Isotopic analysis of cometary organic matter

Carbon isotope ratios have been measured for CN in the coma of comet Halley and for several CHON particles emitted by Halley. Of these, only the CHON-particle data may be reasonably related to organic matter in the cometary nucleus, but the true range of ¹³C/¹²C values in those particles is quite uncertain. The D/H ratio in H₂O in the Halley coma resembles that in Titan/Uranus. The next decade should substantially improve our understanding of the distribution of C, H, N, and O isotopes in cometary organics. The isotopic composition of meteoritic organic matter is better understood and can serve as a useful analog for the cometary case.     

The Solar System d/h Ratio: Observations and Theories

The measured D/H ratios in interstellar environments and in the solar system are reviewed. The two extreme D/H ratios in solar system water - (720±120)×10⁻⁶ in clay minerals and (88±11)×10⁻⁶ in chondrules, both from LL3 chondritic meteorites - are interpreted as the result of a progressive isotopic exchange in the solar nebula between deuterium-rich interstellar water and protosolar H₂. According to a turbulent model describing the evolution of the nebula (Drouart et al., 1999), water in the solar system cannot be a product of thermal (neutral) reactions occurring in the solar nebula. Taking 720×10⁻⁶ as a face value for the isotopic composition of the interstellar water that predates the formation of the solar nebula, numerical simulations show that the water D/H ratio decreases via an isotopic exchange with H₂. During the course of this process, a D/H gradient was established in the nebula. This gradient was smoothed with time and the isotopic homogenization of the solar nebula was completed in 10⁶ years, reaching a D/H ratio of 88×10⁻⁶. In this model, cometary water should have also suffered a partial isotopic re-equilibration with H₂. The isotopic heterogeneity observed in chondrites result from the turbulent mixing of grains, condensed at different epochs and locations in the solar nebula. Recent isotopic determinations of water ice in cold interstellar clouds are in agreement with these chondritic data and their interpretation (Texeira et al., 1999). This revised version was published online in June 2006 with corrections to the Cover Date.     

Composition of Interstellar Neutrals and the Origin of Anomalous Cosmic Rays

Knowledge of the elemental composition of the interstellar gas is of fundamental importance for understanding galactic chemical evolution. In addition to spectroscopic determinations of certain element abundance ratios, measurements of the composition of interstellar pickup ions and Anomalous Cosmic Rays (ACRs) have provided the principal means to obtain this critical information. Recent advances in our understanding of particle acceleration processes in the heliosphere and measurements by the Voyagers of the energy spectra and composition of energetic particles in the heliosheath provide us with another means of determining the abundance of the neutral components of the local interstellar gas. Here we compare the composition at the termination shock of six elements obtained from measurements of (a) pickup ions at ～5 AU, (b) ACRs in the heliosphere at ～70 AU, and (c) energetic particles as well as (d) ACRs in the heliosheath at ～100 AU. We find consistency among these four sets of derived neutral abundances. The average interstellar neutral densities at the termination shock for H, N, O, Ne and Ar are found to be 0.055±0.021 cm^?3, (1.44±0.45)×10^?5 cm^?3, (6.46±1.89)×10^?5 cm^?3, (8.5±3.3)×10^?6 cm^?3, and (1.08±0.49)×10^?7 cm^?3, respectively, assuming the He density is 0.0148±0.002 cm^?3.     

Comet Halley's Gas Composition and Extended Sources: Results from the Neutral Mass Spectrometer on Giotto

The Neutral Mass Spectrometer on the Giotto spacecraft established that H₂O is the dominant species in Comet Halley's volatiles and determined the abundance of more than 10 parent species. The instrument discovered strong extended H₂CO and CO sources in the coma of Comet Halley. Polymerized H₂CO associated with the cometary dust and evaporating slowly as the monomer is most likely the extended H₂CO source. Photodissociation of the H₂CO into CO fully accounts for the extended CO source. This revised version was published online in June 2006 with corrections to the Cover Date.     

