Thermal Balance and Water Ice Sublimation on the Surface of Hyperactive Comet 103P/Hartley 2
https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/2022JE007329
Yuxian Yue et al, 2022
According to the sublimation law, it is estimated that the sublimation temperature of water ice on comet 103P is above 180 K (-93 C). If the dust-to-ice volume ratio is 3:1, the sublimation temperature is about 200 K to 210 K (-73 C to -63 C), which indicates that the water ice may sublimate underneath. This may explain why exposed water ice on the surface can hardly be observed while the active fraction of this comet is up to 100%.
Key Points :
- The sublimation of the water ice on the comet nucleus absorbs energy, which significantly reduces the infrared radiation on the surface
- The high water production rate of hyperactive comets may be due to the sublimation of water ice in the surface layer of the comet nucleus
Plain Language Summary :
Extremely active comets, known as hyperactive comets, release water at very high rates, although observations indicate that they have little exposed water ice on their surfaces. We found a significant energy deficit in the comet surface spectroscopic data, suggesting that the sublimation of water ice may be prevalent in the layers just below the nucleus surface. The sublimation of these water ices may have provided an appreciable water production rate by hyperactive comets.
Temperature dependence of the sublimation rate of water ice: Influence of impurities
https://www.sciencedirect.com/science/article/abs/pii/S0019103514000566
Konrad Kossacki & Jacek Leliwa-Kopystynski, 2014
The sublimation rate of ice is commonly calculated using the simple Hertz–Knudsen formula. This formula is derived from the kinetic theory of gases and ignores microphysical processes determining the actual sublimation rate.
Numerical simulations presented in Kossacki and Markiewicz paper indicate, that derivation of the temperature below the dust mantle from the measured water production rate, ignoring temperature dependence of the sublimation coefficient, can lead to an underestimate of the temperature by more than 10 K. Thus, it is important to know the dependence on the sublimation coefficient of the composition of the real cometary ice, which can be far from purity.
We intended to check whether a small amount of dissolved minerals can affect the temperature dependence of the sublimation coefficient of ice. According to our experiments the answer is positive.
Sublimation of buried cometary ice
https://www.sciencedirect.com/science/article/abs/pii/S0019103518302975
Konrad Kossacki et al, 2019
The sublimation of ice covered by dust is a common way of outgassing of comets. The rate of this process depends on the thermal conductivity and permeability of dust, as well as the properties of the ice itself. It can be calculated using the Hertz-Knudsen equation, corrected for temperature dependent sublimation coefficient αs(T) and the Knudsen equation for the permeability of dust.
In this work we present direct experimental verification of this set of equations using sand with different granulation (grain radii rg) :
- coarse (rg = 0.5 − 1 mm)
- medium (rg = 0.25 − 0.5 mm)
- fine (rg = 0.125 − 0.25 mm)
- unsorted (rg = 0 − 1 mm, rg, av = 0.5 mm)
We have found that at Temps < 220 K (-53 C) the measured rate of recession of the surface is correctly reproduced if αs(T) is the same as determined for the uncovered ice.
The activity of comets is driven by the sublimation of ices. Observations of comets 9P/Tempel 1 and 67P indicate that the bare (uncovered) ice surface can be found only locally. Thus, the sublimation of ice covered by dust is a more common process. When the surface of ice is uncovered, the rate of sublimation is typically calculated using the classical Hertz-Knudsen equation. Originally, the equation was derived assuming equilibrium distribution of the velocities of molecules when the fluxes of molecules condensing on the surface and leaving it are not equal. Thus, the Hertz-Knudsen equation is only approximation. It can be corrected using temperature dependent sublimation coefficient. It has been proved, that presence of a small amount of admixtures in the water ice can significantly affect the sublimation coefficient.
Unfortunately, most of the spectroscopic measurements were made in cometary comas. Therefore, they concern mainly global (average) composition. Local composition of the nucleus was determined after the first touchdown of the probe Philae and at its final landing site.
Remote and in-situ measurements of the composition of comet 67P were presented by Altwegg et al. (2017). The determined values of the average density are always lower than the density of compact ice, e.g. for comet 67P it is 533 kg/m^3 (Patzold et al., 2016). This indicates high porosity and low strength. Concluding, the incomplete knowledge about cometary nuclei means that the samples used in laboratory experiments only approximate the actual cometary material.
