1,721,160 research outputs found
Asteroid detection at millimetric wavelengths with the PLANCK survey
No full textThe PLANCK mission, originally devised for cosmological studies, offers the opportunity to observe Solar System objects at millimetric and submillimetric wavelengths. In this paper we concentrate on the asteroids of the Main Belt, a large class of minor bodies in the Solar System. At present, more that 40000 of these asteroids have been discovered and their detection rate is rapidly increasing. We intend to estimate the number of asteroids that can be detected during the mission and to evaluate the strength of their signal. We have rescaled the instrument sensitivities, calculated by the LFI and HFI teams for sources fixed in the sky, introducing some degradation factors to properly account for moving objects. In this way a detection threshold is derived for asteroidal detection that is related to the diameter of the asteroid and its geocentric distance. We have developed a numerical code that models the detection of asteroids in the LFI and HFI channels during the mission. This code performs a detailed integration of the orbits of the asteroids in the timespan of the mission and identifies those bodies that fall in the beams of PLANCK and their signal strength. According to our simulations, a total of 397 objects will be observed by PLANCK and an asteroidal body will be detected in some beam in 30% of the total sky scan-circles. A significant fraction (in the range from ~50 to 100 objects) of the 397 asteroids will be observed with a high /S/N ratio. Flux measurements of a large sample of asteroids in the submillimeter and millimeter range are relevant since they allow to analyze the thermal emission and its relation to the surface and regolith properties. Furthermore, it will be possible to check on a wider base, the two standard thermal models, based on a nonrotating or rapidly rotating sphere. Our method can also be used to separate Solar System sources from cosmological sources in the survey. This work is based on PLANCK LFI activities
DATING THRUST SYSTEM ON MERCURY.
No full textMercury is a planet dominated by contractional features at a global scale. The age deter-mination of these features will permit to estimate the rate of global contraction and whether limits could be placed on when the contraction occurred. This will give us new clues to better understanding the thermal evolution of the planet
Using Advanced Geometric Models in Image Matching with High Resolution Space Images
No full textHigh resolution Digital Terrain Models (DTMs), suita-ble for geomorphological studies of planets and aster-oids, are today among the main scientific goals of space missions. In the last decades, the great progress in high-resolution imaging of planetary surfaces (with ground-sample-distance that can reach 25 cm/pixel) finds two significant examples in the High Resolution Imaging Science Experiment (HiRISE) on Mars Re-connaissance Orbiter, and in the NAC of the Lunar Reconnaissance Orbiter Camera (LROC) on LRO. These images have provided the widest data-volume ever obtained before from any space mission and are characterized by strong potentialities associated with their astonishing capability of acquire details and fea-tures on the planetary surfaces.
The process for deriving DTMs starting from these raw data is very complex because it has to answer to two main requirements: operate very accurately and work with extremely large data volumes.
Nowadays the most important institutes involved in the planetary mapping are working on developing strategies to fulfil these requests. Despite the introduc-tion of a series of new algorithms for image matching (e.g. the Semi Global Matching: [1]) that yield superior results especially in qualitative terms (smooth and con-tinuous surfaces) and in terms of processing time, the common trend in the planetary-photogrammetry field stays in opting for the established area-based tech-niques and the efforts are more to improving each sin-gle phase of the photogrammetric process (from the image pre-processing stage to the final interpolation of the DTMs).
In this context, the Dense Matcher software (DM) developed at University of Parma has been recently optimized to cope with very high resolution images provided by the most recent missions (LROC NAC and HiRISE) putting the efforts mainly at the correlation phase and at the improvement of the process automa-tion.
In order to improve the performance of the software, a new image correlation code based on advanced Least Squares Matching (LSM) algorithms has been devel-oped.
Perspective changes due to terrain morphology are difficult to accommodate by an area-based stereo cor-relator. The solution has been the use of an iterative algorithm to adapt the correlation window with differ-ent shape functions. Many authors [2] found that the use of a simplified shape function leads to lower com-putational efforts but provides lower accuracy when significant changing in the terrain curvature occurs. Also Bethmann [3] showed that using different shape functions to model the geometric transformation in LSM can bring higher accuracy and solve, in some cases, numerical problems like pixel-locking. In this context, the new DM software uses, rather than the common affine transformation, alternative functional models in the geometrical transformation involved dur-ing LSM to handle perspective differences.
At the same time, working with orbital-space images, usually means to be able to manage large amount of data. Due to that, in order to guarantee good computa-tional performances, efforts have been put in the opti-mization of the processes developing new strategies (grid-matching and tile approach).
Since the stereo-recontruction is strongly correlated to the quality of the image-correlation process (that in many cases can produce outliers and mis-matches) leading to uncorrected interpretations of the topogra-phy, a comparison between uncorrelated data (that don’t participate in the generation of the DTM ) as the laser height points can be used to assess the effective height resolution and the exterior quality (accuracy) of the photogrammetric product.
