1. ACM-2008. #8102. Tuesday, 15 July 2008 Velocities and relative amounts of material ejected after the collision of DI impactor with comet Tempel 1 Sergei I. Ipatov Catholic University of America, USA. The work was initiated at University of Maryland (siipatov@hotmail.com, http://www.dtm.ciw.edu/ipatov, http://www.astro.umd.edu/~ipatov, http://faculty.cua.edu/ipatov/), and Michael F. A’Hearn University of Maryland, College Park, USA The file with this presentation can be found via http://www.dtm.ciw.edu/~ipatov/present.htmsiipatov@http://www.dtm.ciw.edu/ipatovhttp://www.astro.umd.edu/~ipatov

2. 2 Velocities of ejected material and velocities of a level of brightness The mass of impactor was 370 kg. Impact speed was 10.3 km/s. The Deep Impact collision with comet 9P/Tempel 1 was oblique: between 25 and 35 deg from the surface horizontal. We considered velocities and rates of ejection of the particles that give the main contribution to the brightness of the cloud of ejected material, i.e., mainly those of icy particles with d<3 μm. Particles were accelerated by gas. Sublimation. Fragmentation. We considered the projection vp of the velocity on the plane perpendicular to the line of sight and analyzed the velocity of calibrated physical surface brightness (CPSB, always in W m -2 sterad -1 micron -1 ) of a cloud of ejected material on RADREV images. Actual velocities of particles are greater than velocities of the level of brightness: (1) we see only a projection of velocity on the plane perpendicular to a line of sight; real velocity can be greater than the projection by a factor of 1.5-2. (2) if the same amount of material moves from distance D1 from the place of impact to distance D2, then the number of particles on a line of sight (and so the brightness) decreases by a factor of D2/D1 ; (3) ejected particles become cooler with time and so they become less bright. On the other hand, the light from the impact illuminated the dust which was near the comet before the impact.

3. 3 Table. Variation of the relative brightness Br of the brightest pixel with time t (in seconds). The relative brightness at t=4 s is considered to be equal to 1. t |0.06|0.17 |0.22|0.28|0.34|0.40|0.46|0.58|0.70 |0.82| 1 | 2 | 3 Br |0.8 | 1.4 | 2.5 |2.5 |1.2 |2.1 |2.4 |2.5 |2.45 |2.3 |1.7|1.3|1.05 t | 4 | 6-8 | 12 | 16-20 | 25-36 | 42 | 66 | 80-350 |384 |410 Br | 1 |0.95-0.98|0.98| 1.02 | 1.04 |1.02 |0.95| 0.9-0.95 |1.2 |0.95 There were maxima of brightness at t~0.22-0.28 and 0.46-0.7 s. These peaks may correspond to the increase of ejection of material per unit of time and/or to higher temperature of ejected material. At t=0.34 s the maximum brightness was smaller by a factor of two than at t=0.28 and t=0.4 s. One possible explanation of the decrease in the maximum brightness is that a considerable fraction of the brightness at t~0.22-0.28 s was due to the light from the hot place of impact. The brightest pixel could be optically thick (it was not possible to see most of the mass) most of the time considered. Brr is considered to be proportional to Br×R×R (where R is the distance between the cameras and the place of the impact) and equal to 1 at t= 4 s. Brr equals to 0.33, 0.36, and 0.26 at t equal to 350, 384, and 410 s respectively. There could be a local increase of albedo and ejection rate at t~380 s. The direction from the place of impact to the brightest pixel (and probably the mean direction of ejection of the brightest material) was mainly close to the direction of the impact during the first 10-12 s, then quickly changed by about 50 o, and then slowly became closer to the direction of the impact (Fig. 3).

4. Fig. 1. The difference in brightness between MRI (Medium Resolution Instrument) images made 0.06, 0.165, 0.224, 0.282, 0.341 (upper row), 0.400, 0.462, 0.579, 0.697, and 0.814 s (lower row) after the impact and the image at t=-0.057 s. In figure (a) white region corresponds to constant calibrated physical surface brightness CPSB≥3 (in W m -2 sterad -1 micron -1 ), and in figure (b) it corresponds to CPSB≥0.5, but both figures present the same images. 0.28 s →0.34 s, 5 pxl, 7 km/s, bright pixel. 0.46 s (CPSB=1) → 0.58 s (CPSB=0.6), 3 km/s (a) (b)

5. 5 Fig. 2. Contours at CPSB equal to 3, 1, 0.3, and 0.1 for the difference in brightness between MRI images made 0.993, 1.986, 2.978 (upper row), 3.970, 4.962, and 5.720 s (lower row) after the impact and the image at t=-0.057 s. A large cross shows the position of the brightest pixel at t=0.06 s, and a smaller cross, at current time. Velocities v>v(CPSB=0.1)≈1 km/s at 1≤t≤3 s. Production rate is similar at 2≤t≤5 s (based on similar CPSB=3 contours).

