OSIRIS is the main imaging system on Rosetta. The programme started in 1994. At this time Nick Thomas was still working at the Max-Planck-Institut für Aeronomie (now MPS) and participated in the definition of the instrument including the required scientific performance.
The system comprises two separate optical system (a narrow angle camera, NAC, and a wide-angle camera, WAC) which are driven through a common electronics box. Several elements of the two cameras are duplicates of each other. For example, the filter wheels, the mechanical shutters, and the focal planes are identical.
The NAC has a pixel scale of around 18.6 urad/px with a point-spread function of around 1.2 pixels (FWHM). The resolution of the WAC is around five times lower. The NAC contains a series of moderately wide-band filters (typically 60 nm bandpass) designed to provide high-spatial resolution colours of the surface. The WAC contains a series of filters designed to isolate optical gas emissions with a set of continuum filters available to study the dust and its contamination of signal in the gas lines.
UBE Participation in Planning
As co-investigators on OSIRIS, we participate in the planning process. The Rosetta activities at the comet are separated into distinct phases (e.g. lander phase, escort phase, etc.). Within the escort phase, there are several 'fly-bys' where the spacecraft passes very close to the surface for a short time. These fly-bys are intended to give us information on the structure of the surface and its evolution as the comet moves towards perihelion. Furthermore, the in situ instruments (e.g. the mass spectrometer, ROSINA) obtain the highest densities of gas and dust during these fly-bys leading to increased signal to noise and also the likelihood of stronger constraint in inhomogeneous outgassing.
We are currently supporting MPS by working on the planning of several of these fly-bys. We were involved in the planning of the mid-February 2015 fly-by which produced a marvellous set of images of the shadow of the spacecraft.
UBE Participation in the Data Analysis
We are currently working on several aspects of the data analysis including
- Studies of a cometary outburst which occurred just before Rosetta arrived at the comet (see Tubiana et al. 2014 submitted)
- Definition of major regions on the comet (including nomenclature)
- Identification of major features (including those possibly associated with sublimation)
- Initial assessments of gas and dust emission.
We show some examples of our work from recent publications below.
Cometary Science with Rosetta
Dusty-Gas Outflow from the Nucleus
In general, numerical simulations of the outflow from the cometary nucleus tend to be simplified assuming axisymmetric geometry with respect to the Sun and solving the Navier-Stokes equations to derive a gas and dust distribution (e.g. Keller et al., 1994; Crifo and Rodionov, 1999). Because of its small size (typically <10 km in diameter) the cometary nucleus and the innermost coma is best investigated by spacecraft. In 2001, NASA's Deep Space 1 spacecraft flew within 2000 km of the nucleus of comet 19P/Borrelly and acquired images with the MICAS imaging system (Soderblom et al., 2002). These data immediately showed the limitations of current models. The nucleus was highly irregular in shape and the dust emission highly non-isotropic. The data set was limited to only a few good images of the dust coma. Constraining model parameters under these conditions is difficult. Ho et al. (2005) have studied these images and concluded that improved fits to the inner coma distribution are only possible if an acceleration profile for the dust similar to that found in the numerical models of Tenishev and Combi (2003 and priv. communication) is used. Ho (2005) has also attempted to link the structures seen in the coma to those seen in simultaneous ground-based observations. A numerical model of the dust emission has been established which allows us to input the emission parameters seen at the source and extrapolate them to simulate their appearance to a ground-based observer. However, from this work it is evident that further progress can only be made by detailed modelling of the outflow in close collaboration with image analysis (cf Crifo et al., 2002b). The dust scattering properties also play an important role which we ourselves have recently been addressing (Bertini et al., 2006).
For the gas outflow, modellers have moved towards using the Direct Simulation Monte Carlo (DSMC) approach (Bird, 1994; Davidsson and Skorov, 2004; Crifo et al., 2002a, 2005). This has significant advantages in that low gas outflow rates can be examined. This is of particular importance for the Rosetta mission which will observe comet Churyumov-Gerasimenko from an almost inert state through to perihelion covering a wide range of gas outflow regimes.
We have been developing coma simulations to help us interpret measurements from Rosetta. In the past 2 years, a sensitivity analysis method with a minimum number of simulation runs has been developed. The “Polynomial Chaos Expansion” (PCE) method was developed for uncertainty propagation but was recently used for sensitivity analyses. The method allows one to make statements about surface production rates if the position and size of the interaction region (between strong source and weak background) can be established by in-situ measurements. An approach to using PCE to establish non-linear relations between source parameters and the resulting flow field has been published (Finklenburg and Thomas, 2014).
Effects of the complex topography of comet nuclei on the flow field can only be studied in a 3D DSMC simulation. A collaboration with the group of J.-S. Wu (National Chiao Tung Uni., Taiwan) has been initiated and boosted considerably through an SNSF-funded exploratory workshop in Bern in January 2013. They have developed a 3D DSMC code, which runs on a parallel computer (PDSC+). The shape model of Comet 9P/Tempel 1 (P. Thomas et al., 2007) has been used as a first test case for PDSC+ in Bern. The results show that a production rate proportional to the local surface temperature does not explain the observed gas distribution. Non-uniform emission models have been constructed that agree better with observation (Finklenburg et al., 2014).
