Download The Role of Comets in the Late Heavy Bombardment

The Role of Comets in the Late Heavy Bombardment Hans Rickman* Uppsala Observatory Space Research Center, Warsaw * and G.B. Valsecchi, A. Morbidelli, T. Wisniowski, R. Gabryszewski, P. Wajer, K. Wojcikowski, S. Szutowicz The Late Heavy Bombardment The event is now pushed further back in >me (Morbidelli et al. 2012) Current es?mate ê •  Source: Lunar crater counts, and radiometric da?ng of lunar surface strata connected with mare basin forma?on The Nice Model Disk mass es?mated at 20-­‐50 ME Planet instability and migra?on (Gomes et al. 2005) Basic quesIons •  Consider the trans-­‐planetary disk as a source of impacts and cratering during the LHB: − How important was this source for the lunar cratering? − How did the amount of comet impacts vary between the terrestrial planets? − Did these impacts play a great role in the evolu>on of the Mar>an surface? Lunar bombardment issues •  Comets should dominate (Morbidelli et al. 2010) •  But HSE abundances in lunar impact melts resemble high-­‐temperature meteorites (Kring & Cohen 2002) •  Lunar craters share asteroid size distribuIon, different from giant planet satellites (Strom et al. 2005) But note Pluto/Charon impact crater record! •  E-­‐belt model for the LHB (BoXke et al. 2012) Where did all the comets go…? Monte Carlo simulaIons •  Create a large sample of Jupiter Family comet orbital evolu?ons (model for LHB projecIles) •  Calculate the planetary and lunar impact probabili?es for each orbit •  Derive cumula?ve numbers of impacts on the planets and the Moon •  InvesIgate the effects of physical life?me and size distribu?on of comet nuclei Aiming and Iming •  The asteroid orbit has a minimum distance from the target orbit, called MOID •  For “perfect” >ming of an encounter, the asteroid aims at the (MOID,0) point in the b-­‐plane (perpendicular Bg = collisional radius to the relaIve velocity) •  Otherwise, it aims at points displaced in the ζ direc>on due to the target’s mo>on This is like shoo?ng on a moving target! Planetary impact probability •  We idenIfy orbits, where MOID < Rcoll (collisional radius) for a planetary target; these are called collisional orbits •  For each collisional orbit, we find the range of encounter ?ming (ΔTo) that leads to impact and take the raIo with the orbital period of the target (Pt) (Wisniowski & Rickman 2013; Acta Astron. 63, 293) (Rickman et al. 2014: A&A 569, A47) Lunar impact probability •  We treat the lunar orbit during the LHB as circular in the eclipIc plane with one half its present radius (i.e., aM = 182,000 km) •  We look for all projecIle orbits with Earth MOID < aM + RM = 184,000 km, including all possible collisions with the Moon •  For each such case, we evaluate the lunar impact probability for the actual orbit, using a bivariate >ming range (TE ,TM) Dynamical input •  IniIal condiIons using elements of 6014 disk objects reaching perihelia with q < 3.9 AU in the Nice Model (Brož et al. 2013) •  Down-­‐selec>on (P < 20 yr and TJ > 2) ⇒ 5000 sets of elements, and (Ω,ω,M) cloning by P. Wajer ⇒ sample of 105 star?ng orbits for integra?ons •  RA15 integra>ons of all orbits by R. Gabryszewski over a maximum of 105 revolu?ons, using four giant planets with post-­‐LHB (current) orbits (neglecIng TP influences); elements are stored at each perihelion passage Collisional orbits From 100,000 iniIal condiIons, each integrated for a maximum of 100,000 orbits, we obtain the following numbers of potenIally collisional orbits (MOID < Rcoll): Mars is favored and Mercury disfavored by the typical paVern of JF orbital evolu?on Impact probability for collisional orbits Trend: impact probability goes as Rcoll/ap ScaVer: different MOIDs and encounter geometries Finite lifeImes •  EROSION •  SPLITTING •  Mass loss by comet •  Apparently