Mercury’s Global System of Thrust FaultsSarah MattsonPTYS 395March 5, 2008Lobate ScarpsBackground is first image of Mercury sent back from MESSENGERImage Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of WashingtonOverviewWhat are lobate scarps?Where are they found on Mercury?When did they form?Why are they there?What has/will MESSENGER tell us?2What are Lobate Scarps?3Lobate = having or resembling lobesScarp = cliffGlobal tectonic featureNamed “Rupes” from Latin for “cliff”All named after shipsLarge, curved cliffs- Really large!Interpreted as thrust faultsDiscovery Rupes, Mariner 10 imageStrikeDipHanging WallFoot WallCharacteristics of Lobate Scarps4 No preferred orientation Sinuous or arcuate 100’s of meters in height 100’s of kilometers long Asymmetric cross-section Steep scarp face Gently sloping backairandspace.si.edu/etp/mercury/merc_surface.html Discovery RupesFrom Watters et al., 2001Where are they located?5Find global map• Global map of distribution and orientation of scarps based on Mariner 10 imagery• Most total length south of -50olatitude• All scarps south of -50olatitude dip in a northerly direction• No preferred dip direction north of -50olatitudeBlack lines are thrust faults, with dip direction indicated by triangleGreen lines are high-relief ridges(Head et al., 2007)When did they form?6• Probably formed over a few 100 Myr• Cross-cutting relationships• Craters modified by scarps or scarps modified by craters• Infer relative timing of eventsNASA/JHUAPL/Carnegie Institution of Washington• After differentiation, after LHB• Scarps are dated as Tolstoj - Calorian periods~10 km displacement of Guido d’Arezzo crater by Vostok scarp (Strom & Sprague)Mercury’s Timeline7• Mercury forms• Lithosphere forms• Despinning results in shape change and global tectonism• Heavy bombardment• Core shrinks 1-2 km– Global system of thrust faults forms lobate scarps• Caloris impact structure forms• Lighter cratering continues• Bright rayed craters• Polar volatiles accumulateKuiperianPre-TolstojanTolstojanCalorianMansurianHow did the scarps form?8• 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 observedPrevailing Theory9Cooling of large coreGlobal contractionCompressional StressesThrust FaultingPlanetary radius shrinks 1-2 KmImpact Basins10Inferred stress directions to form Adventure, Resolution and Discovery RupesWestern hemisphere, 48S/58W(Watters et al., 2001)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/54WChange in Surface Area11Assumptions (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 measuredAssumptions about displacement lead to estimates of loss of surface area of 31,000 – 63,000 km2(Strom & Sprague)Radius of Mercury = 2439.7 kilometersSurface area today = 4* *R2 = 74,796,748 km2Surface area before contraction = 74,858,076 to 74,919,431 km2Difference is 122,683 to 61,328 km2Need better data to reduce uncertainty.MESSENGER Data Return12• MESSENGER has already imaged a portion of the surface missed by Mariner 10• Discovered more lobate scarps• Mercury Laser Altimeter (MLA)• Topographic mapping of entire planet with • Look for long wavelength folding• Accommodate contraction estimated by models• Mercury Dual Imaging System (MDIS)• New faults not seen before• Detection of smaller scale faults• Better global imagery, mapping of faultsPIA10177 NASA PhotojournalImage approx. 500 km acrossNASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of WashingtonSummary• Global set of thrust faults spanning hundreds of kilometers• Up to a 1.5 km highWhat are lobate scarps?• GloballyWhere are they found on Mercury?• Tolstojan through Calorian• After LHBWhen did they form?• Cooling core shrank• Crust accommodates shrinking core by compressing, creates thrust faultsWhy are they there?• Has already returned many images of new areas, showing more scarps!• Future flybys … and orbit to provide complete picture of scarp