The Surface Layer
For the comet rendezvous phase, UBE is intending to focus scientifically on the dusty-gas outflow (within the first few 100 m) and its relationship to the surface layer (including surface topography and the temperature structure). There is now considerable debate about the structure of the upper layer. The latest data from the Deep Impact spacecraft (A'Hearn et al., 2005) showed that 9P/Tempel 1 has significant inhomogeneity in shape, surface appearance, local topography, and colour. In addition, the first observations of the gas distribution directly above the nucleus surface have been reported (Feaga et al., 2007). The H2O column density was at a maximum close to the sub-solar point with a distribution which suggests emission proportional to the solar irradiance. However, this work has also provided the remarkable conclusion that the distribution of CO2 above the nucleus differs markedly from that of H2O. The surface temperature of 9P/Tempel 1 at 1.5 AU [Groussin et al., 2007] showed a maximum temperature of 336±7 K. Even in regions where water ice was detected on the surface of 9P/Tempel 1 by IR spectroscopy [Sunshine et al., 2006], the observed temperature exceeded 270 K at an image scale of 120 m px-1. The data have allowed an estimate of the thermal inertia of <50 W m-2 s-1/2 K-1 [Groussin et al., 2007] although this has been challenged by Davidsson et al. . The differences have major implications for heat transfer in the upper layers of the nucleus [Thomas, 2009]. A thermal model has therefore been developed and linked to the PDMC+ inputs to test the implications for the inner coma gas distribution.
Although our model has several layers of sophistication, we are currently using very simple systems to model the outgassing of 67P because the shape of the nucleus provides such a strong influence.
Definitions of Regions
We have been leading the definition of morphologically regions on the nucleus. The figure below shows the current definition from our Science paper. This will be updated once we get more data from the southern hemisphere.
We have seen these remarkable structures (figure left) on the nucleus that look like aeolian ripples. Normally these coarse-grained ripples are found on Earth or on Mars in a dense atmosphere, relatively high gravity environment. 67P has gas pressures and almost no gravity. So how can you explain this?
Well, it turns out not to be easy. But we can use models of the gas flow field combined with particle impact on the surface to support the concept of reptation and creep of the particles across the surface. The details are difficult and models are still needed to demonstrate that we fully understand it. But the explanations look promising.
On the right, you can even see wind-tails behind rocks/boulders. So dust particle transport does seem to be important.
Surface Fractures and Ponded Deposits
This fracture in the Aker region of 67P (figure left) is 200 m long. It indicates that the surface layer is more consolidated and possibly stronger than we originally imagined.
To the right we see a smooth flat region which may be what is referred to as a ponded deposit. This type of flat smooth surface has been seen on asteroids and might be the result of electrostatic effects on dust particles.
These figures have been discussed in our paper in Science (Thomas et al., 2015).
This anaglyph was prepared by our group and issued as a press release by ESA.
Bertini, I., N. Thomas, and C. Barbieri, (2004),Modelling of the structure and light scattering of individual dust particles, ASSL Vol. 311: The New Rosetta Targets. Observations, Simulations and Instrument Performances, 167.
Ho, T.-M. (2005) PhD thesis. University of Berne, Switzerland.
Thomas, N. (2009) The nuclei of Jupiter family comets: A critical review of our present knowledge, Planetary and Space Science, 57, 1106-1117.
Finklenburg, S., N. Thomas, J. Knollenberg, and E. Kührt, (2011), Comparison of DSMC and Euler Equations Solutions for Inhomogeneous Sources on Comets, 27th International Symposium on Rarefied Gas Dynamics, 2010 AIP Conf. Proc. 1333, 1151-1156, doi: 10.1063/1.3562799.
Finklenburg, S. and N. Thomas, (2014) Relating in situ gas measurements to the surface outgassing properties of cometary nuclei, Planetary and Space Science, accepted, 11 February 2014.
Finklenburg, S., N. Thomas, C.C. Su , and J.-S. Wu, (2014), The spatial distribution of water in the inner coma of comet 9P/Tempel 1: Comparison between models and observations, Icarus, revised, 31 January 2014.
Thomas, N. et al., (2015) The morphological diversity of comet 67P/Churyumov-Gerasimenko, Science, in press.
The Hardware Team
Six scientific institutes are represented in the OSIRIS consortium. Each institute assigned a lead scientist to represent them. The institutes and lead scientists at the start of the project were
• Max-Planck-Institut fuer Aeronomie (MPAE), Germany (PI: H.U. Keller; replaced by H. Sierks)
• Laboratoire d'Astronomie Spatiale (LAS), France (P. Lamy; L. Jorda)
• University of Padova (UPD), Italy (C. Barbieri)
• Institute for Astrophysics, Granada (IAA), Spain (R. Rodrigo)
• Astronomical Observatory, Uppsala (AOU), Sweden (H. Rickman; B. Davidsson)
• ESTEC (SSD), ESA (K.P. Wenzel; D. Koschny)
Space Research & Planetary Sciences Division Web Site