Sky & Telescope - March 2024 - 9

PLOTS: SCHAYE ET AL. / MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL
SOCIETY 2023 (2); MARS INTERIOR: IPGP / CNES / N. STARTER
SOLAR SYSTEM
Marsquake Reveals Molten Layer Above Martian Core
3 million ly
30 million light-years
Gas surface density
ON SEPTEMBER 18, 2021,
an impact triggered a
marsquake on the planet's
farside from the seismometer
aboard NASA's Insight
lander. For the first time in
three years of operations,
it detected vibrations that
could enable scientists to
probe the deep Martian interior. In
the October 26th Nature, two separate
groups used the impact to probe a layer
of molten magma in the lower mantle.
In 2021, scientists employed Insight
3 million ly
to measure the size of the liquid
Martian core (S&T: Nov. 2021, p. 8),
estimating its diameter to be 3,660 km
(2,270 miles), more than half the planet's
width. The large volume implied a
low average density, just 6 grams per
cubic centimeter compared to at least
10 g/cm3 in Earth's core.
That's problematic in light of Mars's
30 million light-years
formation, says Amir Khan (ETH
Zürich), who led one of the Nature
studies. " It would be very difficult to get
all those light elements into the core. "
Scientists needed more data, and just
Cold dark matter surface density
pThese two simulation stills compare the
surface density of gas (top) and dark matter
particles (bottom) in the modern-day universe.
The insets zoom in on a massive dark
matter halo, which might hold many galaxies.
feedback delivered via ordinary,
baryonic matter might explain this
tension. However, the Flamingo team
concluded that while feedback indeed
affects the universe's large-scale
structure, it's not enough to solve
the clumpiness problem. If unknown
systematic errors in the analysis can
also be excluded, the researchers
write, " then the exciting implication
would appear to be that new physics
[...] is required. "
In other words, the new Flamingo
simulations might indicate that
something is wrong with our standard
model of cosmology.
¢GOVERT SCHILLING
months before Insight lost power, the
planet provided. The September impact
and a second, closer event on December
24, 2021, generated slow seismic waves
that indicated they'd traveled through
a stratified mantle, explains Henri
IN BRIEF
Planets Really Do Start with Pebbles
The James Webb Space Telescope turned up evidence that icy pebbles really do deliver
vital building blocks to the inner regions of planet-forming disks. A team led by Andrea
Banzatti (Texas State University) used Webb's mid-infrared instrument to explore four
disks around young stars in Taurus. Two of them are compact (just 10 to 20 astronomical
units, or a.u., across), while two are bigger (100 to 150 a.u.), with multiple gaps and rings.
Under the pebble scenario, greater friction within more compact disks causes distant,
icy pebbles to drift inward. The rings and gaps of larger disks, on the other hand, impede
such pebble drift. If a pebble drifts across the snow line, the temperature rises enough to
sublimate the veneer of ice into water vapor, leaving behind bare, rocky material for terrestrial
planet formation. In the November 10th Astrophysical Journal Letters, Banzatti's
team reports excess cool water vapor around the snow line, compared with only hot
vapor for the larger ones. However, additional observations could complicate this picture,
says Jane Greaves (Cardiff University, UK), who wasn't involved in the research.
¢COLIN STUART
sk yand tele scope .o r g * MARCH 2024
9
An artist's view of the internal
structure of Mars shows the
propagation of seismic waves
from two events in 2021.
Samuel (University of
Paris), who led the second
Nature study. Both Khan's
and Samuel's teams saw
a layer of molten silicates
at the bottom of the mantle; Samuel's
team also found that a partially molten
layer tops the fully molten region.
Accounting for the stratified mantle
shrinks the measurement of the true
core to 3,300 km in diameter, increasing
the density to about 6.5 g/cm3.
While still not nearly as dense as
Earth's core, the new value reduces the
amount of light elements required.
The results have other implications,
too, Samuel's team argues. A liquid
lower mantle would have insulated the
core, preventing it from cooling and
thus inhibiting internal motions. So
Mars's early magnetic field might have
come not from within but from external
sources, such as giant impacts.
" This shows the power of
seismology, " says Simon Stähler (also at
ETH Zürich), who wasn't involved in the
study. " It can sound the deep interior
and give us otherwise unobtainable
information. "
¢JEFF HECHT

http://skyandtelescope.org

Sky & Telescope - March 2024

Table of Contents for the Digital Edition of Sky & Telescope - March 2024