Photometric analysis of asteroids and comets from space observations

The European space mission Rosetta, during its still ongoing journey to the comet 67P/Churyumov-Gerasimenko, on 10 July 2010 made an intermediate stop flying close to the asteroid 21 Lutetia at a distance of less than 3200 km and observed it from a varying observing point, otherwise inaccessible from Earth. Less than four months later, on 4 November 2010, the EPOXI mission, extension of the NASA Deep Impact mission, offered another unexpected opportunity approaching the small hyperactive Jupiter family comet 103P/Hartley 2 a few days after its perihelion passage at a distance of less than 700 km from its nucleus. Those encounters provided an extremely important possibility for the advance in understanding our Solar System formation and history. Asteroids and comets are indeed the unique left samples of the primordial planetesimals that accreted in the original solar nebula. They are therefore key bodies for understanding the conditions and the processes occurred during the Solar System initial formation phases. The principal aim of this thesis is therefore to provide an advance in the small bodies science, particularly comets, through the photometric analysis of high-resolution observations obtained by the two mentioned space missions. Investigations of asteroid 21 Lutetia, observed on 10 July 2010 through the OSIRIS imaging system (Optical Spectroscopic and Infrared Remote Imaging System) onboard the Rosetta spacecraft, have been focused mainly on its surface physical properties. The integral phase curve analysis and Hapke's modeling showed that the regolith particles constituting Lutetia's surface are highly re ecting, very small, compact and opaque, and form a low-porosity and overall smooth layer over the high-density nucleus of Lutetia. The quite at and featureless spectra observed suggest, together with the high density, that Lutetia is an X-type asteroid for the spectral taxonomy and that it has possibly an enstatite chondrite composition. Moreover the spectral slope is found to vary signicantly with phase angle showing a pronounced reddening. This evidence, still to be completely explained, may be one possible explanation of the continuously changing spectral slope of Lutetia spectrum observed from Earth. We found strong evidences of color variegation over the surface of Lutetia, and in particular on a geologically interesting surface area, called Baetica Region. The variegation of this region, found to be about 10%, suggests the presence of bluer particles on the crater walls, indicative of bigger grains, possibly revealing fresh material, and of redder particles at the bottom of the slant, where debris deposits are potentially present. In view of a future extension of the work to the resolved photometric analysis, a series of complementary processing tools which make use of the high resolution shape model have been implemented. The photometric analysis of comet 103P/Hartley 2, visited by EPOXI spacecraft on 4 November 2010, and pictured through MRI (Medium Resolution Imager) multi-band imaging system, has been focused instead on the cometary atmosphere and its dust and gas features and emission processes. The study of the colors and reddening of the dust, through narrowband continuum observations, shows that dust in Hartley 2 coma is slightly redder in the tailward direction than in the sunward direction. This is tentatively explained considering that ices and refractories are both emitted in the sunward direction, but, while ices sublimate, refractories are pushed away by the Sun's radiation pressure and form a slightly redder tail. A detailed study of OH emission structures in the period spanning from the day of perihelion up to 10 days afterward, has been performed. It shows an overall radial antisunward OH distribution in all observations apart the closest approach (CA) images, where a radial sunward jet coming from the central waist of the nucleus is evident in the very innermost regions of the coma, within 35 km from the nucleus. This OH feature, very close to the nucleus, provided an indication of a possible secondary emission mechanisms. The prompt emission (PE) of excited OH molecules coming from photodissociation of water has been proposed. CN structure analysis in the near-nucleus region shows instead a rounded structure, within 35 km from the nucleus, which is interpreted as an indication that CN is emitted in the coma by grains or particles that are aected by the nucleus rotation. OH observations have been further investigated in order to derive the water production rate in the coma of Hartley 2. A coma model has been adopted, correspondent to vectorial model but extending inside the coma down to the nucleus. A water production rate of 1.17e28 mol/s (logQ = 28.07) has been evaluated, consistent with other authors measurements (see Knight et al., 2013). However the water production rate is found to be varying as function of time with a periodicity that suggests a correlation with the nucleus rotation, which has a period of about 18 hours. However a strong peak in the production rate is observed, correspondent to CA nucleus-resolved observations. The prompt emission mechanism for OH brightness has been invoked as possible responsible and an evaluation of the theoretical observable OH PE flux through MRI-OH narrowband filter has been performed, yielding an intensity of about 26% of the fluorescence emission at about 50 km from the nucleus. However, this is probably an overestimate of the prompt emission, considering indeed a value of about 10%, observations are well reproduced by the cometary model used, even in the innermost coma. All the studies performed in this thesis will have a direct application to the upcoming encounter of Rosetta with the comet 67P/Churyumov-Gerasimenko occurring on August 2014, and lasting more than a year, until December 2015. Rosetta will approach the comet, deliver a lander on its surface and escort the comet along its orbit up to its next perihelion passage. This encounter is expected to revolutionize the cometary science, giving answer to most of the up-to-date still unexplained comets mysteries. The investigations performed on asteroid Lutetia and comet Hartley 2 will be therefore combined together for data reduction, analysis, procedures implementation and results interpretation, with the final aim to obtain a better understanding of comets in all their aspects

