Sublimation Of Ice Research Articles

Temperature and Concentration Affect Particle Size Upon Sublimation of Saline Ice: Implications for Sea Salt Aerosol Production in Polar Regions

AbstractUsing an environmental scanning electron microscope, we quantified for the first time aerosol‐sized salt particles formed during the sublimation of sea ice as a function of temperature and concentration. The sublimation temperature of the ice is a dominating physical factor to determine the size of the residua: Below −20°C, micron‐sized pieces of salt emerge, whereas above the temperature large chunks of salt are detected. Another such aspect influencing the distribution of sizes in salt particles is the concentration: Micron‐sized particles are observed exclusively at salinities below 3.5 psu, while below 0.085 psu particles with a median smaller than 6 μm arise from sea ices at any subzero temperature. Moreover, when a chunk of salt sublimes at less than −30°C to be dried and warmed later, a large number of sub‐micron crystals will appear. We relate our findings to the production of the polar atmospheric sea salt aerosols.

Thermal alteration and differential sublimation can create Phaethon's “rock comet” activity and blue color

In 2010 Jewitt and Li published a paper examining the behavior of “comet-asteroid transition object” 3200 Phaethon, arguing it was asteroid-like in its behavior throughout most of its orbit, but that near its perihelion, at a distance of only 0.165 AU from the sun, its dayside temperatures would be hot enough to vaporize rock (>1000 K, Hanus et al. 2016). Thus it would act like a “rock comet” as gases produced from evaporating rock were released from the body, in a manner similar to the more familiar sublimation of water ice into vacuum seen for comets coming within ~3 AU of the Sun. In this Note we predict that the same thermal effects that would create “rock comet” behavior with Qgas ~ 1022 mol/s at perihelion have also helped to greatly bluen Phaethon's surface via preferential thermal alteration and sublimative removal of Fe and refractory organics, known reddening and darkening agents. These predictions are testable by searching for signs of spectral bluening of the surfaces of other small bodies in Phaethon-like small perihelion orbits, including comets, and by in situ measurements of Phaethon's surface and coma composition near perihelion with the upcoming DESTINY+ mission (Kawakatsu & Itawa 2013, Arai et al. 2018) to Phaethon by JAXA.

Martian atmospheric water vapor abundances in MY26-30 from Mars Express PFS/LW observations

I report on retrievals of water vapor abundances in the Martian atmosphere using observations by Planetary Fourier Spectrometer Long Wavelength channel (PFS/LW) aboard the Mars Express (MEx) spacecraft. PFS/LW observations used in this work cover almost 4 Martian Years from Ls ~ 331° in MY26 (January 2004) to Ls ~ 209° in MY30 (December 2010). Vapor abundances were retrieved from nadir spectra collected over a range of latitudes, seasons and local hours. Most of the retrieved abundances correspond to daytime observations at local hours from 8 to 17 h. The spatial and temporal coverage of retrieved abundances varies significantly between years of observations, with the best coverage in MY27 and MY29. The average annual water vapor cycle observed by PFS/LW is qualitatively similar to the cycle observed by the Thermal Emission Spectrometer (TES) onboard Mars Global Surveyor (MGS), however the PFS/LW abundances are systematically lower. The likely reason for this difference is the difference in the respective water vapor absorption models. Comparison between PFS/LW abundances for time periods with low atmospheric dust opacity between different years of observations does not show significant interannual variability. PFS/LW abundances are significantly lower than the abundances retrieved from observations by Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) on Mars Reconnaissance Orbiter (MRO) in MY29. The difference is likely due to the differences in retrieval models for different spectral regions used by CRISM and PFS/LW to retrieve water vapor abundances and due to different environmental parameters used in the retrievals. PFS/LW abundances are generally similar to abundances retrieved by the SPICAM IR instrument, except in the polar regions during summer. The difference likely arises because of non-uniform vertical distribution of water vapor in the summer polar atmospheres and different sensitivities of the two instruments to vertical distribution of vapor. The PFS/LW observed systematic longitudinal dependence of scaled vapor abundances in the northern tropics with two maxima at longitudes ~30–60°E and ~ 240–270°E. This longitudinal dependence resembles a standing wave forced in the local atmospheric flow by topography and surface thermo-physical properties. The longitudinal non-uniformity of water vapor possibly shows a seasonal dependence increasing around solstices and decreasing around equinoxes. During northern spring in the Northern Polar Region (NPR) PFS/LW observed an increase of vapor near the edge of the seasonal cap interpreted as a buildup of vapor by repeated cycles of water ice freezing and sublimation at the cap edge. Similar, but weaker and longitudinally non-uniform increase in vapor near the edge of the seasonal cap is observed in the Southern Polar Region (SPR) during spring. Tentative signs of water vapor diurnal variability were observed in the PFS/LW data over northern mid-latitudes during late spring and early summer, potentially reflecting vapor exchange between the regolith and the atmosphere.

