Mass Erosion and Transport on Cometary Nuclei, as Found on 67P/Churyumov-Gerasimenko

The Rosetta spacecraft rendezvoused with comet 67P/Churyumov-Gerasimenko in 2014–2016 and observed its surface morphology and mass loss process. The large obliquity (52°) of the comet nucleus introduces many novel physical effects not known before. These include the ballistic transport of dust grains from the southern hemisphere to the northern hemisphere during the perihelion passage, thus shaping the dichotomy of two sides, with the northern hemisphere largely covered by dust layers from the recycled dusty materials (back fall) and the southern hemisphere consisting mostly of consolidated terrains. A significant amount of surface material up to 4–10 m in depth could be transferred across the nucleus surface in each orbit. New theories of the physical mechanisms driving the outgassing and dust ejection effects are being developed. There is a possible connection between the cometary dust grains and the fluffy aggregates and pebbles in the solar nebula in the framework of the streaming-instability scenario. The Rosetta mission thus succeeded in fulfilling one of its original scientific goals concerning the origin of comets and their relation to the formation of the solar system.

AMBITION – comet nucleus cryogenic sample return

We describe the AMBITION project, a mission to return the first-ever cryogenically-stored sample of a cometary nucleus, that has been proposed for the ESA Science Programme Voyage 2050. Comets are the leftover building blocks of giant planet cores and other planetary bodies, and fingerprints of Solar System’s formation processes. We summarise some of the most important questions still open in cometary science and Solar System formation after the successful Rosetta mission. We show that many of these scientific questions require sample analysis using techniques that are only possible in laboratories on Earth. We summarize measurements, instrumentation and mission scenarios that can address these questions. We emphasize the need for returning a sample collected at depth or, still more challenging, at cryogenic temperatures while preserving the stratigraphy of the comet nucleus surface layers. We provide requirements for the next generation of landers, for cryogenic sample acquisition and storage during the return to Earth. Rendezvous missions to the main belt comets and Centaurs, expanding our knowledge by exploring new classes of comets, are also discussed. The AMBITION project is discussed in the international context of comet and asteroid space exploration.

In the context of Comet Interceptor: Unexpected polarimetric properties of some dust particles in cometary comae and on small bodies surfaces

<p>The ESA-JAXA Comet Interceptor mission is expected to flyby a dynamically new comet (or an interstellar one) and better reveal the properties of its dust particles and nucleus surface. We therefore tentatively compare polarimetric properties of dust released by some comets, as well as present on surfaces of some small bodies.</p><p>Phase curves of the linear polarization of cometary dust particles (observed in equivalent wavelength ranges) show analogous trends. Some unique dynamically new comets or fragmenting comets (e.g. C/1995 O1 Hale-Bopp, C/1999 S4 LINEAR) may nevertheless present a higher positive branch than Halley-type or Jupiter-family comets (e.g. 1P/Halley, 67P/Churyumov-Gerasimenko). Such differences are clues to differences in the properties (sizes, morphologies, complex optical indices) of the dust particles. Dust particles, ejected by nuclei frequently plunging in the inner Solar System, might indeed partly come from quite dense a surface layer, as detected on the small lobe of comet 67P by Rosetta [1].</p><p>Although polarimetric observations of surfaces of cometary nuclei are almost impossible, observations of the rather quiescent nucleus of 1P/Encke have been obtained [2].  Similarities between polarimetric properties of 1P/Encke and atypical small bodies (e.g. Phaeton and particularly Bennu [3]), and of dust in cometary comae may be pointed out. Numerical and laboratory simulations could represent a unique tool to better understand such similarities. It may also be added that dust particles originating from comets, with emphasis on those of Jupiter-family, may survive atmospheric entry, as CP-IDPs collected in the Earth’s stratosphere, and that dust found in debris disks of stellar systems shows levels of polarization similar to those of highly-polarized comets [4].</p><p> </p><p>[1] Kofman et al., MNRAS, 497, 2616-2622, 2020, [2] Boehnhardt et al., A&A, 489, 1337-1343, 2008. [3] Cellino et al., MNRAS, 481, L49-L53, 2018. [4] Levasseur-Regourd et al., PSS, 186, 104896, 2020,</p><p> </p>

