<o:p></o:p>

The Valles Marineris is a long
series of canyons east of Olympus Mons, the largest mountain in the solar
system. These canyons are about 4,000 km long, 200 km wide and up to 7 km
(23,000 ft) deep. On terrestrial scales, the Valles Marineris is as long as the
distance from New York to Los Angeles. That’s about the same as Beijing to Hong
Kong or Madrid to Copenhagen for our international listeners. They are as wide
as central Florida, central Italy, or the middle of the Korean peninsula. Two
and a half times deeper than Death Valley, though only about 60 percent of the
depth of the Marianas Trench, the lowest point on earth.<o:p></o:p>

Not to be outdone, our planet, Earth,
has even bigger valleys. These occur at the oceanic ridges, where plate
spreading takes place. The longest rift valley on earth lies in the middle of
the Mid-Atlantic Ridge, and it is more than double the length of the Valles
Marineris. But let’s not belittle Mars. After all, while we have a pretty good
idea for how oceanic rifts form on earth, there is quite a bit of debate about
how Mars’ great valley formed.<o:p></o:p>

The most popular theory suggests
that the Valles Marineris are an analog to our oceanic rifts, and formed by the
same process. As the volcanoes of the nearby Tharsis region developed, the
Martian crust bowed down toward the center of the planet due to the weight of
the new volcanic rocks. In time, the crust began to crack. This crack is what we
see in the Valles Marineris. Unlike on Earth, this rift valley did not continue
expanding, but shut down as the Tharsis Region, and Mars as a whole, cooled.
Remember that unlike Earth, Mars does not have plate tectonics. It doesn’t have
a continual process of hot material (like lava) rising to the surface, while relatively cold material (like the oceanic crust) is brought down towards the planet’s center.<o:p></o:p>

More recent work has used
satellite images, and high resolution elevation data to develop new insight
into how the Valles Marineris formed. While images from the 1970’s Mariner 9
orbiter were quite blurry by today’s standards, new missions in the late 90’s
to early 2000’s have given us a better view of the Martian surface than we have
available for the earth. The Mars Reconnaissance Orbiter can take images where
each pixel is about 0.5 m or 20 inches. That is, the color on each image is an
average of an area of 0.25 square meters, or 2.5 square feet. It can then use
image pairs to estimate the elevation of any point on the Martian surface with
a pixel size of 0.25 m, or about 10 inches.<o:p></o:p>

These new satellite images
include multispectral data, or images that look at different wavelengths of
light. The camera on your phone works in the same way: There are sensors that
pick up, red light, green light, and blue light. Your phone records the
intensity of each color in each part of the image, and then plays it back on
your phone’s screen to create a picture.<o:p></o:p>

Some of the satellites orbiting Mars
take this to the next level. They don’t just take different slices of colored
light, but also longer wavelength, infrared light. If you’ve ever seen an image
from a thermal imaging camera, you know what this is. Parts of you show up as
hotter or colder on the screen. It’s the same with the surface of the earth, or
Mars. Scientists can compare the intensity of different wavelengths of light
from each point on the surface. They can then compare these values, with what
would be expected for different rock types. In other words, we’re able to
roughly determine the types of rocks on the Martian surface without ever
setting a boot, or rover tread, on the red planet.<o:p></o:p>

Data from these images has shown
that the Valles Marineris have layered rock formations both on the sides of the
canyons, and within them. The great valley has seen many landslides over the
last 3.5 Billion years of its existence, as well as new and smaller canyons
carved into it. Scientists now speculate that rather than just forming as a big
crack in the Martian surface, the Valles Marineris have been sculpted by
flowing water, either in its liquid form as rivers, or in its solid form as
glaciers.<o:p></o:p>

An alternative hypothesis
proposes that the Valles Marineris formed as a crack during a massive, planetary
scale landslide. This landslide was about half the size of the US or China. How
do you form a landslide that big? Well, you need a large pile of relatively
weak rock, and high elevations for the landslide to flow from. <o:p></o:p>

A key player here is salt. Salt
is relatively weak as compared to rock, and can deform easier when squeezed. It
can also hold water, which can be driven off by heating. On Earth, weak salt
layers are partly responsible for undersea landslides in the Gulf of Mexico.
The Opportunity rover had found some salt layers during its mission on Mars, so
we know salt is present on the red planet. <o:p></o:p>

Some scientists interpret the
layers on the sides of the Valles Merinaris to be made of salt, and possibly
include pockets of ice. This would imply that those layers are weak, and could
potentially move downhill under the right circumstances.<o:p></o:p>

Heating in the Tharsis region
helped de-water salts under the future landslide, melted ice pockets, and
created high elevations on one side of it. Think of it like putting a can on a
wet metal sheet. If you raise one side of the sheet, the can will slide to the
lower side. Just like that, the salty Martian crust broke, and slid downhill. <o:p></o:p>

A crack in the side of this
landslide allowed massive amounts of underground water to escape. As the water
flowed downhill, it eroded the crack to form a massive canyon. This canyon is
the Valles Marineris. The flood that helped form the Valles Marineris was
probably bigger than any seen on earth. Bigger than the massive glacial outburst
floods that formed the channeled scablands of the northwestern United States.
Dick Gibson discussed outburst flooding in the December 27, 2014 episode. Unlike the Earth, the Martian surface has been relatively quiet since
the Valles Marineris formed 3.5 billion years ago.<o:p></o:p>