A major asymmetric ice trap in a planet-forming disk

The complex organic molecules (COMs) detected in star-forming regions are the precursors of the prebiotic molecules that can lead to the emergence of life. By studying COMs in more evolved protoplanetary disks we can gain a better understanding of how they are incorporated into planets. This paper presents ALMA band 7 observations of the dust and ice trap in the protoplanetary disk around Oph IRS 48. We report the first detection of dimethyl ether (CH3OCH3) in a planet-forming disk and a tentative detection of methyl formate (CH3OCHO). We determined column densities for the detected molecules and upper limits on non-detected species using the CASSIS spectral analysis tool. The inferred column densities of CH3OCH3 and CH3OCHO with respect to methanol (CH3OH) are of order unity, indicating unusually high abundances of these species compared to other environments. Alternatively, the 12CH3OH emission is optically thick and beam diluted, implying a higher CH3OH column density and a smaller emitting area than originally thought. The presence of these complex molecules can be explained by thermal ice sublimation, where the dust cavity edge is heated by irradiation and the full volatile ice content is observable in the gas phase. This work confirms the presence of oxygen-bearing molecules more complex than CH3OH in protoplanetary disks for the first time. It also shows that it is indeed possible to trace the full interstellar journey of COMs across the different evolutionary stages of star, disk, and planet formation.

The impact of the presence of water ice on the analysis of debris disk observations

Context. The analysis of debris disk observations is often based on the assumption of a dust phase composed of compact spherical grains consisting of astronomical silicate. Instead, observations indicate the existence of water ice in debris disks. Aims. We quantify the impact of water ice as a potential grain constituent in debris disks on the disk parameter values estimated from photometric and spatially resolved observations in the mid- and far-infrared. Methods. We simulated photometric measurements and radial profiles of debris disks containing water ice and analyzed them by applying a disk model purely consisting of astronomical silicate. Subsequently, we quantified the deviations between the derived and the true parameter values. As stars in central positions we discuss a β Pic sibling and main-sequence stars with spectral types ranging from A0 to K5. To simulate observable quantities we employed selected observational scenarios regarding the choice of wavelengths and instrument characteristics. Results. For the β Pic stellar model and ice fractions ≥50% the derived inner disk radius is biased by ice sublimation toward higher values. However, the derived slope of the radial density profile is mostly unaffected. Along with an increasing ice fraction, the slope of the grain size distribution is overestimated by up to a median factor of ~1.2 for an ice fraction of 90%. At the same time, the total disk mass is underestimated by a factor of ~0.4. The reliability of the derived minimum grain size strongly depends on the spectral type of the central star. For an A0-type star the minimum grain size can be underestimated by a factor of ~0.2, while for solar-like stars it is overestimated by up to a factor of ~4–5. Neglecting radial profile measurements and using solely photometric measurements, the factor of overestimation increases for solar-like stars up to ~7–14.

The Asteroid 162173 Ryugu: a Cometary Origin

Abstract The Japanese Hayabusa2 mission has revealed in detail the physical characteristics of the C-type asteroid 162173 Ryugu, in particular, its spinning top-shaped rubble-pile structure and potentially high organic content. A widely accepted formation scenario for Ryugu is catastrophic collision between larger asteroids and the subsequent slow gravitational accumulation of collisional debris. An alternative scenario is that Ryugu is an extinct comet that lost its icy components. Here, we numerically simulated the sublimation of water ice from a porous cometary nucleus until the refractory components, such as silicate rocks and organic matter, were left behind as evaporative residues. Such a process represents the transformation from a comet to an asteroid. The spin-up related to the shrinking nucleus, associated with water ice sublimation, was also calculated. The result of the calculation indicates that the cometary origin scenario can account for all the features of Ryugu discussed above. We conclude that organic-rich spinning top-shaped rubble-pile asteroids, such as Ryugu, are comet–asteroid transition objects or extinct comets.

