The atmospheric temperatures over Olympus Mons on Mars: An atmospheric hot ring     

P. Wolkenberg  V. Formisano  G. Rinaldi  A. Geminale 《Icarus》2010,207(1):110-123

We study the thermal fields over Olympus Mons separating seasons (northern spring and summer against southern spring and summer) and local time observations (day side versus night side). Temperature vertical profiles retrieved from Planetary Fourier Spectrometer on board Mars Express (PFS-MEX) data have been used. In many orbits (running north to south along a meridian) passing over the top of the volcano there is evidence of a hot air on top of the volcano, of two enhancement of the air temperature both north and south of it and in between a collar of air that is colder than nearby at low altitudes, and warmer than nearby at high altitudes. Mapping together many orbits passing over or near the volcano we find that the hot air has the tendency to form an hot ring around it. This hot structure occurs mostly between LT = 10.00 and 15.00 and during the northern summer. Distance of the hot structure from the top of the volcano is about 600 km (10° of latitude). The hot atmospheric region is 300-420 km (5-7°) wide. Hot ring temperature contrasts of about 40 K occur at 2 km above the surface and decrease to 20 K at 5 km and to 10 K at 10 km. The atmospheric circulation over an area of 40° × 40° (latitudes and longitudes) is affected by the topography and the presence of Olympus Mons (−133°W, 18°N). We discuss also the thermal stability of the atmosphere over the selected area using the potential temperatures. The temperature field over the top of the volcano shows unstable atmosphere within 10 km from the surface. Finally, we interpret the hot temperatures around volcano as an adiabatic compression of down-welling branch coming from over the top of volcano.Different air temperature profiles are observed in the same seasons during the night, or in different seasons. In northern spring-summer during the night the isothermal contours do not show the presence of the volcano until we reach close to the surface very much, where a thermal inversion is observed. The surface temperature shows higher values (by 10 K) in correspondence of the scarp (an abrupt altimetry variation of roughly 5 km) on south (6°N) and north (30°N) sides of volcano. During the southern spring-summer, on the contrary the isothermal curves run parallel to the surface even on top the volcano, just like the GCM have predicted.  相似文献   

Deep versus shallow origin of gravity anomalies, topography and volcanism on Earth, Venus and Mars     

Bernhard Steinberger  Stephanie C. Werner 《Icarus》2010,207(2):564-577

The relation between gravity anomalies, topography and volcanism can yield important insights about the internal dynamics of planets. From the power spectra of gravity and topography on Earth, Venus and Mars we infer that gravity anomalies have likely predominantly sources below the lithosphere up to about spherical harmonic degree l=30 for Earth, 40 for Venus and 5 for Mars. To interpret the low-degree part of the gravity spectrum in terms of possible sublithospheric density anomalies we derive radial mantle viscosity profiles consistent with mineral physics. For these viscosity profiles we then compute gravity and topography kernels, which indicate how much gravity anomaly and how much topography is caused by a density anomaly at a given depth. With these kernels, we firstly compute an expected gravity-topography ratio. Good agreement with the observed ratio indicates that for Venus, in contrast to Earth and Mars, long-wavelength topography is largely dynamically supported from the sublithospheric mantle. Secondly, we combine an empirical power spectrum of density anomalies inferred from seismic tomography in Earth’s mantle with gravity kernels to model the gravity power spectrum. We find a good match between modeled and observed gravity power spectrum for all three planets, except for 2?l?4 on Venus. Density anomalies in the Venusian mantle for these low degrees thus appear to be very small. We combine gravity kernels and the gravity field to derive radially averaged density anomaly models for the Martian and Venusian mantles. Gravity kernels for l?5 are very small on Venus below ≈800 km depth. Thus our inferences on Venusian mantle density are basically restricted to the upper 800 km. On Mars, gravity anomalies for 2?l?5 may originate from density anomalies anywhere within its mantle. For Mars as for Earth, inferred density anomalies are dominated by l=2 structure, but we cannot infer whether there are features in the lowermost mantle of Mars that correspond to Earth’s Large Low Shear Velocity Provinces (LLSVPs). We find that volcanism on Mars tends to occur primarily in regions above inferred low mantle density, but our model cannot distinguish whether or not there is a Martian analog for the finding that Earth’s Large Igneous Provinces mainly originate above the margins of LLSVPs.  相似文献   

Mineralogy of the Phoenix landing site from OMEGA observations and how that relates to in situ Phoenix measurements     

