SCIENTIFIC TECHNIQUES ASSOCIATED WITH THE INVESTIGATION OF PREVIOUS ACIDIC OCEANS ON MARS

ES2230B Introduction to Geochemistry

Abstract

Acidic oceans whether current or past are important to scientifically investigate. Acidic oceans could hold the key to answers regarding the origin of life as we know it and conversely, the inhibition of life. It is common knowledge that acidic environments make it extremely difficult for life to thrive in, or may altogether inhibit the development of life. However, there are also instances on Earth where life (microbial) can thrive in acidic environments with relatively low pH. Aside from what is currently known regarding acidic aqueous environments on Earth, data is also being collected on planetary bodies elsewhere in the Solar System in order to further investigate extraterrestrial acidic ocean environments. Mars has been the primary focus in this research because of the striking similarities Mars shares with Earth in terms of geology, geochemistry, and meteorology. Since the Viking Landers landing on Mars in 1976 to the current Mars Exploration Rovers (MER) of more recent years, much data has been collected from Mars mainly pertaining to the geology and geochemistry of the planet. One of the commonly researched aspects of Mars has been the possible existence of a previous acidic ocean or acidic aqueous environments.

1.1 Introduction

The Viking Landers and MER rovers on Mars have been equipped with various onboard instruments, allowing them the capability of sampling and data collection of the Martian surface. The predominant instruments onboard are a variety of spectrometers. The Viking Landers were equipped with an Energy-Dispersive X-ray Fluorescence Spectrometer (EDXRF), and the MER rovers were equipped with an Alpha Particle X-ray Spectrometer (APXS) and Mssbauer Spectrometer (MB) (Clark et al., 1976; Greenwood and Blake, 2006). EDXRF, APXS, and MB are all similar instruments, in the sense that they all conduct the process of applying an external force of one species of energy in order to result in the yield of a different species of energy to detect and measure in order to fabricate conclusions.

1.2 Applications

All three spectrometry techniques (EDXRF, APXS, and MB) have been useful for a wide variety of applications other than those regarding exploration on Mars. Spectrometry has been used for compositional analyses within geology, planetary science, food science, forensics, environmental studies, hydrological investigations, and biological studies, among multiple others (Gedcke et al., 2004; Dhara et al., 2010).

One of the most valuable and geochemistry-related applications of spectrometry proposed for the present and future, is further analyses to be conducted on other planetary bodies in and around the solar system. Main proposed targets to obtain spectrometry measurements are the Moon, Mercury, asteroids, and comets (Clark and Trombka, 1997).

2.0 Energy-Dispersive X-ray Fluorescence Spectrometry (EDXRF)

Energy-Dispersive X-ray Fluorescence Spectrometry allowed the Viking Landers to measure the composition of Martian soils; one of the most valuable measurements being the relatively high concentration of chlorine and sulfur within the Martian soil (Greenwood and Blake, 2006). The technique of EDXRF is the firing of x-rays onto a location or sample of interest. This causes the electrons of the samples atoms to be ejected outside of specific electron shells (ie: K and L shells). With electron vacancies within these shells, electrons residing in the higher energy shells will drop down into the vacancies within the K and L electron shells. Electrons dropping down in energy shells results in the emission of energy, and in the case of EDXRF the emitted energy is in the form of x-rays (Fig. 1) (Clark and Baird, 1973; Thermo Scientific, 2007).

Being capable of analyzing a variety of materials such as solids (pellets, and powder particles as small as 12 m) and liquids, makes this technique versatile and thus practical (West et al., 2010). In addition to the versatility of EDXRF, the technique also offers very precise readings possible in parts-per-million (ppm), and to the credit of further technological advancements of the technology, parts-per-billion (ppb) (Khamizov et al., 2005).

EDXRF allowed the Viking Landers to detect elevated concentrations of sulfur, phosphorus, and chlorine within Martian soil. Speculation over the sources and causes of these elevated concentrations have led to propositions that perhaps previous volcanic degassing (source of sulfur and chlorine) due to incorporation into the geochemical cycle (Fig. 2), has led to the weathering of igneous rocks, yielding phosphorus (Greenwood and Blake, 2006).

