On a clear night, with a decent
amateur telescope, Saturn and its series of remarkable rings can be seen from
Earth’s surface. But how did those rings come to be? And what can they tell us
about Saturn and its moons, one of the potential locations NASA hopes to search
for life? A new series of supercomputer simulations has offered an answer to
the mystery of the rings’ origins – one that involves a massive collision, back
when dinosaurs still roamed the Earth.
According to new research by NASA and its partners,
Saturn’s rings could have evolved from the debris of two icy moons that
collided and shattered a few hundred million years ago. Debris that didn’t end
up in the rings could also have contributed to the formation of some of
Saturn’s present-day moons.
“There’s so much we still don’t
know about the Saturn system, including its moons that host environments that
might be suitable for life,” said Jacob Kegerreis, a research scientist at
NASA’s Ames Research Center in California’s Silicon Valley. “So, it’s exciting
to use big simulations like these to explore in detail how they could have
evolved.”
NASA’s Cassini mission helped scientists
understand just how young – astronomically speaking – Saturn’s rings and
probably some of its moons are. And that knowledge opened up new questions
about how they formed.
To learn more, the research team
turned to the Durham University location of the Distributed Research using
Advanced Computing (DiRAC) supercomputing facility in the United Kingdom.
They modeled what different collisions between precursor moons might have
looked like. These simulations were conducted at a resolution more than 100
times higher than previous such studies, using the open-source simulation code,
SWIFT, and giving scientists their best insights into the Saturn system’s
history.
Saturn’s rings today live close to
the planet, within what’s known as the Roche limit – the farthest orbit where a
planet’s gravitational force is powerful enough to disintegrate larger bodies
of rock or ice that get any closer. Material orbiting farther out could clump
together to form moons.
By simulating almost 200 different
versions of the impact, the team discovered that a wide range of collision
scenarios could scatter the right amount of ice into Saturn’s Roche limit,
where it could settle into rings.
And, while alternative explanations
haven’t been able to show why there would be almost no rock in Saturn’s rings –
they are made almost entirely of chunks of ice – this type of collision could
explain that.
“This scenario naturally leads to
ice-rich rings,” said Vincent Eke, Associate Professor in the Department of
Physics/Institute for Computational Cosmology, at Durham University and a
co-author on the paper. “When the icy progenitor moons smash into one another,
the rock in the cores of the colliding bodies is dispersed less widely than the
overlying ice.”
Ice and rocky debris would also
have hit other moons in the system, potentially causing a cascade of
collisions. Such a multiplying effect could have disrupted any other precursor
moons outside the rings, out of which today’s moons could have formed.
But what could have set these events
in motion, in the first place? Two of Saturn’s former moons could have been
pushed into a collision by the usually small effects of the Sun’s gravity
“adding up” to destabilize their orbits around the planet. In the right
configuration of orbits, the extra pull from the Sun can have a snowballing
effect – a “resonance” – that elongates and tilts the moons’ usually circular
and flat orbits until their paths cross, resulting in a high-speed impact.
Saturn’s moon Rhea today orbits
just beyond where a moon would encounter this resonance. Like the Earth’s Moon,
Saturn’s satellites migrate outward from the planet over time. So, if Rhea were
ancient, it would have crossed the resonance in the recent past. However,
Rhea’s orbit is very circular and flat. This suggests that it did not
experience the destabilizing effects of the resonance and, instead, formed more
recently.
The new research aligns with evidence that Saturn’s
rings formed recently,
but there are still big open questions. If at least some of the icy moons of
Saturn are also young, then what could that mean for the potential for life in
the oceans under the surface of worlds like Enceladus? Can we unravel the full story
from the planet’s original system, before the impact, through to the present
day? Future research building on this work will help us learn more about this
fascinating planet and the icy worlds that orbit it.
Still image from a computer
simulation of an impact between two icy moons in orbit around Saturn. The
collision ejects debris that could evolve into the planet's iconic and
remarkably young rings. The simulation used over 30 million particles, colored
by their ice or rock material, run using the open source SWIFT simulation code.
This
graphic shows antihydrogen atoms falling and annihilating inside a magnetic
trap, part of the ALPHA-g experiment at CERN to measure the effect of gravity
on antimatter. Credit: U.S. National Science Foundation
For
those still holding out hope that antimatter levitates rather than falls in a
gravitational field, like normal matter, the results of a new experiment are a
dose of cold reality.
