Follow Slashdot blog updates by subscribing to our blog RSS feed

 


Forgot your password?
Close
typodupeerror
×
Science

There is a 'Gravity Hole' in the Indian Ocean. Scientists Now Think They Know Why (cnn.com) 70

Posted by EditorDavid from the mystery-spot dept.
CNN reports that "There is a 'gravity hole' in the Indian Ocean — a spot where Earth's gravitational pull is weaker, its mass is lower than normal, and the sea level dips by over 328 feet (100 meters)." This anomaly has puzzled geologists for a long time, but now researchers from the Indian Institute of Science in Bengaluru, India, have found what they believe is a credible explanation for its formation: plumes of magma coming from deep inside the planet, much like those that lead to the creation of volcanoes. To come to this hypothesis, the team used supercomputers to simulate how the area could have formed, going as far back as 140 million years. The findings, detailed in a study published recently in the journal Geophysical Research Letters, center around an ancient ocean that no longer exists.

Humans are used to thinking about Earth as a perfect sphere, but that's far from the truth. "The Earth is basically a lumpy potato," said study coauthor Attreyee Ghosh, a geophysicist and associate professor at the Centre for Earth Sciences of the Indian Institute of Science. "So technically it's not a sphere, but what we call an ellipsoid, because as the planet rotates the middle part bulges outward." Our planet is not homogeneous in its density and its properties, with some areas being more dense than others — that affects Earth's surface and its gravity, Ghosh added. "If you pour water on the surface of the Earth, the level that the water takes is called a geoid — and that is controlled by these density differences in the material inside the planet, because they attract the surface in very different ways depending on how much mass there is underneath," she said. The "gravity hole" in the Indian Ocean — officially called the Indian Ocean geoid low — is the lowest point in that geoid and its biggest gravitational anomaly, forming a circular depression that starts just off India's southern tip and covers about 1.2 million square miles (3 million square kilometers).
This discussion has been archived. No new comments can be posted.

There is a 'Gravity Hole' in the Indian Ocean. Scientists Now Think They Know Why More

There is a 'Gravity Hole' in the Indian Ocean. Scientists Now Think They Know Why

Comments Filter:

  • Why doesn't the ocean float up if gravity is less? (Score:4)

    by quenda ( 644621 ) on Tuesday July 25, 2023 @03:17AM (#63712908)

    ... says the 5-year-old in me. How to provide a simple but satisfying explanation for the depression?
    I'm thinking it's like tides. But nobody ever agrees on how tides work.

    • Re:Why doesn't the ocean float up if gravity is le (Score:5, Insightful)

      by NotEmmanuelGoldstein ( 6423622 ) on Tuesday July 25, 2023 @03:35AM (#63712912)

      ... float up if gravity is less?

      It does, then it falls into the gravity well of the surrounding geoid. Thus, normal gravity steals water from this low-gravity spot.

      • ... float up if gravity is less?

        It does, then it falls into the gravity well of the surrounding geoid. Thus, normal gravity steals water from this low-gravity spot.

        "Normal" gravity vs "low" gravity "interacting" in a single spot? Was that supposed to be a joke? Like quenda, I can't picture how water, which seeks its own level and all that, could be drawn closer to the center of the Earth in an area of a lower gravitational field. It's counterintuitive.

        • Re: (Score:3)

          by john83 ( 923470 )
          I think you could picture "finding its own level" to mean that the gravitational force at every point on the surface is the same (i.e. the potential energy due to gravity is the same). If the local gravity in one region is weaker, the water surface needs to be lower to experiece the same gravity as it does at other places. That makes for a dip in the low gravity region.

          • Re:Two independent co-located gravity vectors? (Score:5, Insightful)

            by iMadeGhostzilla ( 1851560 ) on Tuesday July 25, 2023 @06:07AM (#63712996)

            Where would the water level be if the local gravity in the region were stronger? If I imagine cranking up the local gravity volume it seems reasonable that the water would be ... well since it would have nowhere to go, the local region would actually steal extra water from the surrounding areas so the body of water would be thicker near the high gravity local spot. In that case the reverse makes sense as well.