6. Fig. 3. Coordinates x and y of the brightest pixel in MRI (INTTIME=0.05 s, 64×64 pixels) and HRI (INTTIME=0.1 s, 512×512 pixels) images (made with CLEAR filter) relative to the position of the brightest pixel on the MRI image at t=0.001 s and t=0.06 s or the position on the HRI image at t=0.215 s. The differences in brightness between a current image and an image before the impact were considered. Coordinates are given in HRI pixels (i.e., the number of MRI pixels was increased by a factor of 5). HRI y-plot was made relative to the position on the HRI image at t=0.215 s (the place of ejection of material at t≥0.2 s) and then was shifted down by 5 pixels to consider the position of the brightest pixel relative to the place of impact. A jump of direction of ejected material at te~10-12 s.

7. Fig. 4. Contours corresponding to CPSB equal to 3 (a), 1 (b), 0.3 (c), and 0.1 (d) for the difference in brightness between HRI (High Resolution Instrument) images made 1.852, 4.379, 8.00, 12.254, 16.524, and 20.906 s after the impact and the image at t=-0.629 s. The largest cross shows the position of the brightest pixel at t=0.215 s. The size of a cross indicating the position of the brightest pixel at current time is smaller for a greater value of time. At 8 < te < 20 s the rate of ejection could increase with te and could be greater than that at 4 < te < 8 s. Sizes of contours mainly increased with time. (a) (b) (c) (d)

8. 8 Fig. 5. Contours corresponding to CPSB equal to 3, 1, 0.3, and 0.1 for the difference in brightness between HRI images made 1.852 (a), 4.379 (b), 8.00 s (c), 12.254 s (d), 16.524 s (e), and 20.906 s (f) after the impact and the image at t=-0.629 s. The largest cross shows the position of the brightest pixel at t=0.215 s, and a smaller cross, at current time. Sizes of crosses are the same as in Fig. 4. (a) (b) © (d) (e) (f)

9. Fig. 6. Contours corresponding to CPSB equal to 3 (a) and 1 (b), for the difference in brightness between HRI images made 1.008, 1.852, 25.332, 30.00, 35.715, 42.618, and 109.141 s after the impact and the image at t=-0.629 s. The contour at t=109 s is the third from the place of impact, and other contours are larger for larger times. The largest cross shows the position of the brightest pixel at t=0.215 s. The size of a cross indicating the position of the brightest pixel at current time is smaller for a greater value of time. The rate of ejection was about the same at time of ejection te~20-40 s (based on comparison of contours CPSB=3). (a) (b)

10. 10 Fig. 7. Contours corresponding to CPSB equal to 2γ, γ, 0.5γ, 0.3γ, 0.1γ, and 0.03γ (where γ=(Ro/R) 2, Ro and R are distances of HRI from the comet at the time of impact and at a current time, respectively), for HRI images made 39.274 (a), 66.176 (b), 142.118 (c), and 384.561 s (d) after the impact. The position of the brightest pixel in an image is marked by a cross. ykm(t=66 s)>ykm(t=39 s) for γ, 0.5γ, 0.3γ; ykm(t=66 s)<ykm(t=39 s) for 2γ; D(γ)=4 km, D(2γ)=2 km at t=66 s; => ejection rate began to decrease at te~50 s [for vp~200 m/s]; (a) (b) ykm(t=66 s )>ykm(t=142 s ) for all γ => rate decreased at te~50-140 s; ykm(t= 142 s)/ykm(t= 385 s)= 1.2, 1.3, 1.04 for 2γ, γ, 0.5γ => local maximum of ejection rate and/or albedo at te~6 min. Rays of ejected material, especially (c) (d) at t=66 s. Upper-right, upper-upper- right, down-left, left bumps.

11. 11 Fig. 8. Contours corresponding to CPSB equal to 0.5γ, 0.3γ, 0.1γ, 0.03γ, 0.01γ, and 0.003γ (where γ=(Ro/R) 2, Ro and R are distances of MRI from the comet at the time of impact and at a current time, respectively), for MRI images made 77.651 (a), 138.901 (b), 191.53 (c), 311.055 (d), 351.043 (e), and 410.618 s (f) after the impact. The position of the brightest pixel in an image is marked by a cross. (a) (b) © (d) (e) (f)

