Morphometric Analysis of Martian Valley Network Basins Using a Circularity Function

This paper employs a circularity function to quantify the internal morphology of Martian watershed basins in Margaritifer Sinus region and to infer the primary erosional processes that led to their current geomorphologic characteristics and possible climatic conditions under which these processes operated. The circularity function describes the elongation of a watershed basin at different elevations. We have used the circularity functions of terrestrial basins that were interpreted as having been modified by (1) erosion related to primarily groundwater sapping and (2) erosion related to primarily rainfall and surface run-off, as well as the circularity functions of cratering basins on the Moon, in order to formulate discriminant functions that are able to separate the three types of landforms. The spatial pattern of the classification of Martian basins based on discriminant functions shows that basins that look morphologically similar to terrestrial fluvial basins are mostly clustered near the mainstream at low elevation, while those that look morphologically similar to terrestrial basins interpreted as groundwater sapping origin are located near the tributaries and at higher elevation. There are more of the latter than the former. This spatial distribution is inconsistent with a continuous Earth-like warm and wet climate for early Mars. Instead, it is more aligned with an overall early dry climate punctuated with episodic wet periods. Alternatively, the concentrated erosion in the mainstream could also be caused by a change of water source from rainfall to snowfall or erosion cut through a duricrust layer

Computer simulation of the role of groundwater seepage in forming Martian valley networks

The role of groundwater in forming Martian valley networks is simulated in a computer model as seepage erosion by contributing to surface runoff and as seepage weathering by causing accelerated weathering of bedrock, which makes its subsequent erosion and removal easier. Simulation results show that seepage erosion cannot mobilize large grain size sediment and is marginally effective at generating integrated valley networks with realistic rates of aquifer recharge. On the other hand, seepage weathering may play a major role in forming Martian valley networks. Seepage weathering combined with fluvial runoff creates stubby deep canyons with abrupt headwalls that are similar in morphology to terrestrial and Martian valley systems attributed to erosion by groundwater. Depending on the relative contribution of groundwater weathering to surface runoff erosion, a continuum of valley network morphology can be generated. Eolian modification masks the original differences in fluvial landforms, making different scenarios visually more similar. Martian valley networks may have developed through a range of combinations of runoff erosion and seepage weathering, which can complicate the interpretation of the processes based on final landform morphology. Unequivocal identification of seepage involvement of valley incision on Mars may not be possible without knowledge of subsurface properties (hydraulic conductivity, layering, degree of cementation, etc.) and the grain sizes of sediment transported through the valley systems

Effects of surfactants Sb and Bi on the incorporation of zinc and carbon in III/V materials grown by organometallic vapor-phase epitaxy

Journal ArticleThe incorporation of both dopants and background impurities during the organometallic vapor phase epitaxial _x0002_OMVPE_x0003_ growth of GaAs, GaInP, and GaP has been significantly altered by the use of the surfactants Sb and Bi. Sb and Bi are isoelectronic with the group V host elements, and so produce no independent doping. This paper demonstrates that the incorporation of Zn can be increased by an order of magnitude in GaP to a value of approximately 1020 cm−3, the highest value reported to date. Additionally, these same surfactants lead to significant decreases in carbon contamination during growth. At high growth temperatures the reduction can be as large as 100_x0002_ in GaP. The surfactants also result in marked decreases in autodoping due to S and Si from the substrate. A marker that may help identify the mechanism for these effects is H incorporation. Both Sb and Bi surfactants give rise to increased concentrations of H in the GaP layers. Similar effects are observed in GaInP. However, in GaAs, no H is detected in the layers. One possible explanation for these phenomena is that Sb or Bi increases the surface H concentration. The increased H would have two possible effects on the incorporation of dopants and impurities. _x0002_1_x0003_ Passivation of the Zn acceptor by formation of a neutral Zn-H complex would lead to increased incorporation for thermodynamic reasons. _x0002_2_x0003_ Allowing increased desorption of C, S, and Si from the surface by increased formation of volatile hydrides leads to decreased incorporation levels. These results suggest a simple and effective method of controlling the incorporation of dopant and impurity atoms by adding a minute amount of surfactant during OMVPE growth

Effects of Br and Cl on organometallic vapor phase epitaxial growth and ordering in GaInP

Journal ArticleCuPt ordering in GaInP has significant effects on the electrical and optical properties. In fact, band gap reductions as large as 160 meV are potentially useful for devices. Thus, control of ordering is important. This has led to the investigation of surfactants that affect the surface processes during growth with little incorporation into the solid. A potentially interesting class of surfactant elements are those from group VII, that are known not to be incorporated during growth, but to change the dynamics of surface processes