The formation of a layer of dust, even a few millimeters thick, can be sufficient to stop emission of water vapor (Poch et al., 2016).
In typical experimental conditions the samples are kept on a cooling plate and illuminated from above. This induces strong temperature gradient in the forming sublimation deposit. The emission of vapor molecules when the surface is not warmed and the dust layer is nearly isothermal. The results of various types of cometary simulation experiments were recently summarized in Gudipati et al. (2015).
The presence of solid ice under the surface of the comet is unlikely, but in our research we did not intend to reconstruct the material that created the comet. We only study the effect of granulation of the surface layer on the rate of degassing. For this purpose, it is important to keep constant the thickness of the dust layer. This condition may not be met if the ice is mixed with dust grains. In this case, the sublimation of ice releases grains of dust. It, in turn, can lead to a thickening of the dust layer.
The main features of our investigations are:
- The measurements were performed under high vacuum of 0.01 Pa (0.00007501 torr)
(my calculated pressure for AtlasV rocket at avg altitude of 70 km, was 0.03 torr/ 4 Pa)
- We use only two components: pure water ice and sand. A thin layer of fine sand can be blown out during experiments as a result of ice sublimation. Despite this, we do not use any additional material protecting grains against ejecting by gas. To avoid ejection, we use sand instead of dust and we perform measurements at very low temperatures. We do not use protecting materials to avoid unnecessary differences between the sample and the real material composing a comet.
- The theoretical rate of the recession is calculated using the Hertz-Knudsen equation with the temperature dependent sublimation coefficient and the Clausing equation for the flow of gas through the non-volatile layer.
- The accuracy of the calculated sublimation rate is enhanced by including correction for the presence of the actual pressure in the vacuum chamber.
Space Power Facility (SPF)
https://en.wikipedia.org/wiki/Space_Power_Facility
Location : near Sandusky, Ohio
Thermal-Vacuum Test Chamber : The facility can
- sustain a high vacuum (10^−6 torr, 130 μPa)
- simulate solar radiation via a 4 MW quartz heat lamp array
- solar spectrum by a 400 kW arc lamp
- cold environments (-195.6 °C) with a variable geometry cryogenic cold shroud.
Exposed Water Ice Deposits on the Surface of Comet 9P/Tempel 1
https://www.researchgate.net/publication/7318279_Exposed_Water_Ice_Deposits_on_the_Surface_of_Comet_9PTempel_1
JM Sunshine, et al, 2006
Nonetheless, the temperature derived for the water ice regions varies from 285 K to 295 K (12 C to 22 C). These temperatures are significantly above 200 K (-73 C), the free sublimation temperature of water ice at a distance of 1.5 AU from the Sun and an incidence angle of 55 degrees. Thus, the temperatures measured indicate that pure water ice cannot fill a complete IR pixel.
Exposed Water Ice Deposits on the Surface of Comet 9P/Tempel 1 (no pics)
https://www.science.org/cms/asset/9f143f2e-17fb-456a-9f40-4d414d495c66/pap.pdf
JM Sunshine, et al, 2006
An Experimental Study of the Sublimation of Water Ice and the Release of Trapped Gases
https://science.gsfc.nasa.gov/691/cosmicice/reprints/IcGas1.pdf
Hudson & Donn, 1989, 1991
The temperature, thermal inertia, roughness and color of the nuclei of Comets 103P/Hartley 2 and 9P/Tempel 1
https://www.sciencedirect.com/science/article/abs/pii/S0019103512004095
Groussin et al, 2013
The two comets have the same color, moderately red. There are very small variations of the color across the surface, except for regions with water ice that are neutral to blue, and two dark spots with redder materials on Hartley 2.
Moreover, the color temperature of Comets Hartley 2 and Tempel 1 is relatively homogeneous across the surface and does not vary strongly with incidence angle. These two effects mainly result from surface roughness and associated projected shadows.
For Hartley 2 and Tempel 1, the temperature of the regions with exposed water ice was 300+ K / 27+ C. That is more than 100 degrees above the 200 K (-73 C) sublimation temperature of water ice. This observation indicates that the thermal emission is dominated by dust, and that water ice is not intimately mixed with dust at the scale of observation, with water ice patches at the meter or sub-meter scale.
-163 C is too warm for ice to exist on surface or sub-surface on the permanently shadowed floor of Shackleton crater on south pole of our moon, where no sunlight ever reaches. Since, it is almost exactly at the south pole, it is unlikely that any other craters with permanently shadowed region further away from south pole are going to be colder than Shackleton crater.
