<p>Yes, there are known mass anomalies on Ganymede.</p>
<p>From <em><a href="https://www.jpl.nasa.gov/news/scientists-discover-ganymede-has-a-lumpy-interior" rel="nofollow noreferrer">Scientists Discover Ganymede has a Lumpy Interior</a></em>  Aug. 13, 2004</p>
<blockquote>
<p>Scientists have discovered irregular lumps beneath the icy surface of Jupiter's largest moon, Ganymede. These irregular masses may be rock formations, supported by Ganymede's icy shell for billions of years. This discovery comes nearly a year after the orchestrated demise of NASA's Galileo spacecraft into Jupiter's atmosphere and more than seven years after the data were collected.</p>
</blockquote>
<p><a href="https://i.sstatic.net/sdNW0.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/sdNW0.jpg" alt="Ganymede mascons" /></a></p>
<p>Unfortunately, the paper is paywalled, but here's the abstract.</p>
<h3>Discovery of Mass Anomalies on Ganymede</h3>
<p>JOHN D. ANDERSON , GERALD SCHUBERT, AND JENNIFER L. PALGUTA <a href="https://doi.org/10.1126/science.1099050" rel="nofollow noreferrer">https://doi.org/10.1126/science.1099050</a></p>
<blockquote>
<p>We present the discovery of mass anomalies on Ganymede, Jupiter's third and largest Galilean satellite. This discovery is surprising for such a large icy satellite. We used the radio Doppler data generated with the Galileo spacecraft during its second encounter with Ganymede on 6 September 1996 to model the mass anomalies. Two surface mass anomalies, one a positive mass at high latitude and the other a negative mass at low latitude, can explain the data. There are no obvious geological features that can be identified with the anomalies.</p>
</blockquote>
<p>These mascons should not be a problem for JUICE, which is carrying plenty of fuel for the manoeuvres required to visit its targets.</p>
<hr />
<p>Here's some more recent information on Ganymede's gravitational field, from <em>Geophysical Research Letters</em>, <a href="https://astronomy.stackexchange.com/questions/55495/are-there-mascons-on-ganymede-similar-to-the-ones-on-the-moon-and-if-so-how-wi/55496#comment125936_55496">found by <em>planetmaker</em></a>.</p>
<p><a href="https://doi.org/10.1029/2022GL099475" rel="nofollow noreferrer">Gravity Field of Ganymede After the Juno Extended Mission</a>, 12 December 2022, by L. Gomez Casajus, A. I. Ermakov, M. Zannoni, J. T. Keane, D. Stevenson, D. R. Buccino, D. Durante, M. Parisi, R. S. Park, P. Tortora, S. J. Bolton</p>
<blockquote>
<p>On 7 June 2021, Juno performed the first close flyby of Ganymede, the largest satellite of Jupiter (and the largest moon in the Solar System), since the end of the Galileo mission. The gravity field of Ganymede was reconstructed using the radio tracking data from all of the Ganymede encounters of both the Galileo and Juno missions. The data analysis hints at localized gravity anomalies. Interpretation of the gravity data suggests a slightly higher moment of inertia with respect previous publications, indicating a lesser degree of differentiation.</p>
</blockquote>
<p><a href="https://i.sstatic.net/FV49D.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/FV49D.jpg" alt="Ganymede gravity, from Galileo &amp; Juno" /></a></p>
<blockquote>
<p>The gravity field of Ganymede derived from joint analysis of Galileo and Juno flyby data, computed on the reference sphere with radius 2,631.2 km. (a) The full gravity field solution, expanded to degree/order 5. Spacecraft ground tracks are shown in green (Galileo) and yellow (Juno). (b) gravity field solution, expanded to degree/order 5, removing completely the J2 and C22 terms. In b, the opacity of the gravity map is scaled by the signal-to-noise ratio. The white contours enclose regions with signal-to-noise ratio greater than 1 (dashed line contour) and 3 (solid line contour). All maps are in Mollweide projection, centered on the sub-Jupiter point. Latitude and longitude grid lines are in 30° intervals. Maps are draped over a color image mosaic of Ganymede (Collins et al., 2013).</p>
</blockquote>
<p>The acceleration unit used in these figures is the <a href="https://en.wikipedia.org/wiki/Gal_(unit)" rel="nofollow noreferrer">Gal</a>, which is equal to <span class="math-container">$1\,\mathrm{cm/s^2}$</span>.</p>
<p>As well as the having the benefit of additional data of higher precision, this study used more powerful and sophisticated data analysis techniques than the study from 2004.</p>

