Physics of Cometary Nuclei – from Giotto to Rosetta H. Uwe Keller IGEP Universität Braunschweig Institut für Planetenforschung DLR Berlin 感谢 PMO 的邀请 28. 5. 2019 PMO H. Uwe Keller - Physics of Nuclei 1 The source and origin of these capricious appearances are tiny nuclei. They release dust particles by subliming (water) ice when heated up by the sun. What have we learned about their physical nature and activity? Are they as diverse as their tails? 28. 5. 2019 PMO H. Uwe Keller - Physics of Nuclei 2 孛星 彗星 掃星 天攙篷星 Record of Comet Halley from 240 BC (太史公書, 史記 ) Chinese records about 3rd century BC First records 600 BC Comet Halley 1059 BC? 19th-century replica of Du Yu's 3rd-century AD annotated Annals 28. 5. 2019 PMO H. Uwe Keller - Physics of Nuclei 3 Aristotle’s Cosmology From Renaissance to … Moon separates Earth from Parallax measurements by Tycho Brahe Heaven (1546 – 1601) positioned comets beyond the Moon (Great Comet of 1577) making Planets and Stars move in comets planetary objects perfect circles • Kepler(1571-1630)’s view of comets: Heaven is eternal – no change – Coma is globe of transparent Comets (κόμη head of hair) - nebula-like matter, denser than unpredictable and temporary - normal ether – “As many comets as fish in the sea” are exhalation of the • Only at the beginning of the 18th atmosphere rising towards the century comets finally became sphere of fire subjects of the new physics 384 – 322 BC introduced by Newton (1642 – 1726) • Halley (1656 – 1742)’s prediction of the re-appearance of a periodic • Geocentric view prevailed comet demonstrated that the law of for more than 1800 years gravity is valid far beyond the planets But what is the nature of comets? 28. 5. 2019 PMO H. Uwe Keller - Physics of Nuclei 4 Dusty Comets • G. Schiaparelli to connect comets to meteor streams (1866) • A. Bredikhin and Bessel modeled the dust tail by a repulsive force from the sun • Shift in perihelion passage of comet Encke was explained by non-gravitational forces (Bessel 1835) • A. Eddington (1910) described the fountain model • Lyttleton (1953) published his “sandbank” model, where gas is adsorbed on grains 28. 5. 2019 PMO H. Uwe Keller - Physics of Nuclei 5 Gas and Ice • P. Swings (1941): excitation of radicals (CN) by resonance fluorescence by solar radiation • K. Wurm (1943) introduced the concept of “parent” molecules to store the observed radicals • Whipple’s (1950) icy conglomerate model to explain the effects of non-gravitational forces • Water as dominant volatile determined by OH and H Ly α observations with first UV satellites (1972) • Delsemme (1970): water clathrates to store super volatiles 28. 5. 2019 PMO H. Uwe Keller - Physics of Nuclei 6 28. 5. 2019 PMO H. Uwe Keller - Physics of Nuclei 7 Snowballs Delsemme (1982) in Comets (ed. L. L. Wilkening)) Delsemme and Rud (1973) inferred the above albedo and nucleus radius. The water production rate near the sun provides (1-A) R2 and far away the brightness observations of the “bare” nucleus determine A R2. The Comet Halley Armada Giotto - the last and closest flyby The Halley fly-bys changed our perception of cometary nuclei • Cometary nuclei are very dark containing dust, ices, and organic material • Nuclei had to be formed and kept at T < 30 K • There is no (water) ice visible • Their physical properties are rough and determined by the non-volatile (dust) component • The refractory to volatile ratio is > 1 (possibly >>1) • Cometary nuclei are bigger than required to produce the observed activity (restricted and/or limited areas of sublimation activity) • Cometary nuclei are porous and of low density and tensile strength 28. 5. 2019 PMO H. Uwe Keller - Physics of Nuclei 10 Halley Multicolour Camera Chain of Active Hills Region Image size 75x75 px Central Hill Depression Crater Mountain Bright Patch Ridge 28. 5. 2019 PMO H. Uwe Keller - Physics of Nuclei 11 15 years later! Comet 19/P Borrelly Cliff Collapse Britt et al. (2004) 8 x 3.2 km2 Sun Pits 28. 5. 2019 PMO H. Uwe Keller - Physics of Nuclei 12 81P/Wild 2 (Stardust) 2004 The jets appear to be normal to the limb rather than be radial from the nucleus center. (A) A variety of pinnacles and mesas seen on the limb of Wild 2. (B) The location of a 2-km series of aligned scarps that are best seen in the stereo images. 28. 5. 