Blazars的聚束效应

本文对Blazars的聚束效应及相关的理论作了较全面的综述，指出了一 些有待进一步探讨的问题和需要进一步完善的理论，并对其中几个具体问题进行研究，得到了一些新结果。第一章简单介绍了活动星系核的特征、分类及其标准模型。第二章综述了Blazars的基本性质、对Blazars的谱特征、高光度、高偏振、激烈光变、超光速现象和高能辐射等作了介绍。第三章介绍了相对论喷流模型，以及利用相对论喷流模型解释Blazars的极端观测特性，如用相对论喷流模型从理论上解释了Blazars的高光度、剧烈光变及高能量转换率，偏振方向的快速变化，超光速现象，发射线和高能辐等能观测特性。同时介绍了喷流具有相对论性的观测证据并重点介绍了喷流的加速和减速两个理论模型。第四章是聚束效应的几项具体研究工作，首先分析了28个BL Lac天体，24个核优势高偏振类星体，29个核优势低偏振类星体，以及11个瓣优势低偏振类星体的射电和光学流量，证实具有相对论喷流的AGNs的Doppler提升效应确实存在，且很明显，光学和射电是高度聚束的。最小光变时标是一个及其重要的物理理，短时标光变能给人们提供大量的信息。但最小光变时标一般是在不同波段探测到的。利用加速模型，我们从理论上导出了一个联系各波段最小光变时标的公式，可把观测到的不同波段最小光变时标转换为同一波段的最小光变时标。利用该公式，把观测到的29个Blazars的不同波段最小光变时标转换为γ波段的最小光变时标，我们分析了这些对象光度与光变时标的关系，证实了Blazars的γ射线是高度聚束的。我们还研究了Blazars的偏振，发现偏振也是和聚束效应密切相关的，对Blazars的高能辐射也作了一些探讨，发现同步自康普顿模型能较好的解释Blazars天体的γ辐射。另外，提出一个新的喷流的电子注入模型，成功的解释了射电选Blazars天体在爆发时的谱硬化现象。     

高红移耀变体4C 38.41光学多波段光变性质

4C 38.41是一颗红移为1.813、可分类为平谱射电类星体的耀变体,使用一台85 cm望远镜在2018年2月22日至26日期间对其进行了光学V和R波段测光观测,得到两个波段的准同时数据.基于这些数据分析了该源在不同光变时标下的光变特性.结果表明:这颗源在观测期间处于较弱的活动状态, V和R波段星等(V, R)总变化均约为0.20等.在天内时标下,其中3晚探测到了该源的天内光变,另有一晚可能存在天内光变.此外,通过色指数和时延分析发现4C 38.41在不同时标下都展现越亮越红行为,与大多数平谱射电类星体类似,但在最后一晚4C 38.41在颜色-星等图上呈现出v字形,即首先展现越亮越红行为,然后转变为越亮越蓝行为,这可能是由该源的辐射在吸积盘主导和喷流主导之间的转换造成的.此外在第2晚探测到了V、R波段之间的时延,这是首次在高红移耀变体中探测到不同光学波段光变之间的时延,可以用随机同步辐射小结构模型来解释.     

Blazar天体的辐射特性研究

收集了47个Blazar天体的短时标光变资料,估算了中心天体质量和不同波段辐射区域,并对估算结果作了统计分析,发现Blazar天体中心黑洞质量在10~7M_☉到10~(10)M_☉之间,BL Lac天体与平谱射电类星体中心黑洞质量有很大差异,平谱射电类星体中心黑洞质量大于BL Lac天体中心黑洞质量;红外波段和γ射线波段辐射区域大小相似.同时,利用收集到Blazar天体的热光度分析了Blazar天体热光度与短时标光变之间的关系,证实了射电选BL Lac(RBL)天体和平谱射电类星体(FSRQs)的辐射是强成束的,但相对论聚柬效应对X射线选BL Lac(XBL)天体的辐射影响较小.     

射电宁静类星体紫外/光学光变的混合模型研究Ⅱ:来自SDSS DR7的的数据支持

到目前为止射电宁静类星体的光变起源还不清楚,其中一个可能的机制是吸积盘-再辐射模型。通过对斯隆数字巡天第7次释放数据(Sloan Digital Sky Survey data release 7, SDSS DR7)中789个经过多历元观测并且表现出变亮变蓝现象的射电宁静类星体进行分析,发现黑洞质量较小类星体的综合相对光变谱(残余谱/综合谱)的变亮变蓝幅度更大;其次,对于比较长的光变时标,尤其是在紫外波段,类星体变亮变蓝的幅度也更大。吸积盘-再辐射模型对以上两个主要结果中相对光变谱都拟合得很好,为该模型提供了进一步的支持。     

