Volatile Inventory Research Articles

Origin of Noble Gases in the Terrestrial Planets

Identifying the mechanisms that drove the evolution of planetary volatiles from primordial to present-day compositions is one of the classic challenges in the planetary sciences. The field bristles with models of one type or another, but none are without difficulties. Efforts to understand the histories of volatile species in the atmospheres and interiors of the terrestrial planets have concentrated on their noble gases, free of the entanglements of chemical interaction, as evolutionary tracers. The elemental and isotopic compositions of these minor constituents are rich in clues to the chemical characteristics of their source reservoirs, the physics of their evolution, and the nature of the astrophysical and planetary environments in which they evolved. Most workers would agree that atmospheric mass distributions of the nonradiogenic noble gases were probably established very early, through the action of processes operating before, during, or shortly after planetary accretion. But beyond this there are no certainties as yet on the specifics of sources or mechanisms. The question of past and present volatile inventories on and in the terrestrial planets is intrinsically interesting in the more general context of the evolution of the planets themselves. For each individual body it crosscuts issues relating to atmospheric origin and compositional history, emergence of a coupled atmosphere-surface system and climate evolution, surface morphology and its record of past geological processing, and planetary rheology, differentiation, and degassing history. But the volatile problem extends well beyond its relationship to the initial state of a particular planet and the mechanisms that drove it down its specific evolutionary track. Attempts to decipher planetary histories in the broader context of the evolution of the solar system as a whole are focusing more and more attention on the sources and processing of volatiles in the primordial solar accretion disk, in primitive meteorites, and in the terrestrial …

Formation and early evolution of the atmosphere

Abstract The tectonic activity of the Earth allowed exchange of volatile elements (H, C, N, rare gases) between the surface of the Earth (atmosphere, crust, sediments, oceans) and the mantle. However, some of these elements still present elemental and isotopic heterogeneities that allow us to reconstruct the volatile composition of the terrestrial mantle. The protosolar nebula supplied a significant fraction of helium and neon, which were presumably trapped during the major phase of the Earth’s accretion and were possibly hosted by accreting dust and/or small porous planetesimals. Surprisingly, volatile elements are in chondritic proportion despite their drastic (10 −3 ) depletion in the mantle relative to chondrites, in a way that recalls the case of highly siderophile elements. From stable isotope systematics, we find that the contribution of comets to the volatile inventory of the Earth was very limited. The integrated flux of chondritic-like material necessary to provide water, carbon and nitrogen is consistent with that required for the formation of the lunar craters as well as that necessary to account for the inventory of siderophile elements in the mantle. A consequence of this scenario is that the Earth’s surface was oxidized very early. Alternatively, volatile and siderophile elements of the mantle could be the remnant of small patches of chondritic material that did not equilibrate with the core nor drastically degas.

The Galactic Habitable Zone: Galactic Chemical Evolution

We propose the concept of a “Galactic Habitable Zone” (GHZ). Analogous to the Circumstellar Habitable Zone (CHZ), the GHZ is that region in the Milky Way where an Earth-like planet can retain liquid water on its surface and provide a long-term habitat for animal-like aerobic life. In this paper we examine the dependence of the GHZ on Galactic chemical evolution. The single most important factor is likely the dependence of terrestrial planet mass on the metallicity of its birth cloud. We estimate, very approximately, that a metallicity at least half that of the Sun is required to build a habitable terrestrial planet. The mass of a terrestrial planet has important consequences for interior heat loss, volatile inventory, and loss of atmosphere. A key issue is the production of planets that sustain plate tectonics, a critical recycling process that provides feedback to stabilize atmospheric temperatures on planets with oceans and atmospheres. Due to the more recent decline from the early intense star formation activity in the Milky Way, the concentration in the interstellar medium of the geophysically important radioisotopes 40K, 235,238U, and 232Th has been declining relative to Fe, an abundant element in the Earth. Also likely important are the relative abundances of Si and Mg to Fe, which affects the mass of the core relative to the mantle in a terrestrial planet. All these elements and isotopes vary with time and location in the Milky Way; thus, planetary systems forming in other locations and times in the Milky Way with the same metallicity as the Sun will not necessarily form habitable Earth-like planets. As a result of the radial Galactic metallicity gradient, the outer limit of the GHZ is set primarily by the minimum required metallicity to build large terrestrial planets. Regions of the Milky Way least likely to contain Earth-mass planets are the halo (including globular clusters), the thick disk, and the outer thin disk. The bulge should contain Earth-mass planets, but stars in it have a mix of elements different from the Sun's. The existence of a luminosity–metallicity correlation among galaxies of all types means that many galaxies are too metal-poor to contain Earth-mass planets. Based on the observed luminosity function of nearby galaxies in the visual passband, we estimate that (1) the Milky Way is among the 1.3% most luminous (and hence most metal-rich) galaxies and (2) about 23% of stars in a typical ensemble of galaxies are more metal-rich than the average star in the Milky Way. The GHZ zone concept can be easily extrapolated to the universe as a whole, especially with regard to the changing star formation rate and its effect on metallicity and abundances of the long-lived radioisotopes.

