LPSC 2011, March 10, #1317 THE OUTBURST TRIGGERED BY THE COLLISION OF THE DEEP IMPACT MODULE WITH COMET TEMPEL 1, AND CAVITIES IN COMETS. Sergei I. Ipatov 1 Catholic University of America, USA. 2 Space Research Institute, Moscow, Russia This presentation can be found on

Introduction On July 4, kg impactor collided with Comet 9P/Tempel 1 at velocity of 10.3 km/s (A’Hearn et al. 2005). It was an oblique impact, and the angle above the horizon was about o. Evolution of the cloud of ejected material was observed by Deep Impact (DI) cameras, by space telescopes (e.g. Rosetta, Hubble Space Telescope, Chandra, Spitzer), and by over 80 observatories on the Earth. The values of the projection of the velocity v le of the leading edge of the dust cloud of ejected material onto the plane perpendicular to the line of sight were about m/s for observations made 1-40 h after impact. Jorda et al. (2007) concluded that particles with d<2.8 μm represent more than 80% of the cross-section of the observed dust cloud. The velocities discussed in our presentation correspond mainly to particles with d<3 μm, which probably constitute not more than 7% of the total ejected material. The total mass of ejected dust particles with diameter d less than 2, 2.8, 20, and 200 μm was estimated to be about 7.3×10 4 –4.4×10 5, 1.5×10 5 –1.6×10 5, 5.6× ×10 5, and ×10 7 kg, respectively. Our studies were based on analysis of the images made by Deep Impact (DI) cameras during the first 13 minutes. The main aims of our studies: Time variations in velocities and relative amount of material ejected from Comet 9P/Tempel 1. The role of a triggered outburst in the ejection. Excessive ejection in a few directions (rays of ejected material). Cavities in comets as sources of outbursts. 2

SeriesInstru- ment INTTIME, seconds Size, pixelsEXPIDIMPACTM, seconds min, max Ma (dif)MRI × , , MbMRI × , , Ha (dif)HRI0.1512× , , HbHRI × , , Hc (dif)HRI0.6512× , , HdHRI × , , HeHRI × , , We analyzed several series of DI images. In each series, the integration time and the size of image were the same. For series Ma, Ha, and Hc, we analyzed the differences in brightness between a current image and that before the impact. These series are marked by “(dif)”. For other series, we analyzed the brightness in current images. Observed brightness of the cloud of ejected material was mainly due to particles with diameters d<3 micron, and we discuss mainly ejection of such particles.

Contours corresponding to CPSB (calibrated physical surface brightness) equal to 1, 0.3, 0.1, and 0.03, for series Mb. Our studies were based mainly on analysis of distances from the place of ejection to such contours. The bumps on the contours correspond to rays of excessive ejection. 4

Rays of ejected material The excess ejection of material to a few directions (rays of ejected material) was considerable during the first 100 s, took place during several minutes, and was still observed in images at t~ s even close to the place of ejection. It shows that the outburst could continue up to ~10 min. Considerable excessive ejection to a few directions (some rays) began approximately at the same time t e ~10 s when the direction from the place of ejection to the brightest pixel changed, the peak brightness began to increase, and there was a local peak of the rate of ejection. These features could be caused by the outburst triggered by the impact. The sharpest rays were caused by material ejected at t e ~20 s. The upper-right excessive ejection (perpendicular to the direction of impact) began mainly at t e ~15 s (though there was some ejection at t e ~2 s), could reach maximum at t e ~25-50 s, could still be considerable at t e ~100 s, but then could decrease, though it still could be seen at t e ~400 s. The value of t e ~15 s is correlated with the changes of the direction to the brightest pixel at t~12-13 s. The upper bump of the outer contours is clearly seen at 66<t<665 s, especially at t~ s. The direction from the place of impact to this bump is not far from the direction opposite to the impact direction. 5

Time variations in sizes L (in km) of regions inside contours of CPSB=const. The number after a designation of the series in the figure legend shows the value of brightness of the considered contour. The curves have local minima and maxima that were used for estimates of velocities at several moments in time. Based on the supposition that the same particles correspond to different local maxima (or minima) of L (e.g., to values L 1 and L 2 on images made at t 1 and t 2 ), we calculated the characteristic velocities v=(L 1 -L 2 )/(t 1 -t 2 ) at t e =t 1 - L 1 /v. For series Ma, we considered L as the distance from the place of impact to the contour down in y- direction. For other series, we considered the difference between maximum and minimum values of x for the contour. 6