2.1 Alpha Particle X-ray Spectrometry (APXS)

Alpha Particle X-ray Spectrometry (APXS) is a technique very similar to that of EDXRF. APXS is a more modern technology and is an instrument mounted on the Mars Exploration Rovers (Rieder et al., 2013). The APXS instrument mounted on the Curiosity rover is capable of solely on contact mineralogy measurements; requiring no sample preparation such as powdering or grinding (Rieder et al., 2013). Similar to EDXRF, APXS is based on the introduction of an external source of energy onto the specimen of interest. In the case of the application of APXS on Mars, the specimen of interest is primarily rock and/or soil (Gellert et al., 2004). The external source of energy is comprised of alpha particles which are helium nuclei containing two protons and two neutrons. When alpha particles are fired at a sample the electrons of the atoms within the sample are forced to be ejected from the atomic structure. Similar with EDXRF, electrons vacancies are the result of the electrons being ejected. In order to maintain within a stable state, electrons from the outer electron shells then migrate to the lower shell (relax) and into the electron vacancies. The relaxation of electrons also results in the emission of energy, in the form of x-rays. Due to the specificity of x-rays that each individual element is characteristic of, an accurate measurement of the chemical composition of the specimen of interest can be measured.

2.2 Mssbauer Spectrometry (MB)

Modern MER rovers such as Opportunity are also equipped with a Mssbauer Spectrometer in addition to an AXPS spectrometer (Klingelhfer et al., 2004). Unlike APXS spectrometry, Mssbauer Spectrometry is specific to what samples it can measure. MB spectrometry strictly relies on the Mssbauer Effect, a phenomenon that occurs solely in iron-bearing minerals such as hematite (Fe2O3) and jarosite [KFe3(SO4)2(OH)6] (DeVoe and Spijkerman, 1966). Mssbauer Effect is process in which the isotopic decay of 57Fe nuclei results in the ejection of gamma rays. Measuring the gamma rays spectra allows for quantifying the magnitude, abundance, and distribution of iron within the targeted samples (Klingelhfer et al., 2004). The Opportunity rover as part of the Mars Exploration Rover mission, was equipped with a Mssbauer Spectrometer, allowing it to detect jarosite deposits in a Martian locality known as Meridiani Planum (Fig. 3) (Greenwood and Blake, 2006; JMARS v. 3.1.5 computer software, 2014). Jarosite deposits on the Martian surface have warranted speculation that perhaps Mars was once covered with an acidic ocean, or at least contained some form of acidic environment; this is due to jarosites formation to be strictly the result of acidic alterations and reactions (Greenwood and Blake, 2006).

3.0 Conclusions

Spectrometry provides invaluable knowledge pertaining to the elemental composition of a variety of substances and minerals, of which could also be in various form (ie: liquid, powder, solid, etc). The most emphasized geochemical applications of x-ray spectrometry are related to the techniques being used and applied within planetary science disciplines, particularly extraterrestrial exploration. The Viking Landers from 1970s were equipped with Energy-Dispersive X-ray Spectrometers, which allowed them to make the valuable findings of high concentrations of sulfur, chlorine in global Martian soils. In addition, the Mars Exploration Rovers (part of the modern MER mission) have also detected elevated concentrations of phosphorus in Martian soils in specific localities, along with jarosite mineral deposits, via Alpha Particle X-ray Spectrometry and Mssbauer Spectrometry.

These findings have led some in the scientific community to propose that Mars once had an acidic ocean, or some variant of an acidic hydrosphere. With current knowledge of extremophiles such as acidophilic organisms capable of thriving in acidic environments, proposals such as that of a possible previous acidic ocean on Mars could widen our scope of exploration for extraterrestrial life. This opens many more opportunities for discovery due to the elimination or decrease in the narrow criteria often abided by during the search for extraterrestrial life. Proposals such as a previous acidic ocean on Mars are extremely valuable when considering the possible existence of life on Mars and other planetary bodies in the past, present, or future.

Scientific exploration pertaining to the possible existence of extraterrestrial life (as we know it) is one of the most sought to explore. In addition to the instinctiveness of human curiosity regarding life elsewhere, further investigating through the lens of biological science is likely to result in improvements in the discipline on Earth as well. This could ultimately assist human kind with improving the understanding of many scientific disciplines, as well as these disciplines elsewhere in the Solar System and possibly further out into the universe.

FIGURES

Figure 1 - Illustration of EDXRF technique (Thermo Scientific, 2007).

Figure 2 - Illustration of the geochemical cycle of P, Cl, and S on Mars (Greenwood and Blake, 2006).

Figure 3 - A satellite image of Meridiani Planum, Mars (JMARS v. 3.1.5 computer software, 2014).

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