On the Flux of Water and Minor Volatiles from the Surface of Comet Nuclei

Surface temperature and the available effective energy strongly influence the mass flux of H₂O and minor volatiles from the nucleus. We perform computer simulations to model the gas flux from volatile, icy components in porous ice-dust surfaces, in order to better understand results from observations of comets. Our model assumes a porous body containing dust, one major ice component (H₂O) and up to eight minor components of higher volatility (e.g. CO, CH₄, CH₃OH, HCN, C₂H₂, H₂S), The body's porous structure is modeled as a bundle of tubes with a given tortuosity and an initially constant pore diameter. Heat is conducted by the matrix and carried by the vapors. The model includes radially inward and outward flowing vapor within the body, escape of outward flowing gas from the body, complete depletion of less volatile ices in outer layers, and recondensation of vapor in deeper, cooler layers. From the calculations we obtain temperature profiles and changes in relative chemical abundances, porosity and pore size distribution as a function of depth, and the gas flux into the interior and into the atmosphere for each of the volatiles at various positions of the body in its orbit. In this paper we relate the observed relative molecular abundances in the coma of Comet C/1995 O1 (Hale-Bopp) and of Comet 46P/Wirtanen to molecular fluxes at the surface calculated from our model. This revised version was published online in June 2006 with corrections to the Cover Date.     

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.     

Vertical distribution of disequilibrium species in Jupiter's troposphere

Sources of organic matter and inorganic tracers on Jupiter, including solar UV photolysis, lightning discharges, and convective quenching of hot gases from the lower atmosphere, are reviewed in light of Earth-based and Voyager data with the purpose of predicting the tropospheric steady-state abundances and vertical distributions of HCN, CH₂O, and other species.It is concluded that a steady-state mole fraction of HCN in the Jovian troposphere of only 10^-12 could be maintained by vertical transport of hot gases from the deep atmosphere. The observed HCN abundance (roughly X_HCN = 10^-9) appears to be due to photochemical reactions.After HCN, the most abundant organic disequilibrium species in the troposphere is probably C₂H₆, derived from direct photolysis of CH₄ at high altitudes, with a mole fracton of 10^-10 at the H₂O cloud level. Inorganic tracers of disequilibrium processes are also briefly summarized.     

The Planets and Titan Observed by ISO

Infrared spectroscopic observations of planets and Saturn's satellite Titan with the Infrared Space Observatory led to many significant discoveries that improved our understanding on the formation, physics and chemistry of these objects. The prime results achieved by ISO are: (1) a new and consistent determination of the D/H ratios on the giant planets and Titan; (2) the first precise measurement of the ¹⁵N/¹⁴N ratio in Jupiter, a valuable indicator of the protosolar nitrogen isotopic ratio; (3) the first detection of an external oxygen flux for all giant planets and Titan; (4) the first detection of some stratospheric hydrocarbons (CH₃, C₂H₄, CH₃C₂H, C₄H₂, C₆H₆); (5) the first detection of tropospheric water in Saturn; (6) the tentative detection of carbonate minerals on Mars; (7) the first thermal lightcurve of Pluto. Based on observations with ISO, an ESA project with instruments funded by ESA Member States (especially the PI countries: France, Germany, The Netherlands, and the United Kingdom), and with the participation of ISAS and NASA.     

Helium-3: Status and Prospects

We report on our continuing efforts to determine 3He abundances in H II regions and planetary nebulae. Our detections of 3He in some PNe show that some stars produce large amounts of 3He. However the H II region abundances show no evidence for this production. From our sample of > 40 H II regions, the subsample which should yield the most reliable abundances has 3He/H abundances which scatter between 1-2 × 10-5. There is no trend with either galactocentric distance or metallicity. Even if we do not understand the underlying mechanisms, we see empirically that stars neither produce nor destroy 3He in a major way. We thus suggest that the level of the "3He Plateau" (3He/H = 1.5 _-0.5 ^+1.0 × 10^-5) is a reasonable estimate for the primordial 3He.     

