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First direct evidence of ocean mixing across the Gulf Stream –



The “Triaxus” towing platform breaks through the choppy surface of the ocean during a storm. By towing such a platform with monitoring instruments through the water, changing its depth in a ‘yo-yo’ pattern as it traveled, scientists created high-resolution snapshots of how a dye released upstream evolved across the Gulf Stream front. Credit: Craig M. Lee, UW APL

New research provides the first direct evidence for the Gulf Stream blender effect, identifying a new mechanism of mixing water across the swift-moving current. The results have important implications for weather, climate and fisheries because ocean mixing plays a critical role in these processes. The Gulf Stream is one of the largest drivers of climate and biological productivity from Florida to Newfoundland and along the western coast of Europe.

The multi-institutional study led by a University of Maryland researcher revealed that churning along the edges of the Gulf Stream across areas as small as a kilometer could be a leading source of mixing between the waters on either side of the current. The study was published in the Proceedings of the National Academy of Sciences on July 6, 2020.

“This long-standing debate about whether the Gulf Stream acts as a blender or a barrier to ocean mixing has mainly considered big ocean eddies, tens of kilometers to a hundred kilometers across,” said Jacob Wenegrat, an assistant professor in UMD’s Department of Atmospheric and Oceanic Science and the lead author of the study. “What we’re adding to this debate is this new evidence that variability at the kilometer scale seems to be doing a lot of mixing. And those scales are really hard to monitor and model.”

As the Gulf Stream courses its way up the east coast of the U.S. and Canada, it brings warm salty from the tropics into the north Atlantic. But the current also creates an invisible wall of water that divides two distinct ocean regions: the colder, fresher waters along the northern edge of the Gulf Stream that swirl in a counterclockwise direction, and the warmer, saltier waters on the southern edge of the current that circulate in a clockwise direction.

First direct evidence of ocean mixing across the gulf stream
A research crew deployed a float from the R/V Knorr before releasing a fluorescent dye into the water. Scientists then tracked the drift of both dye and float through the Gulf Stream revealing significant mixing of waters across the swift current. Credit: Craig M. Lee, UW APL

How much ocean mixing occurs across the Gulf Stream has been a matter of scientific debate. As a result, ocean models that predict climate, weather and biological productivity have not fully accounted for the contribution of mixing between the two very different types of water on either side of the current.

To conduct the study, the researchers had to take their instruments to the source: the edge of the Gulf Stream. Two teams of scientists aboard two global-class research vessels braved winter storms on the Atlantic Ocean to release a along the northern front of the Gulf Stream and trace its path over the following days.

The first team released the dye along with a float containing an acoustic beacon. Downstream, the second team tracked the float and monitored the concentration of dye along with , salinity, chemistry and other features.

Back on shore, Wenegrat and his coauthors developed high-resolution simulations of the physical processes that could cause the dye to disperse through the water in the manner the field teams recorded. Their results showed that turbulence across areas as small as a kilometer exerted an important influence on the dye’s path and resulted in significant mixing of water properties such as salinity and temperature.

First direct evidence of ocean mixing across the gulf stream
Fluorescent dye provides a unique way to track the evolution and mixing of water across the Gulf Stream. In a recent study fluorescein dye (as pictured here) was released along the north wall of the Gulf Stream, and tracked by ship as it mixed horizontally across the current. Credit: Lance Wills, WHOI

“These results emphasize the role of variability at very small scales that are currently hard to observe using standard methods, such as satellite observations,” Wenegrat said. “Variability at this scale is not currently resolved in global climate models and won’t be for decades to come, so it leads us to wonder, what have we been missing?”

By showing that small-scale mixing across the Gulf Stream may have a significant impact, the new study reveals an important, under-recognized contributor to ocean circulation, biology and potentially climate.

For example, the Gulf Stream plays an important role in what’s known as the ocean biological pump—a system that traps excess carbon dioxide, buffering the planet from global warming. In the surface waters of the Gulf Stream region, ocean mixing influences the growth of phytoplankton—the base of the ocean food web. These phytoplankton absorb carbon dioxide near the surface and later sink to the bottom, taking carbon with them and trapping it in the deep ocean. Current models of the ocean biological pump don’t account for the large effect small-scale mixing across the Gulf Stream could have on phytoplankton growth.