Link - https://www.shadowcam.asu.edu/images/1284
Investigating Mercury’s South Polar Deposits : Arecibo Radar Observations and High-Resolution Determination of Illumination Conditions
https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1002/2017JE005500
Nancy Chabot et a, 2018
There is strong evidence that Mercury’s polar deposits are water ice hosted in permanently shadowed regions. In this study, we present new Arecibo radar observations of Mercury’s south pole, which reveal numerous radar-bright deposits and substantially increase the radar imaging coverage.
Fig 5 shows huge boulders on the floor of Chao Meng-Fu crater (180 km), at Mercury’s south pole. These are not NOT called as water ice boulders in Mercury's case, unlike in the case of "house sized ice boulders" for Enceladus' south pole. In both cases, boulders are exposed to low angle sunlight, on the open surface.
The area of radar-bright deposits in Mercury’s south is roughly double that found in the north, consistent with the larger permanently shadowed area in the older, cratered terrain at the south relative to the younger smooth plains at the north. Radar-bright features are strongly associated with regions of permanent shadow at both poles, consistent with water ice being the dominant component of the deposits.
However, both of Mercury’s polar regions show that roughly 50% of permanently shadowed regions lack radar-bright deposits, despite some of these locations having thermal environments that are conducive to the presence of water ice. The observed uneven distribution of water ice among Mercury’s polar cold traps may suggest that the source of Mercury’s water ice was not a steady, regular process but rather that the source was an episodic event, such as a recent, large impact of a comet on the innermost planet.
Average temperatures are relevant for deposits that are insulated from extreme daytime temperatures, perhaps by burial under centimeters of regolith. Based on the rates shown in Fig. 1, 1 m of water ice evaporates in 1 billion years at a temperature of approximately 110 K (-163 C). Temperatures of approximately 60 K (-213 C), 70 K (-203 K), and 220 K (-53 C) are required to sustain deposits of CO2 (carbon dioxide), NH3 (Ammonia), and Sα (Sulfur), respectively. The values increase by about 20 K if the time scale is reduced to 1 million years or if the initial thickness is increased to 1 km. We use the above temperature limits to predict where radar-detectable deposits could survive the age of the Solar System.
Killen et al. (1997) estimate that at temperatures less than 110 K (-163 C), the influx of water from meteorites and asteroids balances or exceeds all global losses, providing another reason to suspect that this temperature is an appropriate limit for the stability of water ice deposits. We calculate temperatures using bare ground albedos, assuming that all volatiles must migrate to the polar regions and be cold trapped over bare ground.
This reduced area of the permanently shadowed region in Chao Meng-Fu corresponds strongly to the radar-bright area in this crater, which is shown and discussed in section 4.
While radar-bright deposits are located in regions of permanent shadow, roughly half of the permanently shadowed area in Mercury’s south polar region is not mapped as being radar-bright, a percentage that is very similar to that found for Mercury’s north polar region (Deutsch et al., 2016). One explanation is that these permanently shadowed regions do contain water ice, but the water ice was not detected by the radar observations, due to limitations in the radar viewing geometries or the sensitivity of the radar observations.
While radar observations of Mercury’s south polar region are limited to essentially two distinct viewing geometries, the 2005 and 2012 observations provide nearly opposite viewing directions and both views consistently lack or detect radar-bright features for the polar region.
Additionally, the north polar Arecibo radar observations cover an extensive range of viewing directions, and modeling suggests that radar visibility limitations cannot explain the lack of radar-bright features associated with some sizable permanently shadowed regions in Mercury’s north.
Thus, we favor the alternate explanation that these permanently shadowed locations lack water ice deposits or that the water ice deposits are more deeply buried in these locations to not be detected by the radar observations. Either scenario of this explanation would imply that water ice is not distributed evenly among viable cold traps near Mercury’s poles.
Aug 2012
Walls of Lunar Crater May Hold Patchy Ice, LRO Radar Finds
https://www.nasa.gov/mission_pages/LRO/news/shackleton-ice.html
Radar data indicate that the walls of Shackleton crater may hold ice. As much as 5 to 10 percent of material, by weight, could be patchy ice.
