Giant 'Gravity Hole' in the Ocean May Be the Ghost of an Ancient Sea | Hacker News Reader

Giant 'Gravity Hole' in the Ocean May Be the Ghost of an Ancient Sea

scientificamerican.com

128 points_Microft69 comments

Talking about the Indian Ocean, and the first image in the article shows the Atlantic. This is not the Scientific American of my youth.

Further below they show a plain 2D graph with the Indian Ocean dip, but why pick the wrong version of the “Potsdam Potato” (fancy 3D picture at the beginning) when one showing exactly what they are talking about exists in a press release from the European Geosciences Union they link to in their own article?

“The Indian Ocean Geoid Low at a plume-slab overpass” https://blogs.egu.eu/divisions/gd/2021/02/24/the-indian-ocea...

Fascinating topic, but it was as if i was reading a high school report on the subject.

This, so sadly this. The SciAm I subscribed to in the 70's and 80's is long since dead and gone. Back then, articles were routinely dense, tough long reads, with excellent editing and fact-checking. Now...it's filler fluff, and I'd trust Wikipedia more for facts.

Unfortunately, no one buys subscriptions any more, and advertisers have moved to Facebook and Google.

The ad-based internet has conditioned people to expect free content, and the internet ad model incentivizes click-bait low quality content because it's easy to publish and provides a place to hang ads from.

Yeah...though SciAm seemed to be in a long, slow decline for a decade+ before "free with ads on the web" was really a thing. And (that I've found) there is nothing comparable (to the old SciAm) that is free on the web.

Did the demographic that was willing to think hard & long enough to enjoy the old SciAm gradually die off? Or did they just replace eating paid-for healthy brain food with eating free junk food.

(FWIW - I pretty much dropped all the "premium" subscriptions I once had - all dead-tree editions - between ~2000 and ~2010, as either their quality went to crap, or they ceased publication.)

I don't think it's died off, rather it's found in more independent blogs/newsletters at this point. On one hand, the information is generally free and more accessible to a wider global audience, but maybe that exposure is biased towards scientists that are themselves strong writers. Whereas in a world where high-quality science writers are funded, the researcher doesn't have to master this skill to have their work effectively popularized.

> Did the demographic that was willing to think hard & long enough to enjoy the old SciAm gradually die off?

That demographic were always a minority of the general population and I think they still exist. Maybe at one point Scientific American had some pride and catered to this niche because they thought it was socially constructive or something, but clearly they're now gearing towards a more... mass audience.. to get more money of course.

I don't know how else you'd explain an image of the wrong ocean. If they couldn't get the right image, why include an image at all? Of course pictures are appealing for mass audiences, a substantial fraction of whom are barely literate and need pretty pictures (even if irrelevant) to encourage them along.

I think that demographic is now here. I'm mean on HN. (If they find it.)

For me it was when they formally killed off the amateur scientist column. It had itself been in decline for a decade or more before that, but some of their old articles were really excelent. The replacement these days seems to be educational youtubers.

Happening all over. National Geographic just laid off their last staff writers yesterday. https://www.washingtonpost.com/media/2023/06/28/national-geo...

That’s super sad. I have a lifetime subscription to the magazine my grandfather got for me when I was seven.

BBC Science Focus isn’t bad.

The degradation goes back much further than the 1970s, I’m afraid. I’ve got Scientific American volumes from the 1860s and they were incredible back then.

Letters that formed conversations discussing wild theories about comets that lasted for months, sometimes years. Some guy wrote in about his discovery of heat treating asphalt to make it more reliable. Another wrote in about how machine guns were useless in his experience in the Civil War and how the Europeans were about to repeat the same mistake in whatever war was just starting. Lots of ads for with saw blade drawings for Emerson's patent movable teeth made by the American Saw Company.

The prints were gorgeous [1] and the ads, despite covering a significant portion of each issue, were mostly text. Fascinating from a historical perspective but easy to ignore when reading for the rest of the material.

[1] https://imgur.com/a/vF00pdX

This kind of content still exists in print, it just isn't mass marketed. Physics today for instance is excellent and written for practicing physicists at an advanced undergrad level. It's also online:

https://pubs.aip.org/physicstoday

Hey, they were the winner of the 2018 Best Blog Post, so the depth of what that says is as deep as the TFA subject itself

"when one showing exactly what they are talking about exists in a press release from the European Geosciences Union they link to in their own article?"

Cus you can't just jack photos without permission ?

The implication is that you would ask the owners for permission to reproduce the image for the article.

> The implication is that you would ask the owners for permission to reproduce the image for the article.