Cometary Activity beyond the Planets

Abstract Recent observations show activity in long-period comet C/2017 K2 at heliocentric distances beyond the orbit of Uranus. With this as motivation, we constructed a simple model that takes a detailed account of gas transport modes and simulates the time-dependent sublimation of supervolatile ice from beneath a porous mantle on an incoming cometary nucleus. The model reveals a localized increase in carbon monoxide (CO) sublimation close to heliocentric distance r H = 150 au (local blackbody temperature ∼23 K), followed by a plateau and then a slow increase in activity toward smaller distances. This localized increase occurs as heat transport in the nucleus transitions between two regimes characterized by the rising temperature of the CO front at larger distances and nearly isothermal CO at smaller distances. As this transition is a general property of sublimation through a porous mantle, we predict that future observations of sufficient sensitivity will show that inbound comets (and interstellar interlopers) will exhibit activity at distances far beyond the planetary region of the solar system.

ESEM Methodology for the Study of Ice Samples at Environmentally Relevant Subzero Temperatures: "Subzero ESEM".

Frozen aqueous solutions are an important subject of study in numerous scientific branches including the pharmaceutical and food industry, atmospheric chemistry, biology, and medicine. Here, we present an advanced environmental scanning electron microscope methodology for research of ice samples at environmentally relevant subzero temperatures, thus under conditions in which it is extremely challenging to maintain the thermodynamic equilibrium of the specimen. The methodology opens possibilities to observe intact ice samples at close to natural conditions. Based on the results of ANSYS software simulations of the surface temperature of a frozen sample, and knowledge of the partial pressure of water vapor in the gas mixture near the sample, we monitored static ice samples over several minutes. We also discuss possible artifacts that can arise from unwanted surface ice formation on, or ice sublimation from, the sample, as a consequence of shifting conditions away from thermodynamic equilibrium in the specimen chamber. To demonstrate the applicability of the methodology, we characterized how the true morphology of ice spheres containing salt changed upon aging and the morphology of ice spheres containing bovine serum albumin. After combining static observations with the dynamic process of ice sublimation from the sample, we can attain images with nanometer resolution.

The rise time of the change of cometary brightness during its outburst

The paper presents a new method for determining the change in the brightness of a comet during an outburst in which particles are ejected into the coma at a uniform rate, i.e. via a long-lived jet. In the proposed approach, the mass flow rate by a jet and the time it takes to increase the brightness of the comet play key roles. Absolute values adopted for H2O ice and CO2 ice were taken from measurements by Rosetta of the jets observed on 67P/Churyumov–Gerasimenko. These two parameters significantly affect the amplitude of the change in the brightness of a comet during its outburst. The performed numerical calculations concern a hypothetical comet from the Jupiter family for which it was assumed that its sublimation activity was controlled by the sublimation of water ice and carbon dioxide. Based on numerical simulations, it was shown that the difference between the change in brightness for H2O ice and CO2 ice is about −3.4 magnitude for the same fraction of the area that is active and the same mass flow rate. This value corresponds to the rise time of the amplitude of the brightness change t = 3 h. In addition, it should be emphasized that this difference is a consequence of thermodynamic parameters, such as the speed of sublimating gas molecules, the sublimation rate of given ice, which determine the numerical values of the individual scattering cross-sections C(t1) and C(t2).

Near-infrared reflectance spectroscopy of sublimating salty ice analogues. Implications for icy moons

The composition of the surface of the Galilean icy moons has been debated since the Galileo mission. Several chemistries have been proposed to explain the composition of the non-icy component of the moon's surfaces, notably, sulphuric acid hydrates and magnesium and sodium sulphates. More recently, magnesium and sodium chlorides have been proposed to explain features observed in ground-based observations. We have considered four salts (NaCl, Na2SO4, MgSO4 and MgCl2) with various concentrations, to produce salty ice analogues. Granular particles were produced by a flash-freezing procedure. Additionally, compact slabs of salty ices were produced by a slow crystallisation of salty liquid solution. These two methods mimic the end-members (plumes and slow ice block formation) for producing hydrated salty ices on the surface of icy moons such as Europa and Ganymede. We have monitored the near-infrared (NIR) evolution of our salty ices during sublimation, revealing differences between the granular particles and the slabs. The slabs formed a higher amount of hydrates and the most highly hydrated compounds. Granular ices must be formed from a more concentrated salty solution to increase the amount of hydrates within the ice particles. The sublimation of salty ices removed all excess water ice efficiently, but the dehydration of the salts was not observed. The final spectra of the slabs were most flattened around 1.5 and 2.0 μm, especially for the Na2SO4, MgCl2 and MgSO4, suggesting the presence of stable, highly hydrated compounds. We find that Na2SO4, MgCl2 and MgSO4 are most compatible with the non-icy component at the surface of the icy moons as observed by the NIMS instrument on Galileo and by ground-based observations.

