Esa Bulletin September 1999 [antikvár]

tSesa bulletin 99 September 1999ISO's Astronomical Harvest Continues*M.F. Kessler, A. Heske, L. Metcalfe, T. Prusti A. SalamaISO Data Centre, Astrophysics Division, Space Science Department, ESA Directorate for Scientific Programmes, Villafranca, SpainIntroductionDuring Its 29-month operational lifetime in orbit, ESA's Infrared Space Observatory (ISO) provided astronomers with a facility of unprecedented sensitivity, wavelength coverage and spectral resolution for exploring the sky at infrared wavelengths. It carried four scientific instruments: two spectrometers each with a medium and a high spectral resolution mode, a camera, and an imaging photo-polarlmeter. ISO made nearly 30 000 individual scientific observations of all types of astronomical objects and the data have been re-processed homogeneously and are now available to the entire astronomical community via the World Wide Web.Last autumn, nearly 400 astronomers gathered in Paris for a week-long discussion and review of ISO results, as presented in some 300 papers. This article presents a handful of ISO's many results, selected - mainly from the Proceedings (ESA SP-427) of the Paris meeting -both to show the breadth of the scientific fields being impacted by ISO and to complement results that have previously appeared in the 'In Brief' pages of the Bulletin. ISO has delivered a wealth of new results, spanning long-awaited confirmations of models to unexpected surprises and from our own Solar System out to the most distant extragalactic sources.* Additional information on ISO, including results galleries and how to retrieve data, can be found by following links from the ISO Home Page (www.iso.vilspa.esa.es); news items are also posted at www.scl.esa.lnt. Previous ESA Bulletin articles have addressed the data archive (Arviset & Prusti, No. 98, June 1999); the ISO operations (Kessler, Clavel & Faelker, No. 95, August 1998); early ISO results (Kessler. No. 86, May 1996); and pre-launch descriptions of the overall mission (No. 84, November 1995).Solar SystemDeuterium-to-hydrogen (D/H) ratio The abundance of deuterium - the heavy isotope of hydrogen - in the giant planets and in comets is one of the keys to understanding how they formed. Our Solar System Is believed to have formed out of a protosolar nebula composed of gas and grains. In Jupiter and Saturn, the lighter elements of the gas such as hydrogen (H) and deuterium (D) constitute the bulk of the planets' mass, with only a small fraction (3% for Jupiter, 10% for Saturn) being made of an icy/rocky core of solid material. It is therefore likely that the deuterium abundances in these two planets represent those of the gaseous part of the protosolar nebula. On the other hand, Uranus and Neptune may have been enriched in deuterium by the mixing of their atmospheres with comparatively much larger cores (more than 50% of their mass)containing deuterium-rich Icy grains. Therefore, measuring the D/H ratios In the giant planets and using Inferior models provides Important information on the early history of our Solar System.ISO provided a coherent and direct determination of the D/H ratio In the four giant planets through the first detections of HD rotational lines (Fig. 1). The results are In agreement with the value obtained by the Galileo probe for Jupiter and are compatible with independent measurements of the protosolar value. On Uranus and Neptune, the D/H ratio is consistent with the current understanding of planetary evolution. The inferred D/H ratio for their icy cores is three times less than that found on comets, implying a different time and/or location for their formation.Planetary atmosphere composition and structureISO observations have enhanced our knowledge of the composition and turbulence of the atmospheres of the giant planets. New hydrocarbons have been detected (CH3CCH, C4H21 CgHg on Saturn; CH3 on Saturn and Neptune). These components of the planet's stratosphere are produced by photochemistry of methane through complex reaction schemes. Different species have different vertical distributions, so that their relative abundances allow reconstruction of the strength of the vertical transport as a function of altitude. ISO data also show evidence for ammonia clouds on Jupiter, best understood In terms of a two-cloud model, with 10-micron-sized particles in the upper cloud.Water in planets and cometsA surprise obtained with ISO was the first detection of external water in the stratospheres of all of the giant planets and Titan (Fig. 2). Water is expected to be present deep In their atmospheres, because during their formation they Incorporated oxygen, for which H20 is the main carrier. This water, however, condenses out In the troposphere, as the temperature