Wednesday 29 July 2020

The discovery of a new satellite obiting Asteroid (31) Euphrosyne, and how it helps to better understand the nature of the parent asteroid.

The main asteroid belt is a dynamically living relic, with the shapes, sizes, and surfaces of most asteroids being altered by ongoing collisional fragmentation and cratering events. Space probes and ground-based observations have revealed a fascinating variety among asteroid shapes, where large asteroids are nearly spherical and small asteroids are irregularly shaped. Most asteroids with diameters greater than about 100 km have likely kept their internal structure intact since their time of formation because the dynamical lifetime of those asteroids is estimated to be comparable to the age of the Solar System. There are a few exceptions comprising essentially the largest remnants of giant families, e.g., (10) Hygiea, whose shapes have been largely altered by the impact. In contrast, the shapes of smaller asteroids have been determined mainly through collisions, where their final shapes depend on collision conditions such as impact energies and spin rates. Asteroid (31) Euphrosyne is the largest member of its namesake family. Previous studies have noted that the Euphrosyne Family exhibits a very steep size frequency distribution, significantly depleted in large and medium sized asteroids. Such a steep size frequency distribution is interpreted as a glancing impact between two large bodies resulting in a disruptive cratering event. Euphrosyne is a CB-type asteroid (asteroid with a largely carbonaceous composition and a bluish colouration), with an optical albedo 0.045. Euphrosyne’s diameter has been reported as 276 km and 282 km, while its mass has been estimated by various studies leading to an average value of 12 700 000 000 000 kilotonnes, with about 50% uncertainty. These size and mass estimates imply a density estimate of about 1180 kg/m³.

In a paper published on the arXiv database at Cornell University on 16 July 2020, and accepted for publication in the journal Astronomy & Astrophysics, a team of scientists led by Bin Yang of the European Southern Observatory, present the high-angular resolution observations of Euphrosyne with the the Zurich Imaging Polarimeter of the Spectro-Polarimetric High-contrast Exoplanet Research Instrument on the Very Large Telescope, which were obtained as part of the European Southern Observatory large program. Yang et al. discovered a satellite of Euphrosyne during this study, implying that it is one of the few large asteroids for which the density can be constrained with high precision.

The arrival of second generation extreme adaptive-optics instruments, such as the Spectro-Polarimetric Highcontrast Exoplanet Research instrument on the Very Large Telescope and the Gemini Planet Imager at GEMINI-South, of ers a great opportunity to study detailed shape, precise size and topographic feature of large main belt asteroids with diameters greater than 100 km via direct imaging. adaptive-optics-aided observations with high spatial resolution also enable detection of asteroidal satellites that are much smaller and closer to their primaries, which, thus far, could have remained undetected in prior searches. Consequently, physical properties that are not well constrained, such as the bulk density, the internal porosity and the surface tensile strength, can be investigated using adaptive-optics corrected measurements. These are the key parameters that determine crater formation, family formation and/or satellite creation.

Yang et al. used their observations, together with a compilation of available and newly obtained optical lightcurves, to constrain the three dimensional shape of Euphrosyne as well as its spin state and surface topography. They then described the discovery of its small moonlet S/2019 (31) 1 and constrain its mass by fitting the orbit of the satellite. Both the three dimensional shape (hence volume) and the mass estimate allow us to constrain the density of Euphrosyne with high precision. Yang et al. also use the adaptive-optics images and the three dimensional shape model to search for large craters, which may be associated with the family-forming event.

Euphrosyne was observed, between March and April 2019, using the Zurich Imaging Polarimeter of  the Spectro-Polarimetric Highcontrast Exoplanet Research instrument in the direct imaging mode with the narrow band filter (filter central wavelength  645.9 nm, width 56.7 nm). The angular diameter of Euphrosyne was in the range of 0.16–0.17" and the asteroid was close to an equator-on geometry at the time of the observations. Therefore, the Spectro-Polarimetric Highcontrast Exoplanet Research instrument images of Euphrosyne obtained from seven epochs allowed Yang et al. to reconstruct a reliable three dimensional shape model with well defined dimensions. The reduced images were further deconvolved with the MISTRAL (Myopic Iterative. STep Preserving ALgorithm) algorythm using a parametric point-spread function.

