Pre-Rosetta expectations on small scale surface characteristics of comet 67/P C-G Akiva Bar-Nun and Diana Laufer Dept. of Geosciences, Tel Aviv University Thanks to the OSIRIS team 1

The comet is chaotic since it was formed by agglomeration of fluffy ice balls at low speeds, followed by violent gas release events on the comet. Comet 67P/Churyumov-GerasimenkoLutetia 2

How Fluffy Simulation of large (20 cm diameter and 10 cm high) samples of amorphous gas-laden ice at Tel Aviv University. An agglomerate of ~100 µm particles of amorphous gas-laden ice which fall on the surface in vacuum. Warmed from above by IR at a solar constant 3

4 Density  = kg m -3 Porosity p=0.73

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A schematic drawing of the machine producing large (200 cm 2 × 10 cm) gas-laden amorphous ice samples, as a "comet" simulation: (1) vacuum chamber; (2) cold plate at 80 K; (3) 200-μm amorphous gas-laden ice; (4) homogeneous flow of water vapor and gas; (5) water vapor and gas pipes; (6) 200 cm 2 and 5–10-cm thick ice sample; (7) heating dome; (8) 80 K cold knife; (9) thermocouples; (10) density measurements; (11) mass spectrometer; (12) ionization gauge; (13) heating tape; (14) LN2 cooling pipes. 6

How Fluffy Density 250 – 300 kg m -3 Mechanical strength: Tensile strength of 2-4 kPa, like talcum powder. The tensile strength of the upper layer of comet Temple 1 is even lower, kPa. The thermal inertia is very low: I = (KC p  ) 1/2 =80 WK -1 m -2 s 1/2 Like Temple 1: < 100 WK -1 m -2 s 1/2 7

Crust Formation A crust is formed upon heating the surface, by back migration of water vapor inward. Its Tensile strength of 20 kPa. An ice crust which was removed from the top of a several cm thick ice sample, warmed by 1.1 solar constants for 1.5 h. Note the rugged ice structure. I. Pat-El, D. Laufer, G. Notesco, A. Bar-Nun An experimental study of the formation of an ice crust and migration of water vapor in a comet's upper layers Icarus, Volume 201, Issue 1, 2009, 406 –

Mechanical Strength The crust’s mechanical strength is 20 times higher than the fluffy ice underneath. 9

Ice Grain Ejection Gas is released from the ice upon its warming up, when the heat wave penetrates inward and the amorphous ice transforms into crystalline ice, thus releasing the trapped gases. The gas jets carry with them µm ice grains, at a speed of ~1.5 m sec -1 (like in KOSI). These at 1 g, fall back on the surface and cover it. 10

Ice grain ejection from thin ice samples: 2 layers were formed: a ~100 μm layer of frozen CO 2 covered by a ~100 μm layer of amorphous ice. The thick gray line describes the temperature on the right axis. Note the rise in the flux of water vapor during massive ice grain ejection. The gap in the flux from 1800 to 2200 sec is due to the stopping of the measurement, during the warming-up of the cold finger, which cools the sample plate to release the gases trapped on it. In the insert, an extended time scale shows wide water peaks along with narrow ones. Akiva Bar-Nun, Diana Laufer, CO 2 as the driving force of comet Hartley 2’s activity—An experimental study Planetary and Space Science, Volume 86, 2013, 160 – 167, 11

Fig. 5. Size distributions of the ice grains in the different temperature ranges, measured for all grains recorded in Fig

1cm 1 min40 min 65 min 20 cm diameter x 3 cm high 3 Layers: Ice, frozen CO 2, Ice Upon heating, the CO 2 sublimates, breaking the overlying water ice layer gradually and spewing a huge number of ice grains which cover the entire sample with small grains. At the beginning of the heating process (a) small “craters” in the ice layer were already existing (b), which were covered by very small grains at a rate of 0.13 mm 3 min −1 (b) and (c). From (c) the ice grains can be estimated to be several to 100 µm, below the resolution of the camera. Diana Laufer, Akiva Bar-Nun, Igal Pat-El, Ronen Jacovi, Experimental studies of ice grain ejection by massive gas flow from ice and implications to Comets, Triton and Mars, Icarus, Volume 222, Issue 1, 2013, 73 –

Ice grain coverage 14

Behavior of the crust Gases can not escape freely from under the crust and bulge it. Finally the crust is broken. Diana Laufer, Akiva Bar-Nun, Igal Pat-El, Ronen Jacovi, Experimental studies of ice grain ejection by massive gas flow from ice and implications to Comets, Triton and Mars, Icarus, Volume 222, Issue 1, 2013, 73 – Time sequence of swelling (c), breakage (d and e), and collapse (f–h), in 1.5-cm-thick ice samples having a crust. The magnitude of the collapse is best seen in (h). Streaks of ejected ice grains are marked by arrows in (d and e) 15

A chaotic terrain is formed, with small and large boulders. When occurring uphill the shattering of the crust can cause an avalanche. Shattered terrain from the collapse of the ice is seen in (a). An oval “crater” is seen in (b) Diana Laufer, Akiva Bar-Nun, Igal Pat-El, Ronen Jacovi, Experimental studies of ice grain ejection by massive gas flow from ice and implications to Comets, Triton and Mars, Icarus, Volume 222, Issue 1, 2013, 73 –

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Once the gas below is exhausted, the unshattered surface collapses to form cracks. 18