“To make progress on this we need to find ways to quantify these processes on a finer scale using theory, state-of-the-art numerical models and new observational techniques,” Wenegrat said. “We need to be able to understand their impact on large-scale circulation and biogeochemistry of the ocean.”

The research paper, “Enhanced mixing across the gyre boundary at the Gulf Stream front,” Jacob O. Wenegrat, Leif N. Thomas, Miles A. Sundermeyer, John R. Taylor, Eric A. D’Asaro, Jody M. Klymak, R. Kipp Shearman, and Craig M. Lee, was published in the July 6, 2020 issue of the Proceedings of the National Academy of Sciences.

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New opportunities for ocean and climate modelling

More information:
Jacob O. Wenegrat el al., “Enhanced mixing across the gyre boundary at the Gulf Stream front,” PNAS (2020).

First direct evidence of ocean mixing across the Gulf Stream (2020, July 6)
retrieved 6 July 2020

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Ask Ethan: Why Are The Moon And Sun The Same Size In Earth’s Sky? – Forbes



In our Solar System, there’s one overwhelming source of mass that all the planets orbit around: our Sun. Each planet has its own unique system of natural satellites that exist in stable orbits around it: moons. Some moons, like Saturn’s Phoebe or Neptune’s Triton, are captured objects that were once comets, asteroids, or Kuiper belt objects. Others, like Jupiter’s Ganymede or Uranus’s Titania, formed from an accretion disk at the same time the planets of the Solar System formed. But from the surface of Earth, we have just one Moon — likely formed from an ancient, giant impact — and it just so happens to be practically identical in angular size to the Sun. Is that just a wild coincidence, or is there some reason behind this fact? That’s what Brian Meadows wants to know, asking:

“From a scientific point of view, what are the chances that the Moon and the Sun would appear the same size in the sky?”

It’s a great question, and one that still has great uncertainties surrounding it. Here’s what we know so far.

As far as moons of the Solar System go, there are four known ways that they naturally form.

  1. From the initial material that formed the objects of the Solar System; this is where most of the large moons around our gas giant planets come from.
  2. From collisions between a planet and another large body in space that kick up debris, where that material then coalesces into one or more moons around the planet.
  3. From other objects traversing the Solar System that become gravitationally captured by a parent planet.
  4. Or from material in a ring system around a planet that accretes to form a moon all on its own.
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When we examine the moons found in our Solar System, we find strong evidence of all four types.

But three of those types of moons — the ones that form from the initial Solar System material, the ones that get gravitationally captured, and the ones that form from accreted ring systems — are only found around the gas giant worlds in our Solar System. The moons that we find around smaller, terrestrial worlds, including Earth, Mars, and even objects like Pluto, Eris, and Haumea, are all consistent with their moons arising from one source and one source alone: ancient impacts between a large, massive, fast-moving body and the major world itself.

We didn’t always think this was the case, but an enormous suite of evidence now exists to support it. The Apollo missions returned samples of the lunar surface to Earth, where analysis confirmed that the material composing the Moon’s and the Earth’s crust have a common origin. Measurements of the composition and orbital parameters of Mars’s moons not only point to their creation from an impact, but indicate that a third, larger, inner moon was created, and has since fallen back to Mars. And most recently, measurements by New Horizons support a picture that Charon, Pluto’s giant moon (and likely the other, outer moons) all originated from a giant impact as well.

So if you’re asking a question like, “what are the odds that an Earth-like planet would have a Moon that’s approximately the same angular size as the Sun as seen from that same planet,” here are the facts we have to consider.

  • The only way that we know of, so far, to get a moon around a rocky planet like Earth is to have some sort of giant impact in the planet’s past.
  • We’ve only ever detected moons around rocky worlds in our Solar System, never around rocky exoplanets, as the technology to do so isn’t there yet.
  • Of the rocky planets, Mercury and Venus have no moons, Earth has just the one of this “miracle” size, while Mars’s two surviving moons both appear much smaller than the Sun.

And yet, when we consider the parameters of Earth’s moon with respect to how we observe it compared to the Sun, we experience a remarkable set of circumstances that no other known system possesses.