耀变体的光学偏振研究进展

耀变体是一类喷流与视线方向近似平行的活动星系核,高偏振是其区别于其他活动星系核的一个典型特征。耀变体的偏振一般认为是由喷流中相对论电子的同步辐射产生的。通过对光学偏振的观测,可以充分约束喷流性质,也给其他波段的光变信息提供补充。主要介绍耀变体光学偏振的研究情况:首先介绍耀变体及其偏振的一些背景;随后介绍耀变体偏振产生的原因、耀变体的去偏振效应、耀变体偏振随波长变化的研究,以及高能耀发与光学偏振相关性研究;最后,对耀变体光学偏振研究进行展望。     

低峰频blazars的射电光变分析

从大量文献资料中收集了6个低峰频blazars(BL Lacertae、0235+164、OQ530、0716+714、3C 345及3C 273)最近30多年来在4.8 GHz、8 GHz、14.5 GHz、22GHz和37 GHz最完备的光变数据.利用离散相关函数法对这6个源的相关性及延迟进行分析,利用结构函数法对6个源的光变曲线的光变周期和光变时标进行分析并对其光变幅度进行比较分析.分析结果显示:在光变幅度方面,0716+714和0235+164在6个blazars中的光变幅度相对较大,3C 345和OQ 530次之,3C 273和BL Lacertae的光变幅度相对较小;多波段的延迟分析显示0235+164在相邻两波段之间都显示高频波段要超前于低频波段的变化趋势,3C 345整体呈现出高频波段滞后于低频波段的变化趋势.其余blazars分析结果显示在部分射电波段之间呈现出高频波段要超前于低频波段的变化趋势,而在其余射电波段之间却呈现出高频波段滞后于低频波段的变化趋势;结构函数法的分析结果显示3C 345的光变时标、拟合斜率及光变周期与其它5个低峰频blazars相比都偏大,这表明3C 345的活动性与其它5个源相比较弱,这表明在3C 345内部可能存在与其它5个低峰频blazars不同的物理过程.     

γ噪Blazar天体短时标光变的寄主星系影响

利用云南天文台1m光学望远镜2000～2001年对3个GeV和/或TeV γ噪Blazar天体的观测,发现γ噪类星体PKS 1510-089在R波段有一个光变时标为41min的星等变化为2.0mag的剧烈光变,这是迄今为止我们所观测到的变幅最大的一个γ噪Blazar天体短时标光变.对此光变,可估计限制光变辐射模型的一些参数,如辐射区半径R、多普勒因子δ以及吸积转化效率η等.η=59.6的值强烈预示着相对论聚束效应可较好解释γ噪Blazar天体的辐射机制.我们仔细考察了大气视宁度对光变的影响.发现对1ES 2344+514,观测到的光变与当地大气视宁度有一个弱相关,结果表明,对光变参数较小(C＜5)的光变,大气视宁度引起的假光变的贡献较大,需要选择较严格的光变判据.     

γ噪Blazar天体短时标光变的寄主星系影响

利用云南天文台1m光学望远镜2000－2001年对3个GeV和／或TeVγ噪Blazar天体的观测，发现γ噪类星体PKS 1510－089在R波段有一个光变时标为41min的星等变化为2．0mag的剧烈光变，这是迄今为止我们所观测到的变幅最大的一个γ噪Blazar天体短时标光变。对此光变，可估计限制光变辐射模型的一些参数，如辐射区半径R、多普勒因子δ以及吸积转化效率η等。η＝59．6的值强烈预示着相对论聚束效应可较好解释γ噪Blazar天体的辐射机制。我们仔细考察了大气视宁度对光变的影响。发现对1ES 2344＋514，观测到的光变与当地大气视宁度有一个弱相关，结果表明，对光变参数较小（C＜5）的光变，大气视宁度引起的假光变的贡献较大，需要选择较严格的光变判据。     

类星体3C 273的射电流量和偏振度关系

利用UMRAO数据平台,研究了类星体3C273的射电流量密度与偏振度的关系（包括4.8GHz,8GHz和14.5GHz3个频段）,结果发现偏振度与射电流量在3个频段都具有很强的负相关性。这些结果可能表明3C273的射电流量密度和偏振度的变化与聚束效应无关。一般,偏振度与流量密度的强负相关现象可以用喷流成分＋激波成分的双成分模型来解释：在喷流成分与激波成分的偏振角相互垂直并且偏振度基本相同的情况下,激波在喷流中的传播产生了这些偏振度与流量密度的强负相关性。     