Typical projections v model of velocities (in km/s) on the plane perpendicular to the line of sight at time t e of ejection for the model for which velocities v model at t e are the same as velocities v expt =c×(t/0.26) -α of particles at the edge of observed bright region at time t. The values of vy obs and vx obs are based on analysis of local minima and maxima of plots on the previous slide. The distance from the place of ejection to the edge was used to find the dependence of t on t e. As the first approximation, the characteristic velocity at t e >1 s can be considered to be proportional to t e or t e -0.7 (i.e. α~ ; 0.71 corresponds to sand; 0.75, to the ejection mainly governed by momentum). The values of vymin and vxmin show the minimum velocities of particles needed to reach the edge of the bright region (in an image considered at time t) from the place of ejection moving in y or x- direction, respectively. 7

Calculations of the relative rate of ejection Considering that the time needed for particles to travel a distance L r to the edge of the bright region is equal to dt=L r /v expt (where v expt (t)=c×(t/0.26) -α ), we find the time t e =t−dt of ejection of material of the contour of the bright region considered at time t. The volume V ol of a spherical shell of radius L r and width h is proportional to L r 2 h, and the number of particles per unit of volume is proportional to r te ∙(V ol ∙v) -1, where v is the velocity of the material. Here r te corresponds to the material that was ejected at t e and reached the shell with L r at time t. The number of particles on a line of sight, and so the brightness Br, are approximately proportional to the number of particles per unit of volume multiplied by the length of the segment of the line of sight inside the DI cloud, which is proportional to L r. Actually, the line of sight crosses many shells characterized by different r te, but as a first approximation we supposed that Br is proportional to r te (v∙L r ) -1. For the edge of the bright region, Br≈const. Considering v=v expt, we calculated the relative rate of ejection as r te = c ∙L r ∙t -α. 8

Relative rate of ejection at different times t e of ejection for the model in which characteristic velocities of particles constituting the edge of the observed bright region at time t are equal to v expt =c×(t/0.26) -α. At 1<t e <3 and 8<t e <60 s, the plot of time variation in the estimated rate rte of ejection of observed material was essentially greater than the exponential line connecting the values of rte at 1 and 300 s. (The exponential monotonic decrease of the rate is predicted by theoretical models.) There was a local maximum of the rate at t e ~10 s with typical projections of velocities v p ~100 m/s, and there was a sharp decrease of rte at t e ~60 s. Such variations in rte can be mainly due to the triggered outburst. Our studies do not contradict to a continuous ejection of material during at least 10 minutes after the collision. 9

Relative amount f ev of observed material ejected with velocities greater than v vs. v for the model VExp in which characteristic velocities of particles at the edge of the observed bright region in an image at time t are equal to v=v expt =c×(t/0.26) -α for five pairs of α and c. f ev =1 for material ejected before the time t e803 of ejection of particles located at the edge of the bright region in an image made at t=803 s. At velocities of several tens of meters per second, the model amount is greater than for theoretical estimates. For theoretical models, exponents of the velocity dependence of the relative volume f ev of material ejected with velocities greater than v, equal to -1, -1.23, -1.66, and -2, correspond to α equal to 0.75, 0.71, 0.644, and 0.6, respectively (α=0.71 is for sand). 10

‘FAST’ and ‘SLOW’ OUTBURSTS The rates and velocities of material ejected after the DI impact were different from those for experiments and theoretical models. Holsapple and Housen (2007, Icarus 187, ) concluded that the difference was caused by vaporization of ice in the plume and fast moving gas. In our opinion, the greater role in the difference could be played by the outburst triggered by the impact (by the increase of ejection of bright particles). The ‘fast’ outburst could be caused by ejection of material under gas pressure. ‘Slow’ outburst ejection could be similar to the ejection from a ‘fresh’ surface of a comet and could be noticeable at min after the formation of the crater. The jump of the contribution of the DI outburst to the brightness of the DI cloud began at t e ~8 s. The ejection rate at t e =10 s was greater by a factor of 1.4 than that obtained at the supposition that the previous decrease of the rate was prolonged with the same exponent. This factor may characterize the increase of the role of the outburst in the ejection of small particles if at that time v eo ~v e. There could be a sharp decrease of the outburst at t e ~60 s. The contribution of the outburst to the brightness of the cloud could be considerable, but its contribution to the total ejected mass could be relatively small because typical sizes of particles ejected at the outburst probably were smaller than those ejected at the normal ejection. 11

