Tightly closed ecological systems reveal atmospheric subtleties – experience from Biosphere 2

Processes which produce slow changes in air composition in a closed ecological system (CES) may not be noticed if the leak rate of the CES is significant. Dilution of the system’s air with outside air can mask these processes. A tightly closed CES provides the opportunity for slow changes to accumulate over time and be observed and measured. Biosphere 2 (volume 200,000 m³) had a low leak rate of less than 10 percent per year. Oxygen declined slowly at varying rates reflecting seasonal influences, which averaged to about 140 ppm per day during the first 16 months of the two-year closure. Computer simulations of the observed rate of oxygen loss combined with other hypothetical leak rates suggest that the decline would have been hidden by a leak rate as low as one percent per day. Sealing Biosphere 2 involved rigorous design specifications and inclusion of two expansion chambers (called “lungs”) to accommodate expansion/contraction of the atmosphere, which enabled limiting the pressure difference between inside and outside atmospheres to the range of ±8 Pa (0.08 mBar). Measurement of leak rate was by two methods: the first, measuring the rate of deflation of the lungs while holding a constant elevated pressure differential enabled calculation of an estimated leak rate within the usual operating pressure differential range; the second was to measure the progressive dilution of trace gases spiked into the atmosphere. Both methods confirmed leakage to be less than 10 percent per year. Operational data from the 40 m³ Laboratory Biosphere is used to illustrate how normal variations of temperature, humidity and barometric pressure would combine to force leakage and rapidly dilute the internal atmosphere if it were not equipped with a lung. It is demonstrated that very high degrees of closure for a CES enable experimental observation of small imbalances in atmospheric cycles or slow accumulation of trace gases that could otherwise be masked by dilution with atmosphere external to the CES.     

Cometary Deuterium

Deuterium fractionations in cometary ices provide important clues to the origin and evolution of comets. Mass spectrometers aboard spaceprobe Giotto revealed the first accurate D/H ratios in the water of Comet 1P/Halley. Ground-based observations of HDO in Comets C/1996 B2 (Hyakutake) and C/1995 O1 (Hale-Bopp), the detection of DCN in Comet Hale-Bopp, and upper limits for several other D-bearing molecules complement our limited sample of D/H measurements. On the basis of this data set all Oort cloud comets seem to exhibit a similar ratio in H₂O, enriched by about a factor of two relative to terrestrial water and approximately one order of magnitude relative to the protosolar value. Oort cloud comets, and by inference also classical short-period comets derived from the Kuiper Belt cannot be the only source for the Earth's oceans. The cometary O/C ratio and dynamical reasons make it difficult to defend an early influx of icy planetesimals from the Jupiter zone to the early Earth. D/H measurements of OH groups in phyllosilicate rich meteorites suggest a mixture of cometary water and water adsorbed from the nebula by the rocky grains that formed the bulk of the Earth may be responsible for the terrestrial D/H. The D/H ratio in cometary HCN is 7 times higher than the value in cometary H₂O. Species-dependent D-fractionations occur at low temperatures and low gas densities via ion-molecule or grain-surface reactions and cannot be explained by a pure solar nebula chemistry. It is plausible that cometary volatiles preserved the interstellar D fractionation. The observed D abundances set a lower limit to the formation temperature of (30 ± 10) K. Similar numbers can be derived from the ortho-to-para ratio in cometary water, from the absence of neon in cometary ices and the presence of S₂. Noble gases on Earth and Mars, and the relative abundance of cometary hydrocarbons place the comet formation temperature near 50 K. So far all cometary D/H measurements refer to bulk compositions, and it is conceivable that significant departures from the mean value could occur at the grain-size level. Strong isotope effects as a result of coma chemistry can be excluded for molecules H₂O and HCN. A comparison of the cometary ratio with values found in the atmospheres of the outer planets is consistent with the long-held idea that the gas planets formed around icy cores with a high cometary D/H ratio and subsequently accumulated significant amounts of H₂ from the solar nebula with a low protosolar D/H. This revised version was published online in June 2006 with corrections to the Cover Date.     

