopy
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1. Introduction
Gamma-ray spectroscopy has proven to be extremely valuable in space exploration efforts - particularly in the study of bodies within the Solar system. A process that enables gamma-ray spectroscopy is spallation. This is where the nuclei of elements on the surface of a body are battered by galactic cosmic rays as well as sporadic solar energetic particles. Spallation causes the release of neutrons and the interactions between them and the surface nuclei will emit gamma rays through inelastic scattering and neutron capture. The natural decay of radioisotopes such as potassium, uranium and thorium also causes emission of gamma rays. These gamma rays are characteristic of the element that emitted them and thus detection of these gamma rays is the key to determining the elemental and isotopic composition of a body in the Solar system.
Gamma-ray spectrometers designated for use in space are made of germanium - a semiconductor that boasts the best energy resolution when compared to other instruments that are currently available. However, this energy resolution is only achievable at very low temperatures and thus a germanium spectrometer will require cryogenic cooling. This poses a challenge when designing the spectrometer as the cooling mechanism must be able to withstand shock during launch and also not be a burden on the power and mass budget.
Whether it be via Compton scattering, pair production or the photoelectric effect, gamma rays emitted from the surface of a body will create electron-hole pairs in the germanium detector. This induction of charge allows for a reading to be made by a preamplifier that then registers the detection of the gamma rays. A spectrum is produced, where the energy is characteristic of the element that produced the gamma ray and the intensity of the spectrum is correlated to the concentration of said element. The analysis of these spectra allows for mapping of the abundance and distribution of elements on the surface of a body.
As a result, gamma-ray spectroscopy is perfect for studying the formation, composition and habitability of planets within the Solar system. Past missions that have addressed these topics are 2001 Mars Odyssey, MESSENGER and Dawn and their technology solutions in pursuing those science objectives will be discussed in this paper.
2. Planetary Science Missions with Gamma-ray Spectrometers
2.1 2 001 Mars Odyssey
2001 Mars Odyssey, as the name suggests, was launched by NASA in April 2001 and has been active for over 19 years. The mission objectives of 2001 Mars Odyssey include mapping the elemental composition of the Martian surface and determining how abundant hydrogen is under the surface of Mars. The instrument of interest on 2001 Mars Odyssey is GRS - the Gamma Ray Spectrometer. GRS was specifically implemented in order to achieve the aforementioned mission objectives. The instrument had some heritage from the lost Mars Observer probe but it was ultimately a new design developed by the University of Arizona.
Alongside the Martian surface, the 2001 Mars Odyssey spacecraft itself would also emit gamma rays when exposed to cosmic rays. If left unaccounted for, detections of these gamma rays would lead to false readings and thus invalid data. This was solved by separating GRS from the rest of the spacecraft using a boom. The University of Arizona faced budget challenges when developing GRS. At first GRS was allocated a significantly lower budget than what was proposed by the University and after several months of intense negotiations, GRS was finally allocated a larger budget that was closer to what the University of Arizona had originally proposed. The intricacies of GRS made development very complicated. Another challenge came when the detector failed to meet specifications regarding vibrations. This delayed the integration of GRS by half a year, following which more challenges were presented. Failure during performance testing led to many more months of delay to the point where GRS only joined the rest of the spacecraft three months before launch.
By February of 2002, 2001 Mars Odyssey had commenced the mapping of the Martian surface using GRS. This enabled accurate global maps of hydrogen abundance to be produced. GRS was constrained in the sense that it could only observe the surface up to a depth of less than a metre. Despite this, GRS was able to produce the first ever maps of global elemental abundance on Mars. Areas of increased hydrogen abundance confirmed statements in literature regarding the presence of ice under the Martian surface. As a result, GRS has been instrumental in advancing the field of planetary science - specifically regarding the evolution of Mars. The habitability of Mars is very topical and establishing the presence of water is a key factor in investigating the possibility of life on Mars.
