Description: Publication year: 2011 Source: Planetary and Space Science, Available online 29 December 2011 R.P. Kane The interplanetaryBz(min)and geomagneticDst(min) are well related. There is no large negativeBzwithout causing large negativeDst. Similarly, large negativeDstis not expected to occur without a large negativeBz; but recently,reported that theDststorm of Jan. 21–22, 2005 wasanomalousbecause theDststorm main phase developed duringnorthwardinterplanetary magnetic field (positiveBz). Here, we examined this event (storm 3) and compared it with earlier events of Jan. 16–20, 2005 (storms 1 and 2, in quick succession). It was noticed that storms 1 and 2 had large negativeDstdeviations (−121 nT and −93 nT, separated by 26 hours) but theBzfluctuated between +21 nT and −17 nT. For storm 3 where negativeDstdeviations were large (−105 nT), theBzwas not completely positive (northward) as mentioned bybut had negative values (−7 nT) for an hour, followed by positive values (13 nT). Thus, the need of a negativeBz(albeit small) was satisfied. Highlights ► When a ICME engulfs Earth, a geomagnetic storm (Dst) occurs provided ► the ICME hás a substantial negative Bz component of magnetic field ► For Jan. 16−26, 2005, storm development during positive Bz was pointed out. ► This was not true. It was a delayed effect of a short-lived negative Bz

Description: Publication year: 2011 Source: Planetary and Space Science, Available online 19 December 2011 Conor A. Nixon, Ralph D. Lorenz

Description: Publication year: 2011 Source: Planetary and Space Science, Available online 21 December 2011 F. Tosi, F. Capaccioni, A. Coradini, S. Erard, G. Filacchione, ... On 10 July 2010, the Rosetta spacecraft encountered the asteroid 21 Lutetia on its way to the comet 67P/Churyumov-Gerasimenko. This was the second of two planned asteroid fly-bys performed by the probe, the first fly-by being with the much smaller asteroid 2867 Steins in September 2008. The VIRTIS imaging spectrometer (angular resolution 0.250 mrad, overall spectral range 0.25–5.1 μm) onboard Rosetta acquired data of Lutetia well before the closest approach phase, when the target was spatially unresolved, in order to obtain a light curve of the asteroid in the infrared spectral range extending up to 5 μm. Two light curve sequences were obtained, separated by a gap of 3.25 h. During the first light curve, the solar phase angle of the asteroid had a constant value of 10.8°, while in the second light curve it decreased from 10.7° to 7.8°.Assuming the latest value derived for the rotational period of Lutetia (8.168270±0.000001 h,), each of the light curve sequences covered roughly one rotation of the asteroid. In the overall campaign, VIRTIS collected a total of 16 cubes where Lutetia was captured 165 times, both in the visual and in the infrared range. Given the low signal and the unresolved appearance of the source in the first sequence and at the beginning of the second sequence, several wavelengths turn out to be unsuited to sample the light curve. Nevertheless, in both the VIS and IR ranges we find a similar trend, with two different maxima and minima during one rotational period. In the edge-on view with north pole tilted towards the spacecraft, we find that the near infrared spectrum of Lutetia in its norther hemisphere is essentially flat and grey, with no relevant absorption features in the range 1–3.5 μm within the sensitivity of the VIRTIS instrument. We find a marked spectral homogeneity throughout the two explored rotational periods within the 1−σvariability of the data. Consistent with the results obtained at closest approach by, we confirm the absence of OH features at 1.9, 2.7 and 3 μm, and no other absorption features are seen in the near infrared range 1–3.5 μm of VIRTIS data at northern latitudes. Highlights ► We obtained a total of 165 spectra in the overall range 0.25–5.1 μm. ► We find that the NIR spectrum of Lutetia on its norther hemisphere is essentially flat and grey. ► We find a marked spectral homogeneity throughout two rotational periods. ► We confirm the absence of OH features at 1.9, 2.7 and 3 μm within the sensitivity of VIRTIS. ► We conclude that the northern hemisphere of Lutetia is not linked to the most primitive meteorites.