Mars' volatile and climate history.

There is substantial evidence that the martian volatile inventory and climate have changed markedly throughout the planet's history. Clues come from areas as disparate as the history and properties of the deep interior, the composition of the crust and regolith, the morphology of the surface, composition of the present-day atmosphere, and the nature of the interactions between the upper atmosphere and the solar wind. We piece together the relevant observations into a coherent view of the evolution of the martian climate, focusing in particular on the observations that provide the strongest constraints.

Controls on magmatic degassing along the Reykjanes Ridge with implications for the helium paradox

To consider the 3He characteristics of plume-related lavas, we report a detailed survey of helium isotope ( 3He/ 4He) and concentration ([He]) variations along an 800-km transect of the Reykjanes Ridge (RR). 3He/ 4He ratios vary from 11.0 to 17.6 R A (where R A=air 3He/ 4He) whereas [He] ranges over three orders of magnitude from >5 μcm 3 STP/g – in the range of most mid-ocean ridge basalts (MORB) – to lows of 4 ncm 3 STP/g. The lowest [He] and intermediate 3He/ 4He ratios occur along the northern RR (closest to Iceland) where eruption depths are shallow (<1000 m) and water contents of lavas are high (0.3–0.4 wt%). We suggest that low-pressure, pre-eruptive magmatic degassing is extensive in this region with degassed magmas susceptible to addition of radiogenic helium thereby lowering 3He/ 4He ratios. Along the southern RR, [He] reaches maximum values, and 3He/ 4He ratios display strong correlations with lead isotopes ( 206Pb/ 204Pb) consistent with binary mixing. These correlations indicate that the high- 3He/ 4He plume component has higher absolute abundances of the primordial isotope 3He compared to the source of depleted MORB mantle. This finding implies that the so-called ‘helium paradox’ – the observation that plume-derived oceanic glasses apparently have lower 3He contents than MORB glasses – may be an artifact related to considering lavas (e.g. from Loihi seamount, Hawaii) which have not retained their source volatile inventory as well as those erupted along the southern RR.

Stereo topography of the south polar region of Mars: Volatile inventory and Mars Polar Lander landing site

Viking stereo images and topographic maps reveal that the south polar layered deposits of Mars are topographically complex and morphologically distinct from the north polar layered deposits. The dominant feature is a 500‐km‐wide topographic dome that rises 3 km above the surrounding plains. This dome underlies the residual ice cap but is at least 50% larger in area. Erosional scarps and terraces indicate that this dome was once more extensive and has undergone erosional retreat. Adjacent to the dome, layered deposits form a vast plateau 1–1.5 km high extending ∼1000 km beyond and to one side of the residual south polar cap. This plateau is relatively flat at kilometer scales, although it is cut in places by troughs and depressions, which have locally steep scarps up to 2 km high and sloping up to roughly 10°. Contiguously flat kilometer‐scale regions the size of the Mars Polar Lander (MPL) landing ellipse are present. These are in the form of plateaus 100–300 km wide and 1–2 km high. One of the largest of these plateaus has been proposed as a landing site for the Mars Polar Lander (MPL). The volume associated with the south polar layered deposits may be comparable to those of the layered deposits at the north pole. Although this doubles the current probable inventory of surface ice on Mars, it still falls far short of accounting for the inferred volume of water on Mars in the past.