Here on Earth, the Moon orbits our planet in almost exactly the same plane that the Earth rotates on its axis: another piece of evidence that points to our Earth and Moon having a common origin from a giant impact. When the Moon happens to pass directly between the Earth and the Sun, and all three bodies are perfectly aligned, we experience a phenomenon known as a solar eclipse. This is common to all worlds with moons that cross the planet-Sun plane, but Earth and our Moon are unique in a very exciting way.

On Earth, we can experience three different types of solar eclipse with a perfect alignment:

  1. Total solar eclipse — where the Moon appears to entirely block out the disk of the Sun.
  2. Annular solar eclipse — where the Moon fails to block out the Sun’s disk, creating an annulus (or ring) of visible Sun circumscribing the eclipsing Moon.
  3. Hybrid solar eclipse — where the Moon fails to block the entire Sun for a portion of the eclipse, but does successfully block the entire Sun for a different portion.

Earth only experiences all three types of solar eclipse because the Moon, in its elliptical orbit around the Earth, can appear either larger or smaller than the Sun does due to Earth’s elliptical orbit around the Sun. This is no doubt a rarity; neither of Mars’s moons is ever large enough to eclipse the Sun totally, as every eclipse from Mars is annular. Moreover, if Mars did have a third, larger, inner moon at one point, its eclipses would have always been total eclipses; annular or hybrid eclipses would have been impossible.

But there’s another point to consider: these three possibilities weren’t always what Earth experienced, and they won’t always be what Earth experiences, either. The story of our Solar System, as best as we can reconstruct it, tells a tale of an ever-changing relationship between the Earth, Moon, and the Sun. It began some 4.5 billion years ago, where our ancient protoplanetary disk, which gave rise to all the planets, began to fragment into clumps that grew, interacted, and both merged and ejected one another. There were two types of survivors: large, massive planets that held onto hydrogen and helium envelopes, and smaller, less-decisive victors, which become planets and dwarf planets.

These early planets, planetoids, and planetesimals interact and sometimes collide, and those collisions — when they occur — tend to kick up large amounts of debris that surround the major planet. This shroud of post-impact material around the planet is known as a synestia, and although it’s short-lived, it’s incredibly important. Most of that material winds up falling back to the parent planet, while the rest coalesces into one or more moons. In general, the innermost moon will be the largest and most massive, and then you’ll have smaller, less massive moons that can exist at greater distances.

These moons exert differential forces on the planet: they gravitationally attract the portion of the planet that’s closer to the moon with a greater force than the portion that’s farther away. This not only creates tides on the planet, but it also results in what we call tidal braking, which causes the main planet to slow its rotation and the moon(s) to spiral away from the planet. Of course, there’s a competing effect: the planet’s atmosphere can create a drag force on the moon, drawing it closer to the planet. Depending on how the moons initially form, either effect can win.

In the case of Mars, the drag force appears to have won, drawing the innermost moon in; over time, the next moon, Phobos, will eventually fall back onto Mars as well. In the case of Pluto, tidal braking is complete, and the Pluto-Charon system is now a binary planet, where Pluto and Charon are both tidally locked to one another, surrounded by four additional, outer, smaller moons.

But the Earth-Moon system is fascinating. The current thought is that, early on, the Moon was very close to Earth, and there may have been a number of smaller, outer moons beyond our own. Earth, back more than 4 billion years ago, may have been rotating incredibly rapidly, completing a 360° rotation in just 6-to-8 hours. A year, back in Earth’s early history, may have had as many as 1500 “days” in it.

But over time, the tidal friction of the Moon slowed that rotation tremendously, an act which transfers angular momentum from the spinning Earth to the orbit of the Moon. Over time, this causes the Moon to spiral away from the Earth.

For billions of years, until only a few hundred million years ago, all of the solar eclipses on Earth were total eclipses; the Moon was close enough that it always blocked out the Sun from our perspective. In 570 million years, Earth will experience its final total solar eclipse, and in another 80 million years, its final hybrid solar eclipse. After that, all of Earth’s solar eclipses will be annular.