Simulation of Martian Dust Effects on Polar CO2 Ice Caps and Atmospheric Circulation Using the MarsWRF Model

AbstractMartian dust plays an important role in modulating climate. However, the effects of dust on polar CO2 ice caps, atmospheric circulation processes and the relationship between them are not yet clear. In this study, a Mars general circulation model (MarsWRF) was applied to investigate the impacts of the magnitude and timing of Martian dust storms on the spatiotemporal characteristics of polar CO2 ice caps and atmospheric circulation. The results show that dust can inhibit the sublimation of southern CO2 ice through the radiation effect. Under the “High Dust” scenario, the ice edge can extend up to 5° more equatorward than that under the “Standard” scenario around Ls = 220°. The extent of northern CO2 ice cap is hardly affected by dust because the impacts induced by dust mainly appear north of 80°N. By shifting the timing of maximum dust loading during the dust season, the "Early Dust" scenario inhibits the sublimation of the southern ice cap, while the "Late Dust" scenario accelerates the sublimation process. In addition, the variations in dust loading strength can lead to changes in three near‐surface wind belts, which are related to intensified Hardly circulation. The appearance of the warming vortex under the "Early Dust" scenario is delayed, and its intensity is 60 K lower than that observed in the "High Dust" and "Late Dust" scenarios. When dust loading increases, the northern CO2 condensation process in the Western Hemisphere accelerates, which is due to the increased pole‐to‐equator temperature gradient and the increased meridional wind speed.

Five‐Year Analysis of Evaposublimation Characteristics and Its Role on Surface Energy Balance SEB on a Midlatitude Continental Glacier

AbstractAs a principal pathway of moisture loss of cryosphere, the sublimation of snow or ice needs to absorb a high amount of energy to complete the phase transition, while related researches on how sublimation affects surface energy balance (SEB) are still insufficient, causing unclear conclusions. To characterize the seasonal variation of a mountain glacier's sublimation or evaporation and analysis its role on SEB, the meteorological and glaciological data over five‐year measurement period (2016–2020) were obtained on the top of August‐one glacier. In the non‐melt period of a year (January–May and October–December), sensible heat flux H was the dominant energy contributor to glacier surface (76%), followed by net radiation Rnet (22%). Sublimation removed average 85 mm (0.4 mm/day) of water vapor from surface for the dry and windy weather, and the latent heat flux of sublimation LEE accounted for 33% of energy sinks (67% of energy was stored in the glacier Qs). In the melt period (June–September), evaposublimation (mainly composed of evaporation) reached to 36 mm (0.3 mm/day), but LEE only played a small heat sink (14%) and the Qm (the energy for melting) had the largest proportion (70%). Rnet and H contributed 81% and 15% of surface energy, respectively. Obviously, under the global climate warming, the sublimation of non‐melt period seems to be effective in decrease the heating rate of glacier surface.

Observations on the Changing Shape of the Ice Mass and the Determination of the Sublimation End Point in Freeze-Drying: An Application for Through-Vial Impedance Spectroscopy (TVIS).

Models for ice sublimation from a freeze-drying vial rely on the assumption of a planar ice interface up to ~25% loss of ice mass (which is difficult to qualify) whereas single-vial determinations of the sublimation endpoint (by temperature sensors) are based on the point when the observed temperature reaches a plateau, which cannot differentiate between sublimation and desorption-drying. In this work, the real part capacitance of TVIS vial(s) containing frozen water (during sublimation drying) was measured at 100 kHz. This parameter was shown to be highly sensitive to the shape and volume of the ice mass and is therefore a useful parameter for monitoring ice sublimation. By placing a digital camera in front of an isolated TVIS vial containing ice, it was possible to relate the changes in the shape of the ice mass with the changes in the trajectory of the time profile of and determine the point of deviation from a planar ice interface and ultimately determine the point when the last vestiges of ice disappear. Thereafter, the same characteristics of the time-profile were identified for those TVIS vials located out of sight of the camera in a separate full-shelf lyo study, thereby obviating the need for photographic examination.