"Several of the instruments on LRO have made unique contributions to this story, but only the radar penetrates beneath the surface to look for signatures of blocky ice deposits." The observations indicate an enhanced radar polarization signature, which is consistent with the presence of small amounts of ice in the rough inner wall slopes of the crater. "The interior of this crater lies in permanent shadow and is a 'cold trap'-a place cold enough to permit ice to accumulate," says Mini-RF's principal investigator, Ben Bussey of the JHUAPL. "The radar results are consistent with the interior of Shackleton containing a few percent ice mixed into the dry lunar soil."
Numerous lines of evidence from recent spacecraft observations have revised the view that the lunar surface is a completely dry, inhospitable landscape. Thin films of water and hydroxyl have been detected across the lunar surface using several space-borne near-infrared spectrometers. Additionally, orbital neutron measurements indicate elevated levels of near‐surface hydrogen in the polar regions; if in the form of water, this hydrogen would represent an average ice concentration of about 1.5% by weight in the polar regions.
The Shackleton findings are also consistent with those of the recent LCROSS spacecraft's controlled collision with a nearby permanently shadowed polar region near the lunar South Pole, which revealed evidence for water in the plume kicked up by its impact. A radar instrument flown on India's Chandrayaan-1 spacecraft in 2009 found evidence for ice deposits in craters at the lunar North Pole. Measurements of the albedo (surface reflectance) inside Shackleton crater using LRO's laser altimeter and far‐ultraviolet detector are also consistent with the presence of a small amount of ice.
"Inside the crater, we don't see evidence for glaciers like on Earth," says Thomson. "Glacial ice has a whopping radar signal, and these measurements reveal a much weaker signal consistent with rugged terrain and limited ice."
The radar measurements of Shackleton crater were made during three separate observations between December 2009 and June 2010. Radar illuminates shadowed regions and can detect deposits of water or ice, which have a distinctive radar polarization signature compared to the surrounding material. In addition, radar penetrates the terrain to depths of a meter or two and can measure water or ice buried beneath the surface. Radar measurements of Shackleton crater place an upper bound on the ice content of the uppermost meter of loose material of the crater's walls at between five and ten percent ice by weight.
Near-Surface Temperatures on Mercury and the Moon and the Stability of Polar Ice Deposits
https://typeset.io/papers/near-surface-temperatures-on-mercury-and-the-moon-and-the-1wewptadh7
Ashwin Vasavada et al, 1998
The surface temperature at the pole is 174 K / -99 C (Mercury) and 159 K / -114 C (Moon). Buried water ice deposits are stable to thermal sublimation within 2 degrees latitude of the lunar poles. Temperatures at the poles below the extent of the temperature oscillation are 147 K / -126 C (Mercury) and 93 K /-180 C (Moon). These are based on data + temperature models.
Water ice deposits can survive on the floors of 40-km craters as far as 8◦ latitude from the poles, beyond which their permanently shaded area is warmer than 110 K (-163 C).
One of the goals of this study is to better determine the temperatures of surfaces on Mercury that produce ice-like radar responses. Craters very near the poles of Mercury, such as craters C and D, contain large regions in which surface temperatures never exceed 110 K (-163 C). The regions where the model predicts stable surface or subsurface ice deposits are consistent with the sizes of the radar features at those locations.
More surprising, radar features are seen within craters where model surface temperatures greatly exceed 110 K (-163 C), such as craters S and T. The coldest regions within those craters have maximum surface temperatures above 145 K (-128 C) and cover a very small fraction of their crater floors. Most of the floor within each crater is not permanently shaded and experiences much higher temperatures.
Model calculated surface temperatures within craters N, P, Q, R, S, T, and Y on Mercury are significantly above the limit for long-term stability of water ice. Yet these craters contain large radar features on the Arecibo maps. What accounts for this discrepancy ?
Perhaps our assumed temperature limit should be higher. The limit of 110 K was calculated using vapor pressure data extrapolated by several orders of magnitude beyond lab measurements at 132 K. However, the limit would have to be 150 K for our model to predict surface ice deposits within all of the observed craters. A limit of 190 K is more consistent with the sizes of the observed radar features.
Perhaps the deposits are composed of elemental sulfur and the calculated temperature limit is 220 K (-53 C). With this limit our model results match the radar features, but as pointed out by Butler (1997), they also predict a (sunlit) surface polar cap 1◦ latitude wide. A polar cap is neither observed in Mariner 10 imagery nor suggested by the Arecibo radar mapping.