This means that when we look from Earth at the Moon today, and compare its angular size to that of the Sun today, we see three different types of solar eclipses, but that this is a temporary situation. The evidence indicates that, early on, the Moon was much larger in angular size than the Sun was, and that there may have been additional moons farther out. Over time, our Moon has spiraled away, and if there were smaller, more distant moons, they’ve been ejected. In the far future, the Moon will spiral out even farther, and will become eternally smaller in our sky than the Sun will ever be, for the remainder of its lifetime.

When you ask the question, “what are the odds that an Earth-like planet will have a Moon that’s comparable in angular size to the Sun,” you’re really asking what the odds are of:

  • having an Earth-like planet, which is an Earth-sized planet at the right distance from its star for liquid water on its surface,
  • that experienced a giant impact in its early history, creating a synestia,
  • where the planet itself winds up rotating rapidly after that collision,
  • where a large, inner moon gets created but won’t fall back onto the planet,
  • and then spirals away as angular momentum gets transferred from the planet to the Moon.

It’s remarkable that science, despite only having information about moons around terrestrial planets in our Solar System alone, has uncovered the ingredients necessary to create the situation we have today. If you assume you get an Earth-like planet, our best estimates have enormous uncertainties, but may lead to a total probability in the range of around 1-10%. To really know the answer to this question, however, we’ll need more and better data, and for that, we’ll need to wait for the next generation of astronomical observatories.

The answers are out there, written on the face of the Universe itself. If we want to find them, all we have to do is look.

Send in your Ask Ethan questions to startswithabang at gmail dot com!

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The 2020 Perseid meteor shower is still going strong: How to watch the show – MSN Money



It’s mid-August, which means the annual Perseid meteor shower is active, and will be until Aug. 24. The Perseids are one of the best, brightest batches of shooting stars, and it feels like we could really use them now to add a bit of wonder and distraction into these pretty dismal times we’re living through.  

© Provided by CNET
Some 2019 Perseids, as seen from Macedonia. Stojanovski

This famous shower comes around this time every year as the Earth drifts through a debris cloud left behind by the giant comet 109P/Swift-Tuttle. Bits of dust, pebbles and other cosmic detritus slam into our atmosphere, burning up into brief, bright streaks and even the occasional full-blown fireball streaking across the night sky. 

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Some 2019 Perseids, as seen from Macedonia.

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a sky view looking up at night: Some 2019 Perseids, as seen from Macedonia.

© Stojanovski

Some 2019 Perseids, as seen from Macedonia.

Technically, the 2020 Perseids peaked on the evening of Tuesday, Aug. 11 and morning of Wednesday, Aug. 12, but that doesn’t mean the show is over. Far from it, actually. 

The popularity of the shower is a combination of the fact that it’s one of the strongest, with up to 100 visible meteors per hour on average, and it’s coinciding with warm summer nights in the northern hemisphere. The waning moon is likely to wash out many otherwise visible meteors, but that still leaves plenty that should be easy to see if you do a little planning. 

In general, a good strategy is to head out to look for the Perseids as late in the evening as possible, but still before moonrise at your location. (You can look up sunset and moonrise for your location with a site like


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You can also try to block out the moon by situating yourself next to a building, tree or something else that keeps some of that moonlight out of your retinas. 

The moon will begin to totally disappear after mid-month, and although the Perseids will be past their prime, they will still be active and visible. This shower at half-peak with totally dark skies could be about the same as full peak with a bright moon, so don’t think you must go out on the peak night to catch it. 

Once you’ve decided on the perfect time and a place with minimal light interference and a wide view of the sky, just lie back, let your eyes adjust and relax. Pillows, blankets, lounge chairs and refreshments make for the ideal experience. It can take about 20 minutes for your eyes to adjust to the dark, so be sure to be patient. If you follow all my advice, you’re all but guaranteed to see a meteor. 

It doesn’t really matter where in the sky you look, so long as you have a broad view. That said, the Perseids will appear to radiate out from the constellation of Perseus, the Hero. If you want to practice to be an advanced meteor spotter, locate Perseus and try focusing there while you watch. Then try just looking up without focusing anywhere. See if you notice a difference. We’re still dealing with the unpredictability of nature, so results will vary. 

Arguably the best part of the Perseids each year are the gorgeous photos we get from talented astrophotographers spending long nights outside.