2019 PMO H. Uwe Keller - Physics of Nuclei 13 Traces of water ice Deep Impactä (Tempel 1) 2005 Active areas could not be identified Very low thermal inertia: Γ < 50 W m-2 s-1/2 K-1 consequently very low heat conductivity: -1 -1 Surface temperature κ ≈ 0.01 W m K (Groussin et al. 2007) Conclusions from 1. interactive experiment with a comet Cometary nuclei: • of low density • very weak tensile strength • very low thermal inertia • very low heat conductivity • ice not visible (spots with few % H2O) Question: What are the consequences for cometary activity? Heating by Insolation Diurnal temperature variation in the outer layer of cometary nucleus: Temperature profiles shown at 30° hour angle intervals starting with curve 1 at local noon. (Prialnik et al. 2007) Diurnal skin depth only a few centimeters Orbital skin depth several decimeters Modelling Activity Situation at active surface area: • Diurnal skin depth ca. 1.5 cm • Orbital skin depth ca. 70 cm • Eroded surface layer ca. 100-200 cm per orbit • Surface recedes faster than heat wave penetrates! Material, a few centimetres below an active surface, was covered in previous orbits by metres of essentially unaltered nucleus material and is not affected by insolation Activity shows little degradation with cometary age CO2 Hartley 2 EPOXI 2010 Grain Deposition OrganicsH2O Ice CO2 + Ice + Grains • Mass Movement of Material A’Hearn et al., Science, 2011 H2O – Water ice grains dragged out Vapor by CO2 are deposited in lows – Sedimentary processes on comets!! A’Hearn et al., Science, 2011 Cometary Nuclei 2005 (Stardust-Next) 2011 2001 1986 4.1 km 67/P (Rosetta) 2004 6 flybys of 5 nuclei 2010 + 1 rendezvous 19 Rosetta at 67P/Churyumov- Gerasimenko in 2014 • Powerful remote sensing instruments (UV to IR) • Nucleus interior probing sounder • Philae lander: super high resolution imaging and thermal probing Full cycle of cometary activity will be followed during rendezvous Comet 67P/Churyumov-Gerasimenko observed by Rosetta/OSIRIS 2014 28. 5. 2019 PMO H. Uwe Keller - Physics of Nuclei 21 Effects of low gravity • Assumption: a block (density 500 kg m-3) tumbles down the Hathor wall (>900 m). It takes about 1 h to reach the valley floor of Hapi. • Its fresh surfaces are active and sublimation accelerates the spherical block with radius r: b/g ≈ x [Pa] 5/r [m] • Water sublimation can reach a pressure of x = 1 [Pa] • The ball becomes self propelled and can leave the nucleus • Gravity acceleration: 1.5 10-4m s-2 ≈ 10-5 of earth! • Tensile strength about 1 to 100 [Pa] ≈ 10-5 of rocks • Ratio gravity/tensile strength like on earth! 28. 5. 2019 PMO H. Uwe Keller - Physics of Nuclei 22 28. 5. 2019 PMO H. Uwe Keller - Physics of Nuclei Arizona? 23 67P 28. 5. 2019 PMO H. Uwe Keller - Physics of Nuclei 24 Baseline Sublimation Model of 67P We predicted that the southern hemisphere would be much more strongly eroded and as a consequence dust particles would be migrating from there and cover horizontal planes on the north. Hapi: area of minimal insolation Keller et al. (2015) Water ice erosion due to insolation integrated over the whole orbit Erosion potential on the south is about 4 times larger than on the north hemisphere! 28. 5. 2019 PMO H. Uwe Keller - Physics of Nuclei 25 28. 5. 2019 PMO H. Uwe Keller - Physics of Nuclei 26 Dichotomy Dichotomy of the back fall covered north and the consolidated very rough south with strong activity! Cause: limited insolation on the north. Only about 20% of water production during northern summer 28. 5. 2019 PMO H. Uwe Keller - Physics of Nuclei 27 Dichotomy at neck - slanted views Hapi Sobek 28. 5. 2019 PMO H. Uwe Keller - Physics of Nuclei 28 OSIRIS observes back-falling decimeter-sized chunks (Agarwal et al. 2016). Downward acceleration by rocket effect of active chunks Decimeter sized back fall particles contain water ice (H2O) but are too warm to carry CO2 or CO ice 28. 5. 2019 PMO H. Uwe Keller - Physics of Nuclei 29 28. 5. 2019 PMO H. Uwe Keller - Physics of Nuclei 30 WAC image Exposure time 2.4s Irregularly shaped boulder rotates Period ca. 10s Size ca. 1 m Apparent motion from image to image is caused by the spacecraft movement 28. 5. 2019 PMO H. Uwe Keller - Physics of Nuclei 31 NAC_2014-10-01T04:36 Pillar ≈ 30 m Cliffs are strongly insolated Cavity ≈ 15 m deep Compare observations of comets Borelly and Tempel 1 A geologist’s delight 28. 5. 2019 PMO H. Uwe Keller - Physics of Nuclei 32 Activity everywhere! 21.00.01.669 F18 18.59.18.754 19.59.18.781 20.59.18.781 (F22)Activity everywhere –(F22)also from the back fall covered(F22) areas => back fall is “wet”, i. e. contains water ice 28. 5. 2019 PMO H. Uwe2015_05_11 Keller - Physics of Nuclei 33 28. 5. 2019 PMO H. Uwe Keller - Physics of Nuclei 34 Outburst 29 July 2015 Jet base diameter < 40 m, area 1.25 10-3 km2 or about 10-4 of insolated surface 28. 5. 2019 PMO H. Uwe Keller - Physics of Nuclei 35 12 Aug. 2015 14:07 17:35 23:31 Distance to comet 330 km 16:50 no jet 17:05 no strong jet, maybe a beginning of the narrow feature 17:20 narrow jet + broader structure.