Haze aerosols in the atmosphere of early Earth: manna from heaven

An organic haze layer in the upper atmosphere of Titan plays a crucial role in the atmospheric composition and climate of that moon. Such a haze layer may also have existed on the early Earth, providing an ultraviolet shield for greenhouse gases needed to warm the planet enough for life to arise and evolve. Despite the implications of such a haze layer, little is known about the organic material produced under early Earth conditions when both CO(2) and CH(4) may have been abundant in the atmosphere. For the first time, we experimentally demonstrate that organic haze can be generated in different CH(4)/CO(2) ratios. Here, we show that haze aerosols are able to form at CH(4) mixing ratios of 1,000 ppmv, a level likely to be present on early Earth. In addition, we find that organic hazes will form at C/O ratios as low as 0.6, which is lower than the predicted value of unity. We also show that as the C/O ratio decreases, the organic particles produced are more oxidized and contain biologically labile compounds. After life arose, the haze may thus have provided food for biota.     

Deep Impact Mission Design

The Deep Impact mission is designed to provide the first opportunity to probe below the surface of a comet nucleus by a high-speed impact. This requires finding a suitable comet with launch and encounter conditions that allow a meaningful scientific experiment. The overall design requires the consideration of many factors ranging from environmental characteristics of the comet (nucleus size, dust levels, etc.), to launch dates fitting within the NASA Discovery program opportunities, to launch vehicle capability for a large impactor, to the observational conditions for the two approaching spacecraft and for telescopes on Earth.     

Deep Impact: A Large-Scale Active Experiment on a Cometary Nucleus

The Deep Impact mission will provide the first data on the interior of a cometary nucleus and a comparison of those data with data on the surface. Two spacecraft, an impactor and a flyby spacecraft, will arrive at comet 9P/Tempel 1 on 4 July 2005 to create and observe the formation and final properties of a large crater that is predicted to be approximately 30-m deep with the dimensions of a football stadium. The flyby and impactor instruments will yield images and near infrared spectra (1–5 μm) of the surface at unprecedented spatial resolutions both before and after the impact of a 350-kg spacecraft at 10.2 km/s. These data will provide unique information on the structure of the nucleus near the surface and its chemical composition. They will also used to interpret the evolutionary effects on remote sensing data and will indicate how those data can be used to better constrain conditions in the early solar system.     

Mercury’s Atmosphere: A Surface-Bounded Exosphere

The existence of a surface-bounded exosphere about Mercury was discovered through the Mariner 10 airglow and occultation experiments. Most of what is currently known or understood about this very tenuous atmosphere, however, comes from ground-based telescopic observations. It is likely that only a subset of the exospheric constituents have been identified, but their variable abundance with location, time, and space weather events demonstrate that Mercury’s exosphere is part of a complex system involving the planet’s surface, magnetosphere, and the surrounding space environment (the solar wind and interplanetary magnetic field). This paper reviews the current hypotheses and supporting observations concerning the processes that form and support the exosphere. The outstanding questions and issues regarding Mercury’s exosphere stem from our current lack of knowledge concerning the surface composition, the magnetic field behavior within the local space environment, and the character of the local space environment.     