          • You need a physics refresher, A surface of equipotential will be higher in an area of lower field strength.

            I'm still not sure I quite understand it because all my knowledge of how hydrostatic pressure works tells me there should be a bulge in the water level in that region. But if the force of gravity is not just weaker but importantly angled off plumb away from the center of the hole I can see a dip forming.

            • I'm no physicist, but I think you have the right idea, and here's a gedankenexperiment to demonstrate it. Imagine a thick ring of lead with a tank of water resting on top of it so that the ring is near the rim of the tank. The gravity of the lead will pull the water toward the rim, causing a dip in the middle.

              • Re: (Score:2)

                by Chaset ( 552418 )

                That's a good one.
                I was thinking of it this way:
                Think of a pair of hypothetical ultra-dense planets with a very deep ocean. A small solid core is surrounded by a very thick layer of liquid. Now move these two planets closer together. The water between them will merge and will form a figure-8 in cross section. Get to the point where the two cores touch, and you will see that the water distribution is not a sphere, either. There will be a "dent" where the liquid is pulled away to each core. Generalizi

        • Re: (Score:3)

          by ceoyoyo ( 59147 )

          Not at the same spot. The depression is roughly a circle, and there's lower gravity there. There's higher gravity all around. So water is drawn away from the depression. In the depression gravity doesn't point straight at the centre of the Earth, but rather at an angle towards the edge of the depression.

          The force diagram is pretty similar to a glass of water on a turntable. The water piles up at the rim due to centrifugal force in the glass or higher gravity on the rim of the depression. That leaves less wa

      • It does, then it falls into the gravity well of the surrounding geoid. Thus, normal gravity steals water from this low-gravity spot.

        Yeah, because gravity works sideways...

        • It does, then it falls into the gravity well of the surrounding geoid. Thus, normal gravity steals water from this low-gravity spot.

          Yeah, because gravity works sideways...

          Gravity works in all directions...

          • Well, it works on all directions where there's mass. But it's the pressure keeping the water level evened out. Or not, as in this case.
        • The spot directly beneath you accounts for only about half of the total gravitational pull on your body. This is the reason why an object inside of a spherical shell experiences no net gravitational pull from the shell in any direction.

          • Re: (Score:1)

            by jonadab ( 583620 )
            That only works if the object is perfectly centered within the shell. Otherwise, it's pulled more strongly toward whichever side of the object it's closer to, because gravitational force falls off with the square of distance. This is why the first three Lagrange points are unstable.

            • False. It works everywhere inside the shell (note: a shell is hollow).

              There's more stuff farther away, which exactly balances its being farther away.

              I learned this in undergrad math/physics. It was probably discovered by Newton.

        • Re: (Score:1)

          by jonadab ( 583620 )
          The word "sideways" only has meaning once you establish a reference frame, which is pretty much always based on the largest relevant gravitational force being considered "down". Spacetime doesn't naturally have any particular orientation, and so neither does gravity: it just pulls masses towards one another. As Weird Al has written, "My pancreas attracts every other pancreas in the universe with a force proportional to the product of their masses, and inversely proportional to [the square of] the distanc

    • Re: (Score:3)

      by nagora ( 177841 )

      Low gravity can't keep things in place, so the water drifts off to where gravity is high enough to hold it.

      If you imagine high gravity as a deep well, the low spot is a spike sticking up.

    • But nobody ever agrees on how tides work.

      Why are there two tides per day?

      • Why are there two tides per day?

        People imagine the moon orbiting around the Earth, but the moon has a large mass so Earth-Moon is more like a barbell with different size spheres.

        The Earth-Moon system orbits around the center of gravity of the barbell model. The center is still within the Earth, but it's not at the actual center of the Earth.

        The part of Earth on the near side (directly between the center of mass and the moon) gets pulled by the moon, and the point on the far side is "swung around" as the system rotates. The far side experi

      • Re: (Score:2)

        by jonadab ( 583620 )
        > Why are there two tides per day?