F. Poulet  R.E. Arvidson  B. Gondet  Y. Langevin 《Icarus》2010,205(2):712-715

We present an analysis comparing observations acquired by the Mars Express Observatoire pour la Minéralogie l’Eau, les Glaces et l’Activité (OMEGA) and Phoenix lander measurements. Analysis of OMEGA data provides evidence for hydrous and ferric phases at the Phoenix landing site and the surrounding regions. The 3 μm hydration band deepens with increasing latitude, along with the appearance and deepening of a 1.9 μm H₂O band as latitude increases ∼60° polewards. A water content of 10-11% is derived from the OMEGA data for the optical surface at the Phoenix landing site compared to 1-2% derived for subsurface soil by Phoenix lander measurements. The hydration of these regions is best explained by surface adsorbed water onto soil grains. No evidence for carbonate or perchlorate-bearing phases is evident from OMEGA data, consistent with the relatively small abundances of these phases detected by Phoenix. The identification of spectral features consistent with hydrated phases (possibly zeolites) from OMEGA data covering regions outside the landing site and the ubiquitous ferric absorption edge suggest that chemical weathering may play a role in the arctic soils.  相似文献   

Metal-catalyzed degradation and racemization of amino acids in iron sulfate brines under simulated martian surface conditions     

Adam P. Johnson  Lisa M. Pratt 《Icarus》2010,207(1):124-603

Recent lines of evidence indicate that Mars may have been substantially warmer and wetter, raising questions about whether the planet’s surface was once more conducive to the possibility of life. To better constrain the extent of preservation of ancient amino acid biosignatures, the effects of metal ions and UV radiation were investigated with respect to the rates of diagenesis of amino acids in metal-rich sulfate brines. Individual experimental results indicate that the oxidation rates of amino acids loosely followed trends characteristic of a radiolytic oxidation mechanism that is dependent on variations in side-chain constituents. Relative rates of racemization only show a limited dependence on the presence of metals; overall, rates of racemization in the presence of metals in solution are an order of magnitude slower than oxidation and both are several orders of magnitude faster than previously reported. This increase in rate constant is indicative of an extremely efficient catalytic enhancement of this destructive diagenetic pathway. These findings imply that chiral life-detection strategies should focus on specific geochemical environments that exhibit relatively slow rates of both degradation and racemization. Episodic liquid water during periods of increased water activity on the martian surface may have significant implications for the oxidation of organic matter through secondary diagenetic reactions and may act as a leveling mechanism for organic compounds.  相似文献   

Morphology, stratigraphy, and mineralogical composition of a layered formation covering the plateaus around Valles Marineris, Mars: Implications for its geological history     

L. Le Deit  O. Bourgeois  E. Hauber  M. Massé  J.-P. Bibring 《Icarus》2010,208(2):684-703

An extensive layered formation covers the high plateaus around Valles Marineris. Mapping based on HiRISE, CTX and HRSC images reveals these layered deposits (LDs) crop out north of Tithonium Chasma, south of Ius Chasma, around West Candor Chasma, and southwest of Juventae Chasma and Ganges Chasma. The estimated area covered by LDs is ∼42,300 km². They consist of a series of alternating light and dark beds, a 100 m in total thickness that is covered by a dark unconsolidated mantle possibly resulting from their erosion. Their stratigraphic relationships with the plateaus and the Valles Marineris chasmata indicate that the LDs were deposited during the Early- to Late Hesperian, and possibly later depending on the region, before the end of the backwasting of the walls near Juventae Chasma, and probably before Louros Valles sapping near Ius Chasma. Their large spatial coverage and their location mainly on highly elevated plateaus lead us to conclude that LDs correspond to airfall dust and/or volcanic ash. The surface of LDs is characterized by various morphological features, including lobate ejecta and pedestal craters, polygonal fractures, valleys and sinuous ridges, and a pitted surface, which are all consistent with liquid water and/or water ice filling the pores of LDs. LDs were episodically eroded by fluvial processes and were possibly modified by sublimation processes. Considering that LDs correspond to dust and/or ash possibly mixed with ice particles in the past, LDs may be compared to Dissected Mantle Terrains currently observed in mid- to high latitudes on Mars, which correspond to a mantle of mixed dust and ice that is partially or totally dissected by sublimation. The analysis of CRISM and OMEGA hyperspectral data indicates that the basal layer of LDs near Ganges Chasma exhibits spectra with absorption bands at ∼1.4 μm, and ∼1.9 μm and a large deep band between ∼2.21 and ∼2.26 μm that are consistent with previous spectral analysis in other regions of LDs. We interpret these spectral characteristics as an enrichment of LDs in opaline silica or by Al-phyllosilicate-rich layers being overlain by hydroxylated ferric sulfate-rich layers. These alteration minerals are consistent with the aqueous alteration of LDs at low temperatures.  相似文献   

Evidence for Hesperian impact-induced hydrothermalism on Mars     

Giuseppe A. Marzo  Alfonso F. Davila  Livio L. Tornabene  Alberto G. Fairén  Thomas Kneissl  Ted L. Roush 《Icarus》2010,208(2):667-60