Statistical analysis of dust grain tracks in Rosetta/OSIRIS images

<p>In some of the images taken by OSIRIS, pieces of debris can be seen as bright tracks instead of points sources as result of the combination of movements of both particles and spacecraft. The properties of those tracks, such as orientation, length and total brightness, depend on various comets parameters, including the activity on the nucleus surface, capable of lifting and accelerating the particles, and the characteristics of dust grains, as grain sizes, spatial distribution, velocity, density and sensitivity to radiation pressure. Previous works have focused on retrieving some of these grain properties from the mentioned images, but since the images show the 2D projection of the 3D dust motion, they rely on different methods to obtain the distance between the camera and the debris.</p> <p>In this work, a new method to bypass this distance determination requirement is proposed. The main steps involved are <em>(i)</em> analyze a large set of images containing tracks generated by moving dust grains, and obtain distribution for selected track parameters (orientation, length, total brightness, etc.) using an algorithm based on the Hough transform method; <em>(ii)</em> compare these results with the ones obtained from artificial images, generated by modeling the three dimensional motion of the debris in the gas flow field of the comet, under the influence of gravity, radiation pressure and gaseous drag; <em>(iii)</em> iterate this process in order to refine the parameters characterizing the physical properties of the dust emission used by the model.</p>

Polarization of disintegrating Comet C/2019 Y4 (ATLAS)

ABSTRACT We observe Comet C/2019 Y4 (ATLAS) before and after its disintegration while making polarimetric measurements over a wide range of phase angles. The disintegration event was marked with a dramatic growth of the positive polarization branch that is consistent with a large relative abundance of absorbing material of up to (96.5 ± 3.4) per cent. This polarization spike relaxed as the carbonaceous particles are preferentially swept from the coma due to solar-radiation pressure. The observations suggest that the primordial material stored within comets is extremely rich in carbonaceous material. The pristine cometary material is processed by subsequent solar interactions, forming a refractory crust on the nucleus surface. Polarimetry provides a means of measuring the volume ratio of carbonaceous material, and hence the weathering that has occurred on the comet due to these interactions. The polarimetric response of Comet C/2019 Y4 (ATLAS) appears similar to that of Comet C/1995 O1 (Hale-Bopp), except on few epochs that are similar to that of Comet C/1996 B2 (Hyakutake).

CRISPR/Cas9-based inactivation of human papillomavirus oncogenes E6 and E7 induces senescence in cervical cancer cells

ABSTRACTHuman papillomaviruses (HPVs) such as HPV16 and HPV18 can cause cancers of the cervix, vagina, vulva, penis, anus and oropharynx. Continuous expression of the HPV viral oncoproteins E6 and E7 are essential for transformation and maintenance of cancer cells. Therefore, therapeutic targeting of E6 and E7 genes can potentially be used to treat HPV-related cancers. Previous CRISPR/Cas9 studies on inactivation of E6 and E7 genes confirmed cell cycle arrest and apoptosis. Here we report that CRISPR/Cas9-based knockout of E6 and E7 can also trigger cellular senescence in HPV18 immortalized HeLa cells. Specifically, HeLa cells in which E6 and E7 were inactivated exhibited characteristic senescence markers like enlarged cell and nucleus surface area, increased β-galactosidase expression, and loss of lamin B1 with detection of cytoplasmic chromatin fragments. Furthermore, the knockout of HPV18 E6 and E7 proteins resulted in upregulation of p53/p21 and pRb/p21 levels in senescent cells. These senescent cells were devoid of characteristic apoptotic markers and re-introduction of codon-modified HPV18 E6 decreased p53 levels. Taken together, our study demonstrates that cellular senescence is as an alternative outcome of HPV oncogene inactivation by the CRISPR/Cas9 methodology.

CO-driven activity constrains the origin of comets

Context. An open question in the study of comets is the so-called cohesion bottleneck, that is, how dust particles detach from the nucleus. Aims. We test whether the CO pressure buildup inside the pebbles of which cometary nuclei consist can overcome this cohesion bottleneck. Methods. A recently developed pebble-diffusion model was applied here to comet C/2017K2 PANSTARRS, assuming a CO-driven activity. Results. (i) The CO-gas pressure inside the pebbles erodes the nucleus into the observed dust, which is composed of refractories, H2O ice and CO2 ice. (ii) The CO-driven activity onset occurs up to heliocentric distances of 85 au, depending on the spin orientation of the comet nucleus. (iii) The activity onset observed at ≈26 au suggests a low obliquity of the nucleus spin axis with activity in a polar summer. (iv) At 14 au, the smallest size of the ejected dust is ≈0.1 mm, consistent with observations. (v) The observed dust-loss rate of ≈200 kg s−1 implies a fallout ≥30%, a nucleus surface active area ≥10 km2, a CO-gas loss rate ≥10 kg s−1, and a dust-to-gas ratio ≤20. (vi) The CO-driven activity never stops if the average refractory-to-all-ices mass ratio in the nucleus is ≤4.5 for a nucleus all-ices-to-CO mass ratio ≈4, as observed in comets Hale–Bopp and Hyakutake. These results make comet C/2017K2 similar to the Rosetta target comet 67P/Churyumov–Gerasimenko. (vii) The erosion lifetime of cometary planetesimals is a factor 103 shorter than the timescale of catastrophic collisions. This means that the comets we observe today cannot be products of catastrophic collisions.