CC BY-SA 4.0
2023-12-09T14:25:06.587Z
Wow, a single flyby - I'm curious if they say that these are the only major mascons, or just that there is at least two. btw [Science Objectives of the Ganymede Laser Altimeter (GALA) for the JUICE Mission](https://arxiv.org/abs/1803.01511) mentions anomalies, and this paper in Icarus is helpful [Mass anomalies on Ganymede](https://www.sciencedirect.com/science/article/abs/pii/S0019103505003258) it's also paywalled but there be *snippets!*
uhoh
CC BY-SA 4.0
2023-12-10T10:26:08.250Z
A newer take on Ganymed's gravity, and not paywalled: https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2022GL099475 (basically confirming and refining the findings of Andersen, Schubert et al). Fig.1 is very illustrative to this case
planetmaker
CC BY-SA 4.0
2023-12-10T10:56:44.817Z
Nice find, @planetmaker. I didn't think to do a search including Ryan Park's name. Oh well. ;) BTW, you need to be careful with the spelling, since there's an asteroid named [Ganymed](https://en.wikipedia.org/wiki/1036_Ganymed).
PM 2Ring
CC BY-SA 4.0
2023-12-10T11:40:25.247Z
ahh... yeah, my bad. Different spelling in German for Jupiter's moon (without 'e') and English (with trailing 'e')... but identical for the asteroid.
planetmaker
CC BY-SA 4.0
2023-12-10T16:35:00.627Z
Ganymedes, for the name in Norwegian. Looking out for those Ganym and Ganymedese cases :)
SE - stop firing the good guys
CC BY-SA 4.0
2023-12-10T17:09:20.570Z
This isn't really related to the question, but in figure a, shouldn't the colors be reversed? The sub-jovian point on Ganymede (and its antipode) should have the *lowest* surface gravity, due to both the tidal forces exerted by Jupiter being at their maximum and also because those points on Ganymede's surface would be the most distant from the center, due to Ganymede's shape being distorted due to the frozen tidal bulge (as it is tidally locked). Am i missing something?
user267545
CC BY-SA 4.0
2023-12-11T00:07:22.873Z
@user267545 I *suspect* that the higher gravity is due to the extra mass in the tidal bulge, similar to what happens with Earth's equatorial bulge, see https://physics.stackexchange.com/q/8074/123208 I must confess that I only skimmed that Ganymede / Juno paper. It's far too technical for me. Maybe you should ask a new question about this, either here or on Physics.SE.
PM 2Ring
CC BY-SA 4.0
2023-12-11T00:52:33.547Z
@user267545 the sign is as far as I remember correct. The gravity data are derived for a *sphere* of fixed radius, not a reference ellipsoid. Thus any elongated core oriented towards jupiter will create a positive anomaly.
planetmaker
CC BY-SA 4.0
2023-12-09T07:37:34.683Z
2023-12-10T22:38:19.270Z
2023-12-10T22:38:19.270Z
PM 2Ring
<p><a href="https://en.wikipedia.org/wiki/Mass_concentration_(astronomy)" rel="noreferrer">Mascons</a>, also known as mass concentrations, are anomalous features on the Moon that tug on orbiting spacecraft and cause their orbits to decay and crash on short timescales. There are only a handful of known orbits with select inclinations and other parameters that counter the mascons' effects and ensure long-term stability of the spacecraft.</p>
<p>Is it known whether there are similar mascons on Ganymede that might derail <a href="https://en.wikipedia.org/wiki/Jupiter_Icy_Moons_Explorer" rel="noreferrer">JUICE</a> after it enters orbit around the moon? Or is it much smoother than the Moon, and only requires minimal station-keeping to prevent it from crashing?</p>