激变射电源中偏振角快速旋转的解释

在许多激变射电源(Blazar天体)中,观测到一种非常特别的现象,即偏振角的快速旋转.在相对论喷流的框架下,这种偏振角的旋转可以用双成分模型来解释.在这个模型中,一个成分是作为背景喷流本身的辐射,是恒定不变的成分;另一个成分被认为是沿喷流传播的相对论性激波,它产生变化的流量和偏振.这两个成分的偏系辐射的叠加可以产生观测到的偏振角旋转。本文讨论了三个激变射电源(BL Lac,AO0235+164,0727-115)中发生的偏振角旋转.结果表明,用相对论喷流-激波双成分模型可以很好地拟合观测到的偏振角旋转、偏振度和流量变化。说明射电激变源中出现的快速偏振和流量变化可能是由于相对论激波沿喷流传播时,激波辐射区中磁场取向和有序性以及强度和电子密度的变化所引起的。     

Archaean asteroid impacts, banded iron formations and MIF-S anomalies: A discussion

The origin of mass-independent fractionation (MIF-S) of sulphur isotopes (δ³³S) recorded in sediments older than 2.45 Ga is widely interpreted in terms of UV-triggered reactions under oxygen-poor ozone-depleted atmosphere conditions (Farquhar, J., Bao, H., Thiemens, M. [2000] Science, 289, 756; Farquhar, J., Peters, M., Johnston, D.T., Strauss, H., Masterson, A., Wiechert, U., Kaufman, A.J. [2007] Nature, 449, 706-709; Farquhar, J., Wing, B.A. [2003] Earth Planet. Sci. Lett., 213, 1-13; Kaufman, A.J., Johnston, D.T., Farquhar, J., Masterson, A.L., Lyons, T.W., Bates, S., Anbar, A.D., Arnold, G.L., Garvin, J., Buick, R. [2007a] Science, 317, 1900-1903; Kaufman, A.J., Farquhar, J., Johnston, D.T., Lyons, T.W., Arnold, G.L., Anbar, A. [2007b] Deep Time Drilling Project of the NASA Astrobiology Drilling Program). Observed mid-Archaean variability of MIF-S signatures raises questions regarding the extent of atmospheric anoxia (Ohmoto, H., Watanabe, Y., Ikemi, H., Poulson, H.R., Taylor, B. [2006] Nature, 406, 908-991; Farquhar et al., 2007). Late Archaean (∼2.7-2.5 Ga) and mid-Archaean (∼3.2 Ga) sequences in the Pilbara Craton (Western Australia) and Kaapvaal Craton (South Africa), in which MIF-S data were measured, contain asteroid impact ejecta units dated as 2.48, 2.56, 2.63, 3.24, 3.26 and 3.47 Ga old (Lowe, D.R., Byerly, G.R., Kyte, T., Shukolyukov, A., Asaro, F., Krull, A. [2003] Astrobiology, 3, 7-48; Simonson, B.M., Hassler, S.W. [1997] Aust. J. Earth Sci., 44, 37-48; Simonson, B.M., Glass, B.P. [2004] Ann. Rev. Earth Planet. Sci., 32, 329-361; Glikson, A.Y. [2004] Astrobiology, 4, 19-50; Glikson, A.Y. [2006] Earth Planet. Sci. Lett., 246, 149-160; Glikson, A.Y. [2008] Earth Planet. Sci. Lett., 267, 558-570). Mass balance calculations based on iridium and ⁵³Cr/⁵²Cr isotopic anomalies (Byerly, G.R., Lowe, D.R. [1994] Geochim. Cosmochim. Acta, 58, 3469-3486; Kyte, F.T., Shukloyukov, A., Lugmair, G.W., Lowe, D.R., Byerly, G.R. [2003] Geology, 31, 283-286) and on impact spherule size distribution (Melosh, H.J., Vickery, A.M. [1991] Nature, 350, 494-497) suggest projectiles several tens of kilometers in diameter (Byerly and Lowe, 1994; Shukloyukov, A., Kyte, F.T., Lugmair, G.W., Lowe, D.R., Byerly, G.R. [2000]. In: Koeberl, C., Gilmour, I. (Eds.), Impacts and the Early Earth, Springer-Verlag, Berlin, pp. 99-116; Kyte, F.T., Shukloyukov, A., Lugmair, G.W., Lowe, D.R., Byerly, G.R. [2003] Geology, 31, 283-286). Due to incomplete preservation these impacts represent a minimum rate of the Archaean impact flux. High UV radiation associated with low ozone levels in the Archaean atmosphere may have been further enhanced by large impacts, accentuating MIF-S anomalies. The appearance of iron-rich sediments above late and mid-Archaean impact ejecta units (Glikson, A.Y. [2006] Earth Planet. Sci. Lett., 246, 149-160; Glikson, A.Y., Vickers, J. [2007] Earth Planet. Sci. Lett., 254, 214-226) may be related either to microbial oxidation of ferrous iron or, alternatively, photochemical oxidation of ferrous to ferric iron. Given post-2.45 Ga diluting of possible MIF-S anomalies by the oxygenating ocean sulfate reservoir (Pavlov, A.A., Kasting, J.F. [2002] Astrobiology, 2, 27-41), similar MIF-S anomalies may have been associated with Proterozoic and Phanerozoic impacts, although to date little evidence exists in this regard (Marouka, T., Koeberl, C., Newton, J., Gilmour, I., Bohor, B.F. [2002] Geological Society of America Special Paper 356, pp. 337-344; Koeberl, C., Thiemens, M. [2008] Multi-sulfur isotopes in cretaceous-tertiary boundary samples from the Western interior-search for photochemical effects 2008. Joint Meeting of the Geological Society of America, Soil Science Society of America, American Society of Agronomy, Crop Science Society of America, Gulf Coast Association of Geological Societies with the Gulf Coast Section of SEPM. <http://gsa.confex.com/gsa/2008AM/finalprogram/abstract_148134.htm> (abstract)). Detailed sampling and isotopic analyses across the impact ejecta fallout units are required in order to test possible relationships between Archaean impacts and MIF-S anomalies.     

