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Exporting Terrestrial Life out of the Solar System with Gravitational i “Author_tex” — 2019/12/4 — 1:50 — page 1 — #1 i i i International Journal of Exporting Terrestrial Life Out of the Solar Astrobiology System with Gravitational Slingshots of cambridge.org/ija Earthgrazing Bodies Amir Siraj1, Abraham Loeb1 Research Article 1Department of Astronomy, Harvard University, 60 Garden Street, Cambridge, MA 02138, USA Cite this article: Siraj A, Loeb A (2019). Exporting Terrestrial Life Out of the Solar Abstract System with Gravitational Slingshots of Exporting terrestrial life out of the Solar System requires a process that both embeds microbes Earthgrazing Bodies. Submitted to the International Journal of Astrobiology. in boulders and ejects those boulders out of the Solar System. We explore the possibility that Earthgrazing long-period comets and interstellar objects could export life from Earth by collect- Received: xx xxxx xxxx ing microbes from the atmosphere and receiving a gravitational slingshot effect from the Earth. Revised: xx xxxx xxxx We estimate the total number of exportation events over the lifetime of the Earth to be ∼ 1 − 10 Accepted: xx xxxx xxxx for long-period comets and ∼ 1 − 50 for interstellar objects. If life existed above an altitude of 5 Key words: 100 km, then the number is dramatically increased up to ∼ 10 exportation events over Earth’s astrobiology; planets; comets; meteors lifetime. Author for correspondence: Amir Siraj, Email: [email protected], Abraham Loeb, Email: [email protected] arXiv:1910.06414v3 [astro-ph.EP] 2 Dec 2019 © The Authors 2019. i i i i i “Author_tex” — 2019/12/4 — 1:50 — page 2 — #2 i i i 2 A. Siraj & A. Loeb Introduction 0.15 Panspermia is the idea that life can propagate from one planet to another (Wesson, 2010; Wickramasinghe, 2010). Impacts on ∞ the surface of a planet can launch debris at above the escape v 0.1 speed of the planet, thereby allowing debris spread throughout Δ the planetary system and constituting a plausible mechanism for exchanging life between planets orbiting the same star (Fritz et al., 2005; Mileikowsky et al., 2000). However, it is difficult to eject d / dP 0.05 life-bearing material at speeds above the escape speed from a plan- etary system that is effectively shielded from destructive radiation, presenting a significant challenge for spreading life between stars 0 (Wesson, 2010). -6 -3 0 3 6 Life in the Earth’s atmosphere has been detected up to an alti- tude of 77 km (Imshenetsky et al., 1978), constituting a reservoir Δv∞ (km/s) of microbes that objects grazing the atmosphere could draw from. Fig. 1. Distribution of the gravitational ∆v1 for random encounter geometries Long-period comets (LPCs) represent a population of bodies that between Earth and LPCs marginally bound to the Sun near the Earth’s surface. can easily be ejected from the Solar System by gravitational inter- actions with planets due to their low gravitational binding energies and planet-crossing orbits. This makes them ideal, in principle, for as appropriate for LPCs.b The Sun and Earth define the ecliptic both picking up life from Earth and exporting it out of from the plane. For each test particle, we randomly pick an angle within the Solar System. ecliptic plane between 0 and 2π, as well as a zenith angle between In addition, the high speed and abundance of interstellar objects 0 and π. Using these two angles, we set each particle’s position (ISOs) make them, in addition to LPCs, potential exporters of vector relative to Earth. life from Earth to exoplanetary systems. 1I/‘Oumuamua (Meech To ensure that each particle is at its distance of closest et al., 2017; Micheli et al., 2018) was the first interstellar object approach, we require the velocity vector to lie in the plane perpen- detected in the Solar System, CNEOS 2014-01-08 (Siraj & Loeb, dicular to the position vector relative to Earth. For each particle, 2019a) was tentatively the first interstellar meteor discovered we pick a random angle between 0 and 2π to determine in larger than dust, and 2I/Borisov (Guzik et al., 2019) was the first which direction the velocity vector points within this plane. Using confirmed interstellar comet. Ginsburg et al.