Planets are observed to orbit the component star(s) of stellar binary systems on so-called circumprimary or circumsecondary orbits, as well as around the entire binary system on so-called circumbinary orbits. Depending on the orbital parameters of the binary system a planet will be dynamically stable if it orbits within some critical separation of the semimajor axis in the circumprimary case, or beyond some critical separation for the circumbinary case. We present N-body simulations of star-forming regions that contain populations of primordial binaries to determine the fraction of binary systems that can host stable planets at various semimajor axes, and how this fraction of stable systems evolves over time. Dynamical encounters in star-forming regions can alter the orbits of some binary systems, which can induce long-term dynamical instabilities in the planetary system and can even change the size of the habitable zone(s) of the component stars. However, the overall fraction of binaries that can host stable planetary systems is not greatly affected by either the assumed binary population, or the density of the star-forming region. Instead, the critical factor in determining how many stable planetary systems exist in the Galaxy is the stellar binary fraction – the more stars that are born as singles in stellar nurseries, the higher the fraction of stable planetary systems.
<p>The Martian dichotomy features a ~25 km difference in crustal thickness and ~5 km contrast in topography between the southern highlands and northern lowlands [1]. Among various origin hypothesis, a southern impact [2,3] creates a magma pond which, upon cooling, induces crustal thickening and thereby forms the crustal dichotomy within 10s of million years.</p><p>&#160;</p><p>Our previous study [4], which utilizes a head-on parametrized impact in 2D geometry, shows that an impact-induced magma pond in the southern hemisphere is able to not only create a thickened crust in the south, but also a satisfying volcanic history with localized melt production in the equatorial region at geologically recent time.&#160; Depleted material, formed from crystallization of the magma pond, spreads and underplates the thicker and colder Northern lithosphere undisturbed by the impact, reinforcing the lesser extent of volcanism in the northern hemisphere. Our resultant mantle structure is consistent with existing simulation efforts that focus on the post-dichotomy formation evolution history [5], and in addition gives the context of how such thermochemical structure is developed.</p><p>&#160;</p><p>In order to include a more realistic impact scenario, we use smoothed particle hydrodynamics (SPH) simulations [6] to model the first 24-36 hours of a giant impact between proto-Mars and its impactor. The SPH result is then transferred to the mantle convection code StagYY [7], as an initial thermal condition, to simulate the long-term evolution of the crust and mantle for the subsequent 4.5 billion years. We systematically vary the impactor size, impact velocity and pre-impact Martian mantle temperature. Our preliminary results show that a 45-degree impact does not form a Martian dichotomy-like crustal structure, while a 15-degree impact is a better match.&#160; With a realistic impact, the mechanisms reported in our parametrized impact study still hold.</p><p>&#160;</p><p>&#160;</p><p>References:</p><p>&#160;</p><p>[1] Watters, T., McGovern, P., & Irwin III, R. (2007). Hemispheres Apart: The Crustal Dichotomy on Mars. <em>Annual Review Of Earth And Planetary Sciences</em>, <em>35</em>(1), 621-652.</p><p>&#160;</p><p>[2] Reese, C., Orth, C., & Solomatov, V. (2011). Impact megadomes and the origin of the martian crustal dichotomy. <em>Icarus</em>, <em>213</em>(2), 433-442.</p><p>&#160;</p><p>[3] Golabek, G., Keller, T., Gerya, T., Zhu, G., Tackley, P., & Connolly, J. (2011). Origin of the martian dichotomy and Tharsis from a giant impact causing massive magmatism. <em>Icarus</em>, <em>215</em>(1), 346-357.</p><p>&#160;</p><p>[4] Cheng, K.W., Tackley, P.J., Rozel, A.B., Golabek, G.J. (2021). Martian Dichotomy: Impact-induced Crustal Production in Mantle Convection Models, Abstract [DI35B-0023] presented at 2021 Fall Meeting, AGU, New Orleans, LA, 13-17 Dec.