- 4432 - MOON HISTORY - splashed off the Earth? - Scientists recently solved a long-standing mystery surrounding the moon's 'lopsided' geology. About 4.5 billion years ago, a small planet smashed into the young Earth, flinging molten rock into space. Slowly, the debris coalesced, cooled and solidified, forming our moon.
------------------------- 4432 - MOON HISTORY - splashed off the Earth?
- Most of what is known about the origin of
the moon comes from analyses of rock samples, collected by Apollo astronauts
more than 50 years ago, combined with theoretical models. The samples of
basaltic lava rocks brought back from the moon showed surprisingly high
concentrations of titanium. Later satellite observations found that these
titanium-rich volcanic rocks are primarily located on the moon's nearside, but
how and why they got there has remained a mystery..
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- Because the moon formed fast and hot, it
was likely covered by a global magma ocean. As the molten rock gradually cooled
and solidified, it formed the moon's mantle and the bright crust we see when we
look up at a full moon at night. But deeper below the surface, the young moon
was wildly out of equilibrium. Models suggest that the last dregs of the magma
ocean crystallized into dense minerals including “ilmenite”, a mineral
containing titanium and iron.
-
- Because these heavy minerals are denser
than the mantle underneath, it creates a gravitational instability, and you
would expect this layer to sink deeper into the moon's interior.
-
- Somehow, in the millennia that followed,
that dense material did sink into the interior, mixed with the mantle, melted
and returned to the surface as titanium-rich lava flows that we see on the
surface today.
-
- Our moon literally turned itself inside
out. Did this material sink as it formed
a little at a time, or all at once after the moon had fully solidified? Did it
sink into the interior globally and then rise up on the near side, or did it
migrate to the near side and then sink? Did it sink in one big blob, or several
smaller blobs?
-
- The lunar near side with its dark regions,
or "mare," covered by titanium-rich volcanic flows makes up the
moon's familiar sight from Earth. The mare region is surrounded by a polygonal
pattern of linear gravity anomalies interpreted to be the vestiges of dense
material that sank into the interior. Their presence provides the first
physical evidence for the nature of the global mantle overturn more than 4
billion years ago.
-
- Models predicted that the dense layer of
titanium-rich material beneath the crust first migrated to the near side of the
moon, possibly triggered by a giant impact on the far side, and then sunk into
the interior in a network of sheetlike slabs, cascading into the lunar interior
almost like waterfalls.
-
- But when that material sank, it left behind
a small remnant in a geometric pattern of intersecting linear bodies of dense
titanium-rich material beneath the crust.
We see the exact same pattern when we look at subtle variations in the
moon's gravity field, revealing a network of dense material lurking below the
crust.
-
- In the new study, they compared simulations
of a sinking ilmenite-rich layer to a set of linear gravity anomalies detected
by NASA's GRAIL mission, whose two spacecraft orbited the moon between 2011 and
2012, measuring tiny variations in its gravitational pull. These linear
anomalies surround a vast dark region of the lunar near side covered by
volcanic flows known as mare (Latin for "sea").
-
- The gravity signatures measured by the GRAIL
mission are consistent with ilmenite layer simulations, and that the gravity
field can be used to map out the distribution of the ilmenite remnants left
after the sinking of the majority of the dense layer. Ilmenite materials migrated to the near side
and sunk into the interior in sheetlike cascades, leaving behind a vestige that
causes anomalies in the moon's gravity field, as seen by “GRAIL."
-
- The linear gravity anomalies are interrupted
by the largest and oldest impact basins on the near side and therefore must
have formed earlier. Based on these cross-cutting relationships, this suggest
that the ilmenite-rich layer sank prior to 4.22 billion years ago, which is
consistent with it contributing to later volcanism seen on the lunar surface.
-
- Analyzing these variations in the moon's
gravity field show that the anomalies in
the moon's gravitational field match what would be expected for the zones of
dense titanium-rich material predicted by computer simulation models of lunar
overturn.
-
- More than 50 years ago, Apollo astronauts
brought basaltic lava rocks back from the moon with surprisingly high
concentrations of titanium. Later, satellite observations found that these
titanium-rich volcanic rocks are primarily located on the moon's nearside - but
how and why they got there has remained a mystery.
