Thermal consequences of impact bombardments to silicate crusts of terrestrial-type exoplanets
Authors: Mojzsis Stephen J. , Abramov Oleg 
Published: March 2021

&lt;p&gt;&lt;strong&gt;Introduction. &lt;/strong&gt;Post-accretionary impact bombardment is part of planet formation and leads to localized, regional [e.g., 1-3], or even wholesale global melting of silicate crust [e.g., 4]; less intense bombardment can also create hydrothermal oases favorable for life [e.g, 5]. Here, we generalize the effects of late accretion bombardments to extrasolar planets of different masses (0.1-10M&lt;sub&gt;&amp;#8853;&lt;/sub&gt;). One example is Proxima Centauri b, estimated at ~2&amp;#215; M&lt;sub&gt;&amp;#8853;&lt;/sub&gt; [6]. We model a 0.1M&lt;sub&gt;&amp;#8853; &lt;/sub&gt;&amp;#8220;mini-Earth&amp;#8221;&lt;sub&gt;&lt;/sub&gt;and &amp;#8220;super-Earth&amp;#8221; at 10M&lt;sub&gt;&amp;#8853;&lt;/sub&gt;, the approximate upper limit for a &amp;#8220;mini-Neptune&amp;#8221; [7]. Output predicts lithospheric melting and subsurface habitable volumes.&lt;/p&gt;&lt;p&gt;&lt;strong&gt;Methods. &lt;/strong&gt;The model [1,2] consists of (i) stochastic cratering; (ii) analytical thermal expressions for each crater [e.g., 8,9]; and (iii) a 3-D thermal model of the lithosphere, where craters cool by conduction and radiation.&lt;/p&gt;&lt;p&gt;We analyze impact bombardments using our solar system&amp;#8217;s mass production functions for the first 500 Myr [10]. Surface temperatures and geothermal gradients are set to 20 &amp;#176;C and 70 &amp;#176;C/km [2]. Total delivered mass for Earth is 7.8 &amp;#215; 10&lt;sup&gt;21&lt;/sup&gt; kg, and scaled to other planets based on cross-sectional areas, with 1.7 &amp;#215; 10&lt;sup&gt;21&lt;/sup&gt; kg for mini-Earth, 1.2 &amp;#215; 10&lt;sup&gt;22&lt;/sup&gt; kg for Proxima Centauri b, and 3.6 &amp;#215; 10&lt;sup&gt;22&lt;/sup&gt; kg for super-Earth. The impactors' SFD is based on our main asteroid belt [11]. Impactor and target densities are set to 3000 kg m&lt;sup&gt;-3&lt;/sup&gt; and planetary bulk densities are assumed to be 5510 kg m&lt;sup&gt;-3&lt;/sup&gt;, omitting gravitational compression [7]. Impactor velocity was estimated at 1.5 &amp;#215; v&lt;sub&gt;esc&lt;/sub&gt; for each planet, with 7.8 km s&lt;sup&gt;-1&lt;/sup&gt; for mini-Earth, &amp;#160;16.8 km s&lt;sup&gt;-1&lt;/sup&gt; for the Earth, 21.1 km s&lt;sup&gt;-1&lt;/sup&gt; for Proxima Centauri b, and 36.1 km s&lt;sup&gt;-1&lt;/sup&gt; for super-Earth.&lt;/p&gt;&lt;p&gt;&lt;strong&gt;Results. &lt;/strong&gt;We assume fully formed crusts, so melt volume immediately increases due to impacts. Super-Earth reaches a maximum of ~45% of the lithosphere in molten state, whereas mini-Earth reaches a maximum of only ~5%. &amp;#160;This is due to much higher impact velocities and cratering densities on the super-Earth compared to mini-Earth. We also show the geophysical habitable volumes within the upper 4 km of a planet&amp;#8217;s crust as the bombardment progresses. Impacts sterilize the majority of the habitable volume on super-Earth; however, due to its large total volume, the total habitable volume is still higher than on other planets despite the more intense bombardment in terms of energy delivered per unit area.&lt;/p&gt;&lt;p&gt;&lt;strong&gt;References:&lt;/strong&gt; [1] Abramov, O., and S.J. Mojzsis (2009) Nature, 459, 419-422. [2] Abramov et al. (2013) Chemie der Erde, 73, 227-248. [3] Abramov, O., and S. J. Mojzsis (2016) Earth Planet Sci. Lett., 442, 108-120. [4] Canup, R. M. (2004) Icarus, 168, 433-456. [5] Abramov, O., and D. A. Kring (2004) J. Geophys. Res., 109(E10). [6] Tasker, E. J. et al. (2020). Astronom. J., 159(2), 41. [7] Marcy, G. W. et al. (2014). PNAS, 111(35), 12655-12660. [8] Kieffer S. W. and Simonds C. H. (1980) Rev. Geophys. Space Phys., 18, 143-181. [9] Pierazzo E., and H.J. Melosh (2000). Icarus, 145, 252-261. [10] Mojzsis, S. J. et al. (2019). Astrophys. J., 881(1), 44. [11] Bottke, W. F. et al. (2010) Science, 330, 1527-1530.&lt;/p&gt;