Description: Publication year: 2011 Source: Planetary and Space Science, Available online 19 December 2011 Ralph D. Lorenz The amplitudes of seismic signals on a planetary surface are discussed in the context of observable physical quantities – displacement, velocity and acceleration – in order to assess the number of events that a sensor with a given detection threshold may capture in a given period. Spacecraft engineers are generally unfamiliar with expected quantities or the language used to describe them, and seismologists are rarely presented with the challenges of accommodation of instrumentation on spacecraft. This paper attempts to bridge this gap, so that the feasibility of attaining seismology objectives on future missions – and in particular, a long-lived Venus lander, can be rationally assessed.For seismometers on planetary landers, the background noise due to wind or lander systems is likely to be a stronger limitation on the effective detection threshold than is the instrument sensitivity itself, and terrestrial data on vehicle noise is assessed in this context. We apply these considerations to investigate scenarios for a long-lived Venus lander mission, which may require a mechanical cooler powered by a Stirling generator. We also consider wind noise : the case for decoupling of a seismometer from a lander is strong on bodies with atmospheres, as is the case for shielding the instrument from wind loads. However, since the atmosphere acts on the elastic ground as well as directly on instruments, the case for deep burial is not strong, but it is important that windspeed and pressure be documented by adequate meteorology measurements. Highlights ► Presents data on observed seismic amplitudes and wind noise on Earth. ► Summarizes seismometer characteristics and operation. ► Evaluates wind noise on Venus using Viking Mars experience as a guide. ► Evaluates lander noise from a Stirling Radioisotope Generator (ASRG). ► Reconciles instrument performance, mission duration and noise levels.

Description: Publication year: 2011 Source: Planetary and Space Science, Available online 22 December 2011 A.S. Krassilnikov, V. -P. Kostama, M. Aittola, E.N. Guseva, O.S. Cherkashina Coronae and rifts are the most prominent volcano-tectonic features on the surface of Venus. Coronae are large radial-concentric structures with diameters of 100 to over 1000 km. They have varied topographical shapes, radial and concentric fracturing and compressional tectonic structures are common for their annuli. Massive volcanism is also connected with some of the structures. Coronae are interpreted to be the result of updoming and fracturing on the surface due to interaction of mantle diapirs with the lithosphere and its subsequent gravitational relaxation. According to, two types of coronae are observed: type 1 - coronae that have annuli of concentric ridges and/or fractures (407 structures), and type 2 that have similar characteristics to type 1 but lack a complete annulus of ridges and fractures (107 structures). We analyzed 20% of this coronae population (we chose each fifth structure from thecatalogue; 82 coronae of type 1 and 22 coronae of type 2, in total 104 coronae) for the (1) spatial distribution of rift structures and time relationship of rift zones activity with time of regional volcanic plains emplacement, and (2) tectonics, volcanism, age relative to regional plains and relationship with rifts.Two different age groups of rifts on Venus were mapped at the scale 1:50 000 000: old rifts that predate and young rifts that postdate regional plains. Most of young rifts inherit strikes of old rifts and old rifts are reworked by them. This may be evidence of rift-produced uplift zones that were probably mostly stable during both types of rifts formation. Evolution of distribution of rift systems with time (decreasing of distribution and localization of rift zones) imply thickening of the lithosphere with time. Coronae–producing mantle diapirism and uplift of mantle material in rift zones are not well correlated at least in time in most cases, because majority of coronae (77%) of both types has no genetic association with rifts. Majority of coronae (72%) were mostly active before regional plains formation, and only 3% appear to have begun to form after the plains emplacement, which may be also due to thickening of the lithosphere. According to the relationship with regional plains type 2 coronae are in general older than type 1 coronae. Three types of corona-related volcanic activity were observed: shield volcanoes and their clusters, as well as extensive lobate lava flows and smooth volcanic plains. Shield volcanoes during coronae evolution were mostly active before regional plains emplacement. Most active phase of volcanism of corona may not coincide with the time of the major tectonic activity of corona, as majority of coronae (77%) were most active before regional plains formation, but almost half of all coronae have traces of post regional plains volcanism. Detailed mapping and stratigraphic analysis of seven regions with 34 examples of coronae showed a similarity in the sequence of regional geologic units.