It seems implausible that surface ice deposits are present in several craters that contain radar features. Instead we suggest that the deposits are buried beneath the attenuation depth of sunlight driven temperature oscillations and remain at a constant temperature nearly equal to the average surface temperature.
A regolith cover limits evaporative loss by acting as a barrier to diffusing molecules. This could raise the effective temperature limit by tens of degrees Kelvin, as discussed in Salvail and Fanale (1994). A thin regolith cover would also protect the deposits from sputtering by solar wind ions and ablation by interstellar UV.
Although lunar ice deposits have not been unambiguously detected by radar, the results from the neutron spectrometer aboard Lunar Prospector are consistent (although not uniquely) with the presence of relatively pure water ice buried under tens of centimeters of regolith at the Moon’s poles (Feldman et al. 1998).
Nearly every crater on Mercury that has both a relatively undegraded rim and regions where temperatures permit the stability of surface or subsurface water ice also has a radar feature associated with it. In other words, Mercury’s available cold traps are full.
Our model results for craters observed near the Moon’s poles suggest that lunar cold traps are larger and colder than those within craters on Mercury, mostly because of the smaller solar flux at the Moon.
Several factors other than temperature have undoubtedly influenced whether the Moon’s cold traps are as full as Mercury’s. The Moon may have had a significantly larger obliquity in its early history (Ward 1975) which would have greatly reduced the amount of cold, permanently shaded area.
If impacts of large comets are the primary sources of water ice and occur on billion-year time scales (Arnold 1979), the abundance of ice at a given epoch could vary greatly between the bodies.
Our major conclusions are that :
(i) water ice deposits on unshaded surfaces (polar caps) are not stable against sublimation over the age of the Solar System on either body
(ii) unshaded subsurface ice is stable within 2◦ latitude of the lunar poles
(iii) ice deposits within the permanently shaded portions of impact craters are stable as far as 10◦ and 13◦ latitude from the poles of Mercury and the Moon, respectively
(iv) ice deposits are stable within all of the craters observed to produce an ice-like radar response on Mercury, although some deposits must be insulated from extreme daytime temperatures by a regolith cover. Burial under several centimeters of regolith provides protection from several important loss processes and is consistent with observations of ice deposits on both Mercury and the Moon.
Modifications of comet materials by the sublimation process : results from simulation experiments
https://ntrs.nasa.gov/api/citations/19900007305/downloads/19900007305.pdf
E Gruen & KOSI-team
An active comet like comet Halley loses by sublimation a surface layer of the order of 1 m thickness per perihelion passage. In situ measurements show that water ice is the main constituent which contributes to the gas emission although even more volatile species (CO, NH3, CH4, CO2 etc.) have been identified. Dust particles which were embedded in the ices are carried by the sublimating gases. Measurements of the chemical composition of cometary grains indicate that they are composed of silicates of approximate chondritic composition and refractory carbonaceous material. Comet simulation experiments show that significant modifications of cometary materials occur due to sublimation process in near surface layers which have to be taken into account in order to derive the original state of the material.
In-Space Propulsion Engine Architecture Based on Sublimation of Planetary Resources
https://www.nasa.gov/sites/default/files/atoms/files/niac_2011_phasei_sibille_inspacepropulsionfro_isrusublimationtagged.pdf
Simulations of ice chemistry in cometary nuclei
https://arxiv.org/abs/1909.03366
Robin Garrod, 2019
The composition of the ices of dozens of cometary nuclei has been inferred through millimeter-band and infrared observations of the gaseous comae around active comets. These measurements indicate that the nuclear ices are dominated by water, with CO and CO2 comprising up to 20–30% (with respect to water) in some cases, while trace amounts of other organics, such as methanol (CH3OH) and methane (CH4), are detected.
However, so-called complex organic molecules (COMs, typically defined as organics of 6 or more atoms), such as methyl formate (CH3OCHO), glycolaldehyde (CH2OHCHO) and ethylene glycol [(CH2OH)2], have also been identified through remote observations. The Stardust mission to comet Wild 2 returned samples containing the simplest amino acid, glycine (NH2CH2COOH). The same molecule was more recently identified in comet 67P as part of the Rosetta mission (Altwegg et al. 2016). Using Rosetta’s ROSINA spectrometer, Altwegg et al. (2016) detected glycine in the coma of 67P near perihelion.