12. Velocities at t~40-410 seconds We considered a series of MRI images presented in Fig. 8 in order to study the relative rate of ejection at te~3-7 min. For contours CPSB≥0.03γ at 77.65≤t≤410.6 s, the values of ykm (and therefore probably the rate of ejection) mainly decreased with time, exclusive for te~400 s. For contours CPSB=0.5γ and CPSB=0.3γ, ykm equaled 2.7 and 4.0 km at t=351 s and to 2.85 and 4.0 km at t=410.6 s. It shows that there could be a local increase of the rate of ejection and albedo of material ejected at te~400 s. This conclusion is accordance with the similar conclusion based on HRI images and with studies of the brightest pixel. For contours CPSB=0.1γ and CPSB=0.03γ, ykm equaled 11.5 and 16 km at t=77.65 s, and 13 and 29 km at t=139 s. This increase of ykm with time was probably due to particles ejected mainly with vp>200 m/s at te<80 s (29,000/200=58, 139-58=81). At t=77.65 s, ykm equaled 21.6 and 34.7 km for contours CPSB=0.01γ and CPSB=0.03γ, respectively. Such far faint contours could not include low-velocity particles, and velocities vp of all particles constituting these two contours exceeded 278 and 446 m/s, respectively. The above analysis shows that for the model without sublimation, the rate of ejection mainly decreased at te~50-350 s, but there could be a local maximum of ejection rate and albedo (e.g., there could be an ice conglomerate in the crater) at te~6-7 min. Gravity-dominated cratering (i.e., greater amounts of ejected material and greater size of a crater). Ejection with velocities greater than 100 m/s could last for a longer time than it was predicted by theoretical models (e.g., Holsapple & Housen 2007; Richardson et al. 2007) and could take place when there was ejection with smaller velocities.

13. Rays of ejected material There was excessive ejection of material to a few directions (rays of ejected material). The upper-right excessive ejection began mainly at te~15 s (though there was some ejection at te~2 s), could reach maximum at te~25-50 s, could still be considerable at te~100 s, but then could decrease, though it still could be seen at te~400 s. The value of te~15 s is correlated with the changes of the direction to the brightest pixel at t~12-13 s. The bump corresponding to the upper-right excessive ejection is clearly seen for the contours CPSB=1 at 25≤t≤43 s in Fig. 6b. The most sharp bump is seen for CPSB=1 at t=43 s in Fig. 6b, for CPSB=γ at t=39 s in Fig. 7a, and for CPSB=0.5γ and CPSB=0.3γ at t=66 s in Fig. 7b. In Fig. 7c (t=142 s) the bump is clearly seen for CPSB=0.3γ and CPSB=0.5γ. In Fig. 8a (t=78 s) the upper-right bump is most pronounced for the contour CPSB=0.3γ and practically is not seen for the outer contour CPSB=0.003γ. At vp=200 m/s, material of the bump at CPSB=0.3γ (located at D≈6.6 km) could be ejected at te~55 s (78-6600/200). In Fig. 8b (t=139 s) the upper-right bump is most pronounced for the contour CPSB=0.03γ, and it is seen for all other contours. The bump at CPSB=0.03γ was located at D=23 km. The particles with vp=200 m/s that reached this bump were ejected at te~25 s. For particles of the bump ejected at te~55 s, vp=270 m/s. All particles of the bump for CPSB=0.01γ (located at D=30.7 km) had velocities vp≥220 m/s (30700/139). The bump is still seen, but is less pronounced, for the contour CPSB=0.1γ at t=410 s (Fig. 8f, D=4.8 km). So some excess of ejection in this direction could be at te~385 s if we consider vp=200 m/s.

14. Rays of ejected material There was also the bump of a contour in the down-left direction from the place of impact. This direction is opposite to the upper-right direction discussed above. Both directions are approximately perpendicular to the direction of the impact. The sizes of the upper-upper-right, down-left, and left bumps are usually about the sizes of the upper-right bumps if we consider the contours consisting of material ejected during the first 100 s and suppose vp≈200 m/s. For contours corresponding to te>100 s, the down-left and left bumps are usually smaller than the upper-right bump. Directions from the place of impact to the upper-upper-right and left bumps are closer to direction to the center of the comet than those for the upper-right and down-left bumps and are not located close to one line. The right bump is mainly greater than the upper-right bump at t~4-20 s, but it is not practically seen at greater time. The upper bump of the outer contours is clearly seen in Fig. 8b-f, especially, for the CPSB=0.03 in Fig. 8c (t=192 s) and CPSB=0.003 in Fig. 8e (t=351 s). The upper bump is also seen for the contour CPSB=1 at t≥25 s in Fig. 6b. The direction from the place of impact to this bump is not far from the direction opposite to the impact direction (i.e., the bump corresponds to the excessive ejection backwards to the impact direction), but is not exactly perpendicular to the line connecting down- left and upper-right bumps. This work was supported by NASA DDAP grant NNX08AG25G. Main conclusions were presented in Ipatov S.I., A’Hearn M.F., 39th LPSC, #1024, 2008. The paper will be submitted to Icarus (and will be put on astro-ph) at the end of July.