Lobate Scarps Mercury s Global System of Thrust Faults Sarah Mattson PTYS 395 March 5 2008 Background is first image of Mercury sent back from MESSENGER Image Credit NASA Johns Hopkins University Applied Physics Laboratory Carnegie Institution of Washington Overview What are lobate scarps Where are they found on Mercury When did they form Why are they there What has will MESSENGER tell us 2 What are Lobate Scarps Lobate having or resembling lobes Scarp cliff Global tectonic feature Named Rupes from Latin for cliff All named after ships Large curved cliffs Really large Interpreted as thrust faults Strike Hanging Wall Foot Wall Dip Discovery Rupes Mariner 10 image3 Characteristics of Lobate Scarps No preferred orientation Discovery Rupes Sinuous or arcuate 100 s of meters in height 100 s of kilometers long Asymmetric cross section Steep scarp face Gently sloping back airandspace si edu etp mercury merc surface html From Watters et al 2001 4 Where are they located Global map of distribution and orientation of scarps based on Mariner 10 imagery Most total length south of 50o latitude Find global map All scarps south of 50o latitude dip in a northerly direction No preferred dip direction north of 50o latitude Black lines are thrust faults with dip direction indicated by triangle Green lines are high relief ridges Head et al 2007 5 When did they form After differentiation after LHB Scarps are dated as Tolstoj Calorian periods 10 km displacement of Guido d Arezzo crater by Vostok scarp Strom Sprague Probably formed over a few 100 Myr Cross cutting relationships Craters modified by scarps or scarps modified by craters Infer relative timing of events 6 NASA JHUAPL Carnegie Institution of Washington Mercury s Timeline Mercury forms Lithosphere forms Despinning results in shape change and global tectonism Heavy bombardment Pre Tolstojan Tolstojan Core shrinks 1 2 km Global system of thrust faults forms lobate scarps Calorian Caloris impact structure forms Lighter cratering continues Bright rayed craters Polar volatiles accumulate Mansurian Kuiperian 7 How did the scarps form Have to understand Geometry Movements displacements Stresses Compressional stresses in brittle regime upper region of lithosphere Thermal contraction modeling suggests accommodation of shrinking core Reduction in planetary radius by 1 2 km Strom et al 1975 Strain measured indicates radial decrease 1km Watters et al 1998 Some faults may have been obliterated by subsequent impacts Tidal despinning reduction in equatorial bulge Models predict normal faulting in polar areas which has not been observed 8 Prevailing Theory Cooling of large core Global contraction Compressional Stresses Thrust Faulting Planetary radius shrinks 1 2 Km 9 Impact Basins Spudis and Guest 1988 suggest mechanical weakness resulted from large impact basin later with tendency to form faults there as crust shrank Watters et al 2001 43S 54W Inferred stress directions to form Adventure Resolution and Discovery Rupes Western hemisphere 48S 58W Watters et al 2001 10 Change in Surface Area Assumptions Uncertainties Low angle thrust faults 25o more likely than high angle 45o Average vertical displacement 500 m 1 km Rest of planet similar to the 25 where faults measured Assumptions about displacement lead to estimates of loss of surface area of 31 000 63 000 km2 Strom Sprague Radius of Mercury 2439 7 kilometers Surface area today 4 R2 74 796 748 km2 Surface area before contraction 74 858 076 to 74 919 431 km2 Difference is 122 683 to 61 328 km2 Need better data to reduce uncertainty 11 MESSENGER Data Return MESSENGER has already imaged a portion of the surface missed by Mariner 10 Discovered more lobate scarps Mercury Dual Imaging System MDIS New faults not seen before Detection of smaller scale faults Better global imagery mapping of faults Mercury Laser Altimeter MLA Topographic mapping of entire planet with Look for long wavelength folding Accommodate contraction estimated by models PIA10177 NASA Photojournal Image approx 500 km across NASA Johns Hopkins University Applied Physics Laboratory Carnegie Institution of Washington 12 Summary What are lobate scarps Global set of thrust faults spanning hundreds of kilometers Up to a 1 5 km high Where are they found on Mercury Globally When did they form Tolstojan through Calorian After LHB Why are they there Cooling core shrank Crust accommodates shrinking core by compressing creates thrust faults MESSENGER Has already returned many images of new areas showing more scarps Future flybys and orbit to provide complete picture of scarp systems 13