Meteorites from Mars

Figure 4.6 Bulk oxygen isotopic compositions of (a) achondrites and meteorites from Mars, the Moon, and Vesta (b) chondrites (after Yurimoto el al. 2006).

Meteorites come from more massive parents (mainly minor planets - i.e, asteroids - but also Mars and our Moon) and to get to Earth, their immediate predecessors - meteoroids - can only be excavated from its parent body by an explosive impact. This impact almost always generates short-lived but intense shock effects, results of which are present in many individual meteorites and groups of them (7) but which I do not discuss further here. The impact also provides the impulse necessary for the meteoroid to exceed the parent body s escape velocity e.g. 5.4 km/s for Mars. Thus, meteoritic minerals provide barometers for shock pressures up to -60 GPa (6x10 atm) corresponding to post-shock temperatures >1250°C at much higher pressures (temperatures), these materials vaporize. Incidentally, this explains why Earth receives meteorites from Mars but not Venus, which is closer Venus escape velocity is... 

Then came the basic question, still not quite settled, of how many ejection events were required to yield the Martian meteorites that we have. At first, given the perceived difficulty of ejecting any meteorites from Mars, scenarios were suggested with as few... 

The concentrations of chlorine, bromine, and iodine of meteorites measured by Goles et al. (1967) raised the suspicion that non-Antarctic meteorites in museums had been contaminated by being handled with bare hands by collectors and because they had been stored for years in display cases through which air could circulate freely. Therefore, Dreibus and Wanke (1983) and Dreibus et al. (1986) measured the concentrations of halogens in Antarctic meteorites in order to document the contamination of non-Antarctic meteorites. The results in Table 18.7 danonstrated that Antarctic meteorites actually have unexpectedly higher concentrations of fluorine, chlorine, and iodine than similar meteorites of non-Antarctic origin. Even Antarctic martian meteorites have higher iodine concentrations than non-Antarctic meteorites from Mars. 

Nd/ Nd (which is not bracketed in mass by Nd/ Nd) for mass bias render the application of the extinct " Sm— " Nd decay scheme particularly challenging. For example, SNC meteorites from Mars (Table 10.1) define a larger range in " Nd abundances than samples from any other planetary body, but the Nd/ " " Nd ratios still vary by only about 1 e unit (1 part in 10 000). In contrast, the " Nd/ Nd isotope ratios of Martian meteorites display a variability of about 60 . Although Nd can be measured by MC-ICP-MS [73, 74], the stability of the plasma source is generally not sufficient to resolve the small differences in Nd/ Nd of most natural samples, with the possible exclusion of Martian meteorites. Flence Nd analyses are generally conducted by TIMS, which can provide better precision for a comparable sample size [75]. 

The photo below, taken by the Ttl/ng spacecraft, shows that the surface of Mars has been eroded, apparently by liquid water. More recent photos transmitted by Spirit and Opportunity convince scientists that this was the case. Apparently, Mars was once much warmer than it is today. Planetary scientists speculate that at one time the atmosphere of Mars may have contained large amounts of carbon dioxide, setting up a greenhouse effect that made the surface of that planet warmer and wetter. Might there, then, have been life on Mars at some earlier time Molecular stmctures found in meteorites thought to come from Mars have been interpreted to show that there was once life there, but these results are controversial. 

The two rare earth elements niobium (Nb) and tantalum (Ta) were the main subject of study in the investigation referred to. Both elements have very similar properties and almost always occur together in our solar system. However, the silicate crust of the Earth contains around 30% less niobium (compared to its sister tantalum). Where are the missing 30% of niobium They must be in the Earth s FeNi core. It is known that the metallic core can only take up niobium under huge pressures, and the conditions necessary for this may have been present on Earth. Analyses of meteorites from the asteroid belt and from Mars show that these do not have a niobium deficit. 

The experiments were intended to clarify the question as to whether transmission of spores from Mars to Earth could be feasible. To do this, a Mars meteorite was simulated, i.e., the spores were mixed with powdered rock and the mixture pressed together to give a small cube about 1 cm3 in size. The spore concentration was about the same as in normal soil on Earth. The samples were in orbit for around 2 weeks, and their survival ability was determined on their return to Earth, compared with the corresponding samples which had been left on Earth (control experiment). 

A few meteorites have significantly younger ages these are believed to come from the Moon and in some cases from Mars, rather than from asteroids. 

Comparison of trapped gases in impact-melt glass in the EET 79001 Martian meteorite with the composition of the Martian atmosphere as measured by Viking landers. This remarkable agreement is the evidence that convinced most planetary scientists that SNC meteorites came from Mars. 

In this chapter, we review what is known about the chronology of the solar system, based on the radioisotope systems described in Chapter 8. We start by discussing the age of materials that formed the solar system. Short-lived radionuclides also provide information about the galactic environment in which the solar system formed. We then consider how the age of the solar system is estimated from its oldest surviving materials - the refractory inclusions in chondrites. We discuss constraints on the accretion of chondritic asteroids and their subsequent metamorphism and alteration. Next, we discuss the chronology of differentiated asteroids, and of the Earth, Moon, and Mars. Finally, we consider the impact histories of the solar system bodies, the timescales for the transport of meteorites from their parent bodies to the Earth, and the residence time of meteorites on the Earth s surface before they disintegrate due to weathering. 

