Did
flowing water carve the well-known channels on the face of Mars? Or was molten
lava perhaps the instigator? This debate has raged for years, and the answer is
important, because if there was a lot of surface water, that increases the
chances that life may once have existed.
Comparison
of images of Martian channels to lava flows in Hawaii indicates that lava, not
water, may have been the creative force behind at least some of the channels.
So said NASA researcher Jacob Bleacher, speaking before the 41st Lunar and
Planetary Science Conference last week.
"To
understand if life — as we know it — ever existed on Mars, we need to
understand where water is or was," Bleacher said.
The
notion that water once flowed freely on Mars derives from images showing
details resembling fluvial
(water-based) erosion. Fine, delicate terrain features usually aren't
considered products that lava flows can create.
"The
common image [of lava] is of the big, open channels in Hawaii," Bleacher
explains.
More
detailed view
A
single channel on the southwest flank of Mars' Ascraeus Mons volcano, one of
three volcanoes collectively known as the Tharsis Montes, formed the basis of
Bleacher and his colleagues' research. The team pieced together images covering
more than 168 miles (270 kilometers) of this channel utilizing high-resolution
pictures from three cameras: the Thermal Emission Imaging System (THEMIS) on
board Mars Odyssey spacecraft, the Context Imager (CTX) on Mars Reconnaissance
Orbiter and the High/Super Resolution Stereo Color (HRSC) imager on Mars
Express, as well as older data from the Mars Orbiter Laser Altimeter (MOLA)
on Mars Global Surveyor (MGS).
These
data gave a more detailed view of the surface than previously available.
Time
has obliterated the fluid that created the Ascraeus Mons channels, but visual
clues at the source of the channel in question seem to indicate that water is
the culprit. Clues include small islands, secondary channels that branch off
and rejoin the main one and eroded bars on the insides of the curves of the
channels.
But new
close examination of the channel's other end by Bleacher and colleagues
revealed a ridge that appears to have lava flows coming out of it. In some
areas, "the channel is actually roofed over, as if it were a lava tube,
and lined up along this, we see several rootless vents," or openings where
lava is forced out of the tube and creates small structures, he explained.
Water-carved
channels don't typically form these types of features, he notes. Bleacher
argues that one end of the channel forming by water and the other end by lava
is an "exotic" combination. More likely, he thinks, lava formed the
entire channel.
To
compare the Mars features to those created by lava, Bleacher, along with W.
Brent Garry and Jim Zimbelman at the Smithsonian Institution in Washington,
examined the 32 mile (51-kilometer) lava flow from the 1859 eruption of Mauna
Loa on the Big Island of Hawaii. They focused on a mid-channel island almost a
kilometer long. Bleacher says this is much larger than islands typically
identified within lava flows.
To
survey the island, the team used differential GPS, which provides location
information to within about 1.1 to 1.9 inches (3 to 5 centimeters), more accurate
than a car's GPS can offer.
"We
found terraced walls on the insides of these channels, channels that go out and
just disappear, channels that cut back into the main one, and vertical walls 9
meters (~29 feet) high," Bleacher said. "So, right here, in something
that we know was formed only by flowing lava, we found most of the features
that were considered to be diagnostic of water-carved channels on Mars."
The new
results make "a strong case that fluid lava can produce channels that look
very much like water-generated features," Zimbelman said. "So, we
should not jump to a water-related conclusion when we see such channels on
other planets, particularly in volcanic terrain such as that around the Tharsis
Montes volcanoes."
Lunar
evidence, too
Further,
researchers discovered more evidence from the moon by studying a detailed image
of channels in the Mare Imbrium, a large crater filled with ancient lava rock.
In this image, as well, they found channels with terraced walls and branching
secondary channels.
The
conclusion that lava probably made the channel on Mars "not only has
implications for the geological evolution of the Ascraeus Mons but also the
whole Tharsis Bulge [volcanic region]," said Andy de Wet, a co-author of
the study at Franklin & Marshall College, Lancaster, Penn. "It may
also have some implications for the supposed widespread involvement of water in
the geological evolution of Mars."
Bleacher
notes that the team's conclusions do not preclude the possibility of flowing water
on Mars, nor of other channels carved by water.
"But
one thing I've learned is not to underestimate the way that liquid rock will
flow," he said. "It really can produce a lot of things that we might
not think it would."
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