        For exactly the same reason that there are any tides at all: water, being liquid, is much less rigid and thus is deformed much more easily than the crust of the Earth as a whole. Earth's crust is not a perfect sphere, but it doesn't change shape as *much* in the short term due to the moon's ever-moving gravitational pull, and so it remains closer to spherical, than the water, which is more easily deformed by the gravity in the short term and thus is more readily elonga
      • Because the moon's gravity pulls on the earth and its water. The earth is solid, so doesn't deform very much. Water, being liquid, deforms quite readily and bulges out (on both sides!) along the line connecting the moon and the earth. As the earth rotates through these bulges, the tide "rises", and as it rotates out, the tide "falls". Trust me, I'm a doctor.

    • Water won't float up until gravity is reduced from 9.8 m/s2 to 0.03 m/s2 (that's the centrifugal acceleration at the equator).

      In this case, gravity is reduced by 0.01%.

    • Re: (Score:1)

      by jonadab ( 583620 )
      You're hearing "gravity is less" and thinking in terms of the gravitational constant of the entire universe being locally reduced, which is not what's going on. The local region just has less mass, and thus has less gravitational pull of its own; but it does not somehow magically block or reduce the gravitational pull of other mass that is located elsewhere (e.g., the rest of the planet).

      We've known for a couple of centuries that the earth's density is not entirely uniform, which leads to some local variat

    • Re: (Score:2)

      by ceoyoyo ( 59147 )

      It's like water in a glass spinning on a turntable. The water is pulled to the side of the glass by centrifugal force and so there's less in the middle. In this depression, the centrifugal force is provided by higher gravity around the rim of the depression.

    • Because there is less gravity at this 'point' then the surrounding areas, the water is pulled laterally to the higher gravity areas.

    • Gravity pulls things. Less gravity means less things.

    • Gravity is the weakest of the classical four forces. The molecular bonds among the molecules of water with their neighbors are going to keep the same amount of water at the same volume without a much larger gravitational change. Think of how when you draw water into a syringe, the negative pressure doesn't make it foam up. So the gravity is weaker meaning there's less water there, but it's not so weak that there's no atmospheric pressure and the water's foaming up and boiling away. Less water, but the wat

    • Re: (Score:2)

      by ras ( 84108 )

      How to provide a simple but satisfying explanation for the depression?

      The water is responding to the forces it can "feel" locally. There is no way for the water to feel the absolute height from the earth's centre, so it isn't related to that.

      The two forces water can feel are the earth spinning (which leads to the sea level being higher from the centre of the earth at the equator), and gravity. Those two forces combine to create a net force on a body we call weight. At sea level this weight is the same.

  • Do things weigh less in this area?

    • Re: (Score:1)

      by Anonymous Coward
      Of course. That's how weight works.
    • Yes, but not your momma. She weighs more.

    • Yes, by a tiny amount. Gravity seems to be about 0.01% weaker in that area.

    • I am struggling to get my head around this, but I suspect it depends on how you go there, if you went there in a ship then presumably you would weigh the same.

      if you went in a plane that used radar to regulate altitude then presumably the same

      if you went in a plane that used GPS to regulate altitude then perhaps less depending on how much general relativity is accounted for

      If you went there in a plane which used air pressure measurement to maintain altitude then I have no idea, perhaps you would weigh even

      • And apparently submarines are already carefully measuring gravity - hence the big reveal in the hunt for red october [slate.com]

        They too have options for determining how they control their depth and hence, presumably, the gravity felt onboard to a small degree

  • something feels misstated (Score:3)

    by smithcl8 ( 738234 ) on Tuesday July 25, 2023 @06:22AM (#63713018)
    Wouldn't the density changes or the mountains and valleys just move the center of mass, and everything would still get pulled at the exact same rate to the center? I mean, if this is just saying "the hole near India is the deepest" then I understand.
    • Or are there actually higher spots where the water will puddle because it's more dense there than in a lower spots nearby?

