Sky & Telescope - August 2023 - 17

rain. This area was the source for about 90% of all the quakes
Insight detected, accounting for half of the total seismic
energy released on the planet as a whole. The large number of
quakes shows that the faults are still opening today.
" To produce these opening faults you need an active
mechanism, " says Insight team member Simon Stähler (Swiss
Federal Institute of Technology Zurich). " More and more
people are becoming convinced that this could be due to an
upwelling in the mantle, a so-called plume. "
Mantle plumes are powered by heat coming directly from
the mantle-core boundary. They are made by hot but solid
rock - not magma - that over long periods, maybe hundreds
of millions of years, is plastic enough to flow. Once the plume
reaches the bottom of the crust, it pushes from below, opening
the fractures. Its heat can also melt rock, creating magma
pockets that can produce volcanism.
On Earth, mantle plumes are related to volcanism far
from plate boundaries, in places such as Hawai'i. Since there's
no plate tectonism on Mars, mantle plumes seem like the
most plausible driver of volcanism.
Adrien Broquet and Jeffrey Andrews-Hanna (both at University
of Arizona) argue that they've found evidence that this
is what happens at Cerberus Fossae. They decided to look at
the wider picture, puzzled by the intense seismic activity and
recent volcanism.
Using satellite observations, they realized that Elysium
Planitia, the flat region that includes both Cerberus Fossae
Thickest crust: 90 km
(Tharsis volcanic province)
and the Insight landing site, rises up to 2 km over the vast
northern lowlands, forming a very broad dome. They also
spotted subtle gravitational variations, pointing to a build-up
of material coming from the deep. More revealing, they saw
that the floors of impact craters in the area, instead of being
flat, are tilted in the direction of the plume, showing that the
ground has moved after the craters formed.
All the evidence points to a mantle plume that is actively
uplifting today. It could be as large as 4,000 km wide -
roughly the size of the contiguous United States. " That's
not something you'd expect from a planet that is cooling, "
Broquet says.
" It's a bit surprising because it requires quite a bit of activity
in the Martian mantle, and for that you need a certain
amount of heat, " Stähler says. " Since Mars is smaller than
Earth, you'd have expected it to cool quicker and generally be
much colder in the interior, and that's something people will
have to wrap their heads around to explain. "
A Multi-layered Planet
Seismic waves are like a flash of light illuminating the
planet's interior. As they move through different materials,
their speed changes and they are refracted, the same behavior
we see with light waves. If the material's properties change
abruptly, the waves bounce off and produce echoes, revealing
discontinuities that often mark the transitions between the
main layers of a planet.
Thinnest crust: 10 km
(Isidis Planitia)
Insight detected four crustal layers directly
beneath itself. These might or might not
exist elsewhere on the planet.
2-3 km
50 km
0 km
Crust
Basaltic
rocks
25 km
10 km
Mantle
Dense,
olivinerich
rocks
1,580 km
Core
Molten iron
plus lighter
elements
3,390 km
The core is about
10% larger than
expected
Mars shows no sign of a
solid inner core
High pressure
changes rocks
to denser form
below 1,000 km
40 km
*Not to scale
THE MARTIAN INTERIOR Scientists used the
way seismic waves traveled through Mars to
map out its interior structure. The largest shaker
on sol 1222 was particularly helpful.
sk yand tele scope .o r g * AUGUST 2023 17
LEAH TISCIONE / S&T
http://skyandtelescope.org
Sky & Telescope - August 2023

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