On the origin of Titan’s atmosphere

The present atmosphere of Titan exhibits evidence of extensive evolution, in the form of rapid photochemical destruction of methane and a large fractionation of the nitrogen and oxygen isotopes. Attempts to recover the initial inventory of volatiles lead toward a model in which nitrogen was originally supplied as NH 3, essentially unmodified from its relative abundance in the outer solar nebula. Titan’s atmospheric methane, in contrast, appears to have been formed from carbon and other carbon compounds, either by gas phase reactions in the subnebula or by accretional heating during the formation of Titan. These conclusions can be tested by further studies of abundances and isotope ratios in Titan’s atmosphere, augmented by studies of comets. The possible similarity of carbon and nitrogen inventories on Titan to those on the inner planets makes this investigation particularly intriguing.

Water Production and Release in Comet C/1995 O1 Hale–Bopp

Water (H 2O) was detected in Comet C/1995 O1 Hale–Bopp on 10 dates between UT January 21.8 and May 1.2, 1997, using high-resolution infrared spectroscopy. This is the first study of the heliocentric dependence of water released from a comet using direct detection of H 2O itself. Production rates and rotational temperatures were measured, and the derived heliocentric dependence for the water production rate is Q=(8.35±0.13)×10 30 [ R h (−1.88±0.18)] molecules s −1. The spatial distribution of H 2O molecules in the coma is consistent with water being released directly from the nucleus within 1.5 AU of the Sun, although release of a small fraction from icy grains cannot be excluded. When our derived water production rates are compared to the production of native carbon monoxide and dust, we obtain a dust to ice mass ratio of 5.1±1.2 within a heliocentric distance of 1.5 AU. The abundance of H 2O provides a benchmark for the volatile inventory in Hale–Bopp and, when compared to interstellar and nebular material, helps constrain the origin of cometary ices and their processing histories. These production rates derived from the direct detection of H 2O provide a sound basis with which water production rates inferred by indirect methods can be compared.

A Tale of Two Planets

Most of Earth's volcanism occurs at plate boundaries—mid-ocean ridges and island arcs. Most of the 3He and other noble gases also are expelled from the interior at these locations. A small amount, compared to the global budget, of magma (<10%) and 3He («1%) escapes at mid-plate volcanoes, the socalled “hotspots.” Locations of hotspots are controlled by stresses and cracks, not by deep, narrow mantle upwellings. Most, or all, of Earth's volatile inventory appears to have been brought in by a late veneer. The high 3He content and high He/Ne and He/Ar ratios of mid-ocean ridge basalts, plus the excess 129Xe and 136Xe, suggest that the ridge basalt reservoir is relatively undegassed and that there is no primordial undegassed reservoir. The two prevailing views of the origin of Earth are summarized in the context of this information.

Obliquity‐oblateness feedback on Mars

A simple model is presented for the coupled dynamics of the orbit‐rotation‐climate system of Mars. Changes in the orientation of the spin pole, relative to the orbit pole, influence the spatiotemporal pattern of incident radiation and thus drive climatic mass transport into and out of the polar regions on a variety of timescales. Changes in the mass distribution occur from direct climatic forcing and compensating viscous flow in the interior. The net change in mass distribution influences the rate of spin axis precession and thereby influences obliquity. The rate of secular obliquity drift depends on several poorly known parameters, including the magnitudes and response times of volatile inventories and viscosity structure within Mars. Even relatively modest secular obliquity drift can lead to trapping in nearby resonances. The dissipative nature of the coupled dynamical system makes reconstruction of past evolution much more difficult than for a purely inertial system. The long‐term obliquity history of Mars is dominated by climate.

Analytic investigation of climate stability on Titan: sensitivity to volatile inventory

We develop a semiempirical grey radiative model to quantify Titan’s surface temperature as a function of pressure and composition of a nitrogen-methane-hydrogen atmosphere, solar flux and atmospheric haze. We then use this model, together with non-ideal gas-liquid equilibrium theory to investigate the behavior of the coupled surface-atmosphere system on Titan. We find that a volatile-rich Titan is unstable with respect to a runaway greenhouse—small increases in solar luminosity from the present value can lead to massive increases in surface temperature. If methane has been photolyzed throughout Titan’s history, then this runaway can only be avoided if the photolytic ethane is removed from the surface-atmosphere system.