On convection in ice I shells of outer Solar System bodies, with detailed application to Callisto

It has been argued that the dominant non-Newtonian creep mechanisms of water ice make the ice shell above Callisto's ocean, and by inference all radiogenically heated ice I shells in the outer Solar System, stable against solid-state convective overturn. Conductive heat transport and internal melting (oceans) are therefore predicted to be, or have been, widespread among midsize and larger icy satellites and Kuiper Belt objects. Alternatively, at low stresses (where non-Newtonian viscosities can be arbitrarily large), convective instabilities may arise in the diffusional creep regime for arbitrarily small temperature perturbations. For Callisto, ice viscosities are low enough that convection is expected over most of geologic time above the internal liquid layer for plausible ice grain sizes (?a few mm); the alternative for early Callisto, a conducting shell over a very deep ocean (>450 km), is not compatible with Callisto's present partially differentiated state. Moreover, if convection is occurring today, the stagnant lid would be quite thick (∼100 km) and compatible with the lack of active geology. Nevertheless, Callisto's steady-state heat flow may have fallen below the convective minimum for its ice I shell late in Solar System history. In this case convection ends, the ice shell melts back at its base, and the internal ocean widens considerably. The presence of such an ocean, of order 200 km thick, is compatible with Callisto's moment-of-inertia, but its formation would have caused an ∼0.25% radial expansion. The tectonic effects of such a late, slow expansion are not observed, so convection likely persists in Callisto, possibly subcritically. Ganymede, due to its greater size, rock fraction and full differentiation, has a substantially higher heat flow than Callisto and has not reached this tectonic end state. Titan, if differentiated, and Triton should be more similar to Ganymede in this regard. Pluto, like Callisto, may be near the tipping point for convective shutdown, but uncertainties in its size and rock fraction prevent a more definitive assessment.     