Near-UV OH Prompt Emission in the Innermost Coma of 103P/Hartley 2

The Deep Impact spacecraft fly-by of comet 103P/Hartley 2 occurred on 2010 November 4, one week after perihelion with a closest approach (CA) distance of about 700 km. We used narrowband images obtained by the Medium Resolution Imager (MRI) onboard the spacecraft to study the gas and dust in the innermost coma. We derived an overall dust reddening of 15\%/100 nm between 345 and 749 nm and identified a blue enhancement in the dust coma in the sunward direction within 5 km from the nucleus, which we interpret as a localized enrichment in water ice. OH column density maps show an anti-sunward enhancement throughout the encounter except for the highest resolution images, acquired at CA, where a radial jet becomes visible in the innermost coma, extending up to 12 km from the nucleus. The OH distribution in the inner coma is very different from that expected for a fragment species. Instead, it correlates well with the water vapor map derived by the HRI-IR instrument onboard Deep Impact \citep{AHearn2011}. Radial profiles of the OH column density and derived water production rates show an excess of OH emission during CA that cannot be explained with pure fluorescence. We attribute this excess to a prompt emission process where photodissociation of H$_2$O directly produces excited OH*($A^2\it{\Sigma}^+$) radicals. Our observations provide the first direct imaging of Near-UV prompt emission of OH. We therefore suggest the use of a dedicated filter centered at 318.8 nm to directly trace the water in the coma of comets.Comment: 21 page

Cometary science with CUBES

The proposed CUBES spectrograph for ESO's Very Large Telescope will be an exceptionally powerful instrument for the study of comets. The gas coma of a comet contains a large number of emission features in the near-UV range covered by CUBES (305-400 nm), which are diagnostic of the composition of the ices in its nucleus and the chemistry in the coma. Production rates and relative ratios between different species reveal how much ice is present and inform models of the conditions in the early solar system. In particular, CUBES will lead to advances in detection of water from very faint comets, revealing how much ice may be hidden in the main asteroid belt, and in measuring isotopic and molecular composition ratios in a much wider range of comets than currently possible, provide constraints on their formation temperatures. CUBES will also be sensitive to emissions from gaseous metals (e.g., FeI and NiI), which have recently been identified in comets and offer an entirely new area of investigation to understand these enigmatic objects.Comment: Accepted for publication in Experimental Astronom

Near to Mid-infrared Spectroscopy of (65803) Didymos as Observed by JWST: Characterization Observations Supporting the Double Asteroid Redirection Test