      • Gravity isn't a point source in the middle of the earth, instead all points of earth have a gravitational pull. So that water is being pulled towards the center due to the average gravity, meaning it's pulled down but also the side. So at the slightly lower gravitational area, the water doesn't float up because that would require a lot of energy to create vacuums or bubbles in the deep ocean and water doesn't easily expand or compress, instead the water is also being pulled to the sides It's only about

    • Re: (Score:1)

      by jonadab ( 583620 )
      > Wouldn't the density changes or the mountains and valleys just move the center of mass,
      > and everything would still get pulled at the exact same rate to the center?

      You're mentally treating Earth's gravity as coming from a single point mass at the center of gravity. That's a very good approximation when you're talking about things like planetary orbits, which operate at enough distance that the whole Earth is in basically the same direction, but it's imprecise when you get up close (like, say, close

    • Wouldn't the density changes or the mountains and valleys just move the center of mass, and everything would still get pulled at the exact same rate to the center?

      From a far enough distance, the center of gravity is averaged out to be the center of the planet. As you get closer, the gravity can no longer be considered as an averaged whole. This is what is being discussed.

  • It's obviously aliens (Score:4, Funny)

    by nospam007 ( 722110 ) * on Tuesday July 25, 2023 @06:43AM (#63713044)

    Their ship's anti-gravity system messes up the measurements.

    • Re: (Score:1)

      by jonadab ( 583620 )
      Nah, they compensate for that, in order to evade detection. (That's why they deployed the anti-gravity tech in the first place. They knew we'd detected Pluto based on its gravity signature, and they didn't want to be next.) This gravitational effect we're seeing now is just a natural consequence of Earth's non-uniform density after all the unobtanium was mined out to support the galactic economy.

  • So⦠shouldnâ(TM)t Elon be talking with the Indian government about turning the southern tip of India into the preeminent spaceport on Earth? Seems to me that launching from +328ft, out across a low-gravity well, reaching Max-Q over top of it, would be beneficial, eh? I wonder how much so, however.
    With Starship/Superheavy, SpaceX is eeking out percentage point increases of max thrust with the newest engines; offsetting gravity to gain a few more percentage points, off the coast of a low-cost,

  • They've finally found the location of the island. It's somewhere in that gravity hole.

  • The image of a lumpy potato is interesting and helps make the point about why gravity varies in different places. But it's all relative. If the earth were scaled to the size of a bowling ball, the earth would be smoother and more precisely spherical.

    The difference between the diameter at the poles vs. the diameter at the equator is about 27 miles, or 1/300 of the diameter. https://education.nationalgeog... [nationalgeographic.org] Scaled to the size of a bowling ball, this difference would be less than 1 mm.

    • Re: (Score:2)

      by ceoyoyo ( 59147 )

      Bowlingball.com says that a 13 lb bowling ball has a diameter between 8.5 and 8.595 inches, and a roundness tolerance of 0.01". That's about 0.1%. The 43 km difference between the Earth's diameter though the poles and equator, divided by the equatorial diameter is about 0.3%, as you mention, so the bowling ball is rounder.

      Earth is considerably smoother than a bowling ball though, even one that hasn't been dinged up in a bowling alley for years.

      • Thanks for correcting my math. Still, the bowling ball comparison is certainly more apt than a potato!

        • Re: (Score:2)

          by ceoyoyo ( 59147 )

          Yes. The article is also about the geoide, which isn't the physical shape of the Earth anyway, so "a lumpy potato" is doubly inaccurate.

  • I bought 100 kilograms of gold in southern India/Sri Lanka but when I got to the US I had 100.011 kilograms of gold which I sold it for profit. Gold is going for $63 a gram so I made $630.

  • I'm confused why this geoid low would cause a lower sea level. Wouldn't weaker gravity pull the ocean water less and allow for a higher sea level? There's obviously some physics I'm not seeing correctly.

Slashdot Top Deals

He has not acquired a fortune; the fortune has acquired him. -- Bion

Close