Recent low-latitude freeze-thaw on Mars

Outside polar latitudes, features corresponding to surface thaw have yet to be identified on Mars. The youthful gully landforms observed at mid-high latitude [Malin, M., Edgett, K., 2000. Science 288, 2330-2335] are the nearest candidate, but the source (and nature) of the gully carving agent remains controversial [e.g., Musselwhite, D.S., Swindle, T.D., Lunine, J.I., 2001. Geophys. Res. Lett. 28, 1283-1285; Mellon, M.T., Phillips, R.J., 2001. J. Geophys. Res. 106, 1-15; Knauth, L.P., Burt, D.M., 2002. Icarus 158, 267-271; Costard, F., Forget, F., Mangold, N., Peulvast, J.P., 2002. Science 295, 110-113; Christensen, P.R., 2003. Nature 422, 45-48; Treiman, A.H., 2003. J. Geophys. Res. 108]. At higher obliquity than the present epoch, near-surface ground ice should be present globally [Mellon, M.T., Jakosky, B.M., 1995. J. Geophys. Res. 100 (E6), 11781-11799], populated by condensation of atmospheric water vapour in the top few metres of the regolith, or emplaced as dusty ice sheets reaching down towards the equator. The latitudinal restriction of these gullies to regions poleward of ±30° appears to argue against a thaw component to their formation—since ground ice is present and stable at all latitudes at high obliquity, the current (low) obliquity regime should result in ground ice thaw at low latitudes, where insolation and daytime temperatures are currently greatest, and this is not observed. A previously undescribed meltwater sequence in the Cerberus plains, at 20° N/187° E, shows that comparable, but much more continuous, and mappable melting and surface runoff have occurred in the geologically recent past at near-equatorial latitudes on Mars. Polygonal ground in the Cerberus plains is seen by the Mars Global Surveyor Mars Orbiter Camera (MOC) to suffer sequential, regional-scale volatile-loss consistent with thaw of near-surface ground ice under periglacial conditions. This degradation is continuously sampled by a single MOC strip, showing an icy landscape undergoing thaw modification and collapse, and may form the first evidence of equatorial wet-based glaciation during late Amazonian time, with indications of melting within the last million years. The dissolution and re-formation of polygonal ground links this landform to freeze-thaw processes, providing the conclusion to a question that has been the subject of debate for three decades—whether Mars' polygonal grounds require ice to form—and a consistent explanation for the fate of the water that carved the great outflow channels, much of which may still reside as ground ice in the regolith. This thaw occurs in the Cerberus Formation; deposits that are considered to be magmatic in origin, and the type formation for late-stage, “plains-style” volcanism on Mars [Keszthelyi, L., McEwen, A.S., Thordarson T., 2000. J. Geophys. Res. 105, 15027-15049]. By superposing large numbers of small impact craters, polygonal ground in the Cerberus plains sustains previous suggestions of a non-magmatic origin for this and other landforms in the region [Page, D.P., Murray, J.B., 2006. Icarus 183, 46-54]. Together, these periglacial landforms document evidence of climate change much younger than is currently recognised by crater counts, with important implications for age constraints on young surfaces and absolute age determinations by this method. It is tentatively suggested that this melting may be occurring today, with a striking correspondence between permafrost thaw in the Cerberus plains, the high atmospheric methane flux currently observed over this region [Mumma, M.J., Novak, R.E., DiSanti, M.A., Bonev, B.P., Dello Russo, N., 2004. Bull. Am. Astron. Soc. 36, 1127; Krasnopolsky, V.A., Maillard, J.P., Owen, T.C., 2004. Icarus 172, 537-547; Formisano, V., Atreya, S., Encrenaz, T., Ignatiev, N., Giuranna, M., 2004. Science 306, 1758-1761], and the only latitude zone on Mars—equatorward of 30° N—where melting of ground ice is thought possible in the current climate [Haberle, R.M., McKay, C.P., Schaeffer, J., Cabrol, N.A., Grin, E.A., Zent, A.P., Quinn, R., 2001. J. Geophys. Res. 106 (E10), 23317-23326; Lobitz, B., Wood, B.L., Averner, M.M., McKay, C.P., 2001. Proc. Natl. Acad. Sci. 98, 2132-2137]. Low-latitude polygonal ground as transient, and hydrologically active over wide areas transforms our understanding of the recent climatic evolution of Mars, supporting models of atmospheric water-ice migration [Mischna, M., Richardson, M.I., Wilson, R.J., McCleese, D.J., 2003. J. Geophys. Res. 108 (E6). 5062], complex, volatile stratigraphies [Clifford, S.M., Parker, T.J., 2001. Icarus 154, 40-79], and hypothesised, geologically recent ‘ice ages’ [Head, J.W., Mustard, J.F., Kreslavsky, M.A., Milliken, R.E., Marchant, D.R., 2003. Nature 426, 797-802]. The temporal coincidence of glacial epochs on the Earth and Mars during the Quaternary and latest Amazonian would suggest a coupled system linking both [Sagan, C., Young, A.T., 1973. Nature 243, 459].     

2007 Mutual events within the binary system of (22) Kalliope

In 2007, the asteroid Kalliope will reach one of its annual equinoxes. As a consequence, its small satellite Linus orbiting in the equatorial plane will undergo a season of mutual eclipses and occultations very similar to the one that the Galilean satellites undergo every 6 years. This paper is aimed at preparing a campaign of observations of these mutual events occurring from February to May 2007. This opportunity occurs only under favorable geometric conditions when the Sun and/or the Earth are close to the orbital plane of the system. This is the first international campaign devoted to the observation of photometric events within an asynchronous asteroidal binary system. We took advantage of a reliable orbit solution of Linus to predict a series of 24 mutual eclipses and 12 mutual occultations observable in the spring of 2007. Thanks to the brightness of Kalliope (m_v?11), these observations are easy to perform even with a small telescope. Anomalous attenuation events could be observed lasting for about 1-3 h with amplitude up to 0.09 mag. The attenuations are of two distinct types that can clearly be identified as primary and secondary eclipses similar to those that have been previously observed in other minor planet binary systems [Pravec, P., Scheirich, P., Kusnirák, P., Sarounová, L., Mottola, S., Hahn, G., Brown, P., Esquerdo, G., Kaiser, N., Krzeminski, Z., Pray, D.P., Warner, B.D., Harris, A.W., Nolan, M.C., Howell, E.S., Benner, L.A.M., Margot, J.-L., Galád, A., Holliday, W., Hicks, M.D., Krugly, Yu.N., Tholen, D., Whiteley, R., Marchis, F., Degraff, D.R., Grauer, A., Larson, S., Velichko, F.P., Cooney, W.R., Stephens, R., Zhu, J., Kirsch, K., Dyvig, R., Snyder, L., Reddy, V., Moore, S., Gajdos, S., Világi, J., Masi, G., Higgins, D., Funkhouser, G., Knight, B., Slivan, S., Behrend, R., Grenon, M., Burki, G., Roy, R., Demeautis, C., Matter, D., Waelchli, N., Revaz, Y., Klotz, A., Rieugné, M., Thierry, P., Cotrez, V., Brunetto, L., Kober, G., 2006. Photometric survey of binary near-Earth asteroids. Icarus 181, 63-93]. With these favorable circumstances, such photometric observations will provide us tight constraints regarding physical properties of Linus such as the size, shape and synodic spin period.     

