Rubble pile asteroids such as (162173) Ryugu have large bulk porosities, which are believed to result from void spaces in between the constituent boulders (macroporosity) as well as void spaces ...within the boulders themselves (microporosity). In general, both macroporosity and microporosity are estimated based on comparisons between the asteroid bulk density and both the bulk and grain density of meteorite analogs, and relatively large macroporosities are usually obtained. Here we use semiempirical models for the macroporosity of multicomponent mixtures to determine Ryugu's macroporosity based on the observed size‐frequency distribution (SFD) of boulders on the surface. We find that Ryugu's macroporosity can be significantly smaller than usually assumed, as the observed SFD allows for an efficient packing of boulders, resulting in a macroporosity of 16% ± 3%. Therefore, we confirm that Ryugu's high bulk porosity is a direct consequence of a very large boulder microporosity. Furthermore, using estimates of boulder microporosity of around 50% as derived from in situ measurements, the average grain density in boulders is 2,848 ± 152 kg m–3, similar to values obtained for CM and the Tagish lake meteorites. Ryugu's bulk porosity corresponding to the above values is 58%. Thus, the macroporosity of rubble pile asteroids may have been systematically overestimated in the past.
Plain Language Summary
The carbonaceous asteroid (162173) Ryugu formed from fragments which reaccreted after its parent body was disrupted by a catastrophic collision. Asteroids of this type are also known as rubble piles and the reaccumulation process is thought to be one of the causes for their large bulk porosity. We have applied mixing models to determine the amount of interboulder porosity taking the observed abundance of large and small boulders on the surface into account. We find that the relative abundances of differently sized boulders allow for a very efficient packing, such that interboulder porosity in Ryugu is rather small and only 16% ± 3%. This implies that a large part of Ryugu's total porosity must reside inside the boulders themselves. Using estimates of boulder intrinsic porosity, we furthermore constrain the average density of the boulder's constituent minerals to 2,848 ± 152 kg m−3, which is consistent with values measured for carbonaceous meteorites as collected on Earth. Thus, interboulder porosity of rubble pile asteroids may have been systematically overestimated in the past.
Key Points
Ryugu's large bulk porosity is distributed between intrinsic boulder microporosity and macroporosity due to void spaces in‐between boulders
We use the boulder size‐frequency distribution as observed on the surface together with mixing models to estimate Ryugu's macroporosity
We find that macroporosity is 16% ± 3%, indicating that Ryugu's large bulk porosity of close to 50% is governed by microporosity
Context.
After landing on C-type asteroid Ryugu, MASCOT imaged brightly colored, submillimeter-sized inclusions in a small rock. Hayabusa2 successfully returned a sample of small particles from the ...surface of Ryugu, but none of these appear to harbor such inclusions. The samples are considered representative of Ryugu.
Aims.
To understand the apparent discrepancy between MASCOT observations and Ryugu samples, we assess whether the MASCOT landing site, and the rock by implication, is perhaps atypical for Ryugu.
Methods.
We analyzed observations of the MASCOT landing area acquired by three instruments on board Hayabusa2: a camera (ONC), a near-infrared spectrometer (NIRS3), and a thermal infrared imager. We compared the landing area properties thus retrieved with those of the average Ryugu surface.
Results.
We selected several areas and landforms in the landing area for analysis: a small crater, a collection of smooth rocks, and the landing site itself. The crater is relatively blue and the rocks are relatively red. The spectral and thermophysical properties of the landing site are very close to those of the average Ryugu surface. The spectral properties of the MASCOT rock are probably close to average, but its thermal inertia may be somewhat higher.
Conclusions.
The MASCOT rock can also be considered representative of Ryugu. Some of the submillimeter-sized particles in the returned samples stand out because of their atypical spectral properties. Such particles may be present as inclusions in the MASCOT rock.
A review of volatiles in the Martian interior Filiberto, Justin; Baratoux, David; Beaty, David ...
Meteoritics & planetary science,
November 2016, Letnik:
51, Številka:
11
Journal Article
Recenzirano
Odprti dostop
Multiple observations from missions to Mars have revealed compelling evidence for a volatile‐rich Martian crust. A leading theory contends that eruption of basaltic magmas was the ultimate mechanism ...of transfer of volatiles from the mantle toward the surface after an initial outgassing related to the crystallization of a magma ocean. However, the concentrations of volatile species in ascending magmas and in their mantle source regions are highly uncertain. This work and this special issue of Meteoritics & Planetary Science summarize the key findings of the workshop on Volatiles in the Martian Interior (Nov. 3–4, 2014), the primary open questions related to volatiles in Martian magmas and their source regions, and the suggestions of the community at the workshop to address these open questions.