Amino acid survival in large cometary impacts

Abstract— A significant fraction of the Earth's prebiotic volatile inventory may have been delivered by asteroidal and cometary impacts during the period of heavy bombardment. The realization that comets are particularly rich in organic material seemed to strengthen this suggestion. Previous modeling studies, however, indicated that most organics would be entirely destroyed in large comet and asteroid impacts. The availability of new kinetic parameters for the thermal degradation of amino acids in the solid phase made it possible to readdress this question.We present the results of new high‐resolution hydrocode simulations of asteroid and comet impact coupled with recent experimental data for amino acid pyrolysis in the solid phase. Differences due to impact velocity as well as projectile material have been investigated. Effects of angle of impacts were also addressed. The results suggest that some amino acids would survive the shock heating of large (kilometer‐radius) cometary impacts. At the time of the origins of life on Earth, the steady‐state oceanic concentration of certain amino acids (like aspartic and glutamic acid) delivered by comets could have equaled or substantially exceeded concentrations due to Miller‐Urey synthesis in a CO2‐rich atmosphere. Furthermore, in the unlikely case of a grazing impact (impact angle ∼5° from the horizontal), an amount of some amino acids comparable to that due to the background steady‐state production or delivery would be delivered to the early Earth.

On the volatile inventory of Titan from isotopic abundances in nitrogen and methane

We analyze recently published nitrogen and hydrogen isotopic data to constrain the initial volatile abundances on Saturn’s giant moon Titan. The nitrogen data are interpreted in terms of a model of non-thermal escape processes that lead to enhancement in the heavier isotope. We show that these data do not, in fact, strongly constrain the abundance of nitrogen present in Titan’s early atmosphere, and that a wide range of initial atmospheric masses (all larger than the present value) can yield the measured enhancement. The enrichment in deuterated methane is now much better determined than it was when Pinto et al. (1986. Nature 319, 388–390) first proposed a photochemical mechanism to preferentially retain the deuterium. We develop a simple linear theory to provide a more reliable estimate of the relative dissociation rates of normal and deuterated methane. We utilize the improved data and models to compute initial methane reservoirs consistent with the observed enhancement. The result of this analysis agrees with an independent estimate for the initial methane abundance based solely on the present-day rate of photolysis and an assumption of steady state. This consistency in reservoir size is necessary but not sufficient to infer that methane photolysis has proceeded steadily over the age of the solar system to produce large quantities of less volatile organics. Our analysis indicates an epoch of early atmospheric escape of nitrogen, followed by a later addition of methane by outgassing from the interior. The results also suggest that Titan’s volatile inventory came in part or largely from a circum-Saturnian disk of material more reducing than the surrounding solar nebula. Many of the ambiguities inherent in the present analysis can be resolved through Cassini–Huygens data and a program of laboratory studies on isotopic and molecular exchange processes. The value of, and interest in, the Cassini–Huygens data can be greatly enhanced if such a program were undertaken prior to the prime phase of the mission.

Primordial noble gases from Earth's mantle: identification of a primitive volatile component

Carbon dioxide well gases in Colorado, New Mexico, and South Australia show excesses of (124-128)Xe correlated with (129)I-derived (129)Xe and (20)Ne/(22)Ne ratios that are higher than the atmospheric (20)Ne/(22)Ne ratio. The xenon isotopic data indicate the presence of a solarlike component deep within Earth. The presence of this component in crustal and upper mantle reservoirs may be explained by a steady-state transport of noble gases from the lower mantle, which still retains much of its juvenile volatile inventory. These measurements also indicate that the mantle source of these noble gases in the carbon dioxide well gases cannot be the source of Earth's present atmosphere. The variations observed in (129)Xe/(130)Xe between solar wind xenon, Earth's atmosphere, and mantle samples may be generated by variations of iodine/xenon in terrestrial reservoirs, as opposed to rapid early degassing.

