Martian volcanism: Additional observations and evidence for pyroclastic activity

Inspection of the Mariner 9 B-camera (resolution 100–200m) and A-camera (resolution 1–2km) photographs of Mars reveals numerous analogs of terrestrial and lunar volcanic features. In addition to the exceptionally large constructional features in the Tharsis region, many other large and small landforms present probably are related to endogenic processes.     

Magma source transition of lunar mare volcanism at 2.3 Ga

Mare basalts provide insights into the composition and thermal history of the lunar mantle. The ages of mare basalts suggest a first peak of magma activity at 3.2–3.8 Ga and a second peak at ~2 Ga. In this study, we reassess the correlation between the titanium contents and the eruption ages of mare basalt units using the compositional and chronological data updated by SELENE (Kaguya). Using morphological and geological criteria, we calculated the titanium content of 261 mare units across a representative area of each mare unit. In the Procellarum KREEP Terrane, where the latest eruptions are located, an increase in the mean titanium content is observed during the Eratosthenian period, as reported by previous studies. We found that the increase in the mean titanium content occurred within a relatively short period near approximately 2.3 Ga, suggesting that the magma source of the mare basalts changed at this particular age. Moreover, the high‐titanium basaltic eruptions are correlated with a second peak in volcanic activity near ~2 Ga. The high‐titanium basaltic eruptions occurring during the last volcanic activity period can be explained by the three possible scenarios (1) the ilmenite‐bearing cumulate rich layer in the core‐mantle boundary formed after the mantle overturn, (2) the basaltic material layers beneath the lunar crust formed through upwelling magmas, and (3) ilmenite‐bearing cumulate blocks remained in the upper mantle after the mantle overturn.     

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

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.     

Constraints on the resurfacing history of Venus from the hypsometry and distribution of volcanism,tectonism, and impact craters

Improved measurements of the target elevations of 885 impact craters on Venus indicate that they are nearly random with respect to elevation. Although a slight deficit of craters at high elevations and an excess at low elevations is observed, the differences are marginally significant. Using a high-resolution digital map and database of all major volcanic, tectonic and impact features, we examine the distribution of impacts within volcanic and tectonic features, and the distribution of volcanism and tectonism with elevation. We show that the observed crater hypsometry results from resurfacing at higher elevations by volcanic and tectonic features superimposed on less active plains.The distribution of impacts in the map units has two distinct patterns: (1) the plains and shield fields (70%) have high crater densities and low proportions of tectonized or embayed craters; and (2) the remaining volcanic and tectonic features (30%) have low crater densities and high proportions of modified craters. The plains and shield fields appear to represent a much lower level of resurfacing activity. Simple area-balance calculations indicate that resurfacing at higher elevations by tectonic and volcanic features plausibly explains the observed crater hypsometry. However, the subtlety of the effects suggests that either (1) little resurfacing has occurred during the period of crater accumulation, or (2) resurfacing acts almost equally at all elevations. The apparent low activity of the plains and their abundance at lower elevations makes it unlikely that resurfacing is balanced with respect to elevation. It appears that the plains have been mostly quiescent since their emplacement, and that subsequent resurfacing occurs mostly in the highlands as a result of volcanism, corona formation, and rifting. We estimate that since the end of plains emplacement about 14% of Venus has been resurfaced by volcanism and about 6% by tectonic deformation.     

Beta Regio, Venus: Evidence for uplift, rifting, and volcanism due to a mantle plume

Geophysical data have led to the interpretation that Beta Regio, a 2000×25000 km wide topographic rise with associated rifting and volcanism, formed due to the rise of a hot mantle diapir interpreted to be caused by a mantle plume. We have tested this hypothesis through detailed geologic mapping of the V-17 quadrangle, which includes a significant part of the Beta Regio rise, and reconnaissance mapping of the remaining parts of this region. Our analysis documents signatures of an early stage of uplift in the formation of the Agrona Linea fracture belts before the emplacement of regional plains and their deformation by wrinkle ridging. We see evidence that the Theia rift-associated volcanism occurred during the first part of post-regional-plains time and cannot exclude that it continued into later time. We also see evidence that Devana Chasma rifting was active during the first and the second parts of post-regional-plains time. These data are consistent with uplift, rifting and volcanism associated with a mantle diapir. Geophysical modeling shows that diapiric upwelling may continue at the present time. Together these data suggest that the duration of mantle diapir activity was as long as several hundred million years. The regional plains north of Beta rise and the area east and west of it were little affected by the Beta-forming plume, but the broader area (at least 4000 km across), whose center-northern part includes Beta Regio, could have experienced earlier uplift as morphologically recorded in formation of tessera transitional terrain.     