From Coma Abundances to Nucleus Composition

A major goal of comet research is to determine conditions in the outer solar nebula based on the chemical composition and structure of comet nuclei. The old view was to use coma abundances directly for the chemical composition of the nucleus. However, since the composition of the coma changes with heliocentric distance, r, the new view is that the nucleus composition msut be determined from analysis of coma mixing ratios as a function of r. Taking advantage of new observing technology and the early detection of the very active Comet Hale-Bopp (C/1995 O1) allows us to determine the coma mixing ratios over a large range of heliocentric distances. In our analysis we assume three sources for the coma gas: (1) the surface of the nucleus (releasing water vapor), (2) the interior of the porous nucleus (releasing many species more volatile than water), and (3) the distributed source (releasing gases from ices and hydrocarbon polycondensates trapped and contained in coma dust). Molecules diffusing inside the nucleus are sublimated by heat transported into the interior. The mixing ratios in the coma are modeled assuming various chemical compositions and structural parameters of the spinning nucleus as it moves in its orbit from large heliocentric distance through perihelion. We have combined several sets of observational data of Comet Hale-Bopp for H₂O (from OH) and CO, covering the spectrum range from radio to UV. Many inconsistencies in the data were uncovered and reported to the observers for a reanalysis. Since post-perihelion data are still sparse, we have combined pre- and post-perihelion data. The resulting mixing ratio of CO relative to H₂O as a function of r is presented with a preliminary analysis that still needs to be expanded further. Our fit to the data indicates that the total CO release rate (from the nucleus and distributed sources) relative to that of H₂O is 30% near perihelion. This revised version was published online in June 2006 with corrections to the Cover Date.     

The Composition of the Solar Wind in Polar Coronal Holes

The solar wind charge state and elemental compositions have been measured with the Solar Wind Ion Composition Spectrometers (SWICS) on Ulysses and ACE for a combined period of about 25 years. This most extensive data set includes all varieties of solar wind flows and extends over more than one solar cycle. With SWICS the abundances of all charge states of He, C, N, O, Ne, Mg, Si, S, Ar and Fe can be reliably determined (when averaged over sufficiently long time periods) under any solar wind flow conditions. Here we report on results of our detailed analysis of the elemental composition and ionization states of the most unbiased solar wind from the polar coronal holes during solar minimum in 1994–1996, which includes new values for the abundance S, Ca and Ar and a more accurate determination of the ²⁰Ne abundance. We find that in the solar minimum polar coronal hole solar wind the average freezing-in temperature is ∼1.1×10⁶ K, increasing slightly with the mass of the ion. Using an extrapolation method we derive photospheric abundances from solar wind composition measurements. We suggest that our solar-wind-derived values should be used for the photospheric ratios of Ne/Fe=1.26±0.28 and Ar/Fe=0.030±0.007.     