A theory of the Earth: Hutton and Humpty Dumpty and Holmes

Abstract Paradoxes can be viewed as a bother or as a rich source of information. Paradoxes, fallacies and logical inconsistencies, along with the unstated assumptions behind old worldviews often form the foundation for a new worldview. This is where science differs from myth and just-so stories. Mantle dynamicists and geochemists have developed a worldview that, in many respects, is incompatible with modern theories of planetary accretion, geodynamics and geophysics, and with the geological context of volcanoes. A primordial undegassed mantle, as both the furnace and the fuel of midplate volcanoes, occupies a central role in the geochemical models. Although modern data alone are sufficient to overturn this view, it is interesting to address it in terms of its own assumptions, fallacies and inconsistencies, and the history of ideas. This is a vast field, so, in honour of James Hutton, I focus on volcanoes, their products, their causes and their sources. The following axioms are developed which point toward a different kind of world from the pristine primordial worldviews and the plume paradigm. • Linear volcanic chains delineate the stress field, not the displacement field, of the lithosphere. • Lithospheric architecture, not the core-mantle boundary, dictates the planform of convection in the mantle. • Incipient melting is the normal state of the upper mantle. • The lower mantle is depleted in the heat producing elements compared to the crust, upper mantle and the bulk silicate Earth. • The Earth is depleted in volatile elements. • High 3 He/ 4 He ratios in mantle materials imply a deficit of 4 He. • Primordial heat contributes a significant part of the Earth’s heat flow. These axioms [ A ] are the inverse of the assumptions of the plume and primordial mantle paradigms [not- A ] and the new and the old views are to each other as looking-glass worlds. An ‘axiom’ is a self-evident truth or an established rule. It is also an undemonstrated proposition concerning an undefined set of elements and relationships. The above ‘axioms’ [ A ] are not true axioms but [not- A ] provide the axiomatic framework of the current world model. These [not- A ] propositions are treated as self-evident truths so [ A ] can also be called axioms, even though they are falsifiable. The prevailing worldview, the ‘standard model’, attributes both melting and volcanic chains to deep mantle upwellings controlled by the bottom of the system with most of the original radioactivity and volatile inventory of the Earth in the deep mantle. This is not only inconsistent with modern data but also with the previous worldview embedded in the axioms. These axioms have not been pushed out by better ideas, or falsified; they have simply been overlooked or forgotten. Mantle dynamics is controlled from the top, not the bottom.

This Week in Science

Much of the current visible inventory of volatiles on Mars resides in its polar caps. Understanding the history of water on Mars requires an accurate measure of the size of the north polar cap and the elevation of the polar basin with respect to the elevation of lower latitudes. The Mars Global Surveyor (MGS) has been examining the northern hemisphere of Mars closely, and Zuber et al. (p. 2053) now present an analysis of the topography of the north polar region and the ice cap. The data show that the cap is much smaller than previously assumed (about half the size of the Greenland Ice Sheet), is cut by deep fissures, was once somewhat larger, and resides in a topographic basin that is at a lower elevation than low-latitude regions. This result may negate models in which groundwater flows southward from the ice cap toward the equator. MGS also imaged clouds forming above the ice cap.

The composition of the Jovian atmosphere as determined by the Galileo probe mass spectrometer.

The Galileo probe mass spectrometer determined the composition of the Jovian atmosphere for species with masses between 2 and 150 amu from 0.5 to 21.1 bars. This paper presents the results of analysis of some of the constituents detected: H2, He, Ne, Ar, Kr, Xe, CH4, NH3, H2O, H2S, C2 and C3 nonmethane hydrocarbons, and possibly PH3 and Cl. 4He/H2 in the Jovian atmosphere was measured to be 0.157 +/- 0.030. 13C/C12 was found to be 0.0108 +/- 0.0005, and D/H and 3He/4He were measured. Ne was depleted, < or = 0.13 times solar, Ar < or = 1.7 solar, Kr < or = 5 solar, and Xe < or = 5 solar. CH4 has a constant mixing ratio of (2.1 +/- 0.4) x 10(-3) (12C, 2.9 solar), where the mixing ratio is relative to H2. Upper limits to the H2O mixing ratio rose from 8 x 10(-7) at pressures <3.8 bars to (5.6 +/- 2.5) x 10(-5) (16O, 0.033 +/- 0.015 solar) at 11.7 bars and, provisionally, about an order of magnitude larger at 18.7 bars. The mixing ratio of H2S was <10(-6) at pressures less than 3.8 bars but rose from about 0.7 x 10(-5) at 8.7 bars to about 7.7 x 10(-5) (32S, 2.5 solar) above 15 bars. Only very large upper limits to the NH3 mixing ratio have been set at present. If PH3 and Cl were present, their mixing ratios also increased with pressure. Species were detected at mass peaks appropriate for C2 and C3 hydrocarbons. It is not yet clear which of these were atmospheric constituents and which were instrumentally generated. These measurements imply (1) fractionation of 4He, (2) a local, altitude-dependent depletion of condensables, probably because the probe entered the descending arm of a circulation cell, (3) that icy planetesimals made significant contributions to the volatile inventory, and (4) a moderate decrease in D/H but no detectable change in (D + 3He)/H in this part of the galaxy during the past 4.6 Gyr.

