Ilmarë
Moon of 174567 Varda
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Ilmarë,[d] formal designation (174567) Varda I, is the only known moon of the large Kuiper belt object 174567 Varda. It was discovered by Keith Noll et al. in 2009, at a separation of about 0.12 arcsec, using discovery images taken by the Hubble Space Telescope on 26 April 2009, and reported in 2011.[5] At approximately 403 km in diameter, it is the fourth-largest known moon of a trans-Neptunian object, after Pluto I Charon,[6] Eris I Dysnomia,[7] and Orcus I Vanth.[8] Under the assumption of a lower density of 0.7 g/cm3, which is thought to be more typical that of trans-Neptunian objects in the ~400 km size range, the satellite-to-primary mass ratio would be approximately 0.11:1.[3]
Hubble Space Telescope image of Varda and its satellite Ilmarë, taken in 2010 and 2011 | |
| Discovery | |
|---|---|
| Discovered by | Keith S. Noll et al. |
| Discovery date | 2009, based on images taken on 26 April 2009 |
| Designations | |
Designation | Varda I |
| Pronunciation | /ËɪlmÉriË/ |
Named after | Ilmarë (figure by J. R. R. Tolkien)[1] |
| (174567) Varda I | |
| Adjectives | Ilmarëan /ɪlmÉËriËÉn/ |
| Orbital characteristics[2] | |
| 4805±35 km or 4812±35 km[2] | |
| Eccentricity | 0.0181±0.0045 or 0.0247±0.0048 (0.0215±0.0080 adopted) |
| 5.75058±0.00015 d | |
| Inclination | 101.0±1.9 or 85.1±1.8 |
| Satellite of | Varda |
| Physical characteristics | |
| 403±40 km[3] | |
| Mass | â2.2Ã1019 kg[2] (2015) ~16% of Varda (2025)[3][a] ~11% of Varda (2025)[3][b] |
Mean density | 1.24+0.50 â0.35 g/cm3 (system)[2][c] |
| Albedo | 0.068±0.011 (2025)[3] â0.085 (2020)[4] |
Spectral type | BâV = 0.857±0.061 VâI = 1.266±0.052[2] |
| Varda + 1.7[2] | |
History
Discovery
The Hubble Space Telescope, which discovered Ilmarë on 26 April 2009.
Ilmarë was discovered by Keith Noll et al. in 2009, at a separation of about 0.12 arcsec, using discovery images taken by the Hubble Space Telescope on 26 April 2009, and reported in 2011.[5]
Name
Names for Varda and its moon were announced on 16 January 2014. Ilmarë (Quenya: [ËilmarÉ]) is a chief of the Maiar and handmaiden to Varda, the queen of the Valar, creator of the stars, and principal goddess of the elves in J. R. R. Tolkien's fictional mythology.[1]
Orbit
Orbital characteristics
Ilmarë orbits Varda at a distance of 4805±35 km or 4812±35 km[2] and an eccentricity of 0.0181±0.0045 or 0.0247±0.0048 respectively.[2] Ilmarë has an orbital period of approximately 5.75058±0.00015 d.[2] Ilmarë's orbit has an inclination of 101.0±1.9° or 85.1±1.8° with respect to the ecliptic.[2]
Tidal locking
Ilmarë and Varda are tightly bound, with a separation of about 13 Varda radii, and a consequently low angular momentum. Along with the high inclination of Varda's orbit, they are similar in this way to the OrcusâVanth and SalaciaâActaea systems. As of 2015 two mirror orbital solutions are possible with slightly different orbital parameters. The calculated eccentricity is inconsistent with the likely age of the system, suggesting that it might be spurious, but the expected age is also contradicted by suggestions that Varda may not be tidally locked.[2]
Physical characteristics
Comparison of sizes, albedos, and colors of various large trans-Neptunian objects with diameters greater than 700 km (430 mi). Its parent body Varda is shown on the bottom row, first from the left, and ilmarë is located next to it. The dark colored arcs represent uncertainties of the object's size.
Old estimates of Ilmarë's physical properties
If Ilmarë and Varda have the same albedo, Ilmarë would be 163+19
â17 km in radius, or approximately 8.4% the volume of Varda. If the two bodies also have the same density, Ilmarë would then have approximately 8.4% the system mass of (2.664±0.064)Ã1020 kg.
If, however, the albedo of Varda is 50% greater than that of Ilmarë, Ilmarë would have a radius of 191+22
â21 km and the bulk density of the system would be 1.31+0.52
â0.36 g/cm3. If Ilmarë has a 50%-greater albedo, then its radius would be 137+16
â15 km and the bulk density would be 1.18+0.47
â0.33 g/cm3. Because the absolute magnitudes of the two bodies at different wavelengths are similar, it is not likely that their albedos differ by much, so Ilmarë is likely to be in this size range.[2]
More accurate measurements in 2025
Size and albedo
However, more accurate measurements in 2025 that was done by Kiss et al., measured a low albedo of 0.068±0.011 for Ilmarë,[3] which is a bit lower than the albedo of Varda (0.099±0.002),[4] yielding a larger diameter of 403±40 km km, which is about half that of its primary.[3]
Assuming that the following size estimates are correct, Ilmarë is about the fourth-biggest known moon of a trans-Neptunian object, after Charon (1212±1 km),[6] Dysnomia (615+60
â50 km),[7]:â7â and Vanth (442.5±10.2 km).[8]
Mass and density
Assuming equal densities (1.15 g/cm3) for both bodies, the resulting satellite-to-primary mass ratio is 0.16:1,[3] closely matching that of the OrcusâVanth system.[3] Even under the assumption of a lower density of 0.7 g/cm3, which is thought to be more typical that of trans-Neptunian objects in the ~400 km size range,[3] the mass ratio remains high at 0.11:1.[3] This places VardaâIlmarë among the systems with the largest known mass ratios, comparable to that of PlutoâCharon.[3]
Surface and spectrum
The surface of Ilmare's color is dark and moderately reddish,[3][2] with a low geometric albedo of 0.068±0.011,[3] which is a bit lower than that of Varda, and has a measured BâV color index of 0.857±0.061 and a VâI color index of 1.266±0.052,[2] which is considered a moderately red color.[2]
See also
- Actaea â a moon of another large trans-Neptunian object, 120347 Salacia, that is similar in size to Ilmarë
- Hiʻiaka, a similarly sized moon orbiting the dwarf planet Haumea
