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By Igor MITROFANOV, Dr. Sc. (Phys. & Math.), Institute for Space Research, Russian Academy of Sciences
The NASA Odyssey-2001 mission has been a major event in science lately: on October 24, 2001, a US interplanetary vehicle was put in orbit around the Red Planet, Mars. This spacecraft carried the Russian-made device HEND (High- Energy Neutron Detector). The very first studies conducted with its aid brought sensational results. Dr. Igor Mitrofanov supplies the details.
HEND device was developed at our research center on the instructions of the Russian Aerospace Agency. The cost of this project totaled something like ten million rubles. HEND will be employed in studying the Red Planet up until the year 2004; but our and American experts are discussing cooperation prospects until the years 2007 - 2011.
First, let's recall the characteristic features of Mars. This planet is surrounded by a very thin atmosphere, and it has no global magnetic field. Therefore galactic cosmic rays can freely reach the Martian surface and, at a depth of 1 to 2 meters, generate what we call secondary neutrons.
Such kind of processes always occur in a substance exposed to a flux of energized particles (protons for the most part). Now, generation of neutrons off the surface of Mars is the physical phenomenon our device is meant to explore.
A few words about its design. While discussing the HEND idea with neutron physics experts, we saw that making such a device posed no major problems. Corresponding units and parts were there in many research and commercial nuclear setups where neutron fluxes, generated during nuclear reactions, should be monitored.
There was a constraint though: our laboratory "article" weighed around 40 kg, or as much as the entire research kit on Odyssey. But it had to satisfy one rigorous condition-its mass should not be above 4 kg. So we had to make a small and compact device sensitive enough to register high-energy neutrons, shock- and vibration-proof during ground launching and capable of sustaining the heat regime in actually complete isolation from the body of a space vehicle; it was to be control reliable, it was to have artificial intelligence of its own and memory for research data evaluation and storage. In addition, its design was to be based on state-of-the-art physical principles: after all, no one would be testing novel neutron detectors in Martian orbit.
Schematic image of the HEND device. Three detectors, SD, MD and LD, allow to register neutrons in a spectral range of 0.4 eV-1 MeV. Scintillation detector SC registers neutrons from 850 keVto 15 MeV, and "soft" gamma quanta.
To get on top of this problem, we approached physicists of the Dubna-based Joint Institute for Nuclear Research. After rather long deliberations we decided on a device with four detectors capable of "covering" neutron energies in a wide range of 0.4 eV to 15 MeV. Separating them apart as far as possible, we saw to it that these detectors had the best field of vision during Odyssey's orbital flight for the purpose of mapping the Martian neutron radiation.
Besides, our HEND is part of the GRS (gamma rays spectrometer) complex developed by a team of American scientists under Professor William Boynton (University of Arizona, Tucson) for registering high-spectral resolution gamma radiation. This complex also includes a low-energy neutron spectrometer made by Dr. William Feldmann and coworkers of the US Los Alamos National Laboratory. All these three devices measure parameters of the Martian neutron and gamma radiation which are needed for drawing a global map of the mineral composition of the planet's surface. Such a map, we are hoping, should help in piercing the enigma of the Red Planet's evolution from the once warm and humid body to what it is now, cold and dry. This is all-important for understanding how the bodies of the solar system took form (we cannot make such studies here on earth because of the ongoing violent geological processes and biological life distorting the picture, while Mars is like a museum where everything has been preserved in an intact, pristine form).
Our HEND device is also used for solving two other astrophysical problems besides the mapping of the Martian neutron radiation. The first one is related to its capability of registering, nonstop, the background flux of cosmic gamma rays. The point is that galactic gamma bursts detected more than 35 years ago are thought to be caused by huge explosions of stars at cosmological distances. However, we have no physical model of this phenomenon as yet. To elucidate the nature of such bursts it is very important to keep tabs on them in optical and radio-frequency ranges immediately upon registration of their gamma radiation. With HEND orbiting Mars, we can use the method of what we call "interplanetary triangulation" based on measuring the relative lags in the registration of gamma radiation bursts by different space apparatuses. The data picked up from some of them allow to determine the direction of incoming gamma rays to an accuracy of angular minutes of arc.
Already in the initial period of the Odyssey-2001 mission about thirty gamma bursts were registered, and celestial coordinates were computed for many of them. In some instances observations of the sky in the zones of the bursts produced positive results, the optical afterglow.
The HEND's other astrophysical assignment was to observe solar flares jointly with other research and patrol vehicles orbiting the earth. Such kind of monitoring, from Martian and cir-cumterrestrial orbits simultaneously, is characterized by a rather large angle between two lines of sight. In August 2002 both Mars and the earth were "looking" on the sun from nearly opposite directions. Yet to develop a physical model of solar flares it is very important to get their three-dimensional picture; that's where stereoscopic data may come in handy.
Using HEND, we found out that the intensity of the flux of neutrons reflected from the Red Planet's surface may increase dramatically. The natural radiation background on Mars is about 1 neutron per 1 cm 2 (or much higher than here on earth). During one powerful solar flare the flux was up to 400 neutrons. Should a radioactivity level as high as that obtain in a research laboratory on earth, the personnel will be unable to stay in without special protective measures.
The HEND-registered data show: the excited luminary can "splash
out" an annual radiation dose within one day only. Hence the conclusion: spacemen can be safe on Mars only in quiet sun periods. Or explorers of the Red Planet will have to hide like moles in burrows 3 or 4 meters under.