Indeed. The origin of the image is the German Research Centre for Geosciences (GFZ Potsdam, http://icgem.gfz-potsdam.de/home) which has approved use of a very similar render under Creative Commons Attribution (cc-by) as noted in its wikipedia page: https://en.wikipedia.org/wiki/File:Geoid_undulation_10k_scal...

The source of this picture is their interactive renderer: http://icgem.gfz-potsdam.de/vis3d/longtime

The author of the blog post from the European Geosciences Union was at the time a postdoc at GFZ Potsdam.

The top pictures of major news site' articles often come from a DB / library as fallback, where they try to add something relevant, but it's not always directly related to the article... Because not all articles come with content that have usable pictures. But every article still needs one for UI purposes so they sometimes use filler. Old magazines often were just text and didn't have this requirement.

The one they chose seems to be relevant still.

> Why don't they use the picture from the third party source?

Licensing fees

How much did you expect from an article with “ghost” in the title?

Here's a useful "start here" link:

https://en.wikipedia.org/wiki/Geoid

an important concept, the invisible "Geoid" is an equipotential surface of equal gravitational strength .. call it the " 1 G surface ".

Where there is a large amount of dense mass ( a giant iron deposit | all other things being equal ) the immediate gravitational field is stronger .. it's the usual surface 1G PLUS the extra gravity from the mass of the iron .. therefore the constant 1G equipotential surface is much higher 'above' the mean ellipsoid surface.

There's a lot going on here to get your head around - it generally takes a few WTF?!? iterations to fully grasp the various geophysical surfaces ( the WGS84 ellipsoid (and others), the various 'standard' geoids, the (diurnal flucuating) magnetic topography, etc ).

" a GPS receiver on a ship may, during the course of a long voyage, indicate height variations, even though the ship will always be at sea level (neglecting the effects of tides). That is because GPS satellites, orbiting about the center of gravity of the Earth, can measure heights only relative to a geocentric reference ellipsoid. To obtain one's orthometric height, a raw GPS reading must be corrected. Conversely, height determined by spirit leveling from a tide gauge, as in traditional land surveying, is closer to orthometric height. Modern GPS receivers have a grid implemented in their software by which they obtain, from the current position, the height of the geoid (e.g. the EGM-96 geoid) over the World Geodetic System (WGS) ellipsoid. They are then able to correct the height above the WGS ellipsoid to the height above the EGM96 geoid. When height is not zero on a ship, the discrepancy is due to other factors such as ocean tides, atmospheric pressure (meteorological effects), local sea surface topography and measurement uncertainties."

I would think if one needed to measure the altitude of their ship, things have already gone very, very sideways at this point.

Hmmm.

It might be worth thinking a bit harder then.

Can you think of any reason why the current displacement above or below the regional mean sea level might be of crucial and pressing interest to maritime navigation?

If you can't tell your ship is sinking because of water alarms going off in your hold or the boat is visibly sinking, then what is GPS going to do for you, exactly?

Either everyone on the ship is dead, and the entire electrical system except the GPS is shot, or aliens are lifting your ship with a tractor beam. All situations where things have gone very, very sideways. Wouldn't you say?

Sigh.

Have you ever been in a boat or gone fishing?

Think harder .. why would there be interest in the surface level of water rising and falling (and taking all boats and bouys with it)?

This is not a difficult question.

I’m not doing your homework assignment. Get to the point.

You're apparently fixated on ships sinking or rising into the air when you discuss "altitude".

You've apparently not thought much on geoids, ellipsoids, the meaning of "sea level", geodesy, etc.

You might want to reflect upon what "altitude" means and upon why it's been commonplace for centuries to measure the 'altitude' of things that float.

Should you do so you'll see your comments above in a different light.

You're still bloviating.

How many other people have read this and not learned anything? If you're going to teach people, or disagree with them, at least state your premise instead of trying to sound like Yoda.

and https://en.wikipedia.org/wiki/World_Geodetic_System

in particular WGS-84

From what I remember there's also a 'hill' in the Pacific.

According to the image caption this article seems to be based on ESA's GOCE mission (2009-2013)[0].

"Detailed analysis of GOCE's thruster and accelerometer data serendipitously revealed that it had detected the infrasound waves generated by the 2011 Tōhoku earthquake (whereupon it inadvertently became the first seismograph in orbit)." [0][1]

There's also NASA's GRACE mission (2002-2017) which has been relaunched (GRACE-FO, 2018) with almost identical hardware [2]: "GRACE measures time variations while GOCE measures the static gravity field." [3]

[0] https://en.wikipedia.org/wiki/Gravity_Field_and_Steady-State...

[1] GOCE: the first seismometer in orbit (2013) https://www.esa.int/Applications/Observing_the_Earth/FutureE...

