Age of the erosional valley systems: The sheer observation that lava flows are not observed within the erosional valley systems calls for an explanation because from a volcano-tectonic point of view this is presumably the locus of major volcanism. Let us first look at an alternative explanation for the formation and development of the erosional valleys, namely that they could have formed primarily through extensional faulting, i.e. are rift grabens. No doubt components of rifting and down-faulting of the valley center are present, although fresh faults are extremely rare. The valleys are smooth, of irregular morphology with regard to depth, wall relief and thus generally unlike grabens, that elsewhere on Mars are an extremely common feature. Within well developed valley segments, such as on the northeast side of Arsia Mons and southwest side of Ascraeus Mons, the valley systems have developed to the extent that "island" mountains rise within the valley system center. These mountains and associated valley network have irregularly bending walls that bear no resemblance to rift valleys. It is therefore concluded that the primary factor in their morphologic formation is extensive erosion.

Ground ice vs. glacier ice: In the discussion of ice within the Martian megaregolith the term ground ice is frequently used. By definition jokulhlaup refers to the melting ice when intruded by magma. As the jokulhlaups are best known from Iceland an integral part of the definition is that the ice involved is glacier ice, notably water ice. In the present study the term glacier ice has been used in discussing the potential ice beneath a subsurface layer at the Tharsis Montes. By definition ground ice refers to "bodies of more or less clear ice in permanently frozen ground. Deposits that are only temporary features are excluded under this definition, and the term is not applied to deposits that seem to be on top of the ground. Stagnant earth-covered glaciers appear to fall about in the dividing line on this definition. If their glacial origin is evident, they would be excluded."

Glacier ice by definition is: "a body of ice developed from snow which becomes large enough to move from its place of accumulation." The question thus arises: is there any flow of ice at the Tharsis Montes? Flow of glacier ice is not known to have occurred there although features resembling glacial moraines have been described for Arsia Mons (Williams, 1978). As the present model for jokulhlaups at the Tharsis Montes assumes substantial volumes of ice melting the occurrence of this ice under subsurface conditions needs to be discussed. Firstly, it is assumed that the ice volume involved by far exceeds the volume of lava pore voids or scoriaceous lava boundaries. Such void could locally be up to 30-40%. The volume of partly collapsed elongated structures emanating down-dip from collapse rings suggests that the ice volume is confined to massive ice deposits rather than pore voids. It is rather odd that the massive ice has not clearly moved down-dip. Ice that is intercalated on irregular flow boundaries with a high rock/ice ratio could be relatively stable. Furthermore, ice within the large bowl-shaped calderas may have been the principal source of melted ice. Although the term melt-water has been used throughout the text it is clear that the exact composition of the ice involved may only have a relatively small component of water ice.

Active erosional "pathway" system vs. local valley system: For each of the three Tharsis volcanoes there are two erosional valley systems that differ widely. One is far more advanced than the other with regard to degree of erosion. For Arsia Mons the advanced system is on the northeast side whereas the less developed is on the southwest side. Here the northeast system coincides with a gate in the caldera wall. For Pavonis Mons the advanced segment is on the northeast side while the less developed segment is on the southwest side. For Ascraeus Mons the advanced erosional valley system is on the southwest side whereas the northeast system is poorly developed.

It is speculated that, at least for Arsia Mons, the more advanced system may be connected with the caldera and act as a pathway for melt-water generated through volcanism within the caldera walls. The less developed system may be isolated and formed in response to local magma/ice interaction. Alternatively, the degree of erosion within the valley systems may simply express the intensity of volcanism or availability of subsurface ice.

Are the Tharsis Montes being neglected? A new era in Martian research has begun. The attention among scientist seems to be on the polar regions. The Viking era passed without any clear ideas as to what happened to the presumably lost water. Yet all the evidence were available for an entirely different interpretation, i.e. one that assumes an almost incredibly thick ice layer being present beneath a relatively thin surface layer. Based among other things on theoretical calculations of ice stability (e.g. Squyres et al., 1992) and the interpretation of impact crater diameter vs. latitude, it was concluded that water ice could not at present be stable except at higher latitudes than about �40�. In this context there exist strong arguments for looking much closer at the Tharsis Montes. The young age of these volcanoes (less than 10 million years) predicts that they are for practical purposes active. Within this volcanic regime magma en route to the surface will likely have left a surface impression through interaction/melting of a subsurface ice layer. The impression may vary from collapse craters, elongated depressions, erosional valleys, substantial sediment deposition, formation of impact crater remnants, formation of near free impact crater terrains.