Large extensional tectonic surface structures on Mars are investigated on the basis of image and topographic data. These features have lengths of up to 1400
km, typical widths of tens of kilometers, ...and depths of up to 3000
m. Their structural architecture is characterized by segmentation with changing style, the development of asymmetric basins with changing polarity, and accommodation zones between such basins. Most structures are associated with varying degrees of volcanism. Based on these observations and a comparison to terrestrial continental rifts, we interpret these series of basins or grabens as rift systems. Crater counts of rift-related geologic surface units indicate that rifts on Mars are ∼
4.0 to 3.5
Ga old. Modelling of rift flank uplift yields heat flows ranging between 28 and 66
mW
m
−
2
, and a corresponding thickness of the elastic lithosphere in the order of ∼
10
km at the time of rifting. The radial orientation of some rifts with respect to Tharsis is consistent with a rifting mechanism related to regional extensional stresses in the lithosphere. Rifts with other orientations might have formed in response to stress fields induced by horizontal gradients of the gravitational potential energy, modulated by magmatic zones of lithospheric weakness.
The Hayabusa2 mission provided a unique data set of asteroid Ryugu that covers a wide range of spatial scale from the orbiter remote sensing instruments to the returned samples. The MASCOT lander ...that was delivered onto the surface of Ryugu aimed to provide context for these data sets by producing in situ data collected by a camera (MasCam), a radiometer (MARA), a magnetometer (MasMag) and a spectrometer (MicrOmega). In this work, we evaluate the success of MASCOT as an integrated lander to bridge the gap between orbiter and returned sample analysis. We find that MASCOT’s measurements and derivatives thereof, including the rock morphology, colour in the visible wavelengths, possible meteorite analogue, density, and porosity of the rock at the landing site are in good agreement with those of the orbiter and the returned samples. However, it also provides information on the spatial scale (sub-millimetres to centimetres) at which some physical properties such as the thermal inertia and reflectance undergo scale-dependent changes. Some of the in situ observations such as the presence of clast/inclusions in rocks and the absence of fine particles at the landing site was uniquely identified by MASCOT. Thus, we conclude that the delivery of an in situ instrument like MASCOT provides a valuable data set that complements and provides context for remote sensing and returned sample analyses.
Graphical Abstract
We propose the exploration of polar areas on Mars by a next-generation orbiter mission. In particular, we aim at studying the seasonal and regional variations in snow-deposits, which – in combination ...with measurements of temporal variations in rotation and gravity field – will improve models of the global planetary CO
2
cycle. A monitoring of polar scarps for rock falls and avalanche events may provide insights into the dynamics of ice sheets. The mapping of the complex layering of polar deposits, believed to contain an important record of climate history, may help us understand the early climate collapse on the planet. Hence, we propose an innovative next-generation exploration mission in polar circular Low Mars Orbit, which will be of interest to scientists and challenging to engineers alike. Schemes will be developed to overcome atmosphere drag forces acting upon the spacecraft by an electric propulsion system. Based on the experience of missions of similar type in Earth orbit we believe that a two-year mission in circular orbit is possible at altitudes as low as 150 km. Such a mission opens new opportunities for novel remote sensing approaches, not requiring excessive telescope equipment or power. We anticipate precision altimetry, powerful radars, high-resolution imaging, and magnetic field mapping.
MASCOT, the Mobile Asteroid Surface SCOuT, is a small lander jointly developed by the German and French space agencies Ho et al., 2017, that travelled on board of the JAXA Hayabusa2 spacecraft for ...over 3 years to the C-type asteroid Ryugu. The goal of MASCOT was to perform in situ measurements on the surface of the asteroid by means of its four scientific instruments, substantially contributing in this way to the overall scientific return of Hayabusa2 mission.
The objective of the paper is to provide a detailed overview of the Landing Site Selection Process (LSSP) for MASCOT, from the preliminary design phase that started several years before launch, up to the actual execution of the selection process and its operational implementation. The effort that was put on the LSSP by all the teams involved over all these years was one of the key elements, leading to the unprecedented success of this mission.
