Monday, June 13, 2016

Why is Mars So Small?

If we are going to be thinking about where to find life, starting with solar systems, we should first think about the sizes of planets. Mars is much smaller than Earth and has lost almost all of its atmosphere. If there was life there, it likely died out with no source of carbon dioxide to get carbon from. Exposure to near vacuum evaporates water, so that essential component is missing. Protection from hard photons is not provided, so life would have difficulty on the surface.

If the life origination theory promulgated here, organic oceans, is correct, there was likely none of that here. Too small a planet means no life. But the question remains, why is the planet so small? Why didn’t it collect enough mass like Venus and Earth to provide the right gravity and therefore the possibility of the right atmosphere, provided temperatures were tolerable. The Liquid Water Zone could have been right around Mars, but it would have been to no avail.

This doesn’t seem to be a ‘why question’ that is commonly asked. Perhaps the basic concept going around is that planets just form with whatever material is around them and some planets are in dense areas and some planets are is sparse areas, and it’s just too bad for some planet that originates in a sparse area. It has no chance for life.

But for a minute, think about the planetary disk and the processes that shape it. Perhaps the why question is ‘why was that part of the planetary disk sparse?’ The answer may not be that it was just random luck. The answer may be that Jupiter is a mass thief, and took mass, not just from Mars, but from the asteroid belt and even a little from Earth.

Consider the opposite hypothesis. The gas cloud that formed the planetary disk was nice and uniform, stretching out from close to where the star will form out to beyond where the major planets form. It was uniform because rare areas collect more gas over time and become more like other areas. When that cloud starts forming a star somewhere near the center, it begins to shrink from the gravitational pull, but that pull is uniform and should not make these sparse areas that are suggested.

As the gas cloud shrinks, rotation keeps it large in the radial direction, perpendicular to the total angular momentum of the cloud, as it shrinks in the other. It becomes an oblate spheroid, and then the center collapses into the star, being closer, while the remaining doughnut gets thinner as it shrinks in radius. It speeds up as it condenses, maintaining its angular momentum. Still nothing has happened to make the region around Mars and Ceres sparse.

Instead, the density of the cloud forms a smooth curve. Faster infall toward the center provides a thinning toward the center, and slower infall at the edges means the gas thins out there. There is some distribution curve of gas density, if it were projected down onto its central plane, that starts smaller in the center near the newborn star, gets larger to reach some peak, and then declines down to almost nothing out at the farthest edge. Still a smooth curve.

Now local gravity begins to dominate, and at the heaviest area, planetary clumps of gas form and begin the infall of dust to make the cores. In our solar system, probably both Jupiter and Saturn, the two heavyweights among planets, get formed and become to look like planets with metallic cores and huge atmospheres, still condensing. They interact with one another in an interplanetary resonance, where they move around two radii with periods in some ratio of small integers. There is wobble around the resonance radii, but the two giants have formed a stable situation. Resonance radii do not depend on the mass of the planets, so this would work in any solar system for giant planets of any ratio of mass between each other.

Other planets try to form at other resonance orbits corralled by the pair of giants. These resonant orbits are stable, but not deeply stable because a large deviation will send a proto-planet out of resonance, drifting toward the radius of the nearest giant planet. The closer the resonance orbit is to the giant planet, the stronger is the perturbing force. The larger the giant planet, the smaller the region of stability. This means that gas and dust from the closest resonance orbit drift into the capture zone for the nearest giant, and there is less mass left to form a planet at that resonance.

In our solar system, Jupiter has eliminated the chance for a planet to form at the asteroid belt. There is simply not enough mass there to collect into a planet. So instead, the small clumps of condensed matter, mostly dust, simply keep flying around as they are perturbed in orbital parameters by the two gas giants. The perturbations are large, which is another problem with forming a planet there, even if most of the mass had not been subtracted.

For the resonant radius where Mars is, this process was weaker, and did not remove enough mass to prevent a planet from forming, but only from forming a large one. Mars is so small because of the vulnerability of planetestimals to having their orbits perturbed enough to leave the resonance and go to being captured by Jupiter, or perhaps going into an unusual orbit elsewhere in the solar system.

Earth is closer to the peak area of the original disk that is Venus, and so it should be larger, and it is, but only slightly. Earth, if it had not been subjected to the perturbation forces from the gas giants, would likely have been larger than it is, which might have been a bad thing for life. Mars would have been bigger than Earth, and the asteroids would have collected into an even larger planet. But Jupiter was there, and we got what we see.

On the other side of the orbits of the two gas giants, distances are much larger and the effects are not so great. The inner planets are the ones which feel the largest effect. Thus, there is no surprise in the mass ratios of the planets. In other solar systems which form two large gas giants, instead of one, something similar might happen, meaning the depletion of mass from gravitational collection by the giants, on the inner side of the giants.

If the Liquid Water Zone is near the orbital radius of the innermost gas giant, there is probably no use in mounting a large search for an Earth-sized planet in the LWZ, and instead it would be good to push onward to a different solar system. In general, finding planets would be facilitated by looking near the resonance radii of the gas giants, so even if the two of them are inside the LWZ, if there is a principal resonance in the LWZ outside the outermost of them, life might start up there and it would be worth looking for the planet and checking its mass.

We haven’t finished the story about the formation of planets, but most everyone knows it. When the star gets going, it starts to blast a solar wind outward, which drives remaining gas, but not so much the dust, into the far reaches of the solar system. Out there, the material which is not scooped up by planets and satellites on its way outward winds up in the Oort Cloud, a region where the solar wind has long since died out and left what has been pushed out there to condense into multiple icy blobs, which can play with each other and possibly merge. However, there is not much point in looking for life out there. It is far beyond the LWZ, and there is simply not much energy for any exotic form of life to use, even if there is some exotic form which can find a niche in an icy region such as this.

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