2. Target EnvironmentsFrom the Journal of the British Interplanetary Society 1997, 50, 93-102. Michael N. Mautner, adapted for the general public
Star-forming clouds, dense cores and planetary accretion discs.
A typical target environment will be illustrated by choosing a representative interstellar cloud, the Rho Ophiuchus (520 light years from Earth), a cloud that forms long-lived low and medium mass stars. The following passage describes and attempts to put a scale on the events that occur during the birth of stars in such a cloud.
As described by Mezger  (see figure 1), the width of the Rho Ohpiuchus is about 50 ly (light years) and consists of low density gas with a hydrogen atom density of less than 1E3 per cm3, (or < 1.7E-18kg per m3) of a total mass > 3,000 M (solar mass of 2E30kg), and contains a dense fragment of 6 by 6 ly with a density of 1E4per cm3 and mass ~500 M. This fragment contains 78 young stellar objects of low-mass dust-embedded or early accretion stage T Tauri stars. Within this cloud are four cores with diameters of > 1 ly and, densities of 1E6 per cm3 (1.7E-15kg per m3) and masses of 1 - 15 M.One of these cores shows four protostellar condensations with radii of >3E14m, densities of 1E7 per cm3 (1.7E-14kg per m3) and masses of 0.4 to 3 M. Dust temperatures in this region are 15 - 20 K.
Small panspermia capsules captured in a protostellar condensation or the area round a young star in an accreting planetary system, will become part of the dust in the system. The protostellar condensation free-falls in a period of over 4E4 years to cores with radii of 100 au and densities of 1E11 - 1E12 per cm3 (1.7E-10 - 1.7E-9kg per m3). These cores collapse further during 1E5 - 1E6 years into a 1E6m thick, 100 au (about 1E13m) radius dust ring , that comprises 0.01 M (2E28kg) (possibly up to 0.1 M) mass about a 1 M young T-Tari star, and has a temperature of between 50 - 400 K at 1 au (consider 250 K), with possible periodic heating over 1,000 K, and 250a-0.58 at other distances a (in au = 1.5E11m units) . In the ring, the dust accretes rapidly (in 1E3 - 1E4 periods of revolution) from micron-size grains to 1 - 10 km planetisimals; then, in about 1E5 years, to 1E3km radius, 1E21 kg runaway planetary seeds that developing into 1E23kg planetoids; and in the next 1E8 years, to planets . Most of the gas is ejected from the disk in 1E6 - 1E7 years by bipolar outflow and stellar UV radiation . A fraction of the residual materials accrete in a zone of several tens of au from the star to become a 10 km diameter nuclei of 1E14 - 5E14kg of 1E13 comets, most of which are expelled to interstellar space , except 1E11 - 1E12 comets with a total mass of 1E25 - 1E26kg that are retained in the Oort cloud at 1.7E4 - 1E5 au.  Another about 1E23kg materials form the Kuiper belt comets , and 1E22 form the main-belt asteroids .
Cometary mass erroding in transits maintains a Zodiacal dust cloud of 2.5E16kg and mean lifetime of 1E5 years by injecting at present, about 2E4kg per s1 dust near the perihelion passes at <3.5 au . Of this, 0.15 kg per s-1, ie., a fraction of 1E-5, is collected by the Earth [16a]. At higher densities in the prebiotic period between 3 and 4 Gyr (1Gyr = 1E9years) ago, 1E17kg of the cometary dust accreted onto the Earth in the form of 0.6 to 60 mm radius particles in which organic material can be preserved during atmospheric transit . Similar to the Zodiacal dust collection efficiency, 1E-5 of the asteroid fragments produced by collisions eventually impacts on the Earth as meteorites [16b]. Both data suggest that 1E-5 of the objects in orbit within several au of a habitable planet are eventually collected.
Altogether, the 1E17 kg material of cometary origin that was collected by the Earth in the early biotic period between 3 - 4 Gyr ago, constitutes about 1E-13 of the total 1 M (2E30 kg) protostellar condensation, 1E-11 of the mass of the original accretion dust ring, and 1E-9 of the total present Oort cloud cometary mass.
These data from our solar systems are used as models. These data are current, model-dependent estimates with uncertainties up to an order of magnitude, and respective figures may be of course different in other solar systems.
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