The HEND-supplied data are just another proof of the immense importance of the terrestrial atmosphere shielding us from the deadly cosmic rays. Since Mars has no shield like that, there is a lower probability of biological life on it. Even if some organisms might be present there, they should be deep under the ground surface. Or else they have enhanced radiation resistivity and are endowed with properties that are beyond our imagination.
All this evidence will naturally be used toward a solution of one of the foremost tasks of space exploration in the 21st century, a man's flight to the Red Planet.
While mapping neutron radiation, the HEND device may detect areas with enhanced concentration of subsurface water on Mars. Thereby we shall be able to construct a model of the hydrological evolution of the planet's ground and consequently, conceptualize the subsequent exploration of the Red Planet. Knowing the total mass of subsurface water, we can build a scenario of the planet's catastrophic drying. Besides, it is important to locate areas of moisture-saturated soils where future expeditions are to search for vestiges of life. At this stage it is important to locate the most humid tracts on the Martian surface.
Both the earth and Mars are believed to have come into being about 4.6 billion years ago from a protoplanetary cloud in the vicinity of the then young sun. According to one contemporary model, both planets showed about the same development pattern at the initial stage of their evolution. Large oceans covered their warm surfaces, and there were violent volcanic eruptions to enrich the dense atmospheres with gases of complex chemical composition. Yet several hundred million years after, Mars for some reason suddenly changed from its state of a "warm and humid" planet to that of a "cold and dry one". In keeping with one hypothesis, that irreversible transformation was caused by a chock of a large meteorite that reoriented the planet's poles or perhaps even changed its orbit. It might also be that Mars saw a global change of its climate in consequence of the gradual dissipation of the atmosphere due to the weak gravitational field. As the atmosphere became thin, the planet's surface started cooling and drying fast, with frozen water remaining perhaps only in the polar regions.
Finding the causes of that global transition from one state to another is the chief purpose of the HEND-aided Martian studies. This problem is of great significance for basic science in explaining the evolution of planets; and if solved, it will also enable us to assess the probability of a like hazard for the earth.
The present-day data on the Red Planet show it to be still active. Not so long ago the photographs of craters on its surface revealed ravines and canyons formed by water torrents millions of years ago. We cannot explain other formations on the Martian surface either-namely, the flat terraces, circular depressions and regular dunes.
But here's something very remarkable: in February 2002 our device detected vast provinces of "permafrost" on Mars with subsoil water ice. Proceeding from the HEND-obtained data, a research team drew up a map of 0.4 eV-15 MeV neutron fluxes. A neutron flux numerical modeling shows that particles with an energy of several MeV come out from the upper surface layer about 50 cm thick. But a 50 percent decrease of such energized particles indicates the presence of water making up about
First measurements of a Martian neutron flux with the HEND device aboard the American spacecraft (October 25, 2001). The peaks of LD (top), MD (middle) and SC (bottom) signals correspond to the moments of overflight above the Martian surface.
The first map of the neutron radiation ofMars(0.4eV-100keV) obtained by the Russian device HEND (printed with permission from SCIENCE, Vol. 297, No. 5578, p. 79. (c) 2002, American Association for the Advancement of Science).
5 percent of the ground mass. This is much more than one believed earlier (around 0.1 percent of water could have accumulated in the Martian atmosphere). Neutrons energized in a range of 0.4 eV to 100 keVare ejected onto the Martian surface from a layer as deep as 2 meters. Their flux in the southern province is six times as weak as elsewhere, which indicates: under a topsoil layer 50 to 60 cm thick there is a water- rich horizon which may contain a large amount of water ice (a two-digit figure, percentage wise, relative to the total mass). By our estimates, if a stratum of pure ice, merely 20 cm thick, underlies the dry ice layer, it should lead to depression of neutrons detected in the southern province of the Red Planet: 6 fold depression for 0.4 eV-1.0 MeV neutrons, and twofold depression for neutrons energized to several MeV.
The H EN D observation data agree well with the research findings obtained with the American instruments, the neutron spectrometer and the spectrometer of gamma rays. The Red Planet's neutron maps obtained by the Russian and American instruments actually concur; and according to the gamma spectrometer data, the Martian surface above 60 of the southern latitude emits a strong flux of gamma rays of the deuterium nuclear line. This flux occurs when a large number of hydrogen nuclei are irradiated by a powerful neutron flux. Thus the presence of provinces on Mars rich in subsoil water is now a reliably established fact.
The results of the Odyssey-2001 mission prepare the ground for a reasonable search for signs of biological processes on Mars. First, at the early, "warm and humid" stage of the planet's existence there could appear primitive forms of life similar to those that emerged here on earth then. Second, if there are underground water pools on Mars, they might offer adequate conditions (heat and gases from active volcanoes nearby, water, mineral compounds) for some forms of life. Similar media of the earth are a habitat of microorganisms. And third, odd complex fragments found recently in what is known as martian meteorites-couldn't they be fragments of primordial microbes?
Hence the principal goal of the Red Planet's studies-searching for biological life on it now or signs of such life in the past. That will be a great scientific discovery. A direct comparison of biological structures on Mars and on the earth will show their similarity or difference, and that will make it possible to verify how universal biological laws are on different celestial bodies. On the other hand, should biological processes be absent on Mars, that will confirm the idea of losif Shklovsky (1916 - 1985), Corresponding Member of the USSR Academy of Sciences, about the uniqueness of the terrestrial form of life in the Universe.
Let me say this in conclusion: not counting in the earth, Mars is the only planet in the solar system where man can make do with a sufficiently simple life support kit. Surveying for its natural resources (water, heat, minerals) today is an express condition for man's presence on it tomorrow.
Illustrations courtesy of the author, ITAR-TASS and SCIENCE (USA)
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