Full set of Very Large Telescope/Spectro-Polarimetric Highcontrast Exoplanet Research instrument/Zurich Imaging Polarimeter images of (31) Euphrosyne. Images deconvolved by the MISTRAL algorythm. Yang et al. (2020).

A set of 29 individual lightcurves of Euphrosyne was previously used in order to derive a convex shape model of this large body. These lightcurves were obtained between 1980 and 2012. Yang et al. complemented this data with five additional lightcurves from the recent apparition in 2017: four lightcurves were obtained by the TRAPPIST North telescope and the fifth one was obtained via  Gaia-GOSA. Yang et al.'s final photometric dataset utilised for the shape modeling of Euphrosyne consists of 34 individual lightcurves.

A previouslt determined spin state solution served as an initial input for the shape modeling with the All-data Asteroid Modeling algorithm that fits simultaneously the optical data and the disk-resolved images. Yang et al. followed the same shape modeling approach applied in their previous studies based on disk-resolved data from the Spectro-Polarimetric Highcontrast Exoplanet Research instrument large program. First, Yang et al. constructed a low resolution shape model based on all available data, then they used this shape model as a starting point for further modeling with decreased weight of the lightcurves and increased shape model resolution. Yang et al. performed this approach iteratively until they were satisfied with the fit to the lightcurve and disk-resolved data.  Yang et al. also tested two di erent shape parametrizations, octantoids and subdivision.  Yang et the comparison between the Very Large Telescope/Spectro-Polarimetric Highcontrast Exoplanet Research instrument/Zurich Imaging Polarimeter deconvolved images of Euphrosyne and the corresponding projections of the shape model.

Comparison between the Very Large Telescope/Spectro-Polarimetric Highcontrast Exoplanet Research instrument/Zurich Imaging Polarimeter deconvolved images of Euphrosyne (bottom) and the corresponding projections of the All-data Asteroid Modeling algorithm. The red line indicates the position of the rotation axis. A non-realistic illumination is used to highlight the local topography of the model. Yang et al. (2020).

Owing to the nearly equator-on geometry of the asteroid, our images taken at seven di erent rotation phases have a nearly complete coverage of the entire surface of Euphrosyne. The Spectro-Polarimetric Highcontrast Exoplanet Research instrument data enable an accurate determination of Euphrosyne’s dimensions, including the ones along the rotation axis. The uncertainties reflect the dispersion of values obtained with various shape models based on di erent data weighting, shape resolution and parametrization. These values correspond to about 1 pixel, which is equivalent to 5.93 km. Yang et al.'s volume equivalent diameter of 268 km is with previous radiometric estimates. The shape of Euphrosyne is fairly spherical with almost equal equatorial dimensions and only a small flattening along the spin axis. Euphrosyne’s sphericity index is equal to 0.9888, which is somewhat higher than that of (4) Vesta, (2) Pallas and (704) Interamnia, is equal to 0.9888, which is somewhat higher than that of (4) Vesta, (2) Pallas and (704) Interamnia.

Given the rather spherical shape of Euphrosyne and the fact that its a and b axes have similar lengths (within errors), Yang et al. investigated whether the shape of Euphrosyne is close to hydrostatic equilibrium. It appears that Euphrosyne’s shape is significantly di erent from the predicted Maclaurin spheroid (an oblate spheroid which arises when a self-gravitating fluid body of uniform density rotates with a constant angular velocity), which would be much flatter along the c axis. Yang et al.'s Spectro-Polarimetric Highcontrast Exoplanet Research instrument observations show that Euphrosyne is not actually in hydrostatic equilibrium for its current rotation.

Each image obtained with the Spectro-Polarimetric Highcontrast Exoplanet Research instrument/Zurich Imaging Polarimeter was further processed to remove the bright halo surrounding Euphrosyne. The residual structures after the halo removal were minimized using processing techniques where the background structures were removed using a running median in a 50 pixel box in the azimuthal direction as well as in a 40 pixel box in the radial direction. Adopting this method Yang et al. inserted 100 point sources, which known intensity and full width at half maximum, in each science image to estimate flux loss due to the halo removal processes. In five out of of seven epochs, a faint non-resolved source was clearly detected in the vicinity of Euphrosyne. The variation in the brightness of the satellite is mainly due to the di erence in the atmospheric conditions at the time of the observations, which directly a ect the adaptive-optics performance.