[2] https://en.wikipedia.org/wiki/GRACE_and_GRACE-FO

[3] GRACE & GOCE (2008) https://www.science20.com/planetbye/grace_goce

(Edit: fixed link)

why is the sea level lower where the gravity is lower? surely more gravity would create more "pull"? confused by how this works...

Maybe more gravity in other places creates more pull there and pulls the water away from places with less gravity? Similar to how the gravity of the moon pulls at the water, creating high tide? But that's just my guess, I'm not an expert either...

(From the article) "As a result of the low pull of gravity there, combined with the higher gravitational pull from the surrounding areas, the sea level of the Indian Ocean over the hole is a whopping 106 meters lower than the global average"

Probably like iron filings and magnets, the stronger magnet would have a bigger pile of iron filings on it.

Water isn't compressible, so the distribution would always give the stronger region more water.

Water is dense and gravity is proportional to mass, I wonder if there's any chance that water itself is also a contributor to the perceived gravity at that region.

I don't think the mass of the water would have a very noticeable effect on the perceived gravity. The average depth of the ocean is about 4 km, whereas the radius of the Earth is 6,000 km, and stone is denser than water as well.

> more gravity would create more "pull"?

Exactly. There is more mass below the ocean in the areas around where the hole developed and less mass below the hole. That mass is pulling water from the hole so it became a hole. By the way, less water in the hole and more around it further contribute to making the hole deep because water also have a mass. It doesn't go on forever up to draining the center of the hole dry, there is an equilibrium. Calculating the sea levels could be an unusual physics problem for students.

gravity's impact on liquids is much more defined by them being pulled 'sideways' than 'up' or 'down'. For example, the tides are not mostly caused by the water being lighter or heavier due to the tidal forces of the moon and being pulled down by a greater or lesser degree, but by the sideways pull of the water on the 'side' of the earth causing it to bunch up where the sideways pulls converge (which is why you get tides with bodies of water as big as the oceans but not so much with lakes). The same thing happens here: stronger gravity elsewhere pulls the water sideways away from the low-gravity area, causing the depression (in fact, absent dynamic effects and other forces like wind, the surface of a liquid will be an isograv: every point at its surface will have the same gravitational potential).

This is similar to recent discussions here on HN around the effects of melting ice (Antarctica, Greenland) on sea level. Not only does the meltwater itself cause increase, but the gravitational pull of so much ice pulls the water level around it up higher, resulting in lower sea level everywhere else. When the ice melts, that local gravity lessens, letting the surrounding elevated water spread out to the rest of the ocearn.

interestingly the entire country of Maldives falls inside the marked area, a country with islands 2m above sea level.

> A vast expanse of the Indian Ocean is a staggering 100 meters lower than the global average sea level because of a major dip in Earth’s gravity.

who would write such a gibberish in a scientific magazine?

Scientific American mostly ceased being scientific in the latter 90's, and should be more correctly named Political American.

i wouldn't mind this gibberish if the article would at least make clear afterwards what this is supposed to mean. but i can't find any explanation of the gravitational dynamics at play or even a description how exactly this is measured. 100m relative to what? i mean isn't sea level always 0m by definition? even on that "dip" sea level is 0m. so there is some assumed distance between sea surface and something else. an ideal circumference around the equator with the same volume (or technically area) of air under it as water over it? like an x-axis going through a sinus curve?

one might argue that sth like that shouldn't show up on the hn front page. but maybe the meta message and discussion of the quality of sa journalism is worth the +31 points.

> 100m relative to what?

You quoted it yourself:

"the sea level of the Indian Ocean over the hole is a whopping 106 meters lower than the global average"

We already know that sea level isn't a uniform height. If it were tides wouldn't be a thing. The various ocean's waters are going to be at different levels.

> i mean isn't sea level always 0m by definition?

You could make the same argument for ground level but we know that's not uniform.

... Finally, proof of flat-earth! Everywhere on land is 0cm above the land-level! Checkmate, scientists.

the literalist faction of the flat earth movement.

The Earth has a center of Gravity and they are using satellites to measure it so...

https://en.wikipedia.org/wiki/World_Geodetic_System

A large language model trained by…

Or even a small model. AI generated news was a thing well before anyone heard of transformer models.

Wikipedia gives this, from 2012, as the citation for the claim "2006, Thomson Reuters announced their switch to automation to generate financial news stories on its online news platform":

https://www.tandfonline.com/doi/abs/10.1080/17512786.2012.66...

I don't get it- why is this gibberish?

(other than perhaps being counter-intuitive- you would expect low gravity to cause high sea level)

Surely, you're right. Any geologists care to comment?

Water is incompressible and also attracted to higher gravity. Low gravity zones don’t pull in as much water towards them so the water level is lower near them.

It makes sense to me. I’m assuming in this context sea level is distance from the center of the earth.