Dynamical passage to approximate equilibrium shapes for spinning, gravitating rubble asteroids

Many asteroids are thought to be particle aggregates held together principally by self-gravity. Here we study — for static and dynamical situations — the equilibrium shapes of spinning asteroids that are permitted for rubble piles. As in the case of spinning fluid masses, not all shapes are compatible with a granular rheology. We take the asteroid to always be an ellipsoid with an interior modeled as a rigid-plastic, cohesion-less material with a Drucker-Prager yield criterion. Using an approximate volume-averaged procedure, based on the classical method of moments, we investigate the dynamical process by which such objects may achieve equilibrium. We first collapse our dynamical approach to its statical limit to derive regions in spin-shape parameter space that allow equilibrium solutions to exist. At present, only a graphical illustration of these solutions for a prolate ellipsoid following the Drucker-Prager failure law is available [Sharma, I., Jenkins, J.T., Burns, J.A., 2005a. Bull. Am. Astron. Soc. 37, 643; Sharma, I., Jenkins, J.T., Burns, J.A., 2005b. Equilibrium shapes of ellipsoidal soil asteroids. In: García-Rojo, R., Hermann, H.J., McNamara, S. (Eds.), Proceedings of the 5th International Conference on Micromechanics of Granular Media, vol. 1. A.A. Balkema, UK; Holsapple, K.A., 2007. Icarus 187, 500-509]. Here, we obtain the equilibrium landscapes for general triaxial ellipsoids, as well as provide the requisite governing formulae. In addition, we demonstrate that it may be possible to better interpret the results of Richardson et al. [Richardson, D.C., Elankumaran, P., Sanderson, R.E., 2005. Icarus 173, 349-361] within the context of a Drucker-Prager material. The graphical result for prolate ellipsoids in the static limit is the same as those of Holsapple [Holsapple, K.A., 2007. Icarus 187, 500-509] because, when worked out, his final equations will match ours. This is because, though the formalisms to reach these expressions differ, in statics, at the lowest level of approximation, volume-averaging and the approach of Holsapple [Holsapple, K.A., 2007. Icarus 187, 500-509] coincide. We note that the approach applied here was obtained independently [Sharma, I., Jenkins, J.T., Burns, J.A., 2003. Bull. Am. Astron. Soc. 35, 1034; Sharma, I., 2004. Rotational Dynamics of Deformable Ellipsoids with Applications to Asteroids. Ph.D. thesis, Cornell University] and it provides a general, though approximate, framework that is amenable to systematic improvements and is flexible enough to incorporate the dynamical effects of a changing shape, different rheologies and complex rotational histories. To demonstrate our technique, we investigate the non-equilibrium dynamics of rigid-plastic, spinning, prolate asteroids to examine the simultaneous histories of shape and spin rate for rubble piles. We have succeeded in recovering most results of Richardson et al. [Richardson, D.C., Elankumaran, P., Sanderson, R.E., 2005. Icarus 173, 349-361], who obtained equilibrium shapes by studying numerically the passage into equilibrium of aggregates containing discrete, interacting, frictionless, spherical particles. Our mainly analytical approach aids in understanding and quantifying previous numerical simulations.     