The paper first describes the main features of the optimized version of Dense Matcher, with particular regard to the mathematical model implemented in the Least Squares Matching. Then, the performance of the image correlation kernel of the program is evaluated through comparisons with DTMs generated by other well established software like Socet Set by Bae System and Ames Stereo Pipeline (NASA) on HiRISE stereo pairs. Finally a comparison is also made with the DTMs produced on NAC stereo-pairs by the Vicar software by DLR (German Aerospace Center), as well as against the LOLA altimeter tracks.
The results look very promising and represent a concrete proof of the capability of Dense Matcher in dealing with two of the most significant examples of high resolution orbital imaging: HiRISE images and the ones acquired by NAC
FEM MODELING AND FRACTAL ANALYSIS OF CONCENTRIC AND RADIAL STRUCTURES ON ASCRAEUS MONS: IMPLICATIONS FOR MAGMA CHAMBER DEPTH
No full textBeagle Rupes and Rembrandt scarp: a comparison on Mercury surface.
No full textAreas of crustal convergence and shortening on Earth,
Mars and Venus are often characterized by structural
domains where thrust faults are associated with
strike-slip systems. In the case of Mercury most
structures maintain a wide elongated frontal scarp
and only few of them show kinematic indicators of
lateral slip. The comparison between two Mercurian
linked fault systems like Beagle Rupes and
Rembrandt scarp could aid understanding of
whether diverse hermean strike-slip structures are
influenced by geological context such as surface
heterogeneity and crustal layering or have different
deformational history
Self-similar clustering distribution of structural features on Ascraeus Mons (Mars): implications for magma chamber depth
No full textThe occurrence and spatial distribution of monogenic eruptive structures within volcanic areas are linked to
fracture systems and associated stress fields.
Moreover, they testify the presence of deep crustal or subcrustal magma reservoirs directly connected to the surface
by a percolating fracture network. The correlation between vent distribution and fracture network properties (the
so called backbone) can thus be studied in terms of self-similar (fractal) clustering.
Self-similarity in vent distribution is described by a power law distribution with fractal exponent D and defined
over a range of lengths (l) comprised between a lower limit (lower cutoff, Lco) and an upper limit (upper cutoff,
Uco). The upper cutoff (Uco) for fractal clustering was compared with the respective crustal thickness obtained by
existing independent geophysical data in the East African Rift System (Mazzarini and Isola, 2010). The computed
Ucos for this sector well match the crustal thickness in these volcanic fields. More in detail this computational
model verified the strong linear relationship existing between the upper cutoff of the power law distribution and
the magma source depth.
This method was thus applied to Ascraeus Mons on Mars, which displays basaltic magmatism and hundreds
of collapse pits and vents around its flanks, giving a robust statistic to the calculations. Basing on a structural
mapping performed on HRSC (High Resolution Stereo Camera onboard the ESA Mars Express mission) at 12
m/px and CTX (Context Camera, Mars Reconnaissance Orbiter mission) at 6 m/px mosaics, more than 2300
collapse pits and vents were analysed. Data analyses displayed a clustering in the structures distribution, showing
two distinct populations.
The obtained Uco values revealed the presence and the likely depth of both a deep big magma chamber and a
small shallower chamber placed below the main caldera.
Moreover, the resulting magma source depths are completely consistent and comparable with those obtained by
FEM (Finite Element Method) calculations on the stress field caused by magma chamber overpressure. In fact,
the opening of cracks on Ascraeus flanks, suitable for sill injections and vent formation, as well as their peculiar
distribution are the surficial manifestation of depth-dependent magma chamber stress field.
Estimates by two different independent approaches (self-similar analysis and FEM modelling) converge giving
thus a strong constrain on the presence and position of the magma sources below Ascraeus Mons
FEM modelling of concentric and radial structures on Ascraeus Mons
No full textAscraeus Mons is the northernmost of the three large
shield volcanoes on the Tharsis Rise on Mars. In this
work we test the hypothesis that the latest volcanotectonic
evolution of the Ascraeus Mons could have
been driven by an oblate magma chamber inducing
injection of sheeted and radial dikes with distribution
that recall the ones of the Cuillins cone sheet
complex on the isle of Skye (Scotland). Indeed in this
latter case Finite Element Modelling (FEM) has
demonstrated that the distribution of dykes predict an
oblate shallow magma chamber. On the basis of an
accurate mapping of the Ascraeus Mons structures on
high resolution stereo camera (HRSC) image mosaic,
we were able to recognize concentric and radial
structures, and to assess their interactions and
attitudes. These remote sensing observations,
combined with other physical parameters such as
crustal thickness, critical distance of transition
between concentric and radial fracture systems,
gravitational load of the volcanic body, allow to infer,
trough a FEM analysis, the possible presence of an
oblate magma chamber at the time of the youngest
volcano-tectonic event, its dimensions, depth and the
tensional state of the system within particular
overpressure/inflation conditions within the magma
chamber itself. In addition the age of the
deformational event that formed the fracture patterns
was calculated through crater counting
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Ghost images determination for the Stereoscopic Imaging Channel of SIMBIOSYS for the BepiColombo ESA mission
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