(2018) and Lingam the angle within the plane perpendicular to the position vector, & Loeb(2018) demonstrate dynamically that ejected objects can we construct each particle’s velocity vector. At this point, we be gravitationally captured by other star systems. have fully initialized the 6D coordinates of each particle in both In this paper, we study whether it is possible for ISOs and position and velocity. LPCs to have exported life from Earth’s atmosphere out of the In the first stage of the simulation, we integrate all of the parti- solar system. First, we analyze the gravitational slingshot effect cles backward in time. We use the IAS15 integrator in REBOUND of the Earth during such encounters. We then evaluate the effects to trace each particle from t = 0 to an earlier time −t (Rein & of atmospheric drag on the transporting body’s size and minimum i Spiegel, 2014), where t is an amount of time to sample either side encounter altitude. Next, we discuss the collection of microbial i of the closest approach to Earth. The only constraint on t is that life during the transporting body’s passage through the atmo- i it is a time interval at and above which the results of the simu- sphere. We then estimate the total number of exportation events lation do not change, on the order of a few times the encounter since the dawn of life on Earth. Finally, we summarize our main period; in this case, t is on the order of a few days. We record the conclusions. i change in the speed at infinity for the incoming segment of the par- ticle’s trajectory, ∆v1; in. In the second stage of the simulation, we integrate the particles with unbound initial conditions forward Gravitational Slingshots with Earth in time. We use IAS15 to integrate each particle from t = 0 to To gain an understanding of the approximate change in energy that ti. We record the change in the speed at infinity for the outgoing a transporting body receives through a random gravitational inter- segment of the particle’s trajectory, ∆v1; out, and add it to the action with the Earth, we developed a Python code that randomly the incoming and outgoing changes to find the change in speed at initializes and integrates the motions of particles from their points infinity for the entire encounter, ∆v1. of closest approach to Earth in the past or future, computing the We ran our Python code for 105 particles, and the resulting total change in energy over the interaction. The Python code cre- distribution of ∆v1 is shown in Figure1. Half of encounters ated for this work used the open-source N-body integator software result a positive change in energy, as expected from symmetry a REBOUND to trace the motions of particles under the gravitational to time-reversal, and 95% of such encounters result in ∆v1 ≤ influence of the Earth-Sun system (Rein & Liu, 2012). 3 km s−1. This corresponds to objects with perihelion distances We initialize the simulation with the Sun, Earth, and a volume & 200 AU, or LPCs. of test particles surrounding Earth at 80 km from the Earth’s sur- face, with near-zero gravitational binding energies from the Sun bISOs are not included in the simulation, as they are assumed to have binding energies ahttps://rebound.readthedocs.io/en/latest/ significantly below zero. i i i i i “Author_tex” — 2019/12/4 — 1:50 — page 3 — #3 i i i Exporting Terrestrial Life 3 Atmospheric Drag As the transporting object grazes the atmosphere, it encounters 10000 atmospheric drag, giving rise to constraints on its minimum size and minimum encounter altitude for it to escape. 1000 Because the change in energy due to the gravitational slingshot (m) is small relative to the initial kinetic energy, we approximate the 100 transporting object’s path as linear, summarized by the following min tb, expression for altitude, L 10 q 2 2 z(x) = R⊕ + zmin − R⊕ − x ; (1) 1 where R⊕ is the radius of the Earth, zmin is the minimum altitude 0 20 40 60 80 of the encounter, and x is a distance parameter that fulfills z(0) = z (km) zmin and dx = v dt, where v is the instantaneous speed. min The density of the atmosphere as a function of altitude is Fig. 2. Minimum diameter of transporting body to guarantee total expansion of ρ(z) = e−z=8 km kg m−3, and the density of the transporting < 10% as a function of minimum encounter altitude. The red line indicates the result for an encounter velocity of 42 km s−1, and the gray lines represent the body is taken to be that of a typical cometary nucleus, ρtb = upper and lower bounds, corresponding to the maximum and minimum −3 600 kg m . The acceleration of the transporting body is given encounter velocities of 72 km s−1 and 12 km s−1, respectively. by the drag equation (Collins et al., 2010), resemble bacteria, and thus be mistaken for the latter (Wainwright dv 3ρ(z)CD 2 = − v ; (2) et al., 2004; Smith, 2013). dt 4ρtbLtb The change in speed for all successful encounters is . where CD is the drag coefficient, set to the typical value of 2, and 10−5 m s−1, which is reasonable considering the fact that sig- Ltb is the length of transporting body. nificant slow-down will lead to runaway expansion.
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