<orbit><orbital-mechanics>
Are there mascons on Ganymede similar to the ones on the Moon? And if so, how will that impact JUICE when it orbits over Ganymede?
user267545
# Are there mascons on Ganymede similar to the ones on the Moon? And if so, how will that impact JUICE when it orbits over Ganymede?

(Asked by: username_0 on 2023-12-09)

[Mascons](https://en.wikipedia.org/wiki/Mass_concentration_(astronomy)), also known as mass concentrations, are anomalous features on the Moon that tug on orbiting spacecraft and cause their orbits to decay and crash on short timescales. There are only a handful of known orbits with select inclinations and other parameters that counter the mascons' effects and ensure long-term stability of the spacecraft.

Is it known whether there are similar mascons on Ganymede that might derail [JUICE](https://en.wikipedia.org/wiki/Jupiter_Icy_Moons_Explorer) after it enters orbit around the moon? Or is it much smoother than the Moon, and only requires minimal station-keeping to prevent it from crashing?

**Tags:** orbit, orbital-mechanics

---

## 1 answers

### Answer by username_1 on 2023-12-09. Score: 12 | Accepted answer

Yes, there are known mass anomalies on Ganymede.

From *[Scientists Discover Ganymede has a Lumpy Interior](https://www.jpl.nasa.gov/news/scientists-discover-ganymede-has-a-lumpy-interior)* Aug. 13, 2004

> Scientists have discovered irregular lumps beneath the icy surface of Jupiter's largest moon, Ganymede. These irregular masses may be rock formations, supported by Ganymede's icy shell for billions of years. This discovery comes nearly a year after the orchestrated demise of NASA's Galileo spacecraft into Jupiter's atmosphere and more than seven years after the data were collected.

[![Ganymede mascons](https://i.sstatic.net/sdNW0.jpg)](https://i.sstatic.net/sdNW0.jpg)

Unfortunately, the paper is paywalled, but here's the abstract.

### Discovery of Mass Anomalies on Ganymede

JOHN D. ANDERSON , GERALD SCHUBERT, AND JENNIFER L. PALGUTA <https://doi.org/10.1126/science.1099050>

> We present the discovery of mass anomalies on Ganymede, Jupiter's third and largest Galilean satellite. This discovery is surprising for such a large icy satellite. We used the radio Doppler data generated with the Galileo spacecraft during its second encounter with Ganymede on 6 September 1996 to model the mass anomalies. Two surface mass anomalies, one a positive mass at high latitude and the other a negative mass at low latitude, can explain the data. There are no obvious geological features that can be identified with the anomalies.

These mascons should not be a problem for JUICE, which is carrying plenty of fuel for the manoeuvres required to visit its targets.

---

Here's some more recent information on Ganymede's gravitational field, from *Geophysical Research Letters*, [found by *planetmaker*](https://astronomy.stackexchange.com/questions/55495/are-there-mascons-on-ganymede-similar-to-the-ones-on-the-moon-and-if-so-how-wi/55496#comment125936_55496).