The Didymos binary asteroid was the target of the Double Asteroid Redirection Test (DART) mission, which intentionally impacted Dimorphos, the smaller member of the binary system. We used the Near-Infrared Spectrograph and Mid-Infrared Instrument instruments on JWST to measure the 0.6–5 and 5–20 μm spectra of Didymos approximately two months after the DART impact. These observations confirm that Didymos belongs to the S asteroid class and is most consistent with LL chondrite composition, as was previously determined from its 0.6–2.5 μm reflectance spectrum. Measurements at wavelengths >2.5 μm show Didymos to have thermal properties typical for an S-complex asteroid of its size and to be lacking absorptions deeper than ∼2% due to OH or H2O. Didymos’ mid-infrared emissivity spectrum is within the range of what has been measured on S-complex asteroids observed with the Spitzer Space Telescope and is most consistent with emission from small (<25 μm) surface particles. We conclude that the observed reflectance and physical properties make the Didymos system a good proxy for the type of ordinary chondrite asteroids that cross near-Earth space, and a good representative of likely future impactors

Ejecta Evolution Following a Planned Impact into an Asteroid: The First Five Weeks

The impact of the DART spacecraft into Dimorphos, moon of the asteroid Didymos, changed Dimorphos' orbit substantially, largely from the ejection of material. We present results from twelve Earth-based facilities involved in a world-wide campaign to monitor the brightness and morphology of the ejecta in the first 35 days after impact. After an initial brightening of ~1.4 magnitudes, we find consistent dimming rates of 0.11-0.12 magnitudes/day in the first week, and 0.08-0.09 magnitudes/day over the entire study period. The system returned to its pre-impact brightness 24.3-25.3 days after impact through the primary ejecta tail remained. The dimming paused briefly eight days after impact, near in time to the appearance of the second tail. This was likely due to a secondary release of material after re-impact of a boulder released in the initial impact, through movement of the primary ejecta through the aperture likely played a role.Comment: 16 pages, 5 Figures, accepted in the Astrophysical Journal Letters (ApJL) on October 16, 202

The primordial nucleus of comet 67P/Churyumov-Gerasimenko

Context. We investigate the formation and evolution of comet nuclei and other trans-Neptunian objects (TNOs) in the solar nebula and primordial disk prior to the giant planet orbit instability foreseen by the Nice model. Aims: Our goal is to determine whether most observed comet nuclei are primordial rubble-pile survivors that formed in the solar nebula and young primordial disk or collisional rubble piles formed later in the aftermath of catastrophic disruptions of larger parent bodies. We also propose a concurrent comet and TNO formation scenario that is consistent with observations. Methods: We used observations of comet 67P/Churyumov-Gerasimenko by the ESA Rosetta spacecraft, particularly by the OSIRIS camera system, combined with data from the NASA Stardust sample-return mission to comet 81P/Wild 2 and from meteoritics; we also used existing observations from ground or from spacecraft of irregular satellites of the giant planets, Centaurs, and TNOs. We performed modeling of thermophysics, hydrostatics, orbit evolution, and collision physics. Results: We find that thermal processing due to short-lived radionuclides, combined with collisional processing during accretion in the primordial disk, creates a population of medium-sized bodies that are comparably dense, compacted, strong, heavily depleted in supervolatiles like CO and CO2; they contain little to no amorphous water ice, and have experienced extensive metasomatism and aqueous alteration due to liquid water. Irregular satellites Phoebe and Himalia are potential representatives of this population. Collisional rubble piles inherit these properties from their parents. Contrarily, comet nuclei have low density, high porosity, weak strength, are rich in supervolatiles, may contain amorphous water ice, and do not display convincing evidence of in situ metasomatism or aqueous alteration. We outline a comet formation scenario that starts in the solar nebula and ends in the primordial disk, that reproduces these observed properties, and additionally explains the presence of extensive layering on 67P/Churyumov-Gerasimenko (and on 9P/Tempel 1 observed by Deep Impact), its bi-lobed shape, the extremely slow growth of comet nuclei as evidenced by recent radiometric dating, and the low collision probability that allows primordial nuclei to survive the age of the solar system. Conclusions: We conclude that observed comet nuclei are primordial rubble piles, and not collisional rubble piles. We argue that TNOs formed as a result of streaming instabilities at sizes below ~400 km and that ~350 of these grew slowly in a low-mass primordial disk to the size of Triton, Pluto, and Eris, causing little viscous stirring during growth. We thus propose a dynamically cold primordial disk, which prevented medium-sized TNOs from breaking into collisional rubble piles and allowed the survival of primordial rubble-pile comets. We argue that comets formed by hierarchical agglomeration out of material that remained after TNO formation, and that this slow growth was a necessity to avoid thermal processing by short-lived radionuclides that would lead to loss of supervolatiles, and that allowed comet nuclei to incorporate ~3 Myr old material from the inner solar system

