by Acad. Eric GALIMOV, director of the V. I. Vernadsky Institute of Geochemistry and Analytical Chemistry (GEOKHI), Russian Academy of Sciences
Two satellites of Mars. The radius of the Phobos orbit, 9,378 km.
After this country launched the first artificial satellite of the earth, the sputnik, in October 1957, space studies moved to the foreground the world over in the interests of basic and applied sciences. Here in Russia our scientists and engineers are making an immense contribution to problem solving in this area. The present article acquaints our readers with a singular project within the framework of the federal space research program providing for lunar and planetary studies, among other things.
Why Phobos? This is a natural question for our readers to ask. Indeed: Why fly to this small satellite of Mars and waste so much money what with other objectives and problems, including those on this planet of ours, the earth?
In a way the answer comes from the American astrophysicist and planetary scientist Carl Sagan who, back in 1993, wrote about certain fundamental questions, such as the beginnings of life and of our planet, the origins of nature, and the future of the Universe... Space studies are the tools in our search for the clues to these and other enigmas. But they are expensive, these studies. Therefore
it is important to develop most effective projects at the lowest expense. The Russian "PHOBOS-GRUNT" is one such project.
WHAT IS PHOBOS?
Phobos is one of the two Martian satellites, a relatively small body of irregular form 27x22x19 km in size. Its density is rather low, about 1.9 g/cm3 , i.e. that of loose sand (the figure is 3.3 g/cm3 for the moon). Its period of revolution around Mars is equal to 7 h 39 min, and its orbit is synchronized with the period of revolution. That is why Phobos faces the Red Planet with one side only, just like the moon does the earth. The orbital radius of Phobos around Mars is 9,378 km, it flies at an altitude of only 6 thous. kilometers and is seen at an angle of 40°.
The surface of this Martian satellite is rough. It is pockmarked with craters produced by the impact of meteorites. The largest among them is the Stickney crater about 11 km in diameter and more than 1 km deep. It takes up over a third of the satellite's linear dimensions, and is thus the largest reference point of its relief. Phobos is also remarkable for linear structures in the form of extended depressions up to several kilometers in length, 100 - 200 m wide and 10 - 20 m deep. Its land surface is strewn with rocks and boulders, some of them as large as 20 to 30 m.
The reflection capacity of this celestial body (albedo) is comparatively low -0.068 (against 0.367 for the earth), and its surface appears to be coated with a thin layer of dust at least 1 meter thick.
The temperature measurements made by a thermoemission spectrometer on board the American space probe Mars Global Surveyor (1996) show: the temperature of the illuminated (daylight) side -4°C plummets to - 112°C on the dark (nocturnal) side within the transition zone of merely several kilometers.
The orbital radius of Phobos is close to the Roche limit (with reference to the distance of a satellite body from the mother planet at which the tensile forces of gravitational interactions within this body become commensurate with the cohesive forces of matter, i.e. it is on the point of breaking apart). Had this satellite been liquid, it would have collapsed long ago. But this did not happen due to certain features of its relief-linear furrows, for one. The loosening of its soil may be responsible for the higher presence of debris matter in its orbit around Mars.
WHAT MAKES PHOBOS INTERESTING TO US?
The first attempt at studying Phobos was made here in Russia in 1988. We did not plan taking soil samples then. Our home project provided for the launching of two space probes, Phobos-1 and Phobos-2, which were to land on the satellite surface and explore its soil by means of remote-control instruments. However, this project failed to materialize in full: Phobos-1 was gone while on its trajectory to Mars, and Phobos-2, though already in the long-awaited orbit, stopped responding to communication signals. Still and all, using a kit of onboard instruments, it became possible to take photos of this celestial body, carry out spectral studies and assess its mass- (1.082±0.001) 1014 g.
Phobos is said to be of much interest to planetary scientists because it has preserved intact the relict substance
Diagram in the coordinates δ17 O-δ18 O, where δ18 O = ((18 O/16 Oof the object )/( 18 O/16 OSMOW ) - 1)103 %, an analogous formula is for δ17 O; SMOW - isotope-oxygen standard. In mass-dependent effects, δ17 O = 0,51 δ18 O. This correlation agrees with the terrestrial fractionation line.
of the Solar system. At any rate, such were also the objectives of the first missions. But this is not the main thing, though. Similar substances are found in meteorites. Say, detected in carbonaceous (coaly) chondrites of the earth were organic compounds of extraterrestrial origin, including numerous isomers of amino acids, hydroxy acids and hydrocarbons. Likewise identified were diamond, carbide and nitrile granules with isotope compositions not occurring on our planet. Some of the meteorites were found to contain refractory inclusions formed at the earliest stage of the coming-to-be of the Solar system when nothing but gas and interstellar dust surrounded the young Sun. That is why the exploration of Phobos as a source of relict matter is of secondary importance. The costly space experiment was not worthwhile.