[Gravity Field of Ganymede After the Juno Extended Mission](https://doi.org/10.1029/2022GL099475), 12 December 2022, by L. Gomez Casajus, A. I. Ermakov, M. Zannoni, J. T. Keane, D. Stevenson, D. R. Buccino, D. Durante, M. Parisi, R. S. Park, P. Tortora, S. J. Bolton

> On 7 June 2021, Juno performed the first close flyby of Ganymede, the largest satellite of Jupiter (and the largest moon in the Solar System), since the end of the Galileo mission. The gravity field of Ganymede was reconstructed using the radio tracking data from all of the Ganymede encounters of both the Galileo and Juno missions. The data analysis hints at localized gravity anomalies. Interpretation of the gravity data suggests a slightly higher moment of inertia with respect previous publications, indicating a lesser degree of differentiation.

[![Ganymede gravity, from Galileo & Juno](https://i.sstatic.net/FV49D.jpg)](https://i.sstatic.net/FV49D.jpg)

> The gravity field of Ganymede derived from joint analysis of Galileo and Juno flyby data, computed on the reference sphere with radius 2,631.2 km. (a) The full gravity field solution, expanded to degree/order 5. Spacecraft ground tracks are shown in green (Galileo) and yellow (Juno). (b) gravity field solution, expanded to degree/order 5, removing completely the J2 and C22 terms. In b, the opacity of the gravity map is scaled by the signal-to-noise ratio. The white contours enclose regions with signal-to-noise ratio greater than 1 (dashed line contour) and 3 (solid line contour). All maps are in Mollweide projection, centered on the sub-Jupiter point. Latitude and longitude grid lines are in 30° intervals. Maps are draped over a color image mosaic of Ganymede (Collins et al., 2013).

The acceleration unit used in these figures is the [Gal](https://en.wikipedia.org/wiki/Gal_(unit)), which is equal to $1\,\mathrm{cm/s^2}$.

As well as the having the benefit of additional data of higher precision, this study used more powerful and sophisticated data analysis techniques than the study from 2004.

> **Comments:**
> **username_2**: Wow, a single flyby - I'm curious if they say that these are the only major mascons, or just that there is at least two. btw [Science Objectives of the Ganymede Laser Altimeter (GALA) for the JUICE Mission](https://arxiv.org/abs/1803.01511) mentions anomalies, and this paper in Icarus is helpful [Mass anomalies on Ganymede](https://www.sciencedirect.com/science/article/abs/pii/S0019103505003258) it's also paywalled but there be *snippets!*  
> **username_3**: A newer take on Ganymed's gravity, and not paywalled: https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2022GL099475 (basically confirming and refining the findings of Andersen, Schubert et al). Fig.1 is very illustrative to this case  
> **username_1**: Nice find, @planetmaker. I didn't think to do a search including Ryan Park's name. Oh well. ;) BTW, you need to be careful with the spelling, since there's an asteroid named [Ganymed](https://en.wikipedia.org/wiki/1036_Ganymed).  
> **username_3**: ahh... yeah, my bad. Different spelling in German for Jupiter's moon (without 'e') and English (with trailing 'e')... but identical for the asteroid.  
> **username_4**: Ganymedes, for the name in Norwegian. Looking out for those Ganym and Ganymedese cases :)  
> **username_0**: This isn't really related to the question, but in figure a, shouldn't the colors be reversed? The sub-jovian point on Ganymede (and its antipode) should have the *lowest* surface gravity, due to both the tidal forces exerted by Jupiter being at their maximum and also because those points on Ganymede's surface would be the most distant from the center, due to Ganymede's shape being distorted due to the frozen tidal bulge (as it is tidally locked). Am i missing something?  
> **username_1**: @user267545 I *suspect* that the higher gravity is due to the extra mass in the tidal bulge, similar to what happens with Earth's equatorial bulge, see https://physics.stackexchange.com/q/8074/123208 I must confess that I only skimmed that Ganymede / Juno paper. It's far too technical for me. Maybe you should ask a new question about this, either here or on Physics.SE.  
> **username_3**: @user267545 the sign is as far as I remember correct. The gravity data are derived for a *sphere* of fixed radius, not a reference ellipsoid. Thus any elongated core oriented towards jupiter will create a positive anomaly.  

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