A cryo-TSEM with temperature cycling capability allows deep sublimation of ice to uncover fine structures in thick cells

The scanning electron microscope (SEM) has been reassembled into a new type of cryo-electron microscope (cryo-TSEM) by installing a new cryo-transfer holder and anti-contamination trap, which allowed simultaneous acquisition of both transmission images (STEM images) and surface images (SEM images) in the frozen state. The ultimate temperatures of the holder and the trap reached − 190 °C and − 210 °C, respectively, by applying a liquid nitrogen slush. The STEM images at 30 kV were comparable to, or superior to, the images acquired with conventional transmission electron microscope (100 kV TEM) in contrast and sharpness. The unroofing method was used to observe membrane cytoskeletons instead of the frozen section and the FIB methods. Deep sublimation of ice surrounding unroofed cells by regulating temperature enabled to emerge intracellular fine structures in thick frozen cells. Hence, fine structures in the vicinity of the cell membrane such as the cytoskeleton, polyribosome chains and endoplasmic reticulum (ER) became visible. The ER was distributed as a wide, flat structure beneath the cell membrane, forming a large spatial network with tubular ER.

Which molecule traces what: Chemical diagnostics of protostellar sources

Context. The physical and chemical conditions in Class 0/I protostars are fundamental in unlocking the protostellar accretion process and its impact on planet formation. Aims. The aim is to determine which physical components are traced by different molecules at subarcsecond scales (<100–400 au). Methods. We used a suite of Atacama Large Millimeter/submillimeter Array (ALMA) datasets in band 6 (1 mm), band 5 (1.8 mm), and band 3 (3 mm) at spatial resolutions 0.″5–3″ for 16 protostellar sources. For a subset of sources, Atacama Compact Array (ACA) data at band 6 with a spatial resolution of 6″ were added. The availability of low- and high-excitation lines and data on small and larger scales, is important to understand the full picture. Results. The protostellar envelope is well traced by C18O, DCO+, and N2D+, which stems from the freeze-out of CO governing the chemistry at envelope scales. Molecular outflows are seen in classical shock tracers such as SiO and SO, but ice-mantle products such as CH3OH and HNCO that are released with the shock are also observed. The molecular jet is a key component of the system. It is only present at the very early stages, and it is prominent not only in SiO and SO, but occasionally also in H2CO. The cavity walls show tracers of UV-irradiation such as C2H, c-C3H2 and CN. In addition to showing emission from complex organic molecules (COMs), the hot inner envelope also presents compact emission from small molecules such as H2S, SO, OCS, and H13CN, which most likely are related to ice sublimation and high-temperature chemistry. Conclusions. Subarcsecond millimeter-wave observations allow us to identify these (simple) molecules that best trace each of the physical components of a protostellar system. COMs are found both in the hot inner envelope (high-excitation lines) and in the outflows (lower-excitation lines) with comparable abundances. COMs can coexist with hydrocarbons in the same protostellar sources, but they trace different components. In the near future, mid-infrared observations with JWST–MIRI will provide complementary information about the hottest gas and the ice-mantle content, at unprecedented sensitivity and at resolutions comparable to ALMA for the same sources.

A comprehensive method to prevent top-coal spontaneous combustion utilizing dry ice as a fire extinguishing medium: test apparatus development and field application.

There is a high potential for coal spontaneous combustion (CSC) above the roof beams of supports during the mining-stopped period. Early detection of temperature abnormal zones and corresponding measures are necessary to prevent CSC. In this work, a top-coal temperature measurement method was proposed, combining the coal surface temperature detection and the drilling temperature observation. Furthermore, an apparatus was developed that dramatically increases the rate of dry ice sublimation, resulting in the rapid release of cryogenic carbon dioxide gas. The device utilizes water from firefighting pipes in underground coal mines as a heat source for dry ice sublimation without electrical energy and has been applied and validated taking Silaogou Coal Mine in China as a field test site. Specifically, we found that during the stoppage period, the coal above the supports near the air inlet tunnel is more likely to appear hot spots; the carbon dioxide gas generated by the dry ice phase change device can quickly reduce the hot spots temperature, and the coal temperature does not rebound after the gas injection is stopped. Based on the above analysis, this work can effectively prevent the early top-coal spontaneous combustion during the stop mining period.