Mars is believed to have possessed a dynamo that ceased operating approximately 4 Ga ago, although the exact time is still under debate. The scope of this study is to constrain the possible timing of ...its cessation by studying the magnetization signatures of craters. The study uses the latest available model of the lithospheric magnetic field of Mars, which is based on Mars Global Surveyor data. We tackle the problem of nonuniqueness that characterizes the inversion of magnetic field data for the magnetization by inferring only the visible part of the magnetization, that is, the part of the magnetization that gives rise to the observed magnetic field. Further on, we demonstrate that a zero visible magnetization is a valid proxy for the entire magnetization being zero under the assumption of a magnetization distribution of induced geometry. This assumption holds for craters whose thermoremanent magnetization has not been significantly altered since its acquisition. Our results show that the dynamo shut off after the impacts that created the Acidalia and SE Elysium basins and before the crust within the Utopia basin cooled below its magnetic blocking temperature. Accounting for the age uncertainties in the dating of these craters, we estimate that the dynamo shut off at an N(300) crater retention age of 2.5–3.2 or an absolute model age of 4.12–4.14 Ga. Moreover, the Martian dynamo may have been weaker in its early stage, which if true implies that the driving mechanism of the Martian dynamo was not the same throughout its history.
Plain Language Summary
Mars is believed to have possessed a dynamo that ceased operating approximately 4 Ga ago, although the exact time is still under debate. The scope of this study is to constrain the possible timing of its cessation by studying the magnetization signatures of craters. The study uses the latest available model of the lithospheric magnetic field of Mars, which is based on Mars Global Surveyor data. Since the complete magnetization cannot be uniquely recovered by magnetic field measurements, we infer the visible part of the magnetization, that is, the part of the magnetization that gives rise to the observed magnetic field. Further on, we demonstrate that a zero visible magnetization is a valid proxy for craters whose thermoremanent magnetization has not been significantly altered since its acquisition. Our results show that the dynamo shut off after the impacts that created the Acidalia and SE Elysium basins and before the crust within the Utopia basin cooled down. Accounting for the age uncertainties in the dating of these craters, we estimate that the dynamo shut off at an absolute model age of 4.12–4.14 Ga. Moreover, the Martian dynamo may have been weaker in its early stage, which if true implies that that the driving mechanism of the Martian dynamo was not the same throughout its history.
Key Points
A visible magnetization model of Mars is obtained up to SH degree and order 110
A zero visible magnetization is shown to be a valid proxy for a crater's entire magnetization being zero in the absence of major alterations
The Martian dynamo ceased during the Noachian era
The Mars Exploration Program Analysis Group has identified measurements of the state and the variability of the Martian atmosphere as high priority investigations for the upcoming years. ...Balloon-borne instruments could bridge the gap in both temporal and spatial resolution in mesoscale distances between local, stationary landers and global orbiter observations. The idea to use a balloon system for such a purpose is not new in essence and has been proposed already in past decades. While those concepts considered an aerial deployment during entry and descent, the concept outlined in this study revisits a launch off the payload deck of a lander from the Martian surface. This deployment option profits today mainly from the technological advance in micro-electronics and sensor miniaturization, which enables the design of a balloon-probe significantly smaller than earlier proposed systems. This paper presents the feasibility assessment for this instrument and gives further details on the scientific and operational concept, a strawman sensor suite, its system components and the associated size and budget estimates. It is complemented by the analysis scheme proposed to assess, manage and mitigate the deployment risk involved in automatically launching such a balloon-system off a planetary surface.
We have investigated the influence of thin thermally opaque dust layers on the thermal emission of rocks and regolith and determined the thermal response of these dust-covered surfaces to diurnal ...insolation cycles. Results are computed for Hayabusa2’s target asteroid (162173) Ryugu, which was observed by thermal infrared instruments on the orbiter and in situ. We show that even a very thin (10..100 μm) fine-grained porous dust layer with thermal inertia of 25 J m
−2
K
−1
s
−1/2
can have a significant influence on surface temperatures and alter the apparent thermal inertia of the underlying material derived under the simplified assumption of a homogenous half space by more than 20%. The masking of the underlying material is complete at about 1 diurnal skin depth, corresponding to ~ 10 mm on Ryugu. Between 0.1 and 1 diurnal skin depths, we find a thermal lag smaller than what would be predicted for a surface consisting of dust only.
If a dust cover were present on Ryugu, this should be clearly visible in the data returned by the orbiter’s thermal infrared imager (TIR) and the MASCOT lander’s radiometer (MARA), which observed a single boulder at the landing site. However, this appears not to be the case, and dust seems to play a minor role in the thermal emission from the asteroid.
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