Traces of Space Climate Events in the Geologic Record

The solar corona is the outermost region of the Sun's atmosphere. Its expansion induces a flux of protons, electrons and nuclei of heavier elements (including the noble gases). These interplanetary particles are accelerated by the high temperatures of the solar corona, to high velocities that allow them to escape from the Sun's gravitational field. The wind contains approximately five particles per cubic centimetre moving outward from the Sun at velocities of 3 x 105 to 1 x 106 m s-1; this creates a positive ion flux of just over 100 ions cm-2 s-1, each ion having an energy equal to at least 15 eV The solar wind reaches the surface of the Moon modifying its upper surface or regolith. We have considerable information on the lunar regolith thanks to the Apollo Missions.

In the years 1969-1972 these missions retrieved so much material and made it available to many laboratories that influenced much of our preliminary understanding of the origin of life on the early Earth. These missions gave an opportunity for detailed studies of isotopic fractionation of the biogenic elements on the surface of the Moon. In general terms, the preliminary understanding that the Apollo Missions added to the work that was available at the time on meteorites was related to the fractionation of H, C, N and S on the lunar surface. In fact, the preliminary hint that was relevant for the origin of life was that the distribution range of 32S/34S appears to be narrower than the isotopic ratio of hydrogen, carbon or nitrogen. For this reason, it was suggested that the fractionation of S isotopes would be the most reliable parameter for estimating biological effects

(Kaplan, 1975; Chela-Flores, 2007). Deviations of 32S/34S from meteoritic values discovered on the Moon by the Apollo missions can be understood by the fact that the solar wind modifies its structure leaving a tell-tale signal of how it changes over geologic time, since the Moon is an inactive body being modified only by the impacts of meteorites and asteroids.

Much more recently, the Genesis Mission was NASA's first sample return mission sent to space. It was the fifth of NASA's Discovery missions. Genesis was launched in the year 2001 with the intention to bring back samples from the Sun itself. Three years later, after crash-landing, the probe was retrieved in Utah, USA. Genesis collected particles of the solar wind on wafers of gold, sapphire, silicon and diamond. The amount of stardust collected by Genesis was about 1020 ions, or equivalently, 0.5 mg. Preliminary studies indicate that contamination did not occur to a significant extent. The objective is to obtain precise measures of solar isotopic abundances. By measuring isotopic compositions of oxygen, nitrogen, and noble gases we would have data that will lead to better understanding of the isotopic variations in meteorites, comets, lunar samples, and planetary atmospheres. This will lead to a deeper understanding of the early Solar system, and hence an additional opportunity beyond fossils for a closer approach to the mystery of the origin of life on Earth by being able to assess properly potential biomarkers that may be suggested from the point of view of biogeochemistry.

There will be also attempts to use Accelerator Mass Spectrometry (Tuniz et al., 1998) to detect a long-lived radionuclide of solar wind origin, for example such as 10Be and 26Al (Jull and Burr, 2006).

The Moon is depleted of volatile elements such as hydrogen, carbon, nitrogen and the noble gases, consistent with the fact that the most widely accepted theory of its formation is the impact of the Earth by a Mars-sized body during the accretion period. Exceptionally though, volatiles are abundant in lunar soils. The lunar surface evolved during the heavy bombardment period, adding material with a different composition to the Sun, and not derived from the Sun: The variability of 14N and 36Ar in grains (single mineral and glass) from a lunar soil were measured by laser extraction, to study the origin of trapped nitrogen in the lunar regolith (Wieler et al., 1999). The ratio of N is very uniform relative abundances of Ar, Kr, and Xe trapped from the solar radiation observed in mineral grains from the same soil. This strongly suggests that, on average, some 90% of the N in the grains has a non-solar source. This seems to suggest that the non-solar N has not been trapped by ion implantation.

Ions from the solar wind were known to have been directly implanted into the lunar surface (Kerridge et al., 1991). This component was detected during the Apollo missions. The isotopic composition of the noble gases in lunar soils has been established as being subsequent to the formation of the Moon itself.

The production of a long-lived radionuclide on the moon can provide information about the flux of galactic and solar cosmic rays in the past. This can also be done on meteorites from the moon (Gnos et al., 2004).

To gain further insights into the early Solar system, evidence has been sought for a predominantly non-solar origin of nitrogen in the convenient source of information that is represented by the lunar regolith. This search suggests that, on average, some 90% of the N in the grains has a non-solar source, contrary to the view that essentially all N in the lunar regolith has been trapped from the solar wind, but this explanation has difficulties accounting for both the abundance of nitrogen and a variation of the order of 30% in the 15N/14N ratio. The origin of the non-solar component remains a puzzle, but it presumably must have changed its isotopic composition over the past several billion years. The Moon regolith presents a very challenging geological phenomenon. It consists of a very large number of grains with a rich history regarding their exposure to the Sun. Two parameters are useful in the systematic study of the lunar regolith: firstly, its 'maturity namely, the duration of solar wind exposure and, secondly the 'antiquity', namely, how long ago the exposure took place.

For the maturity parameter a useful way to measure it is in terms of the abundance of an element from the solar wind that is efficiently retained. The element nitrogen is a good example. (Alternatively solar noble-gas elements can be used.) Both antiquity and maturity have been used to learn about the evolution of the early Solar system, especially the ancient Sun, the knowledge of which is needed for a comprehensive understanding of the problem of the origin of life on Earth. The exposure age to galactic cosmic rays produce certain nuclides in amounts proportional to the time the sample spends at the topmost part of the surface (some 2 m). The contrast between the known low abundance of a certain nuclide and the one induced by cosmic rays produce an indicator of antiquity. The antiquity parameter has been discussed in detail (Kerridge, 1975). A related question is the search for live 244Pu (half-life = 81 Ma) that is expected to be present in the interstellar medium (ISM) from ongoing nucleosynthesis. The use of resonant ionization mass is capable of detecting extremely low levels of this isotope that may have accreted onto Earth from the ISM (Ofan et al., 2006).

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