Resurfacing on Venus

The resurfacing evolution of Venus has been evaluated through Monte Carlo simulations. For the first time, the sizes of volcanic flows in the models were generated using the frequency-size distribution of volcanic units measured on Venus. A non-homogeneous spatial generation of volcanic units was included in the models reproducing the Beta-Alta-Themis volcanic anomaly. Crater modification is simulated using a 3D approach. The final number of modified craters and randomness of the crater population were used to evaluate the success of the models, comparing the results from our simulations with Venus observations. The randomness of the crater population is evaluated using pair-correlation statistics. On the one hand, a catastrophic resurfacing event followed by moderate volcanic activity covering ≈40% of the planetary surface can reproduce the number of modified craters and the pair-correlation statistics do not reject randomness. On the other hand, the pair-correlation test for equilibrium steady-state resurfacing models rejects the randomness of the crater population when reproducing the observed frequency-size distribution of the volcanic units with a non-homogeneous spatial generation of volcanic units.     

Sulfur volcanism on Io

In February 2003, March 2003 and January 2004 Pele plume transmission spectra were obtained during Jupiter transit with Hubble's Space Telescope Imaging Spectrograph (STIS), using the 0.1″ wide slit and the G230LB grating. The STIS spectra covered the 2100-3100 Å wavelength regions and extended spatially along Io's limb encompassing the region directly above and northward of the vent of the Pele volcano. The S₂ and SO₂ absorption signatures evident in these data indicate that the gas signature at Pele was temporally variable, and that an S₂ absorption signature was present ∼12° from the Pele vent near 6±5 S and 264±15 W, suggesting the presence of another S₂ bearing plume on Io. Contemporaneous with the spectral data, UV and visible-wavelength images of the plume were obtained in reflected sunlight with the Advanced Camera for Surveys (ACS) prior to Jupiter transit. The dust scattering recorded in these data provide an additional qualitative measure of plume activity on Io, indicating that the degree of dust scattering over Pele varied as a function of the date of observation, and that there were several other dust bearing plumes active during the observations. We present constraints on the composition and variability of the gas abundances of the Pele plume as well as the plumes detected by ACS and recorded within the STIS data, as a function of time.     

Life on Venus

A fundamental question in exobiology remains the degree to which habitats on Venus, past and present, were, or are suitable for life. This has relevance for assessing the exobiological potential of extrasolar Venus-like greenhouse planets. In this paper the parameters of the Venusian surface and atmosphere are considered and the biochemical adaptations required to survive them are explored in the light of new information on microbial adaptations to extreme environments. Neither the pressure (9.5 MPa) nor the high carbon dioxide concentrations (97%) represent a critical constraint to the evolution of life on the surface or in the atmosphere. The most significant constraints to life on the surface are the lack of liquid water and the temperature (464°C). In the lower and middle cloud layers of Venus, temperatures drop and water availability increases, generating a more biologically favorable environment. However, acidity and the problem of osmoregulation in hygroscopic sulfuric acid clouds become extreme and probably life-limiting. If it is assumed that these constraints can be overcome, considerations on the survival of acidophilic sulfate-reducing chemoautotrophs suspended as aerosols in such an environment show that Venus does come close to possessing a habitable niche. Conditions on the surface and in the atmosphere may have been greatly ameliorated on early Venus and may also be ameliorated on extrasolar planets with early Venus-like characteristics where temperatures are less extreme and liquid water is available.     