Description: Publication year: 2011 Source: Planetary and Space Science, Available online 17 December 2011 N. Mangold Fluvial valleys provide critical clues to the distribution and state of water throughout the history of the planet Mars. Early in Mars' history (〈3.7 Gy), the climate may have been warmer than at present leading to the development of valley networks. Younger valleys formed on volcanic and glacial landforms under colder conditions than experienced in Mars' early history. Only rare examples of fluvial valleys over fresh impact craters have been reported. In the present study, a survey of hundreds of fresh post-Noachian impact craters (of 12 to 150 km in diameter) has been done to identify fluvial landforms, especially in regions lacking ancient valleys, using images from the High Resolution Stereo Camera (HRSC) instrument onboard Mars Express and from the Context Camera (CTX) instrument onboard Mars Reconnaissance Orbiter. Observations show that these valleys are locally sinuous, display isolated channels, a poor connectivity and frequent braiding. Valleys were most likely formed over a short duration with high discharge rates, estimated from 500 to 40,000 m s. In Arabia Terra, a total of 27 out of the 204 surveyed craters were found to have fluvial landforms on the ejecta blanket, exclusively in the mid-latitude band (25–45°). Dating of impact ejecta gives young ages from the Late Hesperian to the Middle Amazonian, thus providing a temporal constraint for the fluvial activity. Late climatic episodes of snow deposition and subsequent melting scattered in space and time could explain observations. Alternatively, the thermal anomaly of impacts and their ejecta over ice-bearing terrains is a possible triggering mechanism for the observed fluvial valleys. Calculations show that the thermal anomaly can persist in the ejecta over several hundreds of years for mid-size craters (20–40 km). Such a process would not explain all Martian fluvial activity because of the marked difference between the pristine landforms described and Late Noachian valley networks. Nevertheless, fluvial landforms on preserved ejecta blankets can be used as a new proxy for the temporal distribution of water on Mars. Highlights ► Fluvial valleys incise tens of fresh ejecta craters at mid-latitudes on Mars. ► Valleys geometry suggest sporadic fluvial events postdating early Mars period. ► A formation related to warm impact ejecta on shallow ice is suggested. ► Valleys on ejecta can be used as a proxy for the timing of water activity.

Description: Publication year: 2011 Source: Planetary and Space Science, Available online 16 December 2011 S. Gulkis, S. Keihm, L. Kamp, S. Lee, P. Hartogh, ... The European Space Agency's Rosetta spacecraft made a close flyby of asteroid (21) Lutetia on July 10, 2010. The spacecraft carries a dual-band radiometer/spectrometer instrument, named MIRO, which operates at 190 GHz (1.6 mm) and 560 GHz (0.5 mm). During the flyby, the MIRO instrument measured the temperature of Lutetia in both the northern and southern hemispheres. At the time of the flyby, the northern hemisphere was seasonally sun-lit and warmer than the southern hemisphere. Subsurface (depths from ~2 mm to ~2 cm) temperatures ranged from ~200 K on the northern hemisphere to ~60 K on the southern hemisphere. A lunar-like regolith - very low thermal inertia〈20 J/(K m s) in the upper 1–3 cm overlaying a layer of rapidly increasing density and thermal conductivity - is required to explain the observations. A spectroscopic search was made for H2O, CO, CH3OH, and NH3in Lutetia's exosphere but none of the molecules were detected. An upper limit to the water column density was estimated to be 〈5×10 molecules/cmat the time of the flyby. Highlights ► Lutetia's regolith has a very low thermal inertia, similar to the lunar regolith. ► Microgardening of Lutetia's surface is proposed. ► Upper limit on water vapor in Lutetia's exosphere is reported. ► Near surface temperatures of ~60 K measured at Lutetia's winter pole.

