Volcanism Theme Overview

By Laszlo Keszthelyi

 

Overview:

Mars is fundamentally a volcanic planet.  Geologic mapping of Mars shows that about half the surface seems to be covered with volcanic materials that have been modified to some extent by other processes (such as meteorite impacts, blowing wind, and floods of water).  Mars has the largest volcanoes in the entire Solar System.  The great volumes of erupted lava have had a profound impact on the entire planet, extracting heat and selected chemicals from within, adding large amounts of acidic gas to the atmosphere, and providing heat to melt frozen water in the crust.  Mars cannot be understood without studying its volcanoes.

 

Major Science Questions:

There are two fundamental groups of questions related to Mars volcanism that we hope to answer with HiRISE.  The first is related to how the volcanic eruptions took place.  The second is how these eruptions affected the atmosphere and hydrosphere (or water-cycle) on Mars. 

 

In terms of volcanic eruptions, the key questions are those that any good reporter would ask: What?  When?  Where?  How?  Why?  We want to know what type of materials came out of the ground and what type of deposits they formed.  For example, most of the lava on Earth and Mars appears to be chemically similar to what we see erupting in Hawaii today: basalt.  This is not a very viscous (or sticky) type of lava and tends to form black rocks when frozen.  Such lava is indicative of melt that rose from the mantle with relatively few complications.  Finding lavas that are fundamentally different from basalt would be an exciting result!  Volcanoes can erupt in a wide variety of ways, including violent explosions and gentle oozes.  The explosive eruptions should produce fragmented bits of lava that are called "pyroclasts" (meaning "fiery pieces").  The gentler eruptions form lava flows.  We need understand how these different types of eruptions are distributed in time and across the surface of Mars to see the bigger picture of how Mars recycles its interior and loses its internal heat. 

 

While volcanoes are interesting in their own right, the focus of Mars exploration today is "follow the water" to search for evidence of life on Mars.  Therefore, places where lava or magma has interacted with water are of great interest to us.  Such interaction can be quite gentle, producing hot springs. Or it can be explosive, creating craters of various sizes.  By determining when and where lava has met water, we can produce a map of where and when water was near the surface of Mars.  Volcanic eruptions can also add gases to the atmosphere.  Since these gases are typically very acidic, we would expect the pyroclasts to be chemically affected by acid.  By estimating the amount of gas erupted, and determining when these eruptions took place, we can start to predict the some kinds of climate changes that should have taken place on Mars. 

 

Features of interest to HiRISE

While HiRISE is in many ways the best camera to every fly to Mars, it isn't the right tool to answer every question.  It is best suited for taking detailed looks at town-sized pieces of Mars.  For understanding volcanism, some of the most important areas are the vents from which the lava erupted.  We hope to see details that allow us to infer how fast the lava came up – for example, pyroclasts would suggest violent eruptions while lava ponds would suggest gentle eruptions.  We also hope to see into lava flows where cliffs have cut through them.  Features such as "rootless cones" (craters formed when steam explodes through a lava flow) are of special interest because they are indicative of hot lava and water interacting. 

 

One word of caution if you plan to suggest targets on the giant volcanoes: dust!  Large portions of these beautiful volcanoes are covered by huge blankets of dust that hide the lava flows and other underlying features. 

 

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