Martian meteorites and Mars rover analyses suggest that it is a basalt-covered world, a conclusion supported by orbital measurements. Basalts of different ages appear to have distinct compositions. Since its original differentiation, the Martian mantle has remained geochemically isolated, although it is periodically melted to produce basalts. The core has an appreciable amount of sulfide, as inferred from trace elements in basalts. Water, once important in producing clays and sulfates, has now retreated into the subsurface. 

The composition of Martian surface materials can be assessed using laboratory analyses of Martian meteorites, in situ APXS analyses from Mars Pathfinder and the Mars Exploration Rovers, and orbital geochemistry analyzed by GRS and derived from TES spectra. 

HARRY (Hap) Y. McSWEEN Jr. is Chancellor s Professor at the University of Tennessee. He has conducted research on cosmochemistry for more than three decades and was one of the original proponents of the hypothesis that some meteorites are from Mars. He has been a co-investigator for four NASA spacecraft missions and serves on numerous advisory committees for NASA and the US National Research Council. Dr. McSween has written or edited four books on meteorites and planetary science, and coauthored a textbook in geochemistry. He is a former president and Fellow of the Meteoritical Society, a Fellow of the American Academy of Arts and Sciences, recipient of the Leonard Medal, and has an asteroid named for him. 

On the basis of their chemistry, a small number of meteorites are believed to have originated from Mars. Warren, Kallemeyn and Kyte (1999, 2107, 2114) had six suspected Martian meteorites analyzed for arsenic. Arsenic was listed as not detected in two of the samples (ALH77005 and Y-793605). Four others (ALH84001, EET70001, EET79001, and QUE94201) had <0.03 to <0.7 mg kg-1 of arsenic (Warren, Kallemeyn and Kyte, 1999 Table 3.2). Warren, Kallemeyn and Kyte (1999, 2107, 2114) also admit that the arsenic analyses were imprecise (within about 10%) and that they could have been influenced by terrestrial weathering. 

Sheroottites. The shergottite meteorites are a rare class of meteorites which consist essentially of pyroxene and maskelynite. Their composition, petrography, age and gases trapped within the fabric of the meteorite have led to a widespread idea that these meteorites, and the related nakhlites and Chassigny meteorites, were ejected from Mars,... 

Our contribution to this project was to find and characterize these PAHs. In themselves, they could be biomarkers of fossilized organic matter of long ago, or they could have come from some nonliving source, such as metal-catalyzed reactions of organics on hot surfaces. If we could establish these PAHs as indigenous to the meteorite, then they would be the first observation of organic molecules found from Mars. 

Wood C. A. and Ashwal L. D. (1981) SNC meteorites igneous rocks from Mars Proc. Lunar Planet. Sci. Conf. 12B, 1359-1376. 

Figure 10 Oxygen isotopic compositions of whole-rock meteorites from differentiated bodies HED, possibly asteroid Vesta SNC, possibly Mars and Moon. Each body produces a slope-1/2 mass-dependent fractionation line, with values of characteristic of the whole source planet. The isotopic compositions of...

More martian than lunar meteorites appear to have come to Earth per crater. According to Mileikowsky et al. (2000), the total mass of the ejecta increases with the size of the impact event. With more energy required for launch from Mars, and with launches rare, a greater likelihood of the pairing of source craters for martian meteorites seems reasonable. 

Figure 3 The oxygen isotopic compositions of the Earth and Moon are identical to extremely high precision and well resolved from the compositions of meteorites thought to come from Mars and Asteroid 4 Vesta (sources Wiechert et al, 2001, 2003).

Figure 11 Chemical classification of martian volcanic rocks. Squares show basaltic materials in the southern highlands (surface 1) and andesitic materials in the northern lowlands (surface 2), derived from deconvolved TES spectra from Mars Global Surveyor (Hamilton et al., 2001). Analyzed compositions of the Mars Pathfinder dust-free rock (Wanke et al., 2001) and martian meteorites (basaltic shergottites are filled circles and nakhlites are...

Grady M.M. (1999) Meteorites and microfossils from Mars. Geoscientist 9, 4—7. 

Meteorites, which come mainly from various asteroids but also from Mars and our Moon, provide otherwise unobtainable information about objects in the inner Solar System in both space and time. The presence of meteorites on Earth allows application of the full gamut of instrumentation able to analyze materials from the asteroids (minor planets). Mars and the Moon with state of the art sensitivity and accuracy. Many meteorites include material that condensed and accreted in the primitive Solar nebula and were subsequently unaltered some contain evidence for pre-Solar nuclear processes. Information in meteorites tracks the evolution of their parent bodies, both interiors and surfaces, the impacts that ejected them and the nuclear radiation history that occurred as they travelled Earthward. Meteoritic material also allows the dating of all of these episodes and determination of the composition of the Sun s surface and the particles streaming from it. This chapter is a brief tutorial outlining this information and indicating how it is obtained. 

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