The importance of these volcanoes lies in their position that is high above the surrounding plateau. This results in the massive escape of melt-water through magma/wall rock (ice) interaction. For other volcanoes, e.g. Elysium Mons, graben structures have been observed and partly tributed to melting of ground ice through magmatic intrusion (e.g. Mouginis-Mark, 1985). Here, however, the low aspect ratio appears to cause the release of melt-water to be less efficient and erosion insignificant compared with the erosional valleys of the Tharsis Montes.

Why study the Tharsis Montes more closely? The apparent young age of the erosional valleys in the Tharsis Montes, i.e. much less than 10 million years, and their apparent mode of formation through volcano/ice interaction suggests that substantial ice is still present there. This region should therefore have priority over other region, such as Elysium which age is regarded 1 billion years. It is now pertinent to investigate further evidence for recent volcano/ice interactions within the Tharsis Montes that have stared us in the eye ever since Viking images of these volcanoes became available.

What to look for in MOC images? That the pattern of young impact craters at Pavonis Mons is seen to be heavily eroded raises the question what about other similar erosional valleys or depressions? High resolution MOC images could be used to study graben features associated with the various Martian volcanoes to specifically determine if recent erosion of impact craters can be detected there as well. The lava fields of many of these volcanoes have been regarded as 1 billion years old. It may just be that some of the erosional valleys of these volcanoes are considerably younger. Age dating through crater counts for such volcano sub-areas may therefore prove to be a worthwhile effort and provide clues or evidence for how recently these volcanoes possessed excessive ground ice.

Things we need to know about the Tharsis volcanoes: If, in accordance with the present model, there is indeed present volcanic activity in Pavonis Mons it follows that large quantities of ice reside at present beneath the surface layer, either on the volcano flanks or within the caldera of the Tharsis Montes. There is need to thoroughly investigate the morphology of these volcanoes in order to test whether morphological features on the surface can have formed in response to release of massive melt-water. We truly need to know more about the nature of the erosional valley floor. What type of sediment is covering the valley floor? Do they have an unusually high component of hyaloclastite, gravel, boulders, rock blocks or wind blown material? Can the aureole deposits, in particular the finely striated deposits on the lower flank of Arsia Mons, have formed through a gigantic jokulhlaup? How were the striated aureole deposits at Pavonis Mons formed? What would be the composition of the ice involved with the jokulhlaup model?

Summary and conclusions: On the northeast side of Pavonis Mons a deep and broad valley is carved into the volcano flank. Numerous small impact crater remnants on the valley floor have clearly been subjected to severe erosion. The erosive force has attacked the craters in a down-dip direction causing the crater remnants to have a broad upper terminus and a long narrow tail at the lower terminus. The erosive force was of fluid origin. The volcano-tectonic location of the erosional valley is where a volcano is most active volcanically and lavas should be most abundant. The absence of lavas in the erosional valley calls for an explanation. It is concluded that the close relationship between fluvial erosion and volcano-tectonic position of the valley lies in catastrophic floods referred to as jokulhlaups. Such floods form in response to melting of ice through the interaction of magma and ice deposits. The deposits of ice may either or both be located in the volcano flanks or within the caldera walls.

By way of extrapolation results of age dating of Arsia Mons, where lava fields as recent as 40-100 million year old were detected (Hartmann et al., 1999), it is concluded that the northeast erosional valley at Pavonis Mons must be well within 10 million years considering remnant denudation. The age of this volcanically induced event is so young that volcanism must therefore be considered as presently taking place within Pavonis Mons.

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MARSDAILY

Watery Tales From Texas
Martian Science Review 2001
Cameron Park - May 4, 2001
In this eight-part special report Bruce Moomaw takes SpaceDaily readers on a scientific adventure as he reviews the latest in Martian Science as presented at the 2001 Lunar and Planetary Science Conference.

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