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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