 Processed Zurich Imaging Polarimeter images, revealing the presence of the satellite, S/2019 (31) 1, around (31) Euphrosyne in five epochs. The pixel intensities within 0.2200 of the primary have been reduced by a factor of 2000 to increase the visibility of the faint satellite. The images were smoothed by convolving a Gaussian function with full width at half maximum of 8 pixels. The arrow points out the location of the satellite in the image taken on 20 March 2019, when the satellite appeared very dim compared to the other nights. Yang et al. (2020).

Yang et al. measured the relative positions on the plane of the sky between Euphrosyne and its satellite. They then used the GENetic Orbit IDentification algorithm to determine the orbital elements of the satellite. The best solution fits the observed positions with root mean square residuals of 1.5 mas only. The orbit of the satellite is circular, prograde, and equatorial, similar to most known satellites around large main belt asteroids.

From the di erence in the apparent magnitude of 9.0 0.3 between Euphrosyne and S/2019 (31) 1, and assuming a similar albedo for both, Yang et al. estimate the diameter of the satellite to be about 4.0 km. The Euphrosyne binary system has the relative component separation of 5.0 and a secondary-toprimary diameter ratio of 0.005. Compared to other asteroid/satelite systems, S/2019 (31) 1 has one of the smallest secondary-to-primary diameter ratios and is very close to the primary. Given the small size of the satellite, S/2019 (31) is expected to be tidally locked, i. e. its spin period synchronizes to its orbital period on million year timescale.

Owing to the presence of the satellite, Yang et al. derived the mass of the system as 17 000 000 000 000 kilotonnes with a fractional precision of 15%, which is considerably better than all the previous indirect measurements. Combining their mass measurement with the newly derived volume based on their three dimensional-shape, Yang et al. obtained a bulk density of 1.665 g/cm³ for Euphrosyne.

Yang et al. note that the bulk density of Euphrosyne is the lowest among all the other large C-type asteroids measured to date, e.g. (1) Ceres (2.161 g/cm³, with a diameter of about 1000 km), (10) Hygiea (1.94 g/cm³, with a diameter of about 440 km) and (704) Interamnia (1.98 g/cm³, with a diameter of about 300 km). On the other hand, a density around 1.7 g/cm³ or lower is more common among intermediate sized C-type asteroids, such as (45) Eugenia (1.4 g/cm³, with a diameter of about 200 km), (93) Minerva (1.75 g/cm³, with a diameter of about 160 km), (130) Eletra (1.60  g/cm³, with a diameter of about 200 km), and (762) Pulcova (0.8  g/cm³, with a diameter of about 150 km).

For C-complex asteroids the density seems to show a trend with size, where the smaller asteroids have lower densities. Such trend could be explained by increasing porosity in smaller asteroids. Nonetheless, the macroporosity of Euphrosyne is likely to be small ( no more than 20%) due to its relatively high internal pressure owing to its large mass. Given the small macroporosity of Euphrosyne, its density, therefore, is diagnostic of its bulk composition. As for the other large C-type asteroids (Ceres, Hygiea, Interamnia), a large amount of water must be present in Euphrosyne. Assuming 20% porosity, a typical density of anhydrous silicates of 3.4 g/cm³ and a density of 1.0 g/cm³ for water ice, the presence of water ice, up to 50% by volume, is required in the interior of Euphrosyne to match its bulk density.

In terms of topographic characteristics, the surface of Euphrosyne appears smooth and nearly featureless without any apparent large basins. This is in contrast to other objects studied by our large program that show various-sized craters on their surfaces, such as (2) Pallas (B-type), (4) Vesta (V-type), and (7) Iris (S-type). On the other hand, lacking surface features in adaptive-optics images is not unprecedented among large asteroids, especially among C-type asteroids. Ground-based adaptive-optics observations have identified at least three other cases that are lacking prominent topographic structures, namely (1) Ceres, (10) Hygiea, and (704) Interamnia. Although NASA/Dawn observations revealed a highly cratered surface of Ceres, this dwarf planet clearly lacks large craters which suggests rapid viscous relaxation or protracted resurfacing due to the presence of large amounts of water.