Physicists studying antihydrogen—an
anti-proton paired with an antielectron, or positron—have conclusively shown
that gravity pulls it downward and does not push it upward.
At least for antimatter, antigravity
doesn't exist.
The experimental results have been
reported in the Sept. 28 issue of the journal Nature by
a team representing the Antihydrogen Laser Physics Apparatus (ALPHA)
collaboration at the European Center for Nuclear Research (CERN) in Geneva,
Switzerland.
The gravitational acceleration of
antimatter that the team comes up with is close to that for normal matter on
Earth: 1 g, or 9.8 meters per second per second (32 feet per second per
second). More precisely, it was found to be within about 25% (one standard deviation) of normal gravity.
"It surely accelerates downwards,
and it's within about one standard deviation of accelerating at the normal
rate," said Joel Fajans, a UC Berkeley professor of physics who, with colleague
Jonathan Wurtele, a theoretician, first proposed the experiment more than a
decade ago. "The bottom line is that there's no free lunch, and we're not
going to be able to levitate using antimatter."
The result will not surprise most
physicists. Albert Einstein's theory of general relativity, though conceived before antimatter was discovered in
1932, treats all matter identically, implying that antimatter and matter
respond the same to gravitational forces. All normal matter, such as protons, neutrons and
electrons, have anti-particles that bear the opposite electrical charge and,
when they encounter their normal matter counterpart, annihilate completely.
An artist's conceptual rendering of antihydrogen
atoms falling out the bottom of the magnetic trap of the ALPHA-g apparatus. As
the antihydrogen atoms escape, they touch the chamber walls and annihilate.
Most of the annihilations occur beneath the chamber, showing that gravity is
pulling the antihydrogen down. The rotating magnetic field lines in the
animation represent the invisible influence of the magnetic field on the
antihydrogen. The magnetic field does not
rotate in the actual experiment. Credit: Keyi "Onyx" Li/U.S. National
Science Foundation
"The
opposite result would have had big implications; it would be inconsistent with
the weak equivalence principle of Einstein's general theory of
relativity," said Wurtele, UC Berkeley professor of physics. "This
experiment is the first time that a direct measurement of the force of gravity
on neutral antimatter has been made. It's another step in developing the field
of neutral antimatter science."
Fajans noted
that no physical theory actually predicts that gravity should be repulsive for
antimatter. Some physicists claim that, if it were, you could create a
perpetual motion machine, which is theoretically impossible.
Nevertheless, the idea that
antimatter and matter might be affected differently by gravity was enticing
because it could potentially explain some cosmic conundrums. For example, it
could have led to the spatial separation of matter and antimatter in the early
universe, explaining why we see only a small amount of antimatter in the
universe around us. Most theories predict that equal amounts of matter and
antimatter should have been produced during the Big Bang that birthed the
universe.
Gravity is incredibly weak
According to
Fajans, there have been many experiments, all indirect, that strongly suggest
that antimatter gravitates normally, but these experiments have been relatively
subtle.
"You
might ask, why not do the obvious experiment and drop a piece of antimatter, a
sort of leaning tower of Pisa experiment? You know, the experiment that Galileo
didn't actually do—it was apocryphal—where he supposedly dropped a lead ball
and a wooden ball from the top of the tower and proved that they both reached
the ground at the same time," he said.
"The real
problem is that the gravitational force is incredibly weak compared to
electrical forces," Fajans added. "So far, it has proved impossible
to directly measure gravity with a drop-style measurement with a charged
particle, like a bare positron, because any stray electric field will deflect
the particle much more than gravity will."
In fact, the
gravitational force is the weakest of the four known forces of nature. It
dominates the evolution of the universe because all matter—theoretically—is
affected by it over immense distances. But for a tiny piece of antimatter, the
effect is minuscule. A 1 volt/meter electrical field exerts a force on an
antiproton that is about 40 trillion times larger than the force of gravity
exerted on it by planet Earth.
The ALPHA
collaboration at CERN suggested to Wurtele a new approach. By 2010, the ALPHA
team was trapping significant quantities of antihydrogen atoms, and in 2011,
Wurtele insisted to Fajans that since antihydrogen is charge neutral, it would
not be affected by electric fields, and they should explore the possibility of
a gravity measurement.
Fajans
dismissed the idea for many months, but was eventually persuaded to take it
seriously enough to perform some simulations that suggested Wurtele's ideas had
merit. UC Berkeley lecturer Andrew Charman and postdoctoral fellow Andrey
Zhmoginov became involved and realized that a retrospective analysis of prior
data could provide very coarse limits on antimatter's gravitational
interactions with Earth.