Note on the pseudo-Nambu-Goldstone Boson of Meta-stable SUSY Violation

Many models of meta-stable supersymmetry (SUSY) breaking lead to a very light scalar pseudo-Nambu Goldstone boson (PNGB), P, associated with spontaneous breakdown of a baryon number like symmetry in the hidden sector. Current particle physics data provide no useful constraints on the existence of P. For example, the predicted decay rates for both K --> pi + P, b--> s + P and Upsilon --> photon + P are many orders of magnitude below the present experimental bounds. We also consider astrophysical implications of the PNGB and find a significant constraint from its effect on the evolution of red giants. This constraint either rules out models with a hidden sector gauge group larger than SU(4), or requires a new intermediate scale, of order at most 10^{10} GeV, at which the hidden sector baryon number is explicitly broken.Comment: 17 pages, 3 figures. Version 2: minor typographical errors fixed. Version 3: a more reliable estimate for the decay rate of K-->pi+PNGB is provided, and the predicted rate for b-->s+PNGB is now include

Cellular Models for River Networks

A cellular model introduced for the evolution of the fluvial landscape is revisited using extensive numerical and scaling analyses. The basic network shapes and their recurrence especially in the aggregation structure are then addressed. The roles of boundary and initial conditions are carefully analyzed as well as the key effect of quenched disorder embedded in random pinning of the landscape surface. It is found that the above features strongly affect the scaling behavior of key morphological quantities. In particular, we conclude that randomly pinned regions (whose structural disorder bears much physical meaning mimicking uneven landscape-forming rainfall events, geological diversity or heterogeneity in surficial properties like vegetation, soil cover or type) play a key role for the robust emergence of aggregation patterns bearing much resemblance to real river networks.Comment: 7 pages, revtex style, 14 figure

Random Walks in Logarithmic and Power-Law Potentials, Nonuniversal Persistence, and Vortex Dynamics in the Two-Dimensional XY Model

The Langevin equation for a particle (`random walker') moving in d-dimensional space under an attractive central force, and driven by a Gaussian white noise, is considered for the case of a power-law force, F(r) = - Ar^{-sigma}. The `persistence probability', P_0(t), that the particle has not visited the origin up to time t, is calculated. For sigma > 1, the force is asymptotically irrelevant (with respect to the noise), and the asymptotics of P_0(t) are those of a free random walker. For sigma < 1, the noise is (dangerously) irrelevant and the asymptotics of P_0(t) can be extracted from a weak noise limit within a path-integral formalism. For the case sigma=1, corresponding to a logarithmic potential, the noise is exactly marginal. In this case, P_0(t) decays as a power-law, P_0(t) \sim t^{-theta}, with an exponent theta that depends continuously on the ratio of the strength of the potential to the strength of the noise. This case, with d=2, is relevant to the annihilation dynamics of a vortex-antivortex pair in the two-dimensional XY model. Although the noise is multiplicative in the latter case, the relevant Langevin equation can be transformed to the standard form discussed in the first part of the paper. The mean annihilation time for a pair initially separated by r is given by t(r) \sim r^2 ln(r/a) where a is a microscopic cut-off (the vortex core size). Implications for the nonequilibrium critical dynamics of the system are discussed and compared to numerical simulation results.Comment: 10 pages, 1 figur

Persistence of a Continuous Stochastic Process with Discrete-Time Sampling: Non-Markov Processes

We consider the problem of `discrete-time persistence', which deals with the zero-crossings of a continuous stochastic process, X(T), measured at discrete times, T = n(\Delta T). For a Gaussian Stationary Process the persistence (no crossing) probability decays as exp(-\theta_D T) = [\rho(a)]^n for large n, where a = \exp[-(\Delta T)/2], and the discrete persistence exponent, \theta_D, is given by \theta_D = \ln(\rho)/2\ln(a). Using the `Independent Interval Approximation', we show how \theta_D varies with (\Delta T) for small (\Delta T) and conclude that experimental measurements of persistence for smooth processes, such as diffusion, are less sensitive to the effects of discrete sampling than measurements of a randomly accelerated particle or random walker. We extend the matrix method developed by us previously [Phys. Rev. E 64, 015151(R) (2001)] to determine \rho(a) for a two-dimensional random walk and the one-dimensional random acceleration problem. We also consider `alternating persistence', which corresponds to a < 0, and calculate \rho(a) for this case.Comment: 14 pages plus 8 figure

An analysis of the FIR/RADIO Continuum Correlation in the Small Magellanic Cloud

The local correlation between far-infrared (FIR) emission and radio-continuum (RC) emission for the Small Magellanic Cloud (SMC) is investigated over scales from 3 kpc to 0.01 kpc. Here, we report good FIR/RC correlation down to ~15 pc. The reciprocal slope of the FIR/RC emission correlation (RC/FIR) in the SMC is shown to be greatest in the most active star forming regions with a power law slope of ~1.14 indicating that the RC emission increases faster than the FIR emission. The slope of the other regions and the SMC are much flatter and in the range of 0.63-0.85. The slopes tend to follow the thermal fractions of the regions which range from 0.5 to 0.95. The thermal fraction of the RC emission alone can provide the expected FIR/RC correlation. The results are consistent with a common source for ultraviolet (UV) photons heating dust and Cosmic Ray electrons (CRe-s) diffusing away from the star forming regions. Since the CRe-s appear to escape the SMC so readily, the results here may not provide support for coupling between the local gas density and the magnetic field intensity.Comment: 19 pages, 7 Figure