Tuesday, 19 May 2015

Rosetta Images of Comet 67P/Churyumov–Gerasimenko: Icy structures and terrain

Presentation to Royal Astro Soc. National Astronomy Meeting session on Comets/Rosetta Mission, 6thJuly 2015.  
Publicised by the RAS at 
https://www.ras.org.uk/news-and-press/2654-do-micro-organisms-explain-features-on-comets

Icy structures and terrain in comet 67P

Max Wallis and N. Chandra Wickramasinghe

Abstract

Despite the comet’s very black crust, Rosetta’s remarkable images show several indicators of an underlying icy morphology. Comet  67P displays smooth, planar ‘seas’ (the largest 600m X 800m) and flat-bottomed craters, both features seen also on comet Tempel-1.  Comet 67P’s surface is peppered with mega-boulders (10-70km) like comet Hartley-2, while parallel furrowed terrain appears as a new ice feature.  The largest sea (‘Cheops’ Sea, 600 X 800m) curves around one lobe of the 4km diameter comet, and the crater lakes extending  to ~150m across are re-frozen bodies of water overlain with organic-rich debris (sublimation lag) of order 10 cm.  The parallel furrows relate to flexing of the asymmetric and spinning two-lobe body, which generates fractures in an underlying body of ice. The mega-boulders are hypothesised to arise from bolide impacts into ice.  In the very low gravity, boulders ejected at a fraction of 1m/s would readily reach ~100m from the impact crater and could land perched on elevated surfaces.  Where they stand proud, they indicate stronger refrozen terrain or show that the surface they land on (and crush) sublimates more quickly.  Outgassing was already evident in September at 3.3AU, with surface temperature peaks of 220-230K, which implies loosely bound H2O and/or unconsolidated organic mixtures.   Increasing rates of gassing as Rosetta follows comet 67P around its 1.3 AU perihelion will hopefully reveal the activation of possible micro-organisms as well as the nature and prevalence of near-surface ices.

Wallis MK, Wickramasinghe NC (2015) Rosetta Images of Comet 67P/Churyumov–Gerasimenko: Inferences from Its Terrain and Structure. Astrobiol Outreach 3: 127. doi:10.4172/2332-2519.1000127  Open Access
Parallel paper:
Wickramasinghe NC, Wainwright M, Smith WE, Tokoro G, Al Mufti S, et al. (2015) Rosetta Studies of Comet 67P/Churyumov–Gerasimenko: Prospects for Establishing Cometary Biology. Astrobiol Outreach 3: 126. doi:10.4172/2332-2519.1000126 Open Access

David S. McKay 1936-2013 memorial tribute

Pioneer on martian meteorite biomorphs => Life-on-Mars 
To a large number in the scientific community, David McKay is best known as being the point person in the Allan Hills 84001 (ALH84001) Life on Mars hypothesis.  David was the lead author of the 1996 Science manuscript which jarred the scientific community into reexamining our concepts about life on Mars.  The paper was a team effort lead by David, Everett Gibson and Kathie Thomas-Keprta.  The three team leaders and all of the other members of the team (Christopher Romanek, Hojatollah Vali, Simon J. Clemett, Xavier D.F. Chillier, Claud R. Maechling and Richard Zare) contributed to the hypothesis.

The chain of evidence presented in the McKay et al. (1996) manuscript was believed to be compatible with the existence of past life on Mars: (i) an igneous Mars rock from  (age around 4 to 4.5 Ga) which had been penetrated by a low-temperature fluid along fractures and pore spaces, which then became the sites for secondary mineral formations and possible biogenic activity; (ii) a formation age for the carbonate globules (~3.9 Ga) younger than the age of the igneous rock; (iii) SEM and TEM images of carbonate globules and features resembling terrestrial microorganisms, terrestrial biogenic carbonate structures, or microfossils; (iv) magnetite and iron sulfide particles that could have resulted from oxidation and reduction reactions known to be important in terrestrial microbial systems; and (v) the presence of organic molecules associated with the carbonate globules. None of these observations is in itself conclusive for the existence of past life.  Although there are alternative explanations for each of these phenomena taken individually, when they are considered collectively, particularly in view of their spatial association, it provided evidence for primitive life on early Mars (McKay et al., 1996).  This paper has become one of the most heavily cited papers in planetary science.  With continued research, the team remains convinced their evidence is solid and the best interpretation to explain these data is that biogenic processes operated on early Mars.


 McKay et al. (1996) Search for past life on Mars: Possible relic biogenic activity in Martian meteorite ALH84001, Science 273, 924-930.
 McKay D.S., Thomas-Keprta K.L., Clemett S.J., Gibson E.K. Jr., Spencer L. and Wentworth S.J. (2009),  Life on Mars: New evidence from Martian meteorites.  Proc. of SPI,, Instruments and Methods for Astrobiology and Planetary Missions XII, Vol. 7441, 744102-1 to 733102-20 edited by R.B. Hoover, G.V. Levin, A.Y. Rozanov and K.D. Retherford.  , San Diego, CA.

from Memorial tribute prepared by:
Everett K. Gibson, Kathie Thomas-Keprta, Simon Clemett and Penny Morris-Smith
Astromaterials Research Office, NASA Johnson Space Center

Published:  June 2013 issue of Astrobiology