Similarly, as for the cases of Hygiea and Interamnia, the absence of apparent craters in our Euphrosyne images may be due to the flat-floored shape of 40km diameter craters (this diameter corresponds to the minimum size of features that can be recognized on the surface of Euphrosyne), which would be coherent with a high water content for this asteroid, in agreement with Yang et al.'s bulk density estimate.

In an attempt to understand the unexpected nearly spherical shape of Euphrosyne, Yang et al. used hydrodynamical simulations to study the family-formation event. The simulations were performed with a smoothed-particle hydrodynamics code to constrain the impact parameters, such as the impact angle and the diameter of the impactor. Yang et al. assumed the target and the impactor are both monolithic bodies with an initial density of the material of 1.665 g/cm³, corresponding to the presentday density of Euphrosyne. Yang et al,'s smoothed- particle hydrodynamics simulations found that the impact event of Euphrosyne is even more energetic in comparison to that of Hygiea. As such, the parent body of Euphrosyne is completely fragmented by the impact and the final reaccumulated shape of Euphrosyne is highly spherical, which is similar to the case of Hygiea where the nearly round shape is formed following post-impact reaccumulation.

Yang et al. further studied the orbital evolution of the Euphrosyne family and determined the age of the family. N-body simulations further constrained the age of the Euphrosyne family to about 280 million years; which is significantly younger than the previous estimates (between 560 and 1160 million years). The young dynamical age and post-impact re-accumulation, collectively, may have contributed to the apparent absence of craters on the surface of Euphrosyne.

Yang et al.'s new finding about the young age of Euphrosyne makes this asteroid a unique object for us to study the impact aftermath on a very young body that is only 0.3 billion years old. Previously, smoothed-particle hydrodynamics simulations for Hygiea showed that its shape relaxed to a sphere during the gravitational reaccumulation phase, accompanied by an acoustic fluidization. The relaxation process on Hygiea could have settled down on a timescale of a few hours. However, this shape relaxation, in theory, maybe a rather long-term process, which could possibly last as long as the age of the body (three billion years, as suggested by its family). If the physical mechanisms work the same way on both bodies, then the relaxation timescale simply can not be short on one body (diameter 268 km) and be 10 times longer on the other, larger, one (diameter 434 km). To reconcile with both observations, the shape relaxation, if it is a long-term process, should occur on timescales that are comparable to or less than 0.3 billion years.

In addition to the much younger dynamical age, the rotation period of Euphrosyne is also shorter (5.53 hours) than those of Hygiea and Ceres. The spin rate of Euphrosyne is faster than the typical rotation rates of asteroids with similar sizes. This is interpreted as a result of a violent disruption process, where the parent body is re-accumulated into high angular momentum shape and spin configuration. With that spin rate, Euphrosyne would be expected to have a shorter c axis compared to the a axis using MacLaurins equation. However, a MacLaurin ellipsoid represents the hydrostatic equilibrium figure of a homogeneous and intact body, which is not the case for Euphrosyne since it is a reaccumulated body. This may explain why the actual shape of Euphrosyne deviates from that of a MacLaurin ellipsoid.

The disk-resolved images and the three dimensional-shape model of Euphrosyne show that it is the third most spherical body among the main belt asteroids with known shapes after Ceres and Hygiea. Its round shape is consistent with a re-accumulation event following the giant impact at the origin of the Euphrosyne Family. The orbit of Euphrosyne’s satellite, S/2019 (31) 1, is circular, prograde, and equatorial, similar to most known satellites around large main belt asteroids. The estimated diameter of this newly detected satellite is 4 km, assuming a similar albedo for the satellite and the primary. The bulk density of Euphrosyne is 1665 kg/m³, which is the first high precision density measurement via ground-based observations for a CB-type asteroid. Such density implies that a large amount of water (at least 50% in volume) must be present in Euphrosyne. The surface of Euphrosyne is nearly featureless with no large craters detected, which is consistent with its young age and icerich composition.

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