Reaching its target in 2014, the mission culminated in the touchdown of the Philae lander on the comet’s surface. Data from the COSAC mass spectrometer was used to identify 16 molecules that were released from the surface as a result of the impact and heating associated with the landing (Goesmann et al. 2015), with abundances on the order of 0.1–1% of water abundance. Four of these molecules – propanal (C2H5CHO), acetone (CH3COCH3), acetamide (CH3CONH2) and methyl isocyanate (CH3NCO) – represented entirely new detections of complex organic molecules in a comet.
H2S has been detected in various comets with abundances in the range 0.1–1% H2O. For reasons of simplicity, sulfur is therefore omitted from the model, which is nevertheless in keeping with the lack of detection of interstellar solid-phase H2S. Future models will consider the inclusion of sulfur chemistry, by the adoption of more varied initial ice abundances including a range of sulfur-bearing species.
The dust in observed comets in fact displays a distribution of sizes, from submicron to macroscopic scales; as summarized by Agarwal et al. (2007), much of it is composed of aggregates of sub micron-sized subunits. Bieler et al. (2015) also measured hydrogen peroxide abundance in 67P, obtaining a value 6×10−4 for H2O2/O2.
Thermophysical model for icy cometary dust particles
https://arxiv.org/abs/2009.13208
Johannes Markkanen & Jessica Agarwal, 2020
Examining the Potential Contribution of the Hokusai Impactto Water Ice on Mercury
https://agupubs.onlinelibrary.wiley.com/doi/epdf/10.1029/2018JE005552
Carolyn Ernst et al, 2018 (2nd author Nancy Chabot)
Imaging Mercury's polar deposits during MESSENGER's low-altitude campaign
https://lib.jhuapl.edu/media/filer_public/bd/ca/bdca2af3-d24d-460a-8beb-10de7da1da8a/chabot_et_al-2016-geophysical_research_letters.pdf
Nancy Chabot et al, 2016
Additionally, brightness variations across the deposits correlate with variations in the biannual maximum surface temperature across the permanently shadowed regions, supporting the conclusion that multiple volatile organic compounds are contained in Mercury's polar deposits, in addition to water ice. A recent large impact event or ongoing bombardment by micrometeoroids could deliver water as well as many volatile organic compounds to Mercury. Either scenario is consistent with the distinctive reflectance properties and well-defined boundaries of Mercury's polar deposits and the presence of volatiles in all available cold traps.
Two of the main observations from the low-altitude polar deposits campaign provide evidence that the delivery of volatiles to Mercury either occurred in the geologically recent past or is continuing through an ongoing process.
3 main observations :
- low-reflectance boundaries are well defined but also have an approximately 400 m wide transition zone. The nature of the transition zone, whether due to mixing or to small-scale variations in the amount of shadow, is not currently known.
- Every permanently shadowed crater imaged during MESSENGER's low-altitude imaging campaign hosts low-reflectance material, supporting the notion that all of Mercury's available polar cold traps are occupied by volatiles & were emplaced more recently than the formation time of any of the polar cold traps.
- Mercury's polar water ice and volatiles are delivered during impacts from comets, asteroids, and/or micrometeoroids.
There are a large number of organic compounds that are each stable as solids up to a temperature higher than the maximum for water ice but only for temperatures <250 K (-23 C), and primitive objects, such as comets, asteroids, or micrometeoroids, are thought to contain a wide range of such volatile compounds. In particular, Desprez shows evidence for different surface volatiles in areas with maximum temperatures <150 K (-123 C) than are present where maximum temperatures range between 150 K (-123 C) and 250 K (-23 C).
A substantial source of Mercury's polar deposits is from either the impact onto Mercury's surface of a large comet or volatile-rich asteroid, within the last few tens of millions of years or the continuous and ongoing delivery of primitive micrometeoroids. A large recent impact on Mercury would leave a young crater on the surface, and investigations to identify the source crater, such as the study focused on Hokusai crater, are well motivated by the results from MESSENGER's low-altitude polar deposits campaign.
Alternatively, primitive micrometeoroids could also deliver water and organic compounds to Mercury and many more micrometeoroids impact Mercury than the Moon. Micrometeoroid impacts provide a continuing source of volatiles to Mercury, and hence the observations in this study of distinct reflectance properties, well-defined boundaries, and volatiles in all polar cold traps could be consistent with such a source.
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