The Geology of Mercury: The View Prior to the MESSENGER Mission

Mariner 10 and Earth-based observations have revealed Mercury, the innermost of the terrestrial planetary bodies, to be an exciting laboratory for the study of Solar System geological processes. Mercury is characterized by a lunar-like surface, a global magnetic field, and an interior dominated by an iron core having a radius at least three-quarters of the radius of the planet. The 45% of the surface imaged by Mariner 10 reveals some distinctive differences from the Moon, however, with major contractional fault scarps and huge expanses of moderate-albedo Cayley-like smooth plains of uncertain origin. Our current image coverage of Mercury is comparable to that of telescopic photographs of the Earth’s Moon prior to the launch of Sputnik in 1957. We have no photographic images of one-half of the surface, the resolution of the images we do have is generally poor (∼1 km), and as with many lunar telescopic photographs, much of the available surface of Mercury is distorted by foreshortening due to viewing geometry, or poorly suited for geological analysis and impact-crater counting for age determinations because of high-Sun illumination conditions. Currently available topographic information is also very limited. Nonetheless, Mercury is a geological laboratory that represents (1) a planet where the presence of a huge iron core may be due to impact stripping of the crust and upper mantle, or alternatively, where formation of a huge core may have resulted in a residual mantle and crust of potentially unusual composition and structure; (2) a planet with an internal chemical and mechanical structure that provides new insights into planetary thermal history and the relative roles of conduction and convection in planetary heat loss; (3) a one-tectonic-plate planet where constraints on major interior processes can be deduced from the geology of the global tectonic system; (4) a planet where volcanic resurfacing may not have played a significant role in planetary history and internally generated volcanic resurfacing may have ceased at ∼3.8 Ga; (5) a planet where impact craters can be used to disentangle the fundamental roles of gravity and mean impactor velocity in determining impact crater morphology and morphometry; (6) an environment where global impact crater counts can test fundamental concepts of the distribution of impactor populations in space and time; (7) an extreme environment in which highly radar-reflective polar deposits, much more extensive than those on the Moon, can be better understood; (8) an extreme environment in which the basic processes of space weathering can be further deduced; and (9) a potential end-member in terrestrial planetary body geological evolution in which the relationships of internal and surface evolution can be clearly assessed from both a tectonic and volcanic point of view. In the half-century since the launch of Sputnik, more than 30 spacecraft have been sent to the Moon, yet only now is a second spacecraft en route to Mercury. The MESSENGER mission will address key questions about the geologic evolution of Mercury; the depth and breadth of the MESSENGER data will permit the confident reconstruction of the geological history and thermal evolution of Mercury using new imaging, topography, chemistry, mineralogy, gravity, magnetic, and environmental data.     

Martian Seeps and their Relation to Youthful Geothermal Activity

Gullies found on Martian hillsides by Malin and Edgett (2000) appear in many cases to be formed by water seeps produced by underground aquifers. It is proposed that these aquifers result from geologically recent melting of permafrost ice by sporadic, localized geothermal activity. This is consistent with evidence from crater counts and Martian meteorites that much higher-temperature geothermal activity has produced volcanic activity and lava flows within the last 200 Myr, and perhaps within the last 10 Myr. This hypothesis explains an aspect initially described as surprising, namely concentration of the gullies at high latitudes and on shadowed slopes. Similar features are found on Icelandic basaltic hillsides, which may be ideal analogs for further studies that may clarify the Martian phenomena.     

The Contributions of Comets to Planets, Atmospheres, and Life: Insights from Cassini-Huygens, Galileo, Giotto, and Inner Planet Missions

Comets belong to a group of small bodies generally known as icy planetesimals. Today the most primitive icy planetesimals are the Kuiper Belt objects (KBOs) occupying a roughly planar domain beyond Neptune. KBOs may be scattered inward, allowing them to collide with planets. Others may move outward, some all the way into the Oort cloud. This is a spherical distribution of comet nuclei at a mean distance of ～50,000 AU. These nuclei are occasionally perturbed into orbits that intersect the paths of the planets, again allowing collisions. The composition of the atmosphere of Jupiter—and thus possibly all outer planets—shows the effects of massive early contributions from extremely primitive icy bodies that must have been close relatives of the KBOs. Titan may itself have a composition similar to that of Oort cloud comets. The origin and early evolution of its atmosphere invites comparison with that of the early Earth. Impacts of comets must have brought water and other volatile compounds to the Earth and the other inner planets, contributing to the reservoir of key ingredients for the origin of life. The magnitude of these contributions remains unknown but should be accessible to measurements by instruments on spacecraft.