Hawaii Reveals Steamy Martian Underground
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- Date: Wed, 17 Oct 2007 18:05:15 -0700
October 17, 2007
CONTACT:
Nancy Neal-Jones
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Bill Steigerwald
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Hawaii Reveals Steamy Martian Underground
NASA-funded researchers have traced the flow of molten rock (magma)
beneath
three large Martian volcanoes by comparing their surface features to
those
found on Hawaiian volcanoes.
"On Earth, the Hawaiian islands were built from volcanoes that erupted
as
the Earth's crust slid over a hot spot -- a plume of rising magma,"
said Dr.
Jacob Bleacher of Arizona State University and NASA's Goddard Space
Flight
Center in Greenbelt, Md. "Our research raises the possibility that
the
opposite happens on Mars -- a plume might move beneath stationary
crust."
The observations could also indicate that the three Martian volcanoes
might
not be extinct. Bleacher is lead author of a paper on these results
that
appeared in the Journal of Geophysical Research, Planets, September
19.
The three volcanoes are in the Tharsis region of Mars, the same area
as the
giant Olympus Mons volcano, the largest volcano in our solar system at
about
600 kilometers (373 miles) in diameter, or the size of the state of
Arizona.
They are smaller than Olympus, but with each about 300 kilometers
(186
miles) across, they are still huge compared to terrestrial volcanoes.
They
form a chain heading northeast called the Tharsis Montes, from Arsia
Mons
just south of the Martian equator, to Pavonis Mons at the equator, to
Ascraeus Mons slightly more then ten degrees north of the equator.
No volcanic activity has been observed at the Tharsis Montes, but the
scarcity of large impact craters in the region indicates that they
erupted
relatively recently in Martian history. Features in lava flows around
the
Tharsis Montes reveal that later eruptions from large cracks, or rift
zones,
on the sides of these volcanoes might have started at Arsia Mons and
moved
northeast up the chain, according to the new research.
The researchers first studied lava flow features that are related to
the
eruptive history of Hawaiian volcanoes. On Hawaii (the Big Island),
the
youngest volcanoes are on the southeastern end, directly over the hot
spot.
As the Pacific crustal plate slowly moves to the northwest, the
volcanoes
are carried away from the hotspot. Over time, the movement has created
a
chain of islands made from extinct volcanoes.
Volcanoes over the hot spot have the hottest lava. Its high
temperature
allows it to flow freely. A steady supply of magma from the hot spot
means
the eruptions last longer. Lengthy eruptions form lava tubes as the
surface
of the lava flow cools and crusts over, while lava continues to flow
beneath. After the eruption, the tube empties and the surface
collapses,
revealing the hidden tube.
As the volcano is carried away from the hot spot, magma has to travel
farther to reach it, and the magma cools. Cooler magma makes the lava
flow
more slowly compared to lava at the younger volcanoes, like the way
molasses
flows more slowly than water. The supply of magma is not as steady,
and the
eruptions are shorter. Brief eruptions of slowly flowing lava form
channels
instead of tubes. Flows with channels partially or completely cover
the
earlier flows with tubes.
As the volcano moves even further from the hot spot, only isolated
pockets
of rising magma remain. As the magma cools, it releases trapped gas.
This
creates short, explosive eruptions of cinders (gas bubbles out of the
lava,
forming sponge-like cinder stones). Earlier flows become covered with
piles
of cinders, called cinder cones, which form around these eruptions.
"We thought we could take what we learned about lava flow features on
Hawaiian volcanoes and apply it to Martian volcanoes to reveal their
history," said Bleacher. "The problem was that until recently, there
were no
photos with sufficient detail over large surface areas to reveal
these
features on Martian volcanoes. We finally have pictures with enough
detail
from the latest missions to Mars, including NASA's Mars Odyssey and
Mars
Global Surveyor, and the European Space Agency's Mars Express
missions."
Using images and data from these missions, the team discovered that
the main
flanks of the Tharsis Montes volcanoes were all alike, with lava
channels
covering the few visible lava tubes. However, each volcano experienced
a
later eruption that behaved differently. Lava issued from cracks
(rifts) on
the sides of the volcanoes, forming large lava aprons, called rift
aprons by
the team.
The new observations show that the rift apron on the northernmost
volcano,
Ascraeus Mons, has the most tubes, many of which are not buried by
lava
channels. Since tube flows are the first to form over a hot spot,
this
indicates that Ascraeus was likely active more recently. The flow on
the
southernmost volcano, Arsia Mons, has the least tubes, indicating that
its
rift aprons are older. Also, the team saw more channel flows
partially
burying tube flows at Arsia. These trends across the volcanic chain
indicate
that the rift aprons might have shared a common source like the
Hawaiian
volcanoes, and that apron eruptions started at Arsia, then moved
northward,
burying the earlier tube flows at Arsia with channel flows.
Since there is no evidence for widespread crustal plate movement on
Mars,
one explanation is that the plume could have moved beneath the
Tharsis
Montes volcanoes, according to the team. This is opposite to the
situation
at Hawaii, where volcanoes move over a plume that is either stationary
or
moving much more slowly. Another scenario that could explain the
features is
a stationary plume that spreads out as it nears the surface, like
smoke
hitting a ceiling. The plume could have remained under Arsia and
spread
northward toward Ascraeus. "Our evidence doesn't favor either
scenario, but
one way to explain the trends we see is for a plume to move under the
stationary Martian crust," said Bleacher.
The team also did not see any cinder cone features on any of the
Tharsis
Montes rift apron flows. Since cinder cone eruptions are the final
stage of
hot spot volcanoes, the rift apron eruptions might only be dormant,
not
extinct, according to the team. Volcanic eruptions release large
amounts of
greenhouse gasses, like carbon dioxide, into the atmosphere. If the
eruptions are not complete, and future eruptions are large enough,
they
could contribute significant amounts of water and carbon dioxide to
the
Martian atmosphere.
The team includes Bleacher, Drs. Ronald Greeley, David A. Williams,
and
Shelby R. Cave of Arizona State University, and Dr. Gerhard Neukum of
Freie
Universitaet, Berlin, Germany.
For images, refer to:
http://www.nasa.gov/centers/goddard/news/topstory/2007/mars_volcanoes.html
.
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