Distance from ellipsoid surface (eg WGS84 (or closely related model) ) - the earth isn't spherical, the poles are ~ 20km closer to the earth's centre than the mean equatorial radius.

Yeah i read that and I immediately thought BS. The most the tide varies on earth is less 15 meters.

It's in relation to a theoretical ellipsoid shape.

"The geoid can be as low as 106 meters (350 feet) below the ellipsoid or as high as 85 meters (280 feet) above."[1]

[1] https://www2.csr.utexas.edu/grace/gravity/gravity_definition...

Yes. The polar and equatorial diameter of the the earth differ by over 20 miles, but along a latitude line the earth is quite circular.

> quite circular

In this case it’s 106m away from a perfect circle. Almost nothing compared to 20km, but a real thing.

It is not gibberish if you studied physics at school. Water surface remains on the same level of potential energy of gravitational field, othewise it won’t be the state with minimum energy. This means that if the field has complex form, the water surface won’t be ellipsoid. If the pull is stronger, you can expect a bulge, if the pull is weaker, there will be a hole and the depth of that hole can be bigger than the tidal amplitude, because why not.

EDIT: also, in case of Indian Ocean, it is not a hole, just slightly different curvature.

Does anyone know how much impact this has on weight at the highest vs lowest gravity point?

And would it make economical sense to build rocket launch pads at the lowest points?

Not appreciably (the delta is less than 1%, see my response to OP in this thread), but since lowest gravitational acceleration happen to coincide roughly with areas of maximal rotational tangential velocity, that is, of the Earth's rotation about its axis, of roughly 1,000 mph, the optimal siting of rocket launch facilities tends to be near the Earth's equator, where gravitational acceleration itself tends to be lower, and near where the lowest measured accelerations are found.

Other factors would tend to dominate launch location choice, including particularly areas of open water east of the launch pad, useful both for deorbiting stages and in the event of any RUD[1] events in early-boost phase.

________________________________

Notes:

1. See <https://space.stackexchange.com/questions/10022/who-coined-t...>

Interesting question.

This article discusses identifying regions of gravitational variance, and the abstract discusses where gravitational pull at Earth's surface is highest (near the North Pole) and lowest ("at the top of the Huascaran mountain in the South American Andes"), but not by how much ... The project measures local gravitational acceleration across the surface of the Earth at 200m resolution.

<https://www.sciencedaily.com/releases/2013/09/130904105345.h...>

And the linked field map doesn't give specific measurements either. (Archive as original is 404):

<https://web.archive.org/web/20160309210505/http://geodesy.cu...>

The linked PDF article however does give values, on page 5:

<https://web.archive.org/web/20160307144931/http://ddfe.curti...>

Minimum: 9.76392 m * s^-2

Maximum: 9.83366 m * s^-2

Given a 100 kg human (somewhat heavier than typical, but simpler to compute), the difference would amount to a difference of 6.974 newton, or 1.567 pound-force. Or a delta of about 0.712%.

If you were on a boat at water level, there wouldn’t be any weight difference between the two points. The water level follows equipotential lines of gravity.

> As a result of the low pull of gravity there, combined with the higher gravitational pull from the surrounding areas, the sea level of the Indian Ocean over the hole is a whopping 106 meters lower than the global average"

This feels highly suspect. So any area below the average is because of this same condition? That doesn't seem likely. And if so, then check other area now that there's a theory.

The world is a (fairly) big place. Using average as a guide feels cherry pick-y or at the very least random.

Absent any other external forces, liquids (even non-uniform ones like the ocean) will form an isograv, where every part of their surface is at an equal gravitational potential. This is a natural consequence of how pressure works. So you expect this kind of deviation from gravitational anomaly. And while other forces can have a significant impact, there's not a lot else which can cause a persistent change in the average of this size. Dynamic effects will just to make things oscillate around the average but not chang the average much if at all, and even an extremely heavy prevailing wind (rare at the surface) won't be enough to cause much of a change in sea level.

(ok, there is one really big thing left off of this which is important with respect to what 'average sea level' means: the largest other persistent effect is the centrifugal effects from the spinning of the earth, which results in the bulging at the equator (both for the rock itself, which at planet scales and timelines is approximately liquid, and for the oceans). This is way larger: the oceans are actually about 7km higher near the equator than you would expect if the earth was a sphere. The average they are comparing to here is coming from measuring sea level with respect to the shape it would be if the earth were just a ball of liquid with the same mass and spin. There's a bunch of different approximations which get used here and the details get really messy, but the next step after taking into account the centrifugal effect is basically various levels of approximation of the distribution of gravity: https://en.wikipedia.org/wiki/Geoid)

Ok. Now I get it. Sorry.

Thanks for explaining.

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