Deep Impact: Working Properties for the Target Nucleus – Comet 9P/Tempel 1

In 1998, Comet 9P/Tempel 1 was chosen as the target of the Deep Impact mission (A’Hearn, M. F., Belton, M. J. S., and Delamere, A., Space Sci. Rev., 2005) even though very little was known about its physical properties. Efforts were immediately begun to improve this situation by the Deep Impact Science Team leading to the founding of a worldwide observing campaign (Meech et al., Space Sci. Rev., 2005a). This campaign has already produced a great deal of information on the global properties of the comet’s nucleus (summarized in Table I) that is vital to the planning and the assessment of the chances of success at the impact and encounter. Since the mission was begun the successful encounters of the Deep Space 1 spacecraft at Comet 19P/Borrelly and the Stardust spacecraft at Comet 81P/Wild 2 have occurred yielding new information on the state of the nuclei of these two comets. This information, together with earlier results on the nucleus of comet 1P/Halley from the European Space Agency’s Giotto, the Soviet Vega mission, and various ground-based observational and theoretical studies, is used as a basis for conjectures on the morphological, geological, mechanical, and compositional properties of the surface and subsurface that Deep Impact may find at 9P/Tempel 1. We adopt the following working values (circa December 2004) for the nucleus parameters of prime importance to Deep Impact as follows: mean effective radius = 3.25± 0.2 km, shape – irregular triaxial ellipsoid with a/b = 3.2± 0.4 and overall dimensions of ∼14.4 × 4.4 × 4.4 km, principal axis rotation with period = 41.85± 0.1 hr, pole directions (RA, Dec, J2000) = 46± 10, 73± 10 deg (Pole 1) or 287± 14, 16.5± 10 deg (Pole 2) (the two poles are photometrically, but not geometrically, equivalent), Kron-Cousins (V-R) color = 0.56± 0.02, V-band geometric albedo = 0.04± 0.01, R-band geometric albedo = 0.05± 0.01, R-band H(1,1,0) = 14.441± 0.067, and mass ∼7×10¹³ kg assuming a bulk density of 500 kg m⁻³. As these are working values, {i.e.}, based on preliminary analyses, it is expected that adjustments to their values may be made before encounter as improved estimates become available through further analysis of the large database being made available by the Deep Impact observing campaign. Given the parameters listed above the impact will occur in an environment where the local gravity is estimated at 0.027–0.04 cm s⁻² and the escape velocity between 1.4 and 2 m s⁻¹. For both of the rotation poles found here, the Deep Impact spacecraft on approach to encounter will find the rotation axis close to the plane of the sky (aspect angles 82.2 and 69.7 deg. for pole 1 and 2, respectively). However, until the rotation period estimate is substantially improved, it will remain uncertain whether the impactor will collide with the broadside or the ends of the nucleus.     

The Grain Impact Analyser and Dust Accumulator (GIADA) Experiment for the Rosetta Mission: Design, Performances and First Results

The Grain Impact Analyser and Dust Accumulator (GIADA) onboard the ROSETTA mission to comet 67P/Churyumov–Gerasimenko is devoted to study the cometary dust environment. Thanks to the rendezvous configuration of the mission, GIADA will be plunged in the dust environment of the coma and will be able to explore dust flux evolution and grain dynamic properties with position and time. This will represent a unique opportunity to perform measurements on key parameters that no ground-based observation or fly-by mission is able to obtain and that no tail or coma model elaborated so far has been able to properly simulate. The coma and nucleus properties shall be, then, clarified with consequent improvement of models describing inner and outer coma evolution, but also of models about nucleus emission during different phases of its evolution. GIADA shall be capable to measure mass/size of single particles larger than about 15 μm together with momentum in the range 6.5 × 10⁻¹⁰ ÷ 4.0 × 10⁻⁴ kg m s⁻¹ for velocities up to about 300 m s⁻¹. For micron/submicron particles the cumulative mass shall be detected with sensitivity 10⁻¹⁰ g. These performances are suitable to provide a statistically relevant set of data about dust physical and dynamic properties in the dust environment expected for the target comet 67P/Churyumov–Gerasimenko. Pre-flight measurements and post-launch checkouts demonstrate that GIADA is behaving as expected according to the design specifications. The International GIADA Consortium (I, E, UK, F, D, USA).     

Pollution of the upper atmosphere by rockets

This report estimates the amounts of various constituents that would have to be continually injected by rockets into the upper atmosphere in order to double the worldwide natural concentrations there. Involved in the calculations are: (a) the natural atmospheric abundances of constituents such as H₂O, CO₂, NO, Na, K, Li, H, etc.; (b) the residence times in various regions of the atmosphere, since these determine how rapidly a constituent will be removed; and (c) the chemical or photochemical stability of a substance exposed to the upper atmosphere environment. It is concluded that a doubling of the CO₂, H₂O, or NO content would require per year on the order of 10³ to 10⁵ Saturn-type rockets, each injecting 100 tons of exhaust above 100 km. On the other hand, a few hundred small rockets per year, each containing 10 kg of the chemical, would probably double the Na content; similarly, less than two such rockets per year would be expected to double the Li content. These last conclusions have implications for future tracer experiments using these substances.The author is now an Associate Director of the National Center for Atmospheric Research (NCAR) in Boulder, Colorado. The work reported here was supported by the United States Air Force under Contract AF 49(638)-700 with the RAND Corporation. The views or conclusions contained in this paper should not, however, be interpreted as representing the official opinion of the United States Air Force.     