</p><p>&#160;</p><p>[5] Plesa, A., Padovan, S., Tosi, N., Breuer, D., Grott, M., & Wieczorek, M. et al. (2018). The Thermal State and Interior Structure of Mars. <em>Geophysical Research Letters</em>, <em>45</em>(22), 12,198-12,209.</p><p>&#160;</p><p>[6] Emsenhuber, A., Jutzi, M., Benz, W. (2018). SPH calculations of Mars-scale collisions: The role of the equation of state, material rheologies, and numerical effects. <em>Icarus</em>, 301, 247-257</p><p>&#160;</p><p>[7] Tackley, P. (2008). Modelling compressible mantle convection with large viscosity contrasts in a three-dimensional spherical shell using the yin-yang grid. <em>Physics Of The Earth And Planetary Interiors</em>, <em>171</em>(1-4), 7-18.</p><p>&#160;</p>
<p><strong>Introduction:</strong> Sputnik Planitia is a ~1600km-wide basin that dominates the surface of Pluto [1,2]. Observations show this feature to be amongst the deepest on Pluto [2], however its original depth is obscured by a thick layer of N<sub>2</sub> ice [3]. Its quasi-elliptical shape somewhat resembles a large degraded impact basin [4], leading to multiple impact-based investigations [5,6]. However, these works were limited to head-on collisions in two-dimensions, restricting their capacity to reproduce the simulated impact basin&#8217;s precise geometry or explore the oblique impact angles predicted to produce Sputnik Planitia&#8217;s elongated morphology.</p><p>Another common thread in these studies is the inferred presence of a subsurface ocean, as this could reconcile the seeming disparity between the basin&#8217;s gravity contribution and its near-equatorial location. The negative gravity anomaly of such a large basin excavated from a solid mantle would induce true polar wander and drive its position towards one of Pluto&#8217;s poles. A positive anomaly, on the other hand, would provoke the contrary, forcing the basin equatorward [7].</p><p>The currently accepted mechanism to produce such an anomaly is for Sputnik to form in an ice-crust above a global water ocean. The thinning of the crust leads to uplift of denser fluid [8,9], causing a mass concentration or &#8220;mascon&#8221; beneath the basin. This structure must be retained, for if the ice shell relaxes viscously, or the liquid ocean solidifies, the mascon disappears.</p><p>Retaining this structure is no simple task; the ice shell must be cold (and therefore stiff) to avoid rapid viscous relaxation, while the ocean beneath must be warm enough to remain liquid to the present day. Such a scenario may require an unusually high ammonia content to reduce the ocean&#8217;s freezing point [8] and/or a continually-replenished layer of clathrate hydrates to insulate the ice shell from the relatively warm ocean [9].</p><p>Here, we propose a new impact mechanism that introduces a long-lived rocky mascon beneath the Sputnik Planitia basin, while reproducing the topographical shape of the feature in three dimensions, without the need for a present-day subsurface ocean.</p><p><strong>Method:</strong> We simulate the impact using the SPHLATCH smoothed-particle hydrodynamics (SPH) code [10,11]. Shear strength and plasticity are included through a Drucker-Prager-like yield criterion, as such effects have been shown to be important even at planetary scales [11], with their prominence being particularly amplified by the very low temperatures of Pluto. The parameter space explored includes impactors of radii 250-500 km, with impact angles of 0-45&#176; and impact velocities of 1.0-1.4 times the mutual escape speed (~1.2 km/s). We consider undifferentiated impactors of ice, and of rock, and differentiated impactors with core mass fractions from 20-66%.</p><p><strong>Preliminary Results:</strong> The most promising cases lie in the intermediate parameters: core masses 5-30%, impact angles 15-30&#176;, and an impactor radius ~375 km. An example simulation in this range is shown in Fig. 1. Here the impactor initially excavates the material of the immediate impact site (Fig. 1a) before sufficiently slowing such that it can no longer overcome the shear strength of the cold ice and begins to slide along the target mantle towards its surface. While much of the impactor&#8217;s ice is displaced during this first phase, the rocky impactor core remains mostly intact due to its much higher density, melting temperature and strength, forcing out any impactor ice that was initially on the target-facing hemisphere. Most of the transient crater is then filled with infalling impactor ice while the impactor core slides back down towards the target core, losing some of its spherical shape as it does so (Fig. 1b).