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Mascons, 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 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?

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Yes, there are known mass anomalies on Ganymede.

From 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

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.

Gravity Field of Ganymede After the Juno Extended Mission, 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

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, 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.

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    $\begingroup$ 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 mentions anomalies, and this paper in Icarus is helpful Mass anomalies on Ganymede it's also paywalled but there be snippets! $\endgroup$
    – uhoh
    Commented Dec 9, 2023 at 14:25
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    $\begingroup$ A newer take on Ganymed's gravity, and not paywalled: 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 $\endgroup$
    – planetmaker
    Commented Dec 10, 2023 at 10:26
  • $\begingroup$ 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. $\endgroup$
    – PM 2Ring
    Commented Dec 10, 2023 at 10:56
  • $\begingroup$ ahh... yeah, my bad. Different spelling in German for Jupiter's moon (without 'e') and English (with trailing 'e')... but identical for the asteroid. $\endgroup$
    – planetmaker
    Commented Dec 10, 2023 at 11:40
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    $\begingroup$ @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. $\endgroup$
    – planetmaker
    Commented Dec 11, 2023 at 0:52

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