Observations of Comet 9P/Tempel 1 around the Deep Impact event by the OSIRIS cameras onboard Rosetta

Abstract

The OSIRIS cameras on the Rosetta spacecraft observed Comet 9P/Tempel 1 from 5 days before to 10 days after it was hit by the Deep Impact projectile. The Narrow Angle Camera (NAC) monitored the cometary dust in 5 different filters. The Wide Angle Camera (WAC) observed through filters sensitive to emissions from OH, CN, Na, and OI together with the associated continuum. Before and after the impact the comet showed regular variations in intensity. The period of the brightness changes is consistent with the rotation period of Tempel 1. The overall brightness of Tempel 1 decreased by about 10% during the OSIRIS observations. The analysis of the impact ejecta shows that no new permanent coma structures were created by the impact. Most of the material moved with $\sim 200\text{m}{\text{s}}^{\mathrm{-1}}$. Much of it left the comet in the form of icy grains which sublimated and fragmented within the first hour after the impact. The light curve of the comet after the impact and the amount of material leaving the comet ($4.5\text{–}9\times {10}^6\text{kg}$ of water ice and a presumably larger amount of dust) suggest that the impact ejecta were quickly accelerated by collisions with gas molecules. Therefore, the motion of the bulk of the ejecta cannot be described by ballistic trajectories, and the validity of determinations of the density and tensile strength of the nucleus of Tempel 1 with models using ballistic ejection of particles is uncertain.

Introduction

The goal of the Deep Impact mission was to investigate the interior of a comet. When the $\sim 370\text{kg}$ projectile hit Comet 9P/Tempel 1 (hereafter Tempel 1), the Deep Impact spacecraft observed the impact event and the ejecta cloud with high spatial and temporal resolution (A'Hearn et al., 2005). A world-wide campaign of Earth- and space-based observations was initiated to remotely observe the global structure, the composition, and the mid- and long-term evolution of the impact ejecta (Meech et al., 2005).

Among the remote observatories studying Tempel 1, the Rosetta spacecraft, on its way to Comet 67P/Churyumov–Gerasimenko, was located particularly well: It was closer to the comet than Earth-based observers (0.53 AU vs 0.89 AU) and it was able to observe the comet continuously for more than two weeks. The solar elongation of Tempel 1 of slightly more than 90 deg minimized straylight problems for remote sensing instruments.

OSIRIS is the scientific camera system on Rosetta (Keller et al., 2006). It operates from the near ultraviolet to the near infrared spectral range (about 245–1000 nm) and consists of a narrow angle camera (NAC) and a wide angle camera (WAC). The NAC is equipped with 11 medium band width filters (50–90 nm FWHM) which are designed for the study of the physical properties and the mineralogy of the surface of Comet Churyumov–Gerasimenko with high spatial resolution. The WAC is used with two broad band filters (red and green) and 12 narrow band filters (4–14 nm FWHM). Seven of the narrow band filters isolate gas emissions from the cometary coma; the others measure the dust continuum at wavelengths close to that of the gas emissions.

When the opportunity arose to observe Comet Tempel 1 with OSIRIS, an observing program was designed which necessarily had to be different from that for the in situ study of Comet Churyumov–Gerasimenko. The NAC was used in 4 different filters in the red and near infrared spectral region to monitor the dust coma of Tempel 1 as well as the structure and the color of the impact-created ejecta cloud. During the first $\sim 1.5\text{h}$ after the impact, the NAC was effectively used as a photometer that measured the light curve of the inner coma, constraining the processes following the impact. Additional images with a clear filter were taken to be prepared for the case that the effects of the impact on coma brightness would be subtle. The WAC was used to measure the gas production of the comet and to estimate the gas content of the material ejected from the nucleus. Emissions from the OH radical and from atomic oxygen were used as tracers of water production, and the CN production was measured to compare relative abundances between the impact ejecta and the coma created by normal activity of the comet. Sodium D-line emission was looked for to search for signatures of sodium which may be desorbed from warm dust ejected by the impact.

First results from the OSIRIS observations were reported in Küppers et al. (2005) and Keller et al. (2005). The number of water molecules produced in the impact as determined from measurements of the OH emission together with estimates of the dust mass derived from the light curve suggested a dust/ice ratio substantially larger than one for the impact ejecta. This would imply that comets are icy dirtballs (Keller, 1989) rather than dirty snowballs (Whipple, 1950). However, some Earth-based observers derived substantially lower dust masses in the ejecta cloud (Harker et al., 2005, Sugita et al., 2005), so that the dust/ice ratio is still under debate. The abundance ratio between the CN parent molecule(s) and water was somewhat larger in the cloud ejected from the impact than during normal cometary activity. It is, however, not clear yet if this is due to a genuine difference between the surface and the interior of a comet (or between an active and an inactive region) or due to creation of additional CN from organic material which is more likely to happen during the violent impact than during the normal, more gentle activity of the comet driven by sunlight.

The light curve seen by OSIRIS showed a steep increase in the first 40 min and then leveled off. The brightness increase, also measured by Earth-based observers (Meech et al., 2005, Schleicher et al., 2006), is much longer than that expected for crater formation. It is also longer than estimates of the time it takes for the ejecta to become optically thin. The probable cause of the long-lasting brightness increase is ejection of material from the impact crater in the form of icy grains. The ice is subsequently sublimated by sunlight, and the grains fragment and the associated increase in cross section causes brightening of the coma. That interpretation is supported by observations of icy grains from the Deep Impact spacecraft (Sunshine et al., 2006). Ejection of icy grains also explains the expansion velocities of the dust in the ejecta cloud, typically 160 m s⁻¹ and up to $>400\text{m}{\text{s}}^{\mathrm{-1}}$, much larger than typical velocities of crater ejecta (Housen et al., 1983). After sublimation of the ice, the dust is accelerated by collisions with the water molecules.

This paper describes the full data set of OSIRIS observations. We present light curves and color information derived from observations with different filters of the NAC. Coma structures are analyzed and provide information about the interaction between the impact ejecta and the existing coma of the comet. Analysis of the light curves constrains the events during the first minutes after the impact and gives insight into the velocity of the dust. We model the motion of the ejecta cloud with a Monte Carlo code to derive the velocity distribution of the dust and to constrain its size (Jorda et al., this issue). We further present the detection of atomic sodium in the first $\sim 7\text{h}$ after the impact, providing clues about the dust ejected by the impact. We revisit the determination of the water and CN production and provide revised values using the full data set. We discuss implications for the events following the impact and the structure of Comet Tempel 1.

Section 2 summarizes the observations and describes the data reduction. The results are presented in Section 3. Implications for the impact process and the structure of the cometary surface and interior are discussed in Section 4.