What makes Phobos interesting to us is that it holds clues to planetary genesis. If it is indeed the "leftover" orbital material not "captured" by Mars, then we gain a unique chance of determining the structure and composition of the matter from which the planets of the solar system were formed long ago. The point is that these planets, including Mars and the Earth, have been remelted and modified in after processes, and differentiated accordingly. Their large satellites, the Moon among them, have likewise undergone melting processes. Mercury and Venus have no satellites. What they do have are asteroids which, in contrast to Phobos, are not geared to a definite planet. Therefore Phobos is one-of-a-kind celestial body (except Deimos, the other satellite of the Red Planet) that could shed light on the formation mechanism of the solar system's planets.
SCIENTIFIC OBJECTIVES OF THE PROJECT
It is all-important to learn above all if Phobos is a fragment of the matter that went into the making of Mars, or if it is a foreign body drawn into the Martian orbit. To find it out, we should compare some of the parameters of these two bodies, Mars and Phobos, carrying information on their genesis. However, orthodox measurements on their chemical composition-obtainable by remote measurements-are no good for our purpose.
Special tests are practiced in space chemistry: three oxygen isotopes - 16 O, 17 O and 18 O, different as to their correlations-appeared in various parts of the Solar system still at the preplanetary stage. In the process of its formation a planet or a satellite inherit the correlation of these three isotopes proper to their feed zone. Subsequently there occur processes changing the makeup of the planet's minerals and compounds. But these events have been observed proportionally in the pairs 18 O/16 O and 17 O/16 O. Deviations measured in thousandth fractions (%o) are designated δ18 O and δ17 O and correlate approximately as 2:1. Hence the isotope composition of oxygen in the coordinates δ18 O versus δ17 O is aligned in what is known mass-dependent fractionation. The substances of common space chemistry genesis fit into the general fractionation line on the δ18 O - δ17 O diagram. Planetary scientists, true, know of some exceptions to this pattern, but in this very context such details are immaterial. It is not accidental that the fractionation line superposed by the magnitudes δ18 O and δ17 O, determined for most different minerals, water and gases on the earth,
Relative occurrence of the noble gases - Ne, Ar, Kr, and Xe - on the Sun, planets and in coaly (carbonaceous) chondrites.
The Phobos reflection spectrum in the UV band compared with reflection spectra obtained in model experiments: 1(a) and 1(b), graphite irradiated with protons of different intensity; 2 - condensed products of kerogen sublimation: c) at 400o C, d) at 600°C, e) at 1,000°С
is similar to the selfsame parameters of δ18 O - δ17 O in lunar samples, a fact that points to the same source of matter both for our planet and for its natural satellite.
The correlation of two chromium isotopes, 53 Cr/52 Cr, plays about the same role as that of the oxygen isotopes.
These fine correlations of oxygen, chromium, manganese and other elements can be assayed by modern high-precision laboratory mass spectrometers. Therefore soil samples from a planetary body should be brought to a ground-based laboratory here on earth. But even if a "PHOBOS-GRUNT" probe brings in a soil sample from Phobos, the Martian satellite, we shall get no wiser on the structure of the Red Planet's rock.
What is remarkable about the "PHOBOS-GRUNT" project is this: there is a group of meteorites thought to be Martian fragments. These are the SNC-meteorites (the three letters stand for three typical meteoric groups-Shergotti, Nakhla, Chassigny). We have weighty arguments to suppose these are the rocks knocked out from the surface of the Red Planet by the impact of fairly large bodies hitting Mars. Although we cannot be 100 percent certain that the SNC-meteorites are really samples of Martian rock, there is still a high chance-80 percent probability-of their being so. The literature often cites data on the composition of Martian rock, namely on the presence of isotopes of noble gases - 13 C/12 C, 142 Nd/14 Sm, among others. But we should make sure that in all these cases we are dealing indeed with substances contained in SNC-meteorites.
