Size, Shape, and Geometric Albedo of Dwarf Planet Quaoar's Largest Satellite Weywot

summary

The gist

The gist Weywot’s volume-equivalent radius to be req = 73 ± 5 km, with principal axes a = 82+5−3, b = 73+5−3, c = 65+8−12 km, indicating a triaxial shape Size and Shape Determination The

In short

Researchers used Hubble Space Telescope data to study Quaoar's largest satellite, Weywot, to determine its size, shape, and albedo. They found Weywot is a triaxial rubble-pile with a radius of 73 ± 5 km. Its low geometric albedo suggests it may not be made entirely of water ice and could have formed from debris rather than a differentiated progenitor.

Key concepts

Volume-equivalent radius (req): This is a measure of the total volume an object occupies, calculated based on its measured size. For Weywot, this value was found to be 73 ± 5 km. It helps scientists estimate the physical scale of the satellite's body, which is crucial for understanding its structure and mass.
Triaxial shape: This describes a three-dimensional shape that is not symmetrical in all three dimensions (like a perfect sphere or ellipsoid). Weywot was found to have principal axes of 82 km, 73 km, and 65 km. This non-spherical shape is consistent with it being made of rubble piled material.
Geometric albedo (pV): This is the fraction of incident sunlight that a surface reflects back into space. Weywot's low albedo (0.036 ± 0.010 - 0.006) is very dark compared to other small icy bodies like Pluto's moons, suggesting its surface composition or structure differs significantly from typical bright ice.
Roche-ellipsoid rubble-pile: This model suggests that a satellite can maintain a specific triaxial shape if it is made of loose, non-cohesive material (a rubble pile) and the internal friction between the pieces is moderate. This interpretation explains Weywot's observed shape without requiring perfect fluid equilibrium.

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This episode discusses

The paper

Size, Shape, and Geometric Albedo of Dwarf Planet Quaoar's Largest Satellite Weywot · Read on arXiv

Cameron Collyer, Estela Fernandez-Valenzuela, Benjamin Proudfoot, Bryan J. Holler, Jose Luis Ortiz, Ishan Sharma

Florida Space Institute Research Parkway, Orlando, FL 32826, USA · Department of Physics, University of Central Florida, 4111 Libra Drive, Orlando, FL 32816, USA · Space Telescope Science Institute Baltimore MD 21218 · Instituto de Astrofísica de Andalucía Glorieta de la Astronomía s/n, 18008 Granada, Spain · Department of Mechanical Engineering IIT Kanpur

The small satellites of dwarf-planet size worlds in trans-Neptunian space are thought to be produced through giant impacts, and are typically observed to have bright water-ice-dominated surfaces. Here we focus on Quaoar's small primary satellite, Weywot. We build on previous work by re-analyzing the June 2023 stellar occultation by Weywot using results from 11 orbits of HST imaging with the Wide Field Camera 3. Weywot's rotational lightcurve was constrained along with its independent photometric phase curve, which we used to obtain more robust measurements of Weywot's size, shape, and geometric albedo. We found Weywot's volume-equivalent radius to be req = 73+-5 km, with principal axes a = 82+5-3, b = 73+5-3, c = 65+8-12 km, indicating a triaxial shape. This shape is consistent with a Roche-ellipsoid rubble-pile interpretation requiring only moderate internal friction at Weywot's current semimajor axis, and Weywot can retain this shape down to Quaoar's co-rotation radius. We also found Weywot's geometric albedo to be quite low, pV = 0.036+0.010-0.006, akin to that of Eris's satellite Dysnomia, but over an order of magnitude lower than Pluto's minor satellites and Haumea's satellites. We propose this may indicate that Weywot's ice-mass fraction < 100%, and that Weywot is not sourced from the water-ice mantle of a differentiated pre-impact progenitor as suggested for Pluto's minor satellites and Haumea's satellites.

Transcript

Introduction to the show: ident: Astrophysics Radio. Generated commentary on the latest astrophysics papers.

Vera: Today's paper: "Size, Shape, and Geometric Albedo of Dwarf Planet Quaoar's Largest Satellite Weywot".