2.2 MESSENGER
The MESSENGER space probe launched in August 2004 and reached Mercury in March 2011 where it orbited for four years. MESSENGER also had a gamma-ray spectrometer (GRS) onboard its payload. The mission objectives for this specific instrument were similar to that of the GRS on Mars Odyssey. GRS on the MESSENGER probe aimed to determine the elemental composition of Mercury’s surface, the abundance of major elements and to verify the presence of ice water in Mercury’s polar craters. In particular, GRS aimed to determine the abundance of elements that are integral to the evolution of a planet.
Due to the limited mass budget, MESSENGER was unable to host GRS on a boom as had been done before with 2001 Mars Odyssey. A germanium detector was used on GRS as is the tradition with gamma-ray spectrometers. The germanium crystal was not as big as the one present on 2001 Mars Odyssey and this allowed for better cooling in the high temperature environment of Mercury. This particular detector was suitable for the space environment around Mercury due to its radiation hardness and ability to anneal. The germanium detector was coupled with a cooling mechanism and so GRS on the MESSENGER probe marked the first ever use of such a device on an interplanetary mission. The required cryogenic cooling was achieved by encapsulating the germanium crystal in order to keep it at a temperature of 90 K.
As with 2001 Mars Odyssey, GRS was constrained to measuring less than a metre under the surface of Mercury but it was used to map important elements. GRS mapped the abundance of hydrogen in the northern hemisphere of Mercury. Data retrieved from GRS found that the concentration of hydrogen, and thus ice water, increased in the polar regions and this was consistent with existing literature as well as data from neutron spectrometers.
2.3 Dawn
Dawn was a NASA mission that was launched in September 2007 and ended over 11 years later in October 2018. Dawn aimed to characterise the surface of two protoplanets in the Solar system: the dwarf planet Ceres and the asteroid Vesta. By doing so, their formation and evolution could be understood further which would in turn allow for better understanding of the formation of the Solar system. On the payload of Dawn was GRaND - the Gamma Ray and Neutron Detector. As a means of achieving the mission objectives, GRaND was developed using heritage technology from 2001 Mars Odyssey and Lunar Prospector. Vesta and Ceres are similar to Mars and the Moon in regards to their composition and so instrument design was based on the aforementioned missions. Gamma-ray spectroscopy was made possible with a bismuth germanate scintillator that was enveloped by boron loaded plastic scintillators.
While this design is nearly identical to that of the instruments on 2001 Mars Odyssey and Lunar Prospector, GRaND introduced improvements via a 16 element cadmium zinc telluride semiconductor array which allowed for a gamma-ray spectroscopy at a much higher energy resolution. Another improvement came in the form of a phosphor sandwich that was later known as ‘phoswich’. This was implemented into the design of GRaND in order to measure thermal neutrons at low orbital velocities. In total, GRaND was made up of four spectrometers.
The Bismuth Germanate Scintillator
This instrument had heritage as a bismuth germanate scintillator was used prior to the Dawn mission on Lunar Prospector. The device is a bismuth germanate crystal that is able to detect gamma rays up to 10 MeV.
The Cadmium Zinc Telluride Semiconductor Array
This array had a high energy resolution and could thus measure gamma rays from spallation and natural decay. The development of this array was an innovation and had never been implemented in a mission before Dawn.
The Boron Loaded Plastic Scintillator
These scintillators enveloped both the bismuth germanate scintillator and the cadmium zinc telluride crystal. Not only did it negate the effect of cosmic radiation, it also allowed for neutron capture and neutron spectroscopy. This type of scintillator had already been used on 2001 Mars Odyssey and MESSENGER.
The Phoswich Detector
This particular detector was designed to measure gamma rays that were at lower intensities and energies. Phoswich detectors had not been used in a space mission till Dawn but some parts of their design were already heritage in the space industry.
Dawn’s investigation of Vesta found that howardites, eucrites and diogenites meteorites drew their origin from Vesta. Dawn was able to determine that Ceres could be classified as an ‘ocean world’ with ammonia and water interacting with silicates. GRaND produced maps depicting the concentration of hydrogen in the subsurface of Ceres and found it to be present across the whole body in ice form. This proved that ice can last for billions of years under the surface of a body.