</p><p>Finally the site relaxes, settling into the desired teardrop shape of Sputnik Planitia, with the impactor core remaining as a buried mascon near Pluto&#8217;s core-mantle boundary (Fig. 1c). Fig. 2 shows the final distribution of the impactor material, with a remarkable resemblance to Sputnik Planitia. The near-spherical shape in the northern hemisphere corresponds to the initial point of impact where a more classical crater forms and collapses, whereas the pointed, triangular shape in the south corresponds to the sliding region of the impactor core.</p><p>We propose that the overall shape of this feature could remain intact if the impactor mantle&#8217;s precise composition had a slightly higher density than that of the primordial Pluto, as its greater load on the silicate core would lead to a local depression via isostasy. Furthermore, the dense buried mascon provided by the impactor&#8217;s rocky core would produce a significantly deeper region of the basin. N<sub>2</sub> ice would quickly accumulate in the basin [3], culminating in the positive mass anomaly that drove Sputnik Planitia into its current position through true polar wander. The region of the basin with the strongest positive anomaly &#8211; the southernmost, narrow section directly above the mascon &#8211; would be forced closest to the equator, matching present day observations. Finally, as true polar wander best explains the distribution of extensional features on Pluto [7], a more dominant positive anomaly provided by a differentiated impactor and a fully frozen ice shell may better fit the current observational constraints.</p><p><img src="https://contentmanager.copernicus.org/fileStorageProxy.php?f=gnp.037e01abc48269400782561/sdaolpUECMynit/2202CSPE&app=m&a=0&c=b65a5632dcba716f80b045aa1fea2601&ct=x&pn=gnp.elif&d=1" alt="" width="1066" height="300"></p><p><em><strong>Fig. </strong><strong>1</strong><strong>:</strong> Example simulation of the impact mechanism. (a) Shortly after the impact, where the transient crater is still present and the impactor core&#8217;s velocity has slowed to a near stop. The faded object indicates the impactor&#8217;s initial size and velocity. (b) After the collapse of the transient crater, now filled with impactor mantle. The impactor core has begun falling back towards Pluto&#8217;s core-mantle boundary, (c) The post-impact state of Pluto, 6 hours after the collision.</em></p><p><em><strong><img src="https://contentmanager.copernicus.org/fileStorageProxy.php?f=gnp.4aa0b37bc48263000782561/sdaolpUECMynit/2202CSPE&app=m&a=0&c=b7c5bae3be06e2c4b9efb043d512e2e8&ct=x&pn=gnp.elif&d=1" alt="" width="816" height="480"></strong></em></p><p><em><strong>Fig. </strong><strong>2</strong><strong>:</strong> A global map of the final impactor material distribution down to depths of ~120km. This corresponds to the same simulation as Fig. 1.</em></p><p>&#160;</p><p><strong>References:</strong></p><p>[1] Stern et al., 2015. <em>Science</em>,&#160;<em>350</em>(6258).</p><p>[2] Schenk et al., 2018. <em>Icarus</em>,&#160;<em>314</em>:400-433.</p><p>[3] Bertrand & Forget, 2016. <em>Nature</em>,&#160;<em>540</em>(7631):86-89.</p><p>[4] Moore et al., 2016. <em>Science</em>,&#160;<em>351</em>(6279):1284-1293.</p><p>[5] Johnson et al., 2016. <em>Geophysical Research Letters</em>,&#160;<em>43</em>(19):10068-10077.</p><p>[6] Denton et al., 2021. <em>Geophysical Research Letters</em>,&#160;<em>48</em>(2).</p><p>[7] Keane et al., 2016. <em>Nature</em>,&#160;<em>540</em>(7631):90-93.</p><p>[8] Nimmo et al., 2016. <em>Nature</em>,&#160;<em>540</em>(7631):94-96.</p><p>[9] Kamata et al., 2019. <em>Nature Geoscience</em>,&#160;<em>12</em>(6):407-410.</p><p>[10] Reufer et al., 2012. <em>Icarus, 221</em>:296&#8211;299.</p><p>[11] Emsenhuber et al., 2018. <em>Icarus</em>,&#160;<em>301</em>:247-257.</p>
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.