Field reconnaissance of the Anti-Atlas coastline, Morocco: Fluvial and marine evidence for Late Cenozoic uplift

The available evidence regarding the disposition and chronology of Pliocene–Pleistocene fluvial terraces, coastal rock flats, raised beaches and lacustrine sediments adjoining the Anti-Atlas coastline of Morocco has been reviewed and supplemented by additional information from our own field reconnaissance. It is thus suggested that the study region has experienced uplift by  130 m since the Mid-Pliocene climatic optimum ( 3.1 Ma), by  90 m since the latest Pliocene ( 2 Ma), and by  45 m since the Mid-Pleistocene Revolution ( 0.9 Ma). Each of these phases of uplift correlates with a phase of global climate change known independently, and it is thus inferred that the observed uplift is being driven by climate through mechanisms such as erosional isostasy and the associated induced lower-crustal flow. Numerical modelling of the observed uplift history indicates that the mobile lower-crustal layer in the study region is  9 km thick, with a temperature at its base of  500 °C. The base of this mobile layer is inferred to be at  24 km depth, the deepest crust consisting of a layer of mafic underplating that does not flow under ambient conditions. The principal landform in the study region, the coastal rock platform at  60 m a.s.l., thus formed during a succession of interglacial marine highstands in the late Early Pleistocene when uplift rates were low. Although control on the ages of young sediments and landforms is currently extremely limited, being dependent on regional correlation schemes rather than on absolute dating, the study region fits the pattern, emerging worldwide, that climate change is driving the systematic growth of topographic relief evident during the Late Cenozoic.     

A thermodynamic model of high temperature lava vaporization on Io

We modified the MAGMA chemical equilibrium code developed by Fegley and Cameron (1987, Earth Planet. Sci. Lett. 82, 207-222) and used it to model vaporization of high temperature silicate lavas on Io. The MAGMA code computes chemical equilibria in a melt, between melt and its equilibrium vapor, and in the gas phase. The good agreement of MAGMA code results with experimental data and with other computer codes is demonstrated. The temperature-dependent pressure and composition of vapor in equilibrium with lava is calculated from 1700 to 2400 K for 109 different silicate lavas in the ONaKFeSiMgCaAlTi system. Results for five lavas (tholeiitic basalt, alkali basalt, Barberton komatiite, dunite, and a molten type B1 Ca, Al-rich inclusion) are discussed in detail. The effects of continuous fractional vaporization on chemistry of these lavas and their equilibrium vapor are presented. The predicted abundances (relative to Na) of K, Fe, Si, Al, Ca, and Ti in the vapor equilibrated with lavas at 1900 K are lower than published upper limits for Io's atmosphere (which do not include Mg). We predict evaporative loss of alkalis, Fe, and Si during volcanic eruptions. Sodium is more volatile than K, and the Na/K ratio in the gas is decreased by fractional vaporization. This process can match Io's atmospheric Na/K ratio of 10±3 reported by Brown (2001, Icarus 151, 190-195). Silicon monoxide is an abundant species in the vapor above lavas. Spectroscopic searches are recommended for SiO at IR and mm wavelengths. Reactions of metallic vapors with S- and Cl-bearing volcanic gases may form other unusual gases including MgCl₂, MgS, MgCl, FeCl₂, FeS, FeCl, and SiS.     

Titan's 3-micron spectral region from ISO high-resolution spectroscopy

The near-infrared spectrum of Titan, Saturn's largest moon and one of the Cassini/Huygens' space mission primary targets, covers the 0.8 to 5 micron region in which it shows several weak CH₄ absorption regions, and in particular one centered near 2.75 micron. Due to the interference of telluric absorption, only part of this window region (2.9-3.1 μm) has previously been observed from the ground [Noll, K.S., Geballe, T.R., Knacke, R., Pendleton, F., Yvonne, J., 1996. Icarus 124, 625-631; Griffith, C.A., Owen, T., Miller, G.A., Geballe, T., 1998. Nature 395, 575-578; Griffith, C.A., Owen, T., Geballe, T.R., Rayner, J., Rannou, P., 2003. Science 300, 628-630; Geballe, T.R., Kim, S.J., Noll, K.S., Griffith, C.A., 2003. Astrophys. J. 583, L39-L42]. We report here on the first spectroscopic observations of Titan covering the whole 2.4-4.9 μm region by two instruments on board the Infrared Space Observatory (ISO) in 1997. These observations show the 2.75-μm window in its complete extent for the first time. In this study we have also used a high-resolution Titan spectrum in the 2.9-3.6 μm region taken with the Keck [Geballe, T.R., Kim, S.J., Noll, K.S., Griffith, C.A., 2003. Astrophys. J. 583, L39-L42; Kim, S.J., Geballe, T.R., Noll, K.S., Courtin, R., 2005. Icarus 173, 522-532] to infer information on the atmospheric parameters (haze extinction, single scattering albedo, methane abundance, etc.) by fitting the methane bands with a detailed microphysical model of Titan's atmosphere (updated from Rannou, P., McKay, C.P., Lorenz, R.D., 2003. Planet. Space Sci. 51, 963-976). We have included in this study an updated version of a database for the CH₄ absorption coefficients [STDS, Wenger, Ch., Champion, J.-P., 1998. J. Quant. Spectrosc. Radiat. Transfer 59, 471-480. See also http://www.u-bourgogne.fr/LPUB/TSM/sTDS.html for latest updates; Boudon, V., Champion, J.-P., Gabard, T., Loëte, M., Michelot, F., Pierre, G., Rotger, M., Wenger, Ch., Rey, M., 2004. J. Mol. Spectrosc. 228, 620-634]. For the atmosphere we find that (a) the haze extinction profile that best matches the data is one with higher (by 40%) extinction in the atmosphere with respect to Rannou et al. (2003) down to about 30 km where a complete cut-off occurs; (b) the methane mixing ratio at Titan's surface cannot exceed 3% on a disk-average basis, yielding a maximum CH₄ column abundance of 2.27 km-am in Titan's atmosphere. From the derived surface albedo spectrum in the 2.7-3.08 micron region, we bring some constraints on Titan's surface composition. The albedo in the center of the methane window varies from 0.01 to 0.08. These values, compared to others reported in the other methane windows, show a strong compatibility with the water ice spectrum in the near-infrared. Without confirming its existence from this work alone, our data then appear to be compatible with water ice. A variety of other ices, such as CO₂, NH₃, tholin material or hydrocarbon liquid cannot be excluded from our data, but an additional unidentified component with a signature around 2.74 micron is required to satisfy the data.     

