Contents — Mi through Mu Characterizing Polar Layered Deposits at the Martian North Pole: An Assessment of Local Variations

A great diversity in the eruptive style of ArsiaMons volcano can be identified from THEMIS, MOCand MOLA data. Collectively, these data sets providethe first opportunity to study lava shields within thesummit caldera, kilometer-scale pit craters on theupper flanks, and the entire length of lava flows fromvent to distal flow lobes. As such, a more completepicture of the history of this volcano is emergingwhich has both similarities to the other Tharsis Ridgevolcanoes (Pavonis and Ascraeus Montes) anddifferences with Olympus Mons. IntroductionRecent observations [1] of extensive ashdeposits near the summit of Arsia Mons volcano(9.5°S, 238.8°E) have indicated that this volcano hashad a more complex history, including large explosiveeruptions, than had previously believed from analysisof Viking Orbiter images [2, 3]. Data from the MarsOrbiter Camera (MOC), the Thermal EmissionImaging System (THEMIS), and the Mars OrbiterLaser Altimeter (MOLA) have now provided extensivecoverage of the volcano, and have enabled a greaterdiversity of volcanic features to be studied along theNE – SW structural trend through the volcano that isinterpreted to be a rift zone [3]. The transition fromthe summit caldera (elevation ~16.3 km above datum),to the upper flanks (~12 – 16 km) and lower flanks (~7– 12 km) is striking. Along this potential rift zone, it ispossible to identify numerous collapse pits that arebelieved to be the vents for the ash deposits [1], over adozen small shields on the caldera floor [2, 3], and acomplete sinuous rille/lava flow complex. The newinsights into these features are now described. Lower Flank Lava FlowsDaytime THEMIS IR images show almost theentire length of a lava flow, which originates from avent at an elevation of ~11.3 km on the southern lavafan (Fig. 1, 2). Historically, it has been difficult toidentify the vent areas for lava flows on Mars [4]. Wehave speculated that sinuous rilles, such as the onesnear the summit of Elyisum Mons [5], could be thesource areas for lava flows which had high effusionrates. However, until now it has not been possible totrace the downslope flow(s) associated with a rillebecause the rille either disappears into compound flowunits or the image does not cover a sufficiently largearea. But on the SW flank of Arsia Mons, both thevent and the associated distal flow fronts can beidentified in the same image. The sinuous rille formed~34 km from the base of the main shield and is ~65 kmlong. No enlarged source crater or topography can beidentified around the vent. The flow extends a further135 km downslope. Close to the distal end, the flowsplits into four segments, which have widths of ~2.3 –3.2 km and pronounced central lava channels. Figure 1: Shaded relief image derived from MOLA data forthe southern flank of Arsia Mons volcano. The summit isjust off the top of the image. Contours are at 500 meterintervals, and the marked elevations are in kilometers aboveMars datum. Notice that the upper flanks have a shallowerslope than the lower flanks. The area of coverage of Figure2 is also shown. North is towards the top. Unfortunately, MOLA data are insufficient tomeasure the thickness of the flow otherwise this flowwould be an excellent candidate for modeling therheology of the lava as well as study the effects ofslope on flow and channel width and depth.Sixth International Conference on Mars (2003)3001.pdf