-
- The moon is fundamentally lopsided in every
respect. The near side facing the Earth,
and particularly the dark region known as Oceanus Procellarum region, is lower
in elevation, has a thinner crust, is largely covered in lava flows, and has
high concentrations of typically rare elements like titanium and thorium.
-
- The far side differs in each of these
respects. Somehow, the overturn of the lunar mantle is thought to be related to
the unique structure and history of the near side Procellarum region.
-
- When the “Artemis” astronauts eventually
land on the moon to begin a new era of human exploration, we will have a very
different understanding of our neighbor than we did when the Apollo astronauts
first set foot on it.
-
- For decades, astronomers have said that one
of the most optimal places to build large telescopes is on the surface of the
Moon. The Moon has several advantages over Earth and space-based telescopes
that make it worth considering as a future home for giant observatories.
-
- A new paper lists all the advantages,
including how telescopes on the lunar surface wouldn’t be blocked by an
atmosphere or impacted by wind, and how the low gravity would allow gigantic
structures to be built that could be upgraded over time by astronauts.
-
- Progress on the big questions in astronomy,
such as life on certain exoplanets or dark matter, will ultimately require high
angular resolution, a large collecting area and access to the full optical
spectrum. All astronomy will benefit
from the advantages provided by the localization on the Moon.
-
- Over the years, scientists and engineers
have proposed various ideas for lunar observatories as part of the NASA
Innovative Advanced Concepts program. In
2005 there was a proposal for a deep-field infrared observatory near one of the
lunar poles using a rotating liquid mirror.
-
- Earlier this year, 2024, NASA’s Goddard Space Flight Center proposed a
design for a lunar Long-Baseline Optical Imaging Interferometer (LBI) for
imaging at visible and ultraviolet wavelengths.
-
- Astronomers have advocated building radio
telescopes on the far side of the Moon, since this “radio-quiet” zone always
faces away from Earth and would provide the perfect location to study a variety
of astronomical phenomena that can’t be observed in low radio frequencies from
our planet, or even by Earth-orbiting space telescopes.
-
- The Moon offers a combination of three
distinct advantages for astronomical observing. Its lack of atmosphere allows
access to the entire spectrum, including the visible, ultraviolet, and
infrared. Astronomers wouldn’t have to deal with atmospheric turbulence, and
the Moon’s low gravity and absence of wind make it possible to install
extremely large telescopes with very large instruments. This is impossible for
satellites in orbit.
-
- Telescopes on the Moon would allow for the
instruments to be upgraded and to have a very long lifetime, which is
impractical for space satellites due to their limited amount of fuel.
-
- The Moon offers the possibility of
installing large telescopes or interferometers with instruments larger than
those on orbiting telescopes. Very high
angular resolution would allow for the imaging of exoplanet transits or even
the stellar glint on exo-ocean.
-
- In the extragalactic domain, telescopes on
the Moon could allow for the study of the distribution of dark matter or
observing the gravitational lensing of quasars. Other lunar telescopes could
investigate the unexplored Dark Ages of the early universe, a time before and
during the formation of the very first stars and galaxies.
-
- The best location for a lunar telescope
depends on two factors: the physical conditions of the site (temperature, soil
quality, solar illumination) and its scientific objectives. Telescopes pointing towards Earth must be
placed on the near side of the Moon.
From the point of view of target observability, they can be placed
almost anywhere. At the lunar poles, only half the sky is visible, but all the
time. At the lunar equator, the whole sky is visible, but only half the time.
For observations of or towards the Earth, the optimum is not far from the lunar
equator.
-
- Putting observatories on the lunar surface
does present several problems, including incessant dust, the proclivity for
incoming meteoroids, or dealing with other surprises like lunar seismology
(Moonquakes).
-
- A small 30 cm to 1 meter class telescope
will explore significant science cases.
Even though this first lunar telescope would be a prototype, it would
still be astronomically valuable. It could perform observations that would be
complimentary to the James Webb Space Telescope or the Hubble Space Telescope.
-
- But for the future, a 20-meter mirror would
provide resolution 3 times greater than the JWST, and by integrating, or
leaving the “shutter” open for long periods, objects 100 times fainter could be
observed
-
-
April 14, 2023 MOON HISTORY
- splashed off the Earth? 4432
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--------------------- --- Tuesday, April 16, 2024
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