Several hydrated silicate deposits on Mars are observed within craters and are interpreted as excavated Noachian material. Toro crater (71.8°E, 17.0°N), located on the northern edge of the Syrtis Major Volcanic Plains, shows spectral and morphologic evidence of impact-induced hydrothermal activity. Spectroscopic observations were used to identify extensive hydrated silicate deposits, including prehnite, chlorites, smectites, and opaline material, a suite of phases that frequently results from hydrothermal alteration in terrestrial craters and also expected on Mars from geochemical modeling of hydrothermal environments. When combined with altimetry and high-resolution imaging data, these deposits appear associated predominantly with the central uplift and with portions of the northern part of the crater floor. Detailed geologic mapping of these deposits reveals geomorphic features that are consistent with hydrothermal activity that followed the impact event, including vent-like and conical mound structures, and a complex network of tectonic structures caused by fluid interactions such as fractures and joints. The crater age has been calculated from the cumulative crater size-frequency distributions and is found to be Early Hesperian. The evidence presented here provides support for impact-induced hydrothermal activity in Toro crater, that extends phyllosilicate formation processes beyond the Noachian era.  相似文献   

Mapping Medusae Fossae Formation materials in the southern highlands of Mars     

S.K. Harrison  M.R. Balme  J.B. Murray 《Icarus》2010,209(2):405-415

The Medusae Fossae Formation (MFF) is an extensive deposit (2.2 × 10⁶ km², Bradley, B.A., Sakimoto, S.E.H., Frey, H., Zimbelman, J.R. [2002]. J. Geophys. Res. 107, 5058) of wind-eroded material of widely debated origin, which unconformably overlies a considerable area of the crustal dichotomy boundary on Mars. The MFF shows a variety of layering patterns, erosional styles and channel-like forms and has been mapped into five main outcrops and three geological members according to exposure and stratigraphy (Scott, D.H., Tanaka, K.L., 1986. USGS Map I-1802-A; Greeley, R., Guest, J.E., 1987. Map I-1802-B; Zimbelman, J.R., Crown, D., Jenson, D., 1996. Lunar Planet. Sci. XXVII. Abstract #1748.). Away from the three main lobes are numerous outliers of MFF materials. These have mainly been reported in the northern lowlands regions (Keszthelyi, L., Jaeger, W.L., and HiRISE team, 2008. Lunar Planet. Sci. XXXIX. Abstract #2420.) but few studies have examined the possibility of MFF outliers on high ground south of the dichotomy boundary. We have searched Mars Orbiter Camera Narrow Angle (MOC NA) images for outliers in this region. Our observations show that there are many MFF outliers on the southern highlands. The characteristics of the outliers indicate materials which overlie the underlying terrain for they appear widely in dips, craters and topographic lows. The surfaces are typified by yardang fields and have a similar patchy and discontinuous nature to materials of the upper member of the MFF. Most have consistent lineation orientations across the wider area which match the dominant orientation of yardangs in the main MFF outcrops. Furthermore, elevation data shows that the maximum, minimum and mean elevations of these newly discovered outliers are closest to those of the upper member of the MFF. We therefore conclude that these deposits are MFF outliers and that they probably represent remnant upper member material. We suggest that there might be two possible explanations for these outliers: (1) the MFF had a much greater pre-erosional extent than previously estimated, or (2) materials from the main outcrops were eroded and then blown south to accrue in the highland areas, where they were subsequently reworked. We suggest that the topography of the region favors the first option. We outline an “overflowing” layer-cake deposition model, in which layers of sediment stacked up against the dichotomy boundary until they reached the topographic level of the highlands. Further materials (that went onto become upper-member MFF material and outliers) were then deposited across a wider area, including south of the dichotomy boundary. Severe erosion subsequently removed much of this material.  相似文献   

Do young martian ray craters have ages consistent with the crater count system?     

William K. Hartmann  Cathy Quantin  Olga Popova 《Icarus》2010,208(2):621-21

McEwen et al. (McEwen, A.S., Preblich, B.S., Turtle, E.P., Artemieva, N.A., Golombek, M.P., Hurst, M., Kirk, R.L., Burr, D.M., Christensen, P. [2005]. Icarus 176, 351-381) developed a useful test for the internal consistency of crater-count chronometry systems. They argued that certain multi-kilometer, fresh-looking martian craters with prominent rays should be the youngest or near-youngest craters in their size range. The “McEwen et al. test” is that the ages determined from crater densities of the smallest superimposed craters (typically diameter D ∼ 5-20 m) should thus be comparable to the expected formation intervals of the host primary. McEwen et al. concluded from MOC data that crater chronometry failed this test by factors of 700-2000. We apply HiRISE and other imagery to eight different young craters in order to re-evaluate their arguments. We use existing crater chronology systems as well as the reported observed production rate of 16 m craters (Malin, M.C., Edgett, K., Posiolova, L., McColley, S., Noe Dobrea, E. [2006]. Science 314, 1573-1557; Hartmann, W.K., Quantin, C., Mangold, N. [2007]. Icarus 186, 11-23; Kreslavsky [2007]. Seventh International Conference on Mars, 3325). Every case passes the McEwen et al. test. We conclude that the huge inconsistencies suggested by McEwen et al. are spurious. Many of these craters show evidence of impact into ice-rich material, and appear to have ice-flow features and sublimation pits on their floors. As production rate data improve, decameter-scale craters will provide a valuable way of dating these young martian geological formations and the processes that modify them.  相似文献