Description: Publication year: 2011 Source: Planetary and Space Science, Available online 14 December 2011 Konstantinos E. Themelis, Frédéric Schmidt, Olga Sykioti, Athanasios A. Rontogiannis, Konstantinos D. Koutroumbas, ... This article presents a comparative study of three different types of estimators used for supervised linear unmixing of two MEx/OMEGA hyperspectral cubes. The algorithms take into account the constraints of the abundance fractions, in order to get physically interpretable results. Abundance maps show that the Bayesian maximum a posteriori probability (MAP) estimator proposed inoutperforms the other two schemes, offering a compromise between complexity and estimation performance. Thus, the MAP estimator is a candidate algorithm to perform ice and minerals detection on large hyperspectral datasets. Highlights ► Supervised unmixing of an OMEGA hyperspectral image of the south polar cap of Mars. ► ENVI-SVD, a quadratic programming technique and a Bayesian estimator are used for unmixing. ► The Bayesian estimator offers the best compromise between estimation performance and complexity.

Description: Publication year: 2011 Source: Planetary and Space Science, Available online 14 December 2011 Guido Valerio, Alessandro Galli, Pier Matteo Barone, Sebastian E. Lauro, Elisabetta Mattei, ... In this work, the ability of Ground Penetrating Radar (GPR) to detect rocks buried in composite soil is studied in connection with the planned ExoMars mission, as GPR will be used during this mission to scan the Martian subsurface to help define feasible sites for shallow drilling. A realistic model of the operating environment is implemented through a full-wave electromagnetic simulator, taking into account the antenna system and the signal features. The flexibility and efficiency of this numerical approach has allowed for the analysis of a great variety of configurations. The regolith is modeled based on data from recent explorations, while various kinds of embedded rocks are considered that have different geometrical and physical characteristics. The simulated results are compared with ad-hoc GPR measurements performed on basalts buried in a mixture of glass beads, as an analogue of a dry sandy Martian soil. A very good agreement between theoretical and experimental results is found, thus validating the proposed numerical approach. This research has defined useful and reliable information concerning the prediction of scattering effects from buried objects in the environment where the ExoMars rover will operate. Highlights ► Preliminary studies for future on-site investigation of the Martian surface. ► GPR detectability of basaltic rocks buried in Martian sandy soil simulants. ► Numerical simulations of realistic bistatic GPR systems in different soil conditions. ► Experimental measurements on mixtures simulating Martian equatorial shallow surface. ► Detectability is granted for typical rock sizes and compositions.

Description: Publication year: 2011 Source: Planetary and Space Science, Available online 16 December 2011 A. Carbognani, P. Tanga, A. Cellino, M. Delbo, S. Mottola, ... It is common practice nowadays to derive spins and 3D shapes of asteroids from the inversion of photometric light curves. However, this method requires, in general, a good number of photometric points and dedicated observing sessions. On the other hand, the photometric observations carried out by the Gaia mission will be sparse and their number relatively small.For this reason, a multi-parametric shape described by a large number of elementary facets cannot probably be derived from Gaia data alone. Therefore, the Data Processing and Analysis Consortium (DPAC), implemented a simpler solution as an unattended data reduction pipeline which relies on three axial ellipsoids for the shape representation.However, overall accuracy of such triaxial shape solutions has to be assessed. How adequate is an ellipsoidal approximation to represent the overall properties of an irregular body? Which error is made on the volume in comparison a more accurate model with irregular shape?To answer these questions, we have implemented numerical procedures for comparing ellipsoids to more complex and irregular shapes, and we performed a full simulation of the photometric signal from these objects, using both shape representations. Implementing the same software algorithm that will be used for the analysis of Gaia asteroid photometry, rotation period, spin axis orientation and ellipsoidal shape were derived from simulated observations of selected Main Belt asteroids assuming a geometric scattering model (work is in progress for more complex scattering models).Finally, these simulated Gaia results were compared to check the relevance of the ellipsoidal solution in comparison to multi-parametric shapes. We found that the ellipsoids by photometry inversion are closely similar to the best-fitting ellipsoids of the simulated complex shapes and that the error on the asteroid volume (relative to a complex shape) is generally low, usually around 10%. Highlights ► The photomety carried out on asteroids by the Gaia mission will be sparse. ► We have implemented numerical procedures for comparing ellipsoids with irregular shapes. ► We found that the ellipsoids by photometry inversion are closely similar to the best-fitting ellipsoids. ► We found that the volume error on asteroid is low, usually around 10%.