Deuterium Fractionation in Interstellar Clouds

This paper reviews the chemical processes responsible for fractionating deuterium in interstellar molecules. I show that this process is intrinsically a low temperature phenomenon and discuss how the degree of enhancement of the deuterium content of molecules is related to the physical conditions, particularly abundances, in molecular clouds. If significant amounts of abundant species, such as CO, are frozen out on to interstellar dust grains, the resulting enhancement in H₂D⁺ can result in its abundance being greater than that of H ₃ ⁺ at 10K. Transfer of the deuteron from H₂D⁺ can then lead to the efficient formation of multiply deuterated species, such as NHD₂ and ND₃. Fractionation can also occur in grain surface reactions and some simple models are discussed. This revised version was published online in August 2006 with corrections to the Cover Date.     

Tailored analyses of 24 Galactic WN stars

The fundamental properties of 24 Galactic WN stars are determined from analyses of their optical, UV and IR spectra using sophisticated model atmosphere codes (Hillier, 1987, 1990). Terminal velocities, stellar luminosities, temperatures, mass loss rates and abundances of hydrogen, helium, carbon, nitrogen and oxygen are determined. Stellar parameters are derived using diagnostic lines and interstellar reddenings found from fitting theoretical continua to observed energy distributions.Our results confirm that the parameters of WN stars span a large range in temperature (T_*=30–90,000 K), luminosity (log L_*/L=4.8–5.9), mass loss (M=0.9–12×10^–5 M yr^–1) and terminal velocity (v =630–3300 km s^–1). Hydrogen abundances are determined, and found to be low in WNEw and WNEs stars (<15% by mass) and considerable in most WNL stars (1–50%). Metal abundances are also determined with the nitrogen content found to lie in the range N/He=1–5×10^–3 (by number) for all subtypes, and C/N 0.02 in broad agreement with the predictions of Maeder (1991). Enhanced O/N and O/C is found for HD 104994 (WN3p) suggesting a peculiar evolutionary history. Our results suggest that single WNL+abs stars may represent an evolutionary stage immediately after the Of phase. Since some WNE stars exist with non-negligible hydrogen contents (e.g. WR136) evolution may proceed directly from WNL+abs to WNE in some cases, circumventing the luminous blue variable (LBV) or red supergiant (RSG) stage.     

The Coma of Comet 9P/Tempel 1

As comet 9P/Tempel 1 approaches the Sun in 2004–2005, a temporary atmosphere, or “coma,” will form, composed of molecules and dust expelled from the nucleus as its component icy volatiles sublimate. Driven mainly by water ice sublimation at surface temperatures T > 200 K, this coma is a gravitationally unbound atmosphere in free adiabatic expansion. Near the nucleus (≤ 10² km), it is in collisional equilibrium, at larger distances (≥10⁴ km) it is in free molecular flow. Ultimately the coma components are swept into the comet’s plasma and dust tails or simply dissipate into interplanetary space. Clues to the nature of the cometary nucleus are contained in the chemistry and physics of the coma, as well as with its variability with time, orbital position, and heliocentric distance. The DI instrument payload includes CCD cameras with broadband filters covering the optical spectrum, allowing for sensitive measurement of dust in the comet’s coma, and a number of narrowband filters for studying the spatial distribution of several gas species. DI also carries the first near-infrared spectrometer to a comet flyby since the VEGA mission to Halley in 1986. This spectrograph will allow detection of gas emission lines from the coma in unprecedented detail. Here we discuss the current state of understanding of the 9P/Tempel 1 coma, our expectations for the measurements DI will obtain, and the predicted hazards that the coma presents for the spacecraft. An erratum to this article is available at .     