random acIvity (shedding of events, observed quite material, mostly near frequently in SP comets the Sun) •  The physical mechanism is not understood •  Eventually, the comet shrinks away (perhaps, •  Wide range of scales, 46P/Wirtanen) from fragmentaIon of •  We model this mass loss the surface layer to bulk by H2O surface separaIon into a few sublima>on large pieces (3D/Biela) Our physical models •  Infinite life?mes (reference model): No physical evoluIon occurs; the nuclear radius is constant •  Erosional model: We tune the acIvity level to agree with observed gas producIon rates and comet dormancy •  Di Sisto model: We use Model 4 of (Di Sisto et al 2009); best fit including very high split rate Dynamical heaIng in the JF Obs. Init. obs SS SS inf. SS ero. ini The Jupiter Family is dynamically quite “young” − cf. Levison & Duncan (1997) The lifeIme issue •  Our disk is more puffed up than the one of Levison & Duncan •  This would require the JF to be extremely young − probably, unreasonably young •  A more realisIc model of physical evoluIon may be called for, including dormancy and rejuvenaIon (e.g., Rickman et al. 1991) •  TentaIve consequences: longer lifeImes; low capture rate from the SD; low-­‐acIve JF comets hiding with large perihelion distance CumulaIve LHB collision rates Expected number of collisions per million ini?al comets These values were obtained by summing impact probabili>es over the life>mes of the comets – either dynamical or limited by physical effects (depending on size distribu>on) The primordial disk Popula?on: total mass ~ 15-­‐50 ME; uncertain CSD power-­‐law index for comets Kinema?cs: significantly hoper than in the early Nice Model papers •  We take a lower total mass of 18 ME and an upper one of 36 ME; in both cases 2 ME are added as Pluto-­‐sized objects (too few to maper) •  We take a shallow CSD with α = -­‐1.5 and a steep one with α = -­‐2.5 (bracke>ng the range of observed slopes for the Jupiter Family comets) α = -­‐4 (Rickman et al. 2015) Our influx models •  Maximum model: − High-­‐mass disk with shallow CSD slope for small objects (max. number of large objects) − Prob. = 1/3 to reach the inner Solar System •  Minimum model: − Low-­‐mass disk with steep CSD slope for small objects (min. number of large objects) − Prob. = 1/10 to reach the inner Solar System The largest lunar impactor Ÿ = max ✖ = min ∞ Subl. çTrojans Subl.+ Split. The minimum model works best! The largest impact basins Trus?ng the minimum model, the largest basins cannot be formed by comets! The total comet mass Crude es?mate LHB volaIle veneer •  Consider the case of infinite life>me and the low-­‐mass primordial disk! •  The Earth acquired a total of M ≈ 3x1019 kg, much less than the ocean mass •  Mars acquired a total of M ≈ 7x1018 kg •  This is about 10 m Global Equivalent Layer of water (20%), much less than usually esImated •  For 10% CO2/H2O by mass, we get about 30 mb CO2 surface pressure (insufficient to support liquid surface water) ImplicaIons •  The LHB was asteroid dominated in support of the proposed E-­‐belt as the main source •  The Earth and Mars acquired their vola?les early, mainly as part of their formaIon process •  There is no evidence against a late instability, since comets did not cause too much cratering •  Is there posi>ve evidence that comets did contribute to the LHB in the way predicted by a late planet instability? (Marty et al. 2016: Earth’s Ar budget and 67P’s Ar abundance) Collisional evoluIon of 67P? •  A low-­‐mass disk seems OK, but it does not save comets from collisional destrucIon (Rickman et al. 2015) •  And 67P does appear to be primordial! (highly porous, full of volaIles) … − Could the disk be slim enough to save the comets? − Can comets stay primordial in spite of collisions? Comet 67P (OSIRIS) An early instability might help…