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Herschel/HIFI mission – water observations in planetary and cometary atmospheres

The Herschel Space Observatory – was one of the most technically advanced and largest telescopes operating in space. HIFI (Heterodyne Instrument for Far-Infrared) – a spectrometer operating in the submillimeter part of the spectrum, within two frequency ranges: 480-1250 GHz and 1410-1910 GHz was the most advanced instrument carried by this telescope. The high spectral resolution of the spectrometer allowed for very accurate measurements of the rotational spectra of molecules emitted by the observed objects. The HIFI consortium granted Poland 3% of the observation time of the whole mission research program, thanks to the high assessment of the construction input of engineers from the Space Research Centre PAS (Polish acronym: CBK PAN).

The observations with the HIFI were mostly carried within the framework of the Herschel guaranteded time key programme ”Water and Related Chemistry in the Solar System”. Members of the planetological group (Polish acronym: ZDUSiP) from the CBK PAN were engaged in the common observational project of the international scientific team HssO (Herschel Solar System Observations).  Leader of the team was Paul Hartogh from Max-Planck-Institut for Solar System Research (Germany). The observational project addressed the broad topic of water and its isotopes in planetary and cometary atmospheres in order to study their formation and evolution. The scientific objectives of the Polish group were related to the implementation of all three elements of the observation program using the HIFI instrument, i.e. research: (i) the cometary atmosphere, (ii) the atmosphere of Mars, (iii) the atmospheres of Giant Planets (Jupiter, Saturn, Uranus, and Neptune) and their moons. The cometary program included four short-period comets belonging to the Jupiter Family Comets (103P/Hartley 2, 10P/Tempel 2, 81P/Wild 2, 45P/Honda-Mrkos-Pajdušáková), four long-period comets (C/2008 Q3 Garradd,   C/2009 P1 Garradd, C/2006 W3 Christensen, C/2011 L4 PANSTARRS), one distant comet (Centaur – 29P/ Schwassmann–Wachmann 1), and one object from Main Belt Comets (176P/LINEAR). These observations were the first, or one of the first, attempts to direct measurements of the water content in these objects. Such ground-based observations are difficult due to the high content of water vapor in the Earth’s atmosphere.

The high sensitivity of the HIFI instrument enabled the detection of many water rotational lines and accurate determination of the water production rate in the comets. An important result was learning the nature of cometary activity by: studying the mechanism of excitation of water molecules based on observation of its emission lines, studying the spatial distribution and dynamics of water molecules in the comae, monitoring the rate of water production as a function of distance from the Sun, and searching for water in distant or faint comets. Water played an important role in the formation of planets and comets, hence the study of water and its isotopes is a source of information about the evolution of the solar system bodies. One of the more spectacular discoveries made by the Herschel telescope involves measuring the relative content of deuterium to hydrogen for comets. The D/H ratio, the key parameter for constraining the origin and evolution of Solar System species, was measured for the first time in two Jupiter-family comets. Herschel/HIFI observations showed that water evaporating from Comet 103P/Hartley 2 has almost the same isotopic composition as Earth’s oceans and also water of Comet 45P/Honda-Mrkos-Pajdušáková has very similar D/H ratio as our planet’s water.  These D/H measurements reopened debate on water delivery on Earth by impacting water-rich bodies. More recently, the ESA Rosetta mission to 67P/Churyumov–Gerasimenko (also Jupiter family comet) has shown that water observed on 67P has an isotope composition very different from that in oceans on Earth.

Mars observations carried out as a part of the Herschel mission provided information on the presence and concentration of various gaseous components in the planet’s atmosphere, that is: water, carbon monoxide (CO), molecular oxygen (O2), and allowed the determination of altitude profiles of  various thermodynamic parameters as for example a temperature.

Principal Investigator: Sławomira Szutowicz
Funding source: Ministerstwa Nauki i Szkolnictwa Wyższego
Timeframe: May 7, 2008 – May 21, 2013

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