An Analysis of Regional H2O and CH3OH Production rates of Comet 67P from the MIRO Measurements

<p>The coma structure and gas production rates of H<sub>2</sub>O, CO, CH<sub>3</sub>OH, and NH<sub>3</sub> of comet 67P/Churyumov-Gerasimenko were investigated in detail by different instruments onboard the Rosetta spacecraft between August 2014 and September 2016. We analyzed the nadir-pointing microwave spectroscopic data from the MIRO experiment before and during the perihelion passage in June, July, and August 2015 in order to examine the solar zenith angle dependence of the gas production rates (Q) of H<sub>2</sub>O and CH<sub>3</sub>OH, respectively. From a detailed analysis of the spectral shapes of H<sub>2<sup>18</sup></sub>O at a frequency of 547.676 GHz and those of CH<sub>3</sub>OH at 553.146 GHz, the surface distributions of H<sub>2</sub>O and CH<sub>3</sub>OH can be mapped. We found that the Q-values generally increased with solar insolation. However, there also existed large variations that might be related to chemical heterogeneity of the cometary nucleus surface (or subsurface in individual geomorphological areas and at different spatial scales). A comparison of the CH<sub>3</sub>OH/H<sub>2</sub>O mixing ratio in the two lobes will also be attempted.</p>

How comets work: nucleus erosion versus dehydration

ABSTRACT We develop an activity model based on ice sublimation and gas diffusion inside cm-sized pebbles making-up a cometary nucleus. Our model explains cometary activity assuming no free parameters and fixing the nucleus surface temperature Ts, its gradient below the nucleus surface at thermal equilibrium, the pressure inside the porous pebbles, and the gas flux from them. We find that (i) the nucleus erosion rate and water vapour flux are independent of the nucleus refractory-to-ice ratio, which affects the dehydration rate only; (ii) water-driven dust ejection occurs in thermal quasi-equilibrium at Ts > 205 K; (iii) the smallest and largest ejected dust sizes depend on the nucleus surface temperature and its gradient at depths of few cm; and (iv) the water-driven nucleus erosion rate is independent of the water vapour flux. Regarding comet 67P/Churyumov–Gerasimenko, we find that (i) during the northern and southern polar summers, the nucleus active areas are ≈5 km2; (ii) >95 per cent of the southern pristine nucleus has a refractory-to-water-ice mass ratio >5; and (iii) the different temperature dependences of the dehydration and erosion rates explain the seasonal cycle: at perihelion, dm-sized chunks ejected by the sublimation of CO2 ices are rapidly enveloped by an insulating crust, preserving most water ice up to their fallout on the northern dust deposits; the inbound water-driven activity at low temperatures triggers a complete erosion of the fallout if its water-ice mass fraction is >0.1 per cent.

Pinnacles on the surface of the comet 67P/Churyumov-Gerasimenko: regional distribution and morphology

Pinnacles are local topographic promontories of different shapes considered to be formed due to uneven surface erosion. In the case of comets, areal changes in the degree of erosion could be related to inhomogeneities of the nucleus. However, the amount of solar radiation and the thermal gradient is different across the orbit for geomorphological regions, which can result in different erosion and shape for a similar composition among two differently illuminated areas. Therefore, a study of the areal distribution of pinnacles on the nucleus surface and their morphology may help to understand the structure and properties of the nucleus material. We mapped 166 pinnacles on the comet nucleus surface of 67P/Churyumov-Gerasimenko. About a third of them have planimetrically rounded shape (rounded pinnacles) and the rest are planimetrically elongated (local ridges). In the southern hemisphere, number of both round pinnacles and local ridges is larger than in the northern hemisphere. This difference possibly indicates the higher effectiveness of the pinnacles’ formation in the southern hemisphere. At the same time the mean values of the measured parameters, including the height, show no statistically reliable difference between the north and south. We found that the maximum height of the pinnacles is about a hundred meters. Suggesting that they have been formed by sublimational erosion, this value allows estimating the minimum thickness of the eroded material and thus the degree of the evolutionary changes of the nucleus. In our future study, we will model pinnacles formation based on the here presented analysis of observations.