Preservation by basalt of a staircase of latest Pliocene terraces of the River Murat in eastern Turkey: Evidence for rapid uplift of the eastern Anatolian Plateau

Unspiked K–Ar dating makes the age of the Çakmaközü basalt in eastern Turkey 1818 ± 39 ka (± 2σ). This basalt overlies a staircase of four terraces of the River Murat, a Euphrates tributary, each separated vertically by  20 m. We deduce from the relationship with the basalt that these fluvial deposits aggraded during successive  40 ka climate cycles around the Pliocene–Pleistocene boundary (probably MIS 72-66). The incision and rock uplift at  0.5 mm a^− 1, thus indicated, are roughly consistent with the  500 m of entrenchment of this  1.8 Ma Murat palaeo-valley into a former lake basin since the Mid-Pliocene climatic optimum. We infer that the  130 m of incision in this locality since  1.8 Ma dramatically underestimates the associated rock uplift, estimated as  600 m. The  1100 m of rock uplift and  800 m of surface uplift thus estimated since the Mid-Pliocene indicate (assuming Airy isostatic equilibrium)  5 km of thickening of the continental crust, from  37 km to the present 42 km. Eastern Anatolia was thus at a much lower altitude in the Mid-Pliocene than at present, consistent with the low-relief lacustrine palaeo-environment. We infer that the subsequent development of topography and excess crustal thickness are being caused by coupling between surface processes and induced flow in the lower crust: climate change following the Mid-Pliocene climatic optimum resulted in faster erosion that has drawn mobile lower crust beneath the study region.     

Effects of plasticity on convection in an ice shell: Implications for Europa

Europa's surface exhibits numerous pits, uplifts, and disrupted chaos terrains that have been suggested to result from convection in the ice shell. To test this hypothesis, we present numerical simulations of convection in an ice shell including the effects of plasticity, which provides a simple continuum representation for brittle or semibrittle deformation along discrete fractures. Plastic deformation occurs when stresses reach a specified yield stress; at lower stresses, the fluid flow follows a Newtonian, temperature-dependent viscosity. Four distinct modes of behavior can occur. For yield stresses exceeding ∼1 bar, plastic effects are negligible and stagnant-lid convection, with no surface motion and minimal topography, results. At intermediate yield stresses, a stagnant lid forms but deforms plastically, leading to surface velocities up to several millimeters per year. Slightly smaller yield stresses allow episodic, catastrophic overturns of the upper conductive lid, with (transient) stagnant lids forming in between overturn events. The smallest yield stresses allow continual recycling of the upper lid, with simultaneous, gradual ascent of warm ice to the surface and descent of cold, near-surface ice into the interior. The exact yield stresses over which each regime occurs depend on the ice-shell thickness, melting-temperature viscosity, and activation energy for viscous creep. To form hummocky matrix and translate chaos plates by several kilometers, substantial surface strain must accompany chaos formation, and the three plasticity-dominated convection modes described here can provide such deformation. Our simulations suggest that, if yield stresses of ∼0.2-1 bar are relevant to Europa, then convection in Europa's ice shell can produce chaos-like structures at the surface. However, our simulations have difficulty explaining Europa's numerous pits and uplifts. When plasticity forces the upper lid to participate in the convection, dynamic topography of ∼50-100-m amplitude results, but the topographic structures generally have diameters of 30-100 km, an order of magnitude wider than typical pits and uplifts. None of our simulations produced isolated pits or uplifts of any diameter.