Doublet craters on Venus

Of the impact craters on Earth larger than 20 km in diameter, 10-15% (3 out of 28) are doublets, having been formed by the simultaneous impact of two well-separated projectiles. The most likely scenario for their formation is the impact of well-separated binary asteroids. If a population of binary asteroids is capable of striking the Earth, it should also be able to hit the other terrestrial planets as well. Venus is a promising planet to search for doublet craters because its surface is young, erosion is nearly nonexistent, and its crater population is significantly larger than the Earth's. After a detailed investigation of single craters separated by less than 150 km and “multiple” craters having diameters greater than 10 km, we found that the proportion of doublet craters on Venus is at most 2.2%, significantly smaller than Earth's, although several nearly incontrovertible doublets were recognized. We believe this apparent deficit relative to the Earth's doublet population is a consequence of atmospheric screening of small projectiles on Venus rather than a real difference in the population of impacting bodies. We also examined “splotches,” circular radar reflectance features in the Magellan data. Projectiles that are too small to form craters probably formed these features. After a careful study of these patterns, we believe that the proportion of doublet splotches on Venus (14%) is comparable to the proportion of doublet craters found on Earth (10-15%). Thus, given the uncertainties of interpretation and the statistics of small numbers, it appears that the doublet crater population on Venus is consistent with that of the Earth.     

Aeolian processes on Venus

In this pre-Magellan review of aeolian processes on Venus we show that the average rate of resurfacing is less than 2 to 4 km/Ga, based on the impact crater size frequency distribution derived from Venera observations, reasonable values of the impact flux, and the assumption of steady state conditions between crater production and obliteration. Viscous relaxation of crater topography, burial by volcanic deposits, tectonic disruption, chemical and mechanical weathering and erosion, and accumulation of windblown sediments probably all contribute to resurfacing. Based on the rate of disappearance of radar-bright haloes around impact craters, the rate of removal of blocky surfaces has been estimated to be about 10^–2 km/Ga. Pioneer-Venus altimetry data show that the average relative permittivity (at 17 cm radar wavelength) of the surface is too high for exposure of soils 10 cm deep, except for ~5% of the planet located primarily in tessarae terrains. The tectonically disrupted tessarae terrains may be sites of soil generation caused by tectonic disruption of bedrock and the presence of relatively steep slopes, or they may be terrains that serve as traps for windblown material. The overall impression is that Venus is a geologically active planet, but one dominated by volcanism and tectonism. On the other hand, theoretical considerations and experimental data on weathering and transport of surface materials suggest rather different conditions. Thermochemical arguments have been advanced that show: (1) CO₂ and SO₂ incorporate into weathering products at high elevation, (2) transport of weathered material by the wind to lower-elevation plains, and (3) re-equilibration of weathered material, releasing both CO₂ and SO₂. In addition, kinetic data suggest a rate of anhydrite formation of 1 km/Ga, a value comparable to the soil erosion rate on Mars, a planet with an active aeolian environment. Experiments and theoretical studies of aeolian processes show that measured surface winds are capable of moving sand and silt on Venus. Assuming that there is a ready sand supply, the flux could be as high as 2.5 × 10^–5 g/cm/s, a value comparable to desert terrains on Earth. In an active aeolian abrasion environment, sand grains could have lifetimes <10³ years. In addition, comminuted debris may be cold-welded to surfaces at the same time as abrasion is occurring. Magellan altimetry and SAR observations should allow assessment of which model for venusian surface modification (active vs. inactive surficial processes) is correct, given the global coverage, high spatial resolution, the calibrated nature of the data, and the potential during extended missions of acquiring multiple SAR views of the surface.Geology and Tectonics of Venus, special issue edited by Alexander T. Basilevsky (USSR Acad. of Sci. Moscow), James W. Head (Brown University, Providence), Gordon H. Pettengill (MIT, Cambridge, Massachusetts) and R. S. Saunders (J.P.L., Pasadena).     

Impact craters on Venus

Calculations are made to determine the sizes of stone and iron meteoroids which could penetrate the atmosphere of Venus and cause hypervelocity impact craters on the planet's surface. Using scaling relationships based on kinetic energy, impact crater size is related to meteoriod size. Finally, it is determined that the smallest impact craters that might exist on Venus are on the order of 150 to 300 meters in diameter.     

