Day 12
Love and War
Our cannons’ malice vainly shall be spent
Against the invulnerable clouds of heaven
The Life and Death of King John, Act II. Scene I.
William Shakespeare (1564–1616)
The Oxford Shakespeare, 1914
Where else can we look for life? I am going to double back now and recheck places that astrobiologists often cross off the life list. These include Venus, planets around small red dwarf stars, and planets around large red giants.
Purgatory. The surface of Venus is hell, 730 K (Figure 1). Nothing that we can imagine can live there. To boot, the logistics of probing the surface are terrible. Yet Venus does interest astrobiologists.
Figure 1: The Soviet probe Venera 14 imaged the hellish surface of Venus. It revealed cracked rock similar to that erupted on oceanic islands. http://nssdc.gsfc.nasa.gov/image/planetary/venus/venera14.jpg
First, we have considerable hope of finding evidence of past clement climates from the ancient past when our Sun was dim. The products of weathering by liquid water deposited in shallow seas survive in the chemical composition of rocks. Shales (mudstones) accumulate from clays left over after weathering removes more soluble components. They are rich in aluminum oxide. The excess aluminum oxide remains even after the shale melts to form granite (Figure 1). Small aluminum-rich inclusions in crystals indicate that liquid water was present on the Earth by 4.3 billion years ago. Limestone loses its carbon dioxide but the excess of calcium over normal igneous rocks remains. I have seen the chemical remnants of limestone in rocks heated to 1200°C at 22-km depth in an outcrop in Alaska.
We need merely find outcroppings of ancient rocks on the Ishtar region of Venus, which resembles an Earth continent. Then we are in business as the chemical signature survives even melting of the rock.
We have much less hope of finding preserved macrofossils and none for microfossils. Morphological fossils do survive high temperatures on the Earth but only when the rock does not get deformed in the process. We would need a well-exposed section that we could image from various angles. We are not likely to get this from a drop and die probe.
Figure 2: New Hampshire is the Granite State. Here is granite on the right intruded by basalt on the left. Note the thin stringer of basalt above and to the right of the pen. Chemical analyses for aluminum oxide would quickly tell if the granite formed from melted sedimentary rocks. This is a way to look for past clement environments on Venus. Photo by the author.
What about extant life? The surface is hell but the clouds are merely purgatory. The cloud tops are clement and contain drops of sulfuric acid (Figure 3). (Brimstone without fire.) We would die if we drank a glass of Venus cloud drops, but some terrestrial microbes thrive in very acid environments.
Figure 3: Ultraviolet image of Venus cloud tops reveals some structure. The clouds tops are very acidic but clement. http://nssdc.gsfc.nasa.gov/photo_gallery/photogallery-venus.html
David Grinspoon has been actively urging that NASA check out the Venus clouds for life. You may be thinking that clouds are not green on the Earth. True, there is not much photosynthesis in Earth clouds, but they do contain microbes. Drops rain out or evaporate quickly so they are not much of a niche for life.
The Venus cloud drops are much more long lived. Air currents rarely sweep the drops down into the hell that lies below. (Getting swept into the underlying hell is what keeps clouds on the giant planets sterile.) There is a problem in how life evolved to live in the clouds before Venus’ surface became too hot. There may be a problem of the supply of nutrients like phosphorus, which is scarce in Earth clouds. There is some evidence that phosphorus is present in Venus air. As in all astrobiology, the problem may be our own lack of imagination. In any case, the logistics are much kinder than the surface of Venus. An acid-safe balloon would last a long time. There is sunlight for solar cells. A nitrogen-filled balloon would supply buoyancy in the carbon dioxide atmosphere if helium leakage poses a problem. Even if no life is found, we will learn a lot about the chemistry and physics of the Venus atmosphere.
Red dwarf planets. Red dwarf stars are the most common type, yet they are so dim that a person with average eyesight cannot see any with the naked eye. They have the attractive astrobiological feature that their luminosity changes very slowly over time. A planet once in the habitable zone stays in the habitable zone. No red dwarf has yet entered the red giant stage and the Sun will be long gone before any do.
The planets within the habitable zone of red dwarf stars experience intense tides. To see this physically, the energy of starlight to a planet scales inversely to the square of its distance and proportionally to luminosity (the fourth power of the star’s mass). The tidal force scales inversely to the third power of the distance and linearly with star mass. (For those with calculus, tidal forces scale with the derivative of gravity away from the star. You need math to get this result.) With some algebra not given here, the tidal force at a point of constant starlight (representing the habitable zone) scales inversely with the fifth power of the star’s mass. The energy dissipated that tidally heats the planet scales to the inverse of the tenth power of the mass. This yields the attractive astrobiological feature that habitable planets around red dwarfs are tidally heated. Like the satellites of Jupiter, their tectonics do not run down as their radioactivity wanes.
The less attractive astrological feature is that tides de-spin planets, so that one face may end up facing the star, like with the Moon around the Earth. Science fiction writers love civilizations living at the twilight edge. But what happens in reality? An earthlike atmosphere does not freeze out on the backside. Water does freeze, but cannot freeze all the way through to trap an ocean mass of water. Glaciers flow and geothermal heat keeps the bottom of the ocean liquid. There is no problem in habitability to microbes or even multicellular life.
With soon to be available technology, we will be able to image planets around red dwarfs, both directly and in transit. (A 10-earth-mass one has been found by line-of-site measurements.) The ozone in an oxygen atmosphere is a nice biomarker. It forms by photosynthesis on the Earth. Can photosynthesis occur on a red dwarf planet? The oxygen-producing photosynthesis on the Earth requires blue light, which is in short supply around red dwarfs. Solar flares produce intense pulses of ultraviolet light that continually hit red dwarf planets. Our plants would be in deep trouble if shipped there. However, shallow water provides shielding from UV light. Evolution would tend to adapt photosynthetic organisms for the light actually present.
The transit mode of detection is favorable to red dwarfs. The habitable zone is close to the star so that the geometrical probability of transit is much higher than with the Earth and Sun observed from a nearby star. There are lots of red dwarfs to check out.
Red giants. The Earth gets incinerated when the Sun becomes a red giant. However, the Jovian satellites become clement for around a hundred million years before they become too hot. This is enough time for life already present beneath the ice to evolve. It might even become photosynthetic. The Saturn satellites become clement later and stay clement for a shorter period of time, like ten million years. Further out, the intense luminosity at the peak of the red giant stage increases rapidly to a few thousand times the current luminosity of the Sun. The Sun, however, loses mass so that the planets move out. The Neptune satellite Triton is only briefly clement and if it does become clement it will be quickly cooked. The view is spectacular for an observer in a clement place. The Sun is intense enough to flare the inner comets and probably Pluto. The ice of the Jovian satellites evaporates but the gas stays bound to Jupiter. It forms a red-hot disk bigger than the size of the present Sun.
Present technology is not much good for searching around red giants for habitable planets. Astronomers have not given the matter much thought. There are not a lot of nearby red giants. Stars do not linger long in that stage, so statistically this star class is rare. I return to red dwarfs and giants when I consider intelligent life
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