The fate of almost everything on Earth’s surface is determined
by infernal engines deep below.
Mars is no different. Now, thanks to an
intrepid robot parked on the Martian surface by
NASA in November 2018,
scientists have a map of our neighboring world’s geologic abysses, the first
ever made of another planet.
اضافة اعلان
NASA’s InSight lander has been listening to marsquakes and
tracking their seismic waves as they journey through the planet. A trio of
papers published Thursday in the journal Science, using data InSight has
collected, reveals the red planet to be something like a colossal candy treat
imagined by a ravenous deity. Its crust is split into two or three layers of
volcanic chocolate. The mantle below has a surprisingly sizable and rigid
toffeelike filling. And the planet’s core is surprisingly light — less nougaty
center, more syrupy heart.
Paired with recent activities at the surface by new NASA and
Chinese robotic rovers, these missions highlight stark differences between our
blue world and the red one next door.
This survey of the Martian insides has been a long time coming.
Earth’s solid-but-squishy mantle was first glimpsed in 1889, when seismic waves
from a quake in Japan dove in and out of the layer before emerging in
Germany.
Earth’s liquid outer core was discovered in 1914, and the solid inner core was
revealed in 1936. Similar measurements of the moon were made when the Apollo
astronauts left seismometers on its surface.
Now the same basic and foundational measurements have been made
on Mars. This work, conducted with one of the most technologically advanced
seismometers ever built, represents “a major leap in planetary seismology,”
said Paula Koelemeijer, a seismologist at Royal Holloway, University of London
who was not involved in the research but co-wrote a perspective article in
Science.
Earlier missions to Mars have provided rough estimates of the
dimensions and properties of its innards. But InSight’s seismological surveys
provide precision. Models used to simulate the evolution of Mars can now be
built on the foundations of these ground truths.
Revelations from the InSight mission will also be useful for
studying other worlds by providing scientists with an example that differs from
Earth.
“If you’re a doctor, and you only practice on one patient,
you’re not going to be a very good doctor,” said Mark Panning, a planetary
seismologist at NASA’s Jet Propulsion Laboratory in Pasadena, California, and a
co-author on all three papers.
Mars is more like a cousin of our planet than a sibling. Six
times less voluminous, it is strangely small — and geochemical evidence
suggests that “it’s this really ancient relic of the early solar system,” said
Christine Houser, a seismologist at the Earth-Life Science Institute in Tokyo
who was not involved with the research.
Why is diminutive Mars so physically different from Earth and
Venus, a planet thought of as Earth’s geologic twin? InSight’s forensic
examination improves scientists’ chances at finding an answer — and, in the
process, better understanding our planet’s place in the solar system.
Over the past two years, the InSight lander has studied the red
planet’s magnetism, its wobble as it orbits the Sun and the seismic waves
created by its marsquakes.
Most marsquakes occur at shallow depths. But a handful emanate
from deeper locales, ricocheting through the planet before reaching InSight.
Seismic waves change speed and direction as they traverse different materials,
so scientists could use these deep-seated quakes to see what is going on inside
Mars.
It has not been easy going. Working with a solitary seismometer
means scientists get a decent look at just one region on Mars rather than the
entire planet. And to construct a detailed picture of the subsurface, plentiful
powerful quakes that pass through much of the planet’s depths would be ideal.
Unfortunately, Mars’ seemingly infrequent quakes are never more potent than a
magnitude 4.0.
“We just had to push forward and see what we could do with this
data,” said Brigitte Knapmeyer-Endrun, a planetary seismologist at the
University of Cologne and lead author of the paper on the Martian crust.
Despite the challenges, the team managed to make a detailed X-ray of Mars’
guts.
Scientists confirmed that the crust is thicker in the southern
highlands and thinner in the northern lowlands, where ephemeral oceans may have
pooled long ago. On average, the planetary crust is between 15-45 miles thick.
It is also split into a top layer mostly made of volcanic rock shattered by
meteorites, a middle layer of more coherent volcanic rock and, perhaps, a lower
layer whose properties cannot be made out for the time being.
Like Earth’s, Mars’ mantle is far thicker than its crust. But
the rigid part of the upper mantle, which on Earth forms the base of
ever-shifting tectonic plates, is perhaps twice as thick on Mars, maybe more.
This “might be the simple explanation why we don’t see plate
tectonics on Mars,” said Amir Khan, a geophysicist at ETH Zürich in Switzerland
and co-author on all three studies. Such rigidity may have prevented the
fragmentation of Mars’ upper layers into individual tectonic plates, robbing it
of the sculptor that gave Earth such diverse mountains, ocean basins, volcanoes
and continents.
InSight also found that Mars’ mantle as a whole is about half as
thick as Earth’s — a lack of insulation that would have exacerbated Mars’ heat
loss as it erupted heavily in its youth. (Mars’ small size also allowed plenty
of its primordial heat to radiate into space.)
This thin mantle may also partly explain why Mars lost its
protective magnetic field in the first 700 million years of its history.
Earth’s magnetic field is powered by the circulation of iron-nickel currents
within its liquid outer core. Presumably, Mars had a similar circulation, but
the speedy cooling of its innards caused those currents to seize up, shutting
off its magnetic dynamo.
Without a magnetic bubble to shield Mars from the sun’s
radiation, its atmosphere was blown away like confetti. Water that once
frequented its surface — if it was not soaked up by the rocks below — escaped
into space, turning it into a frigid, irradiated desert.
InSight also saw Mars’ core. With a radius of 1,140 miles, it is
bigger than expected. It is also not very dense, which is “one of the most
intriguing results we’ve found so far,” Khan said.
Earth’s core is rather dense because the planet is much larger
than Mars, so all that weight squashes the core together. Mars, being tiny, was
expected to have a slightly less compressed core. But InSight found that it is
half the density of Earth’s, something planetary compaction cannot explain.
This means that Mars’ core must be made of different stuff. Like
Earth, it still contains a preponderance of iron and nickel, but it also
features a sizable fraction of lighter elements, like oxygen, carbon, sulfur
and hydrogen. The Martian nucleus’ unusual chemistry is another hint of the red
planet’s distinctive formation history.
Despite the successes of humanity’s first interplanetary seismic
survey, many questions remain to be answered. No detected marsquake has been
powerful enough to reach the very center of the planet, so scientists do not
know if, like Earth, Mars has a solid inner core. All that can be said for now
is that it has a liquid outer core — albeit a more slurrylike, sluggishly
moving one compared to Earth’s own.
Marsquakes themselves remain confounding. They may be the key to
seeing inside the planet, but their origins are heavily debated. Many shallow
quakes, for example, are more intense and more frequent during Martian winters.
“That is strange, because on Earth you don’t have clear seasonal quakes,” said
Simon Stähler, a seismologist at ETH Zürich in Switzerland and co-author on all
three papers.
InSight’s scientific expedition has been extended to December
2022, so as more marsquakes come in, these puzzles may be solved. But buildup
of dust on the lander’s solar arrays may kill off the robot within the year.
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