Hydrous Silicates and Water on Venus

We used thermochemical equilibrium calculations to predict stabilities of pure rock-forming hydrous silicates on Venus' surface as a function of elevation, atmospheric H₂O and SO₂concentrations, and oxygen fugacity (f_O2). About 50 different hydrous silicates were included in our calculations. We find that many of these are unstable on Venus's surface because of the low atmospheric H₂O content of 30–45 parts per million by volume (ppmv) and the high surface temperatures (660 K on Maxwell Montes to 740 K in the plains). Hydrous Fe²⁺-bearing silicates are unstable due to oxidation to magnetite and/or hematite at the f_O2of the near-surface atmosphere. Ca-bearing hydrous silicates are unstable because of sulfatization to anhydrite. Some Fe-free micas (e.g., eastonite, eastonite–phlogopite micas), and some alkali amphiboles might be stable on Venus' surface, especially in the lower temperature highlands. We discuss hydrous mineral formation in the interior and on the surface of Venus. We review the literature on mica and amphibole thermal decomposition and find that dehydration of phlogopitic micas and fibrous amphiboles produces (metastable) dehydroxylated anhydrides that decompose to more stable minerals at temperatures hundreds of degrees higher than the onset of dehydroxylation. These observations raise the possibility that anhydrides formed from hydrous silicates, which may have been present during a wetter period in Venus' history, may persist somewhere on Venus' present surface. We discuss experiments that could be used on future spacecraft missions to detect hydroxyl in rocks and hydrous silicates on Venus. Finally, we review estimates of the amount of water and OH (hydroxyl) in the Earth's mantle. Based on this review, we suggest that even if no hydrous silicates are stable on Venus, significant amounts of water are plausibly present in surface rocks as OH in nominally anhydrous minerals.     

Lightnings and nitric oxide on Venus

In an updating of energy characteristics of lightnings on Venus obtained from Venera-9 and -10 optical observations, the flash energy is given as 8 × 10⁸ J and the mean energy release of lightnings is 1 erg cm^?2 s which is 25 times as high as that on the Earth. Lightnings were observed in the cloud layer. The stroke rate in the near-surface atmosphere is less than 5 s^?1 over the entire planet if the light energy of the stroke exceeds 4 × 10⁵ J and less than 15 s^?1 for (1–4) × 10⁵ J.The average NO production due to lightnings equals 5 × 10⁸ cm^?2 s^?1, the atomic nitrogen production is equal to 7 × 10⁹ cm^?2s^?1,the N flux toward the nightside is 3.2 × 10⁹ cm^?2s^?1, the number densities [N] = 3 × 10⁷cm^?3 and [NO] = 1.8 × 10⁶cm^?3 at 135 km. Almost all NO molecules in the upper atmosphere vanish interacting with N and the resulting NO flux at 90-80 km equals 5 × 10⁵cm^?2s^?1, which is negligibly small as compared with lightning production. If the predissociation at 80–90 km is regarded as the single sink of NO, its mixing ratio, f_NO, is 4 × 10^?8, for the case of a surface sink f_NO = 0.8 × 10^?9 at 50 km. Excess amounts, $\text{f}{}_{\mathrm{<mtext>NO</mtext>}}\text{? 4 × 10}{}^{\mathrm{?8}}$, may exist in the thunderstorm region.     

Topographic comparisons of uplift features on Venus and Earth: Implications for Venus tectonics

Venus and Earth display different hypsography. We use topographic profiles to search for well-understood terrestrial analogs to venusian features. Specifically, by using cross-correlation, we correlate average profiles for terrestrial rifts (slow and fast, “ultra-slow,” incipient and inactive) and also hotspots (oceanic and continental) with those for venusian chasmata and regiones, to draw inferences as to the processes responsible for shaping Venus’ surface. Correlations tend to improve with faster spreading rates; Venus’ correlations rank considerably lower than terrestrial ones, suggesting that if chasmata are analogous to terrestrial spreading centers, then spreading on Venus barely attains ultra-slow rates. Individual features’ normalized average profiles are correlated with profiles of other such features to establish the degree of similarity, which in turn allows for the construction of a covariance matrix. Principal component analysis of this covariance matrix shows that Yellowstone more strongly resembles Atla, Beta and W. Eistla regiones than it does the terrestrial oceanic hotspots, and that venusian chasmata, especially Ganis, most closely resemble the ultra-slow spreading Arctic ridge.     

Heavy metal frost on Venus

Chemical equilibrium calculations of volatile metal geochemistry on Venus show that high dielectric constant compounds of lead and bismuth such as PbS (galena), Bi₂S₃ (bismuthite) or Pb-Bi sulfosalts condense in the venusian highlands and may be responsible for the low radar emissivities observed by Magellan and Pioneer Venus. Our calculations also show that elemental tellurium is unstable on Venus' surface and will not condense below 46.6 km. This is over 30 km higher than Maxwell Montes, the highest point on Venus' surface. Elemental analyses of Venus' highlands surface by laser induced breakdown spectroscopy (LIBS) and/or X-ray fluorescence (XRF) can verify the identity of the heavy metal frost on Venus. The Pb-Pb age of Venus could be determined by mass spectrometric measurements of the Pb²⁰⁷/Pb²⁰⁴ and Pb²⁰⁶/Pb²⁰⁴ isotopic ratios in Pb-bearing frosts. All of these measurements are technologically feasible now.     