Jocelyn: The gist Weywot’s volume-equivalent radius to be req = 73 ± 5 km, with principal axes a = 82+5−3, b = 73+5−3, c = 65+8−12 km,

Vera: First, who's behind it and why it matters.

Paper summary: Vera: Now moving on to what this paper actually does. The authors build on previous work by re-analyzing the June two thousand twenty-three stellar occultation by Weywot using results from eleven orbits of HST imaging with the Wide Field Camera three <ref:2610.11988#pg1,build on previous work by re-analyzing the June 2023 stellar occultation>.

Jocelyn: They used that rotational lightcurve data along with an independent photometric phase curve to get more robust measurements of Weywot's size, shape, and geometric albedo.

Vera: The main finding they report is the volume-equivalent radius of req = seventy-three plus or minus five kilometers.

Jocelyn: And they detail the triaxial shape parameters: a = eighty-two plus or minus three, b = seventy-three plus or minus three, and c = sixty-five plus eight minus twelve kilometers.

Subrahmanyan: The shape they found is consistent with a Roche-ellipsoid rubble-pile interpretation requiring only moderate internal friction at Weywot's current semimajor axis.

Vera: They also determined the geometric albedo to be pV = zero point six plus or minus one point zero minus zero point six, which they compare to Eris’s satellite Dysnomia but note it’s over an order of magnitude lower than Pluto's or Haumea's minor satellites <ref:2610.11988#pg2>.

Jocelyn: This low albedo, combined with the spectral type support they found, is what leads them to suggest that Weywot's ice-mass fraction fice is less than one hundred percent.

Subrahmanyan: That means they are questioning whether Weywot formed from a water-rich mantle of a differentiated pre-impact progenitor, which was the idea for some of those other satellites.

Vera: They also found an upper limit to Weywot’s mass was five point zero times ten to eighteen kilograms <ref:2610.11988#pg1>.

Jocelyn: That small mass constraint is important because it weakens the hypothesis that Weywot's inclination relative to the ring system might be causing bending waves there, which is why they say it's much lower than previous results.

Subrahmanyan: And finally, they suggest that Weywot may have formed as a result of a massive collision in Quaoar’s past, possibly from the same material making up the ring system.

Conclusion: Vera: So what does this all boil down to regarding the paper "Size, Shape and Geometric Albedo of Dwarf Planet Quaoar's Largest Satellite Weywot"? The authors are really pushing back on how we understand these small bodies.

Jocelyn: It seems the main thing is that Weywot isn't necessarily a fully ice-covered satellite sourced from a differentiated planet's mantle.

Subrahmanyan: Instead, the evidence points toward it possibly being a surviving collisional fragment of a smaller TNO whose surface was depleted in volatiles.

Vera: The paper gives us concrete dimensions: req = seventy-three plus or minus five kilometers, and that specific triaxial shape parameters we discussed.

Jocelyn: And the low geometric albedo, pV = zero point six plus or minus one point zero minus zero point six is a key piece of data that supports this new formation theory <ref:2610.11988#pg2>.

Subrahmanyan: The paper does a lot of work here by tying the rotational dynamics to the orbital period, estimating that Weywot’s spin synchronization took less than one fifty million years as a result of tidal torques.

Vera: So we're looking at a satellite that’s shaped in a specific way, has low reflectivity, and its origin might be from debris rather than deep mantle material.

Jocelyn: And this paper gives us new constraints on Weywot’s tidal state and three dee shape through the lightcurve and occultation analysis <ref:2610.11988#pg1>.

Subrahmanyan: The overall picture is that Weywot likely formed as a result of a massive collision in Quaoar's past, possibly forming from the same material making up the ring system.

Vera: We’ve seen what this research on Quaoar's largest satellite actually tells us about its formation and surface properties.

Jocelyn: It gives us new constraints on Weywot’s tidal state and three dee shape through the lightcurve and occultation analysis <ref:2610.11988#pg1>.

Subrahmanyan: The overall picture is that Weywot likely formed as a result of a massive collision in Quaoar's past, possibly forming from the same material making up the ring system.

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