A&A 448, L53-L56 (2006)
R. Schulz1 - A. Owens2 - P. M. Rodriguez-Pascual3 - D. Lumb2 - C. Erd2 - J. A. Stüwe4
1 - ESA Research and Scientific Support Department, ESTEC, Postbus 299, 2200 AG Noordwijk, The Netherlands
2 - ESA Science Payloads and Advanced Concepts Office, ESTEC, Postbus 299, 2200 AG Noordwijk, The Netherlands
3 - ESA, XMM-Newton Science Operations Centre, ESAC, Apartado 50727, 28080 Madrid, Spain
4 - Sterrewacht Leiden, Postbus 9513, 2300 RA Leiden, The Netherlands
Received 28 September 2005 / Accepted 11 January 2006
Context. Icy grains in the inner coma of a comet may play an important role in the energy balance and in the production of certain gas coma species. Their existence has therefore been assumed repeatedly to explain a variety of observed phenomena. However, owing to their extremely short life time no evidence for the presence of icy grains had been found in any active comet close to the Sun.
Aims. We observed Comet 9P/Tempel 1 during the Deep Impact mission to look for phenomena induced by the impact.
Methods. The comet was observed with the XMM-Newton Observatory. We used the EPIC camera for X-ray imaging and the Optical Monitor for monitoring in the ultraviolet and visible spectral range.
Results. An outburst of the comet nucleus was observed as a result of the impact and the evolution of the coma was monitored in gas and dust. Our observations led to the first detection of icy grains in a comet at 1.5 AU from the Sun.
Conclusions. We showed for the first time that the material ejected from the nucleus of a comet contains icy grains, even at small heliocentric distance.
Key words: comets: general
On 4 July 2005, 5:52 UT the D EEP I MPACT mission impacted a 370 kg projectile onto the nucleus of Comet 9P/Tempel 1 leading to an induced activity outburst of the nucleus. This provided the unique opportunity to directly observe fresh coma material ejected from the nucleus under non-steady-state conditions. Theoretical considerations have led to the suggestion that the dust ejected from the impact site has been broken up by the energetic impact process and thus has a different size distribution from the dust which is normally lifted off the surface by gentle sublimation processes (Hughes 2005). Here we present observations which provide direct evidence that icy grains were ejected from the comet nucleus as a result of the impact and that they subsequently disappeared due to evaporation of the icy component.
|Figure 1: Comet 9P/Tempel 1 observed with the EPIC instrument after the impact. X-rays were detected in the sunward hemisphere at a nucleus distance of km. The cross near the center marks the position of the comet nucleus. The projected Sun direction is also indicated. The image has been smoothed with an adaptive filter to enhance the weak signal.|
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The O PTICAL M ONITOR measured the evolution of photometric fluxes in
3 different filters with a time resolution of 2 h.
We used two filters in the ultraviolet (UVM2 and UVW1) and a broadband B filter
in the visible spectral range.
Figure 2 shows the bandpasses and throughput curves for all three
|Figure 2: The band passes and throughput curves for the UVM2 (dashed), UVW1 (dotted) and B (solid) filter, folded with the O PTICAL M ONITOR detector sensitivity.|
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|Figure 3: The two-dimensional coma evolution observed as the result of the impact (impact time: 4 July 2005 at 5:52 UT). Before impact (right hand side images) the comet could be well detected only in the B filter, while there is no detection in UVM2 and only a faint detection in the UVW1 filter. After impact the coma is however clearly detected in all three filters. In the B and UVW1 filter it remains visible for the next 8 h until the end of the observing run and the coma reached a spatial extension of about 6 arcsec (3900 km) approximately 1.2 h ( UVW1) and 3 h (B) after the impact. In the UVM2 filter the comet was clearly detected only up to about 2 h after the impact, with a diameter of only about 1.8 arcsec (1200 km).|
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|Figure 4: The light curves measured in the UVM2 (), UVW1 () and B () filter before, during and after the impact. The light curve in the UVM2 representing the rise and decay of small coma grains reaches its maximum more than 1 h before the light curves in UVW1, representing the OH abundance and the B filter covering a mixture of gas emission bands and continuum. The UVM2 light curve also decays at a pace that is about twice a high as that of the other two curves. For each observation, the fluxes were integrated over a circular region of 6 arcsec (equivalent to a coma diameter of 3900 km). This unfortunately increases the error bars of the fluxes measured in the UVM2 filter, but this area was selected to ensure that the spatial extension of the coma in the B and UVW1 filter is covered.|
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From the fact that after impact the light curves of OH and broadband B peak at >1.2 h after the UV continuum light curve we conclude that the small particles seen in the UV were icy grains that disintegrated, at least in part, by evaporating water (and probably also other gaseous species). It has been known for a long time that water is the most abundant volatile species in a comet and accounts for about of its gas production rate. It is also well known that inside a heliocentric distance of 3 AU, water is the main driver of activity of a comet nucleus. Whether icy grains do or do not exist in the coma of a comet close to the Sun has therefore been of major importance for understanding the physical and chemical processes in the near-nucleus region of an active comet. However, as the lifetime of micron and sub-micron size icy grains is very short at heliocentric distances smaller than 2 AU, they have not been detectable by remote-sensing observations of a cometary coma, which is at steady-state conditions. Attempts have therefore been made to detect water ice in the coma of comets, which are active at large heliocentric distances. After a number of inconclusive searches, evidence for the presence of water ice was eventually found in infrared spectra obtained of comet Hale-Bopp (C/1995 O1) at heliocentric distances of 7 AU (Davies et al. 1997) and 2.9 AU (Lellouch et al. 1998). The observations of the D EEP I MPACT induced outburst of comet 9P/Tempel 1 provided the unique opportunity to characterize the properties of fresh cometray material coming off the comet nucleus under non-steady state conditions. Hughes (2005) demonstrated that the mass loss during and immediately after the impact was probably not in form of gentle water sublimation, which is consistent with the discovery reported here. Rapidly disintegrating small icy grains were detected shortly after the impact. The maximum abundance of these small grains was observed as early as 1.5 h after the impact, while larger particles reflecting in the visible spectral range reached their maximum only 1.5 h later. This proves the long standing assumption that the water in comets does not exclusively sublimate directly from the comet nucleus, but is also ejected in form of small icy grains at least during outbursts. As we have no measurements of the small grains at the time of the impact, we cannot determine whether they have reached their maximum abundance only when we took our first observation or earlier. For grains intrinsic to the nucleus this would be immediately after impact. If these icy grains came directly from the nucleus this would have strong implications for the formation of a comet nucleus and lead to constraints for the accretion models of the solar system.