This week’s image comes from Io, jupiters volcanic moon.
This image shows a mountain named Tohil Mons and was captured by the NASA Galileo
Spacecraft. Credit goes to the Galileo team. There is a link to NASA’s
description of this image below. They report that the mountain is 5.4
kilometres in height. “North is to the top and the Sun illuminates the surface
from the upper right.” Tohil Mons is located on the Antijovian hemisphere of Io.
The sharp shadows in this image are one way to estimate the height of this mountain.
But the Galileo spacecraft also performed stereo observations where images were
taken from multiple angles and combined to form a 3D model. This allows us to
get a good look at this mountain in 3D.
The surface of Io is covered with volcanic features,
including many large volcanic craters, called “calderas”. There are several
such craters located around this peak, including one just to the east of the
mountain however the mountain in the centre of this image is not itself a
volcano. Very few of Io’s volcanic features have strong positive relief, most
consist of low lying cones and craters.
Instead most of the highest mountains are believed to form through
tectonic processes, although the precise mechanism is still being debated. This
is an area where research is still very much ongoing so I don’t have a
definitive explanation to share. Even on Earth where we have, or can get all
the data we could possibly want it can take a lot of hard work to reconstruct
the formation history of geological features. On moons like Io, which have been
visited quite infrequently, and where limited data is available it becomes even
harder.
Tohil Mons appears to be a complex “massif” with quite
rugged terrain and a straight ridge running to the southwest. A massif is a coherent
block of the crust which has been uplifted or displaced by tectonic forces to
form one or more mountains. It may exhibit layering, as do several other mountains
on Io. It is also surrounded by a basal scarp. This is a steep cliff, or scarp
at the bottom (or base) of the mountain which separates the upland area from
the surrounding volcanic plains. These features are consistent with a tectonic
origin.
Io lacks plate tectonics of the sort seen on Earth, instead
it is undergoing extreme volcanism due to its proximity to Jupiter and the
resonance between its orbit and those of the other Galilean satellites. This
volcanic activity is likely what has caused these tectonic features. As
resurfacing by lava flows takes place the thick blocks of crust between
volcanic sites are buried or displaced as they are forced into a smaller area
by the formation of new terrain. Thrust faulting occurs resulting in uplifted areas
such as these massifs.
This means that while Tohil Mons is not a volcano in its own
right its history is deeply tied to the volcanism which shapes every aspect of
Io’s surface.
Further Reading:
Image Credits: NASA/JPL/University
of Arizona
https://photojournal.jpl.nasa.gov/catalog/PIA03600
https://en.wikipedia.org/wiki/Tohil_Mons#/media/File:Tohil_Monsstructure.jpg
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