Laser solid-phase synthesis of single-atom catalysts

Single-atom catalysts (SACs) with atomically dispersed catalytic sites have shown outstanding catalytic performance in a variety of reactions. However, the development of facile and high-yield techniques for the fabrication of SACs remains challenging. In this paper, we report a laser-induced solid-phase strategy for the synthesis of Pt SACs on graphene support. Simply by rapid laser scanning/irradiation of a freeze-dried electrochemical graphene oxide (EGO) film loaded with chloroplatinic acid (H2PtCl6), we enabled simultaneous pyrolysis of H2PtCl6 into SACs and reduction/graphitization of EGO into graphene. The rapid freezing of EGO hydrogel film infused with H2PtCl6 solution in liquid nitrogen and the subsequent ice sublimation by freeze-drying were essential to achieve the atomically dispersed Pt. Nanosecond pulsed infrared (IR; 1064 nm) and picosecond pulsed ultraviolet (UV; 355 nm) lasers were used to investigate the effects of laser wavelength and pulse duration on the SACs formation mechanism. The atomically dispersed Pt on graphene support exhibited a small overpotential of −42.3 mV at −10 mA cm−2 for hydrogen evolution reaction and a mass activity tenfold higher than that of the commercial Pt/C catalyst. This method is simple, fast and potentially versatile, and scalable for the mass production of SACs.

Volatility of Sodium in Carbonaceous Chondrites at Temperatures Consistent with Low-perihelion Asteroids

Abstract Solar system bodies with surface and subsurface volatiles will show observational evidence of activity when they reach a temperature where those volatiles change from solid to gas and are released. This is most frequently seen in comets, where activity is driven by the sublimation of water, carbon dioxide, or carbon monoxide ices. However, some bodies (notably the asteroid (3200) Phaethon) show initiation of activity at very small heliocentric distances, long after they have reached the sublimation temperatures of these ices. We investigate whether the sodium present in the mineral matrix could act as the volatile element responsible for this activity. We conduct theoretical modeling which indicates that sodium has the potential to sublimate in the conditions that Phaethon experiences, depending on the mineral phase it is held in. To test this, we then exposed samples of the carbonaceous chondrite Allende to varying heating events, similar to what would be experienced by low-perihelion asteroids. We measured the change in sodium present in each sample and find that the highest temperature samples show a significant loss of sodium from specific mineral phases over a single heating event, comparable to a day on the surface of Phaethon. Under specific thermal histories possible for Phaethon, this outgassing could be sufficient to explain this object’s observed activity. This effect would also be expected to be observed for other low-perihelion asteroids as well and may act as a critical step in the process of disrupting small low-albedo asteroids.

Evaluation of infrared assisted freeze drying for strawberry snacks: drying kinetics, energy efficiency and quality attributes.

Feasibility of infrared assisted freeze drying (IRAFD) was evaluated for production of the strawberry snacks. Infrared (IR) radiation provided the driving force of ice sublimation during freeze drying (FD). Different IRAFD conditions were tested, including the continuous IRAFD-1.6kW/m2 and IRAFD-1.6kW/m2 at different weight reductions (20%, 40%, and 60%). Conventional FD had a total drying time of 691 ± 19min, whereas continuous IRAFD significantly reduced the drying time to 309 ± 32min. Continuous IRAFD also reduced the amount of consumed electrical energy by 42% compared to that of FD. A long duration of IR radiation produced a soft texture in the snacks. Drying kinetics were analyzed using various models, including the Page model, exponential model, and Henderson and Pabis model. The Page model provided the best fit to the experimental drying curve. This study showed the potential of IRAFD in producing value-added fruit snacks with good textural quality and efficient use of energy.

Disintegration of Long-period Comet C/2019 Y4 (ATLAS). I. Hubble Space Telescope Observations

Abstract The near-Sun comet C/2019 Y4 (ATLAS) is the first member of a long-period comet group observed to disintegrate well before perihelion. Here we present our investigation into this disintegration event using images obtained in a three-day Hubble Space Telescope campaign. We identify two fragment clusters produced by the initial disintegration event, corresponding to fragments C/2019 Y4-A and C/2019 Y4-B identified in ground-based data. These two clusters started with similar integrated brightness but exhibit different evolutionary behavior. C/2019 Y4-A was much shorter-lived compared to C/2019 Y4-B and showed signs of significant mass loss and changes in size distribution throughout the three-day campaign. The cause of the initial fragmentation is undetermined by the limited evidence but crudely compatible with either the spin-up disruption of the nucleus or runaway sublimation of subsurface supervolatile ices, either of which would lead to the release of a large amount of gas as inferred from the significant bluing of the comet observed shortly before its disintegration. Gas can only be produced by the sublimation of volatile ices, which must have survived at least one perihelion passage at a perihelion distance of q = 0.25 au. We speculate that Comet ATLAS is derived from the ice-rich interior of a nonuniform, kilometer-wide progenitor that split during its previous perihelion. This suggests that comets down to a few kilometers in diameter can still possess complex, nonuniform interiors that can protect ices against intense solar heating.