Velocities of ejected particles Projections of velocities of most of observed material ejected at t e ~0.2 s were about 7 km/s. Analysis of DI observations that used different approaches showed that at 1<t e <100 s the time variations in the projections v p of characteristic velocity onto the plane perpendicular to the line of sight can be considered to be approximately proportional to t e -α with α~ For the VExp model with v p proportional to t e -α at any t e >1 s, the fractions of observed material ejected (at t e ≤6 and t e ≤15 s) with v p ≥200 and v p ≥100 m/s were estimated to be about and , respectively, if we consider only material observed during the first 13 min. The ‘fast’ outburst with velocities ~100 m/s probably could last for at least several tens of seconds, and it could significantly increase the fraction of particles ejected with velocities ~100 m/s, compared with the estimates for the VExp model and for the normal ejection. 12

Velocities and acceleration by gas Our estimates of velocities of particles presented in slide 7 are in accordance with the estimates ( m/s) of projection of velocity of the leading edge of the DI dust cloud that were based on various ground-based observations and observations made by space telescopes. Destruction, sublimation, and acceleration of particles did not affect much on our estimates of velocities because we considered the motion of particles along a distance of a few km during not more than a few minutes. During the considered motion of particles with initial velocities v p ≥20 m/s, the increase of their velocities due to the acceleration by gas did not exceed a few m/s. 13

Cavities with material under gas pressure Analysis of observations of the DI cloud and of outbursts from different comets testifies in favor of that there can be large cavities with material under gas pressure below a considerable fraction of a comet’s surface. Internal gas pressure (e.g. due to crystallization of amorphous ice and/or sublimation of the CO ice) and the material in the cavities can produce natural and triggered outbursts and can cause splitting of comets. The outburst ejection of material from a cavity could be greater for a specific direction. Therefore, its role in the direction from the place of ejection to the brightest pixel could be greater than in the total ejection rate. 14

Location of the upper border of the main excavated cavity Our estimates testify in favor of the location of the upper border of the main excavated cavity at a depth d cav ~5-10 meters. For example, at time t eb =8 s, the depth of a crater d cr =d f ×d h /(T e /t eb ) γ =12.5 m for d f =100 m, d h =0.25, T e =80 s, and (T e /t eb ) γ =10 γ =2; for the same data and T e =400 s, d cr ≈12.5/1.62≈8 m. For theoretical models (Holsapple & Housen 2007), radius of a crater is proportional to t e γ, where γ is about For small cavities excavated at t e =1 s, the value of d cr (~4-5 m) was smaller by a factor of 8 γ (i.e. by about a factor of 2) than at t e =8 s. The distance d cav between the pre-impact surface of the comet’s nucleus and the upper border of the cavity could be smaller than d cr because the excavated cavity could be located at some distance from the center of the crater (not below the center). On the other hand, due to cracks caused by the impact, the outburst from the cavity could begin before excavation of the upper border. The distances from the upper borders of large cavities to the surface of a comet of about 5-10 m, and sizes of particles inside the cavities of a few microns are in a good agreement with the results obtained by Kossacki and Szutowicz (Icarus, 2011, in press) 15

Conclusions At time of ejection t e ~10 s, there was a local maximum of the rate of ejection of observed particles (mainly with diameter d<3 μm) with typical projections of velocities v p ~100 m/s. At 1<t e <3 s and 8<t e <50 s the estimated rate of ejection of observed material was essentially greater than that for theoretical monotonic exponential decrease. Such difference was caused by that the impact was a trigger of an outburst. At t e ~55-72 s, the ejection rate sharply decreased and the direction from the place of ejection to the brightest pixel quickly returned to the direction that was before 10 s. It could be caused by a sharp decrease of the outburst that began at t e ~10 s. Analysis of observations of the DI cloud and of outbursts from different comets testifies in favor of that there can be large cavities with material under gas pressure below a considerable fraction of comet’s surface. The upper boarder of relatively large cavities can be located at about 5-10 meters below the surface of the comet. 16

Results of our studies are presented in the below papers: [1] Ipatov S.I. and A’Hearn M.F., The outburst triggered by the Deep Impact collision with Comet Tempel 1, Mon. Not. R. Astron. Soc., 2011, 32 journal pages, in press. mnras.pdf, mnras.pdfhttp://arxiv.org/abs/ [2] Ipatov S.I., Cavities as a source of outbursts from comets, In “Comets: Characteristics, Composition and Orbits”, Nova Science Publishers, accepted for publication, [3] Ipatov S.I. and A'Hearn M.F., Deep Impact ejection from Comet 9P/Tempel 1 as a triggered outburst, Proc. IAU Symp. S263 "Icy bodies in the Solar System“, Cambridge University Press, 2010, pp A full list of Ipatov’s publications (and copies of most of them) can be found on Acknowledgements The visit to LPSC and this research were supported in part by NASA through the American Astronomical Society's Small Research Grant Program. 17