The magnitudes δ17 O and δ18 O, which correspond to the SNC-meteorites, take a definite position on the diagram 16 O - 17 O - 18 O. We hope that analysis of a Phobos rock sample will show whether the magnitudes δ17 O and δ18 O will coincide on the SNC-meteorites line. If they do, several fundamental questions will be solved. First, the relatedness of Phobos and Mars rock will be established. Consequently it will make sense to investigate the rock sample with the object of uncovering the mechanism for the accumulation of Mars and the other terrestrial planets (Mercury, Venus, Earth). Second, we shall be 100 percent sure in practical terms about the Martian genesis of the SNC-meteorites. Their significance as genuine representatives of the Red Planet's matter will thus increase. In our opinion, samples of the SNC-mete-
Bar chart of stone blocks distribution according to dimensions. The overall number of counted blocks in an area of 16.8 km2 makes up 6429.
orites are present in the Russian collection of GEOKHI. If so, we are lucky to have in our possession precious specimens indeed.
But what if the results prove to be negative? In that case two options will be possible: either Phobos is alien to Mars (as a foreign body)-and then the "capture" scenario will move to the fore (Phobos being drawn, captured into the Martian orbit); or the SNC-meteorites are not fragments of the Martian body at all. Then we shall have to wait for conclusive proof-a rock sample from the Red Planet.
Our doubts are not at all groundless. In its external characteristics (low albedo and density) Phobos has much in common with the aforementioned coaly (carbonaceous) chondrites. But Mars has a different concentration and isotope composition of noble gases (as shown by measurements of the SNC-meteorites!). It is pertinent to ask: is the composition of noble gases on Phobos related closer to Martian rock or to coaly chondrites? Resolving this alternative is important for an understanding of the nature of zonality in the composition of noble gases within the Solar system, and this, in
Landing site on the Phobos map.
Space probe "PHOBOS-GRUNT" designed by the S. A. Lavochkin R&D Association.
turn, is of exceptional significance with respect to processes that occurred at the early stage of its evolution.
Determining the absolute age of Phobos is an essential element in the reconstruction of its history. This involves an analysis of isotope taxonomies in present-day geochronological systems: U/Pb, Sm/Nd, Rb/Sr, etc.
Some radioactive elements have short half-life periods (like, for instance, 26 Al, 146 Sm, 182 Hf) equal to millions of years. These are no more than moments on the scale of the solar system's evolution taking 4.56 billion years. True, short-lived isotopes decayed in full during the initial period of dozens of millions of years. However, their daughter isotopes 26 Mg, 142 Nd and 182 W have survived. Therefore their corresponding correlations - 182 W/183 W, 26 Mg/24 Mg, and 14 Nd/143 Nd - carry information on processes that took place in the nascent solar system.
Analysis of organic compounds is one of the most important and intriguing chapters of Phobos. Its low albedo indicates that its body is composed of dark, light-absorbing matter. This characteristic is proper to coaly chondrites-that is meteorites containing high concentrations of organic compounds and aromatic polymers. So the Martian satellite must be rich in suchlike substances. As we have demonstrated in our previous works, its absorption spectrum is close to one observed in products of the dry sublimation of kerogen, a composite polymer substance. But on the other hand, the spectral characteristics of Phobos is close to that of anhydrous (arid, waterless) black chondrites devoid of organic matter, and basaltic achondrites. The question is still open. To clarify it we should identify organic compounds, isolate them in preparations, and assay them for 13 C/12 C, 15 N/14 N, D/H, 34 S/32 S and 18 O/16 O. If these isotopes are found in Phobos, their study may become a major stride toward resolving the problem of life genesis in the solar system.
For this purpose a sample of Phobos soil should be brought to earth-such is the aim of the project reviewed in the present article. Yet some of the work could be done by remote-control techniques on space probes. Above all it is important to learn about the physical and mechanical characteristics of Phobos and its structure. Namely, how homogeneous is its matter? What is its heat flux and flow? Is there any zonality in its structure? Sample-taking operations are only a part of visual observations- these are likewise significant for a detailed description of relief features. Definite physical and chemical procedures should be carried out then and there, on and about Phobos, such as assessing its humidity level, collecting general data on the chemical composition of rock samples in situ, along with tentative analyses of organic components.