Could Mars be dark and altered?

There is a long known dichotomy in the martian albedo, with an associated, but mostly assumed, mineralogical split as well. The bright red regions are inferred to be weathered, oxidized dust and the dark grey regions unaltered volcanic material. A number of recent analyses suggest this division is unnaturally simplistic and the association of many dark regions with the former presence of water requires a re‐examination of the spectra in light of potential alteration minerals. I present an alternate interpretation of the reflectance spectral characteristics of some dark regions on Mars that includes dark layer silicates. If their presence is confirmed on Mars this will have implications for sequestration of current and past volatile inventories, clues to the extent and type of geochemical weathering, and potential zones where bacterial life forms may have emerged.

CO 2 fluxes from mid-ocean ridges, arcs and plumes

Estimates of CO 2 emissions at spreading centres, convergent margins, and plumes have been reviewed and upgraded using observed CO 2/ 3He ratios in magmatic volatiles, 3He content estimates in the magmatic sources, and magma emplacement rates in the different tectonic settings. The effect of volatile fractionation during magma degassing, investigated using new rare gas and CO 2 abundances determined simultaneously for a suite of Mid-Ocean Ridge (MOR) basalt glasses, is not the major factor controlling the spread of data, which mainly result from volatile heterogeneity in the mantle source. The computed C flux at ridges (2.2±0.9)×10 12 mol/a, is essentially similar to previous estimates based on a more restricted data base. Variation of the C flux in the past can be simply scaled to that of spreading rate since the computed C depends mainly on the volatile content of the mantle source, which can be considered constant during the last 10 8 a. The flux of CO 2 from arcs may be approximated using the CO 2/ 3He ratios of volcanic gases at arcs and the magma emplacement rate, assuming that the 3He content of the mantle end-member is that of the MORB source. The resulting flux is ∼2.5×10 12 mol/a, with approx. 80% of carbon being derived from the subducting plate. The flux of CO 2 from plumes, based on time-averaged magma production rates and on estimated contributions of geochemical sources to plume magmatism, is ≤3×10 12 mol/a. Significant enhancements of the CO 2 flux from plumes might have occurred in the past during giant magma emplacements, depending on the duration of these events, although the time-integrated effect does not appear important. The global magmatic flux of CO 2 into the atmosphere and the hydrosphere is found to be 6×10 12 mol/a, with a range of (4–10)×10 12 mol/a. Improvement on the precision of this estimate is linked to a better understanding of the volatile inventory at arcs on one hand, and on the dynamics of plumes and their mantle source contribution on the other hand.

Mars: was There an Ancient Eden?

AbstractThe clear evidence of water erosion on the surface of Mars suggests an early climate much more clement than the present one. Using a model for the origin of inner planet atmospheres by icy planetesimal impact, it is possible to reconstruct the original volatile inventory on Mars, starting from the thin atmosphere we observe today. Evidence for cometary impact can be found in the present abundances and isotope ratios of gases in the atmosphere and in SNC meteorites. If we invoke impact erosion to account for the present excess of129Xe, we predict an early inventory equivalent to at least 7.5 bars of CO2. This reservoir of volatiles is adequate to produce a substantial greenhouse effect, provided there is some small addition of SO2(volcanoes) or reduced gases (cometary impact). Thus it seems likely that conditions on early Mars were suitable for the origin of life – biogenic elements and liquid water were present at favorable conditions of pressure and temperature. Whether life began on Mars remains an open question, receiving hints of a positive answer from recent work on one of the Martian meteorites. The implications for habitable zones around other stars include the need to have rocky planets with sufficient mass to preserve atmospheres in the face of intensive early bombardment.