Rocky Cometary Particulates: Their Elemental,Isotopic and Mineralogical Ingredients

The determination of the chemical composition of solid cometary dust particles was one of the prime objectives of the three missions to Comet Halley in 1986. The dust analysis was performed by time-of-flight mass-spectrometry. Within the experimental uncertainty the mean abundances of the rock-forming elements in cometary dust particles are comparable to their abundances in CI-chondrites and in the solar photosphere, i.e. they are cosmic. H, C, and N, on the other hand, in cometary dust are significantly more abundant than in CI-chondrites, approach solar abundances, are to some extent related to O, and reside in an omnipresent refractory organic component dubbed CHON. Element variations between individual dust grains are characterized by correlations of Mg, Si, and O, and to a lesser extent of Fe and S. From particle-to-particle variations of the rock forming elements information on the mineralogy of cometary dust can be obtained. Cluster analysis revealed certain groups that partly match the classifications of stratospheric interplanetary dust particles. About half of Halley's analyzed particles are characterized by anhydrous Fe-poor Mg-silicates, Fe-sulfides, and rarely Fe metal. The Fe-poor Mg-silicates link Halley's dust to that of Hale-Bopp as shown by recent IR observations. No significant deviation from normal of the isotopic composition of the elements is unequivocally present with the notable exception carbon: ¹²C-rich grains with ¹²C/¹³C-ratios up to ≈ 5,000 link cometary dust to presolar circumstellar grains identified in certain chondrites. This revised version was published online in June 2006 with corrections to the Cover Date.     

Atomic Deuterium/Hydrogen in the Galaxy

An accurate value of the D/H ratio in the local interstellar medium (LISM) and a better understanding of the D/H variations with position in the Galactic disk and halo are vitally important questions as they provide information on the primordial D/H ratio in the Galaxy at the time of the protosolar nebula, and the amount of astration and mixing in the Galaxy over time. Recent measurements have been obtained with UV spectrographs on FUSE, HST, and IMAPS using hot white dwarfs, OB stars, and late-type stars as background light sources against which to measure absorption by D and H in the interstellar medium along the lines of sight. Recent analyses of FUSE observations of seven white dwarfs and subdwarfs provide a weighted mean value of D/H = (1.52±0.08) × 10⁻⁵ (15.2 ± 0.8 ppm), consistent with the value of (1.50 ± 0.10) × 10⁻⁵ (15.0 ± 1.0 ppm) obtained from analysis of lines of sight toward nearby late-type stars. Both numbers refer to the ISM within about 100 pc of the Sun, which samples warm clouds located within the Local Bubble. Outside of the Local Bubble at distances of 200 to 500 pc, analyses of far-UV spectra obtained with the IMAPS instrument indicate a much wider range of D/H ratios between 0.8 to 2.2 ppm. This portion of the Galactic disk provides information on inhomogeneous astration in the Galaxy. This revised version was published online in August 2006 with corrections to the Cover Date.     

Characterization of Solar Coronal Holes and Plumes Using Spectroscopic Diagnostic Techniques Applied to SOHO/CDS Observations

Results from a series of SOHO/Coronal Diagnostic Spectrometer (CDS) observations of coronal holes and plumes are presented, including analysis of a low-latitude plume observed in August 1996. Spectroscopic diagnostic techniques using the CHIANTI atomic database are applied to derive the plasma parameters: electron density, temperature, and element abundances. The results are compared with quiet sun values. Coronal electron densities in the holes are found to be about 2 × 10⁸ cm^-3, a factor of two to three lower than in the quiet sun. The plasma thermal distribution exhibits differences between coronal holes, the quiet sun and plumes. For example, the peak of the emission in coronal holes is at a lower temperature (T ⋍ 8 × 10⁵ K) than in the quiet sun (T ⋍ 1 × 10⁶ K), while plumes are cooler (T ⋍ 7.6 × 10⁵ K) and show a different distribution, closer to an isothermal state. This revised version was published online in June 2006 with corrections to the Cover Date.