And yet the project will pay off only if a soil sample has been brought to earth. Therefore a space probe should not be overloaded with arrays and sets of instruments of supplementary and secondary designation, for in the end they will decrease the reliability of solution of the main problem.
CHOOSING THE LANDING SITE
Clearly, the landing site on Phobos should be chosen on the side facing the mother planet, Mars, since our project provides for observations of the Red Planet's surface from its natural satellite. Absolutely safe landing is therefore the prime condition. GEOCHE experts have designed an engineering model of Phobos-small as it is, it guarantees success.
Craters and stone blocks (clumps) are a major obstacle for the landing module. An accurate count of the number of such blocks per unit of the area allows to avoid patches having a high density of clumpy matter. Usually
A drilling rig designed at the S. A. Lavochkin R&D Association.
Profile of the "PHOBOS-GRUNT" spacecraft trajectory computed at the S. A. Lavochkin R&D Association jointly with the M. V. Keldysh research center.
the concentration of debris increases near volcanic ejecta (fragments) about the craters. These ejecta range in size from 1 meter to several kilometers. By plotting a cumulative dependence of their number per 1 km2 as a function of their diameter we can calculate the density of occurrence for craters as follows: about 10 craters per 1 km2 with a diameter above 100 m; and as many as 1,000 with a diameter exceeding 10 per 1 km2 . This means that the probability of encountering steep and heavy gradients (slopes) above 3° totals 4 - 5 percent. We have pinpointed the appropriate landing site with an eye to all these limitations.
SPACE VEHICLE FOR PHOBOS EXPLORATION
Our S. A. Lavochkin R&D Association is designing a spacecraft for the Phobos mission. Its centerpiece will be a drilling rig for taking regolith core samples. The same procedures are going to be used as those employed in taking lunar rock samples. Apart from the regolith it is important to pick at least small pieces of intact rock, the mineral granules (grains). Therefore a remote-controlled manipulator is provided additionally - to enable an operator on earth to select suitable objects via visual observation.
Descent and landing trajectory of the space module on Phobos surface.
The total mass of the soil sample is to be around 100 g (given the present high-precision and high-sensitivity analytical technology, this mass will suffice).
Needless to say, adequate arrays and competent personnel should be prepared beforehand. Core samples will naturally be studied in many laboratories of the world. But there should be a head laboratory for data analysis and evaluation. It was GEOKHI that did this job in studying lunar rock samples years ago.
Ballistics experts of the Lavochkin R&D Association have worked out scenarios for the flight trajectory, touchdown and return of the module with a sample back to earth. The whole project will take three years-from launching the probe up to the moment of its return to the earth.
The same is true of taking core samples from other celestial bodies as the priority task. As it is, we know not so little about the Solar system surrounding us. The next step toward expanding our knowledge is through detailed analysis of extraterrestrial matter in earth-based laboratories. The moon, Phobos, comets and asteroids-such is the sequence of the objects for sample taking. In my view, the terrestrial planets of the Solar system should not come first in this series. Their surface, built of differentiation products, varies in composition. Therefore any particular sample is not representative of this or that planet. The same applies to the moon, too. But since it has been studied well enough, we can pick and choose there depending on a specific objective-say, take volatile components that might have concentrated in cold sinks (on the bottom of not sunlit craters) and on the lunar poles.
We might add as well: orbital stations, landers, penetrometers, moon rovers and the like enable observations for a short time only. But rock samples brought to earth are a good object for long-term studies that could be resumed time and again with the further advancement of technology and analytical methods.
Unfortunately the "PHOBOS-GRUNT" project's future looks bleak. Our planetary research program adopted right after the abortive launching of the Russian Martian probe "Mars-96" (1996) provided for two space probes to be launched-one to the Moon (1999) and the other - to Phobos (2003). But thereupon the lunar project was scratched off the list, while "PHOBOS-GRUNT" was shelved up until the year 2005 (that was done in the interests of the astrophysical program whereby a series of "Spektr" satellites was to be launched). Today, once again, our project was put off till 2009 without any plausible excuses at all. What makes the injury double sure is that the astrophysical satellites (for the sake of which our planetary program was sacrificed) have never been launched. I have misgivings about the future of our project: closer to the deadline of 2009 the sample-taking project might be replaced by a simplified program of instrumental observations, that is without bringing core samples to our home planet, the earth. I wish my premonition could never come true.
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