3. Discussion
The three missions were similar in the sense that they made use of germanium based gamma-ray spectrometers. Heritage technology from 2001 Mars Odyssey was vital in the development of GRaND for the Dawn mission and the science objectives of MESSENGER were not too different from that of 2001 Mars Odyssey. The missions were launched roughly 3 years apart from each other and so their mission durations overlapped for a significant amount of time. Despite this, there were many differences in their use.
2001 Mars Odyssey stands out for its use of a boom in preventing GRS from detecting background signals generated by the spacecraft. Due to the mass constraints on MESSENGER, this was not feasible for it’s variation of the GRS. MESSENGER did sport a boom, however this was used to host the magnetometer instead of the GRS. MESSENGER mitigated the effect of background signals by determining radiation levels during cruise, scaling them for GRS and then taking the values into consideration when analysing data from GRS. Dawn was constrained by costs and so a boom was not feasible for hosting GRaND. As a result, GRaND was segmented instead so that it would be able to detect gamma rays from Ceres and Vesta separately from rays emitted by the spacecraft.
The heritage technology from 2001 Mars Odyssey allowed for quicker development of new instruments. Space missions have strong constraints on mass as a means of reducing costs and so by saving mass, money is also saved. Thus the need for more compact instruments was an important aspect in the development of missions following 2001 Mars Odyssey. The GRS on 2001 Mars Odyssey had a mass of 30.5kg. This is much higher than the gamma-ray spectrometers that followed, especially on MESSENGER and Dawn. MESSENGER’s GRS had a mass of 9.2kg which was similar to the mass of Dawn’s GRaND (9.4kg). In just a few years after 2001 Mars Odyssey, gamma-ray spectrometers became significantly more compact which made them a valuable asset on mission payloads. This is reflected in the dimensions of these instruments. GRS on Mars Odyssey measured 46.8 x 53.4 x 60.4 cm, the germanium sensor on MESSENGER measured 5.0 x 5.0 cm and the bismuth germanate sensor on GRaND measured 7.6 x 7.6 x 5.1 cm.
All three of the instruments featured some form of neutron detector. 2001 Mars Odyssey had the High Energy Neutron Detector (HEND) integrated onto GRS. Detection of neutrons allowed GRS to locate water in the Martian soil through the presence of hydrogen. MESSENGER also included a neutron spectrometer (NS) with it’s GRS. The role of NS was to map the distribution of hydrogen as well as basalts in order to further develop knowledge from GRS on the surface composition of Mercury. Like the gamma-ray spectrometer, the neutron detector on GRaND was used to map the elemental composition of protoplanetary surfaces in the Solar system.
4. Conclusion
Planetary science missions have made extensive use of gamma-ray spectrometers in order to map the distribution of elements and isotopes on a body's surface. By taking measurements of gamma ray emissions, a global map can be put together and studied to determine the nature of Solar system bodies. This adds to the existing body of knowledge and it also serves as a driver for future space exploration missions. The successful implementation of gamma-ray spectrometers allows for heritage technology to be passed on and developed further by future missions. This is evident in the advances made from 2001 Mars Odyssey’s GRS to the instrumentation used on MESSENGER and Dawn.
Gamma-ray spectrometers yield significant science results when utilised in space missions. GRS on 2001 Mars Odyssey made advances in the study of the habitability of Mars through its observations of hydrogen and subsurface ice water. MESSENGER’s GRS was able to verify information in existing literature on Mercury, especially regarding the distribution of ice water in Mercury’s polar region. GRaND determined the origin of HED meteorites and found significant evidence that designated Ceres as an ‘ocean world’. This gave further insight into the formation of the early Solar system. Through its extensive history in space missions, gamma-ray spectroscopy has yielded important science results and resulted in heritage technology that continues to improve. The differences between instruments on 2001 Mars Odyssey, MESSENGER and Dawn are indicative of an innovative push for space exploration.
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