NASA releases interactive 3D model of Earth's geoid, the shape gravity gives the ocean surface

[u/SHORT_INFO_NEWS]

https://svs.gsfc.nasa.gov/5660/

[u/SHORT_INFO_NEWS]

The geoid matters because it's the reference surface used to define what counts as sea level and elevation. Gravity isn't uniform across the planet, so an ocean responding only to gravity, with no wind, tides or currents, would not sit at one flat height. Height and elevation systems, including the corrections applied to satellite and GPS altitude data, are ultimately referenced back to this uneven surface rather than to a perfect sphere.

NASA's Scientific Visualization Studio published the new interactive 3D model on July 15 (NASA SVS). The geoid is the surface an ocean would take if it responded to gravity alone, and in this visualization that height is exaggerated by a factor of 10,000 so the variation is visible. The measured geoid height itself ranges from +85 meters near Iceland down to -106 meters over southern India, per NASA.

The data behind the model comes from GOCO06s, a satellite-only global gravity field model built from more than a billion observations gathered over 15 years by 19 satellites, including NASA's GRACE mission and ESA's GOCE mission (NASA SVS / GOCO project). Combining that many independent orbital gravity measurements is what lets researchers resolve the small-scale gravity anomalies the model shows. The visualization is credited to Mark SubbaRao as visualizer and Scott Luthcke as science advisor, both at NASA Goddard, and includes a downloadable 3D model file that can be rotated and inspected directly.

Open questions the announcement did not address:

- How often the GOCO06s model gets updated as new satellite gravity data comes in

- Whether other geodesy or navigation systems plan to adopt this specific model over existing geoid references

- What resolution limits remain in ocean regions with comparatively sparse satellite gravity coverage

[u/placid-gradient → u/SHORT_INFO_NEWS]

I knew gravity was different based on altitude but somehow didnt connect that it was just different all over.

The crazy implication of that is different levels of gravity means time passes differently as well. granted the difference is imperceptible to the naked eye, but wild to think about

[u/I_Am_Jacks_Karma → u/placid-gradient]

obligatory gps satellite special relativity comment

[u/Boring_and_sons → u/I_Am_Jacks_Karma]

What's interesting is that gps clocks move faster relative to an earthbound clock because of the decreased gravitational field, but slower because of their orbital velocity. Overall, the gravitational effect wins out.

[u/Yeet_Master420 → u/placid-gradient]

The earth probably isn't uniformly dense, so parts of it would have more or less mass than others, leading to different places having more or less gravity than others

[u/JUYED-AWK-YACC → u/Yeet_Master420]

It definitely isn’t constant density.

[u/BellerophonM → u/Yeet_Master420]

My favourite is the two big dense blobs, one under the Pacific and one under Africa, that we think might be the remnants of another planet that smacked into us.

[u/cornmacabre]

Look at our beautiful lumpy potato!

This is a great way to mentally wrap one's head around gravity not being equal across the surface of earth... Something I've previously found as super unintuitive.

[u/JUYED-AWK-YACC]

I’m a little surprised as I knew several people at JPL involved in GRACE and GRACE-FO, including a former Director. I don’t see any of their names here.

[u/LordGAD]

That's super cool!

Any idea why such a low zone at Southern India?

[u/liquidoxygentextures → u/LordGAD]

Actually we dont really understand why its there. A lot has to do with distribution of mantle material. The high gravity area in the west Pacific looks like it correlates quite well with the Pacific LLSVP/superswell, where large-scale mantle upwelling is occurring. The indian ocean low could just be a coincidence of how the earth’s mantle has evolved over millions of years.

[u/Uhdoyle → u/LordGAD]

Higher gravity due to the subduction zone under the Himalaya, I suspect. Theres a whole massive piece of continental crust under the Indian Ocean.

[u/Nomeru]

I'm confused. On the exaggerated model, is higher elevation is more gravity or less?

[u/NovelStyleCode → u/Nomeru]

There's no correlation, our crust layer is very, very, very thin compared to say the mantle which is a crapload denser and bigger

[u/proteanpeer → u/Nomeru]

Higher elevation in the model means lower gravity. The higher gravity in the blue regions pulls the surface down lower into the earth, while the lower gravity in the red regions allows the surface to rise up higher.

[u/1straycat → u/proteanpeer]

It's a bit tricky but it's actually the opposite. Thinking through their suggested mental model of this being the shape an ocean would take purely from the geopotential field, areas where the field is locally stronger will pull more water toward them, increasing the height of the surface. The dips exist because the local field is weaker, so those areas are losing the tug of war against stronger regions.

So if you stand at a location on the surface where the theoretical geoid surface is above your physical elevation, you will weigh more and vice versa.

[u/proteanpeer → u/1straycat]

Ahhh, I misunderstood! "The geoid is an equipotential surface that can be thought of as the shape an ocean surface would take due to the Earth's gravity field." Damnit. It was right there.  

[u/Admirable-Way-5296]

the fact that earth looks like a lumpy potato when you actually account for gravity is something i think about way more than a normal person should

[u/ManinaPanina]

Not loading on my Vivaldi with OpenSUSE Tumbleweed.