Crustal tectonic zone on Venus

Various tectonic structures to the south and southeast of Ishtar Terra indicate areal stresses. Compression from east-southeast against Ishtar Terra has resulted in ridge belt formation and surface bending at Salme Dorsa, probably along the seam between two crustal units. En echelon fault zone indicates dextral strike-slip shear(s) resulted in the westward movement of planitia crust related to Ishtar Terra. Meshkenet Tessera displays differential dextral strike-slip faulting where the southernmost bar-like blocks have had largest relative movements. Compression against Tusholi Corona has resulted in foreland surface bending similar to that of Salme Dorsa. The tectonic zone as a whole resembles a dextral transform fault extending from a concave arc in the west to another concave arc in the east. The Cytherean surface, crust or uppermost lithosphere seems to be able to transmit stresses over distances. Deeper understanding of these processes is needed to gain a new idea of the crustal deformation on terrestrial planets.     

Modeling crater populations on Venus and Titan

We describe a model for crater populations on planets and satellites with dense atmospheres, like those of Venus and Titan. The model takes into account ablation (or mass shedding), pancaking, and fragmentation. Fragmentation is assumed to occur due to the hydrodynamic instabilities promoted by the impactors’ deceleration in the atmosphere. Fragments that survive to hit the ground make craters or groups thereof. Crater sizes are estimated using standard laws in the gravity regime, modified to take into account impactor disruption. We use Monte Carlo methods to pick parameters from appropriate distributions of impactor mass, zenith angle, and velocity. Good fits to the Venus crater populations (including multiple crater fields) can be found with reasonable values of model parameters. An important aspect of the model is that it reproduces the dearth of small craters on Venus: this is due to a cutoff on crater formation we impose, when the expected crater would be smaller than the (dispersed) object that would make it. Hydrodynamic effects alone (ablation, pancaking, fragmentation) due to the passage of impactors through the atmosphere are insufficient to explain the lack of small craters. In our favored model, the observed number of craters (940) is produced by ∼5500 impactors with masses , yielding an age of (1-σ uncertainty) for the venusian surface. This figure does not take into account any uncertainties in crater scaling and impactor population characteristics, which probably increase the uncertainty to a factor of two in age.We apply the model with the same parameter values to Titan to predict crater populations under differing assumptions of impactor populations that reflect present conditions. We assume that the impactors (comets) are made of 50% porous ice. Predicted crater production rates are ≈190 craters . The smallest craters on Titan are predicted to be in diameter, and ≈5 crater fields are expected. If the impactors are composed of solid ice (density ), crater production rates increase by ≈70% and the smallest crater is predicted to be in diameter. We give cratering rates for denser comets and atmospheres 0.1 and 10 times as thick as Titan's current atmosphere. We also explicitly address leading-trailing hemisphere asymmetries that might be seen if Titan's rotation rate were strictly synchronous over astronomical timescales: if that is the case, the ratio of crater production on the leading hemisphere to that on the trailing hemisphere is ≈4:1.     

Landing on Venus: Past and future

We briefly describe the history of landings on Venus, the acquired geochemical data and their potential petrologic interpretations. We suggest a new approach to Venus landing site selection that would avoid the potential contamination by ejecta from upwind impact craters. We also describe candidate units to be sampled in both in situ measurement and sample return missions. For the in situ measurements, the “true” tessera terrain (tt) material is considered as the highest priority goal with the second priority given to transitional tessera terrain (ttt), shield plains (psh) and lobate plains (pl) materials. For the sample return mission, the material of regional plains with wrinkle ridges (pwr) is considered as the highest priority goal with the second priority given to tessera terrain (tt) material. Combining the desire to study materials of specific geologic units with the problem of avoiding potential contamination by ejecta from upwind impact craters, we have suggested several candidate landing sites for each of the geologic units. Although spacecraft ballistics and other constraints of specific mission profiles (VEP or others) may lead to the selection of different candidate sites, we believe that the approaches outlined in this paper can be helpful approach in optimizing mission science return.