With help from
their ALPHA colleagues, this led to a paper that concluded that antihydrogen
experiences no more than about 100 times the acceleration—in the up or down
direction—due to Earth's gravity, compared to regular matter.
That
underwhelming start nevertheless convinced the ALPHA team to build an
experiment to make a more precise measurement. In 2016 the collaboration began
to construct a new experiment, ALPHA-g, which conducted its first measurements
in the summer and fall of 2022.
The results
published in Nature are based on simulations and a statistical
analysis of what the team observed last year and puts the gravitational
constant for antimatter at 0.75 ± 0.13 ± 0.16 g, or, if you combine the
statistical and systematic errors, 0.75 ± 0.29 g, which is within error bars of
1 g. The team concluded that the chance of gravity being repulsive for
antimatter is so small as to be meaningless.
UC
Berkeley postdoctoral fellow Danielle Hodgkinson, right, running the ALPHA-g
experiment from the control room at CERN in Switzerland. Credit: Joel Fajans,
UC Berkeley
At least a dozen UC Berkeley
undergraduate physics majors participated in the assembly and running of the
experiment, Fajans and Wurtele said, many of them from groups not well
represented in the field of physics.
"It's been a great opportunity
for many Berkeley undergraduates," Fajans said. "They're fun
experiments, and our students learn a lot."
A balance
The plan for ALPHA-g that Wurtele
and Fajans proposed was to confine about 100 antihydrogen atoms at a time in a
25-centimeter-long magnetic bottle. ALPHA can only confine antihydrogen atoms
that have a temperature less than half a degree above absolute zero, or 0.5
Kelvin.
Even at this extremely low
temperature, the antiatoms are moving at speeds averaging 100 meters per
second, bouncing hundreds of times per second off the strong magnetic fields at the ends of the bottle. (The magnetic dipole
moment of an antihydrogen atom is repelled by the pinched 10,000 Gauss magnetic
fields at each end of the bottle.)
If the bottle is oriented
vertically, the atoms moving downward will accelerate due to gravity, while
those moving upward will decelerate. When the magnetic fields at each end are
identical, that is, balanced, those atoms moving downward will have, on
average, more energy. Thus, they will be more likely to escape through the
magnetic mirror and hit the container, annihilating in a flash of light and
producing three to five pions. The pions are detected to determine whether the
antiatom escaped upward or downward.
The experiment is like a standard
balance used to compare very similar weights, Fajans said. The magnetic balance
makes the relatively tiny gravitational force visible in the presence of much
larger magnetic forces, much the same way that a normal balance makes visible
the difference between 1 kilogram and 1.001 kilograms.
The mirror magnetic fields are then
very slowly ramped down, so that all the atoms eventually escape. If antimatter behaves like normal matter, more antiatoms—about
80% of them—should escape out the bottom than the top.
"The balancing allows us to
ignore the fact that the antiatoms are all of different energies," Fajans
said. "The lowest energy ones escape last, but they're still subject to
the balance, and the effect of gravity is enhanced for all antiatoms."
The experimental setup also allows
ALPHA to make the bottom magnetic mirror stronger or weaker than the top
mirror, which gives each antiatom a boost in energy that can cancel or overcome
the effects of gravity, allowing equal or greater numbers of antiatoms to go
out the top than the bottom.
"This gives us a powerful
experimental knob that allows us, basically, to believe the experiment actually
worked because we can prove to ourselves that we can control the experiment in
a predictable manner," Fajans said.
The results had to be treated
statistically because of the many unknowns: The researchers couldn't be certain
how many antihydrogen atoms they'd trapped, they couldn't be sure they detected
every annihilation, they couldn't be sure there were not some unknown magnetic
fields that would have affected the antiatom trajectories, and they couldn't be
sure they'd measured the magnetic field in the bottle correctly.
"ALPHA's computer code
simulating the experiment could be subtly wrong because we don't know the
precise initial conditions of the antihydrogen atoms, it could be wrong because our magnetic fields aren't
correct, and it could be wrong for some unknown unknown," Wurtele said.
"Nonetheless, the control provided by adjusting the balance knob lets us
explore the extent of any discrepancies, giving us confidence that our result
is correct."
The UC Berkeley physicists are
hopeful that upcoming improvements to ALPHA-g and to the computer codes will
improve the instrument's sensitivity by a factor of 100.