As always, if you capture any beauties yourself, please share them with me on Twitter or Instagram @EricCMack

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Astronomers kill all the fun, blame dust for Betelgeuse’s dimming – Ars Technica



Enlarge / This image is a colour composite made from exposures from the Digitized Sky Survey 2 (DSS2). It shows the area around the red supergiant star Betelgeuse.

Betelgeuse is one of the closest massive stars to Earth. It’s also an old star, and it has reached the stage when it glows a dull red and expands, with the hot core only having a tenuous gravitational grip on its outer layers. This combination means that we’re actually able to resolve different areas on the star’s surface, despite the fact that it resides over 700 light years away.

That ability came in handy late last year when Betelgeuse did something unusual: it dimmed so much that the difference was visible to the naked eye. Telescopes pointed at the giant were able to determine that—rather than a tidy, uniform drop in luminance—Betelgeuse’s dimming was unevenly distributed, giving the star an odd, squished shape when viewed from Earth. That raised lots of questions about what was going on with the giant, with some experts speculating that, because of Betelgeuse’s size and advanced age, the strange behavior was a sign of a supernova in the making.

Now, an international team of international observers is here to throw cold water on the possible explosion. Said researchers were lucky to have the Hubble pointed at Betelgeuse before, during, and after the dimming event. Combined with some timely ground observations, this data indicates a rather mundane reason for the star getting darker: a big burp that formed a cloud of dust near the star.

Stellar indigestion

There’s a couple of things going on at the same time with Betelgeuse. One is that the star has something akin to a heartbeat, albeit an extremely slow and irregular one. Over time, the star cycles through periods when its surface expands and then contracts. One of these cycles is fairly regular, taking a bit over five years to complete. Layered on that is a shorter, more irregular cycle that takes anywhere from under a year to 1.5 years to complete.

We can track these cycles because the surface of the star moves toward or away from us, depending on whether it’s in the expansion or contraction phases. This motion imparts a Doppler shift to the light coming to us from Betelgeuse’s surface, and it causes some features in the star’s light to shift to the red or blue end of the spectrum depending on whether it’s contracting or expanding, respectively.

While they’re easy to track with ground-based telescopes, these shifts don’t cause the sort of radical changes in the star’s light that would account for the changes seen during the dimming event.

But these shifts aren’t the only things going on with Betelgeuse. As we mentioned earlier, the star is a huge, bloated structure, with all the convection and magnetic field activity that we see driving behavior on other stars. These can drive unusual behavior, and that’s just the sort of thing that Hubble was being used to track. The researchers behind the new work had gotten time on the telescope to do observations in the UV area of the spectrum, which struggles to get through Earth’s atmosphere.

These UV observations showed that the photosphere—an outer layer of the star that produces the light we see—started accelerating outward in advance of the dimming event. By the time this acceleration reached its peak, the photosphere was moving at about seven kilometers a second. Critically, this peak came somewhat before the dimming, and the outward push had reversed by the time the dimming became dramatic. Also notable was the fact that the indications of this acceleration weren’t evenly distributed across the star; only part of the star’s surface experienced the sudden expansion.

The big burp

Using all these observations, the researchers put together a picture of what was going on at Betelgeuse leading up to its dimming event. Things started with the star expanding as part of one of its usual cycles. Layered on top of that, however, a portion of Betelgeuse’s surface erupted at speeds that simply aren’t possible due to the cyclical expansion. The researchers suggest that this is the product of a convection cell in the star’s interior reaching its surface.

The result of the combination of these two events is that a lot of material got pushed far enough out from the star that it could cool down, allowing the star to form dust. This dust then accounts for the dimming. The timing makes sense, given that the dimming is delayed relative to the changes in UV light seen by the Hubble.

Fortunately (or unfortunately), all that means is that the strange behavior of Betelgeuse can’t be ascribed to an impending supernova—or aliens, or any other fascinating-but-implausible explanations. But our ability to watch these events from a (astronomically speaking) close distance may help us understand the behavior of other far-off stars, which isn’t a bad thing.

The arXiv. Abstract number: 2008.04945  (About the arXiv). To be published in The Astrophysical Journal.

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