CUBES : the Cassegrain U-band Efficient Spectrograph

In the era of Extremely Large Telescopes, the current generation of 8-10m facilities are likely to remain competitive at ground-UV wavelengths for the foreseeable future. The Cassegrain U-Band Efficient Spectrograph (CUBES) has been designed to provide high-efficiency (> 40%) observations in the near UV (305-400 nm requirement, 300-420 nm goal) at a spectral resolving power of R >20, 000 (with a lower-resolution, sky-limited mode of R ~7, 000). With the design focusing on maximizing the instrument throughput (ensuring a Signal to Noise Ratio (SNR) ~20 per high-resolution element at 313 nm for U ~18.5 mag objects in 1h of observations), it will offer new possibilities in many fields of astrophysics, providing access to key lines of stellar spectra: a tremendous diversity of iron-peak and heavy elements, lighter elements (in particular Beryllium) and light-element molecules (CO, CN, OH), as well as Balmer lines and the Balmer jump (particularly important for young stellar objects). The UV range is also critical in extragalactic studies: the circumgalactic medium of distant galaxies, the contribution of different types of sources to the cosmic UV background, the measurement of H2 and primordial Deuterium in a regime of relatively transparent intergalactic medium, and follow-up of explosive transients. The CUBES project completed a Phase A conceptual design in June 2021 and has now entered the detailed design and construction phase. First science operations are planned for 2028

Ejecta Evolution Following a Planned Impact into an Asteroid: The First Five Weeks

The impact of the Double Asteroid Redirection Test spacecraft into Dimorphos, moon of the asteroid Didymos, changed Dimorphos’s orbit substantially, largely from the ejection of material. We present results from 12 Earth-based facilities involved in a world-wide campaign to monitor the brightness and morphology of the ejecta in the first 35 days after impact. After an initial brightening of ∼1.4 mag, we find consistent dimming rates of 0.11–0.12 mag day−1 in the first week, and 0.08–0.09 mag day−1 over the entire study period. The system returned to its pre-impact brightness 24.3–25.3 days after impact though the primary ejecta tail remained. The dimming paused briefly eight days after impact, near in time to the appearance of the second tail. This was likely due to a secondary release of material after re-impact of a boulder released in the initial impact, though movement of the primary ejecta through the aperture likely played a role

Achievement of the planetary defense investigations of the Double Asteroid Redirection Test (DART) mission

NASA's Double Asteroid Redirection Test (DART) mission was the first to demonstrate asteroid deflection, and the mission's Level 1 requirements guided its planetary defense investigations. Here, we summarize DART's achievement of those requirements. On 2022 September 26, the DART spacecraft impacted Dimorphos, the secondary member of the Didymos near-Earth asteroid binary system, demonstrating an autonomously navigated kinetic impact into an asteroid with limited prior knowledge for planetary defense. Months of subsequent Earth-based observations showed that the binary orbital period was changed by –33.24 minutes, with two independent analysis methods each reporting a 1σ uncertainty of 1.4 s. Dynamical models determined that the momentum enhancement factor, β, resulting from DART's kinetic impact test is between 2.4 and 4.9, depending on the mass of Dimorphos, which remains the largest source of uncertainty. Over five dozen telescopes across the globe and in space, along with the Light Italian CubeSat for Imaging of Asteroids, have contributed to DART's investigations. These combined investigations have addressed topics related to the ejecta, dynamics, impact event, and properties of both asteroids in the binary system. A year following DART's successful impact into Dimorphos, the mission has achieved its planetary defense requirements, although work to further understand DART's kinetic impact test and the Didymos system will continue. In particular, ESA's Hera mission is planned to perform extensive measurements in 2027 during its rendezvous with the Didymos–Dimorphos system, building on DART to advance our knowledge and continue the ongoing international collaboration for planetary defense