"This result is a group
effort, although the genesis of this project was at Berkeley," Fajans
said, "ALPHA was designed for spectroscopy of antihydrogen, not
gravitational measurements of these antiatoms. Jonathan's and my proposal was
completely orthogonal to all the plans for ALPHA, and the research would likely
not have happened without our work and years of lonely development."
And while the null result could be
dismissed as unexciting, the experiment is also an important test of general
relativity, which to date has passed all other tests.
"If you walk down the halls of
this department and ask the physicists, they would all say that this result is
not the least bit surprising. That's the reality," Wurtele said. "But
most of them will also say that the experiment had to be done because you never
can be sure. Physics is an experimental science. You don't want to be the kind
of stupid that you don't do an experiment that explores possibly new physics
because you thought you knew the answer, and then it ends up being something
different."
Undergraduate students who participated
include Josh Clover, Haley Calderon, Mike Davis, Jason Dones, Huws Landsberger,
Nicolas Kalem James McGrievy, Dalila Robledo, Sara Saib, Shawn Shin, Ethan
Ward, Larry Zhao and Dana Zimmer.
Credits: X-ray: NASA/SAO/GSFC/M.
Corcoran et al; HST: NASA/ESA/STScI; Image Processing: NASA/CXC/SAO/L.
Frattare, J. Major, N. Wolk)
A new movie made from over two
decades of data from NASA’s Chandra X-ray Observatory shows a famous star system
changing with time, as described in our latest press release. Eta Carinae contains two
massive stars (one is about 90 times
the mass of the Sun and the other is
believed to be about 30 times the Sun’s mass).
In the middle of the 19th century,
skywatchers observed as Eta Carinae experienced a huge explosion that was
dubbed the “Great Eruption.” During this event, Eta Carinae ejected between 10
and 45 times the mass of the Sun. This material became a dense pair of
spherical clouds of gas, now called the Homunculus nebula, on opposite sides of
the two stars. The Homunculus is clearly seen in a composite image of the Chandra data with
optical light from the Hubble Space Telescope (blue, purple, and white).
A new time-lapse sequence
contains frames of Eta Carinae taken with
Chandra from 1999, 2003, 2009, 2014, and 2020. Astronomers used the Chandra
observations along with data from ESA’s XMM-Newton to watch as the stellar
eruption from about 180 years ago continues to expand into space at speeds up
to 4.5 million miles per hour. The two massive stars produce the blue,
relatively high energy X-ray source in the center of the
ring. They are too close to each other to be seen individually.
A bright ring of X-rays (orange)
around the Homunculus nebula was discovered about 50 years ago and studied in
previous Chandra work. The new movie of Chandra, plus a deep, summed image
generated by adding the data together, reveal important hints about Eta
Carinae’s volatile history. This includes the rapid expansion of the ring, and
a previously-unknown faint shell of X-rays outside it.
This faint X-ray shell is
highlighted in an additional graphic showing the summed image. The image on the
left emphasizes the bright X-ray ring, and the image on the right shows the
same data but emphasizing the faintest X-rays. The shell is located in between
the two contour levels, as labeled.
Credits: NASA/SAO/GSFC/M. Corcoran
et al.
Because the newly discovered outer
X-ray shell has a similar shape and orientation to the Homunculus nebula,
researchers concluded both structures have a common origin. The idea is that
material was blasted away from Eta Carinae well before the 1843 Great Eruption
— sometime between 1200 and 1800, based on the motion of clumps of gas
previously seen in Hubble Space Telescope data. Later this slower material was
lit up in X-rays when the fast blast wave from the Great Eruption tore through
space, colliding with and heating the material to millions of degrees to create
the bright X-ray ring. The blast wave has now traveled beyond the bright ring.
The authors of the paper are
Michael Corcoran (NASA’s Goddard Space Flight Center), Kenji Hamaguchi (GSFC),
Nathan Smith (University of Arizona), Ian Stevens (University of Birmingham,
UK), Anthony Moffat (University of Montreal), Noel Richardson (Embry-Riddle
Aeronautical University), Gerd Weigelt (Max Planck Institute for Radio
Astronomy), David Espinoza-Galeas (The Catholic University of America), Augusto
Damineli (University of Sao Paolo, Brazil), and Christopher Russell (Catholic
University).
NASA's Marshall Space Flight Center
manages the Chandra program. The Smithsonian Astrophysical Observatory's
Chandra X-ray Center controls science operations from Cambridge, Massachusetts,
and flight operations from Burlington, Massachusetts.
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