ASTRO-BLOG: 8月 2015

56 Bootes

Life is not easy, but to create life is even harder. The igniter of life depends on numerous factors. Up till now, the only place where life is found in the big universe is the Earth. However, the universe is just too big that this does not make sense at all. Let us start a journey to the space and explore the planets outside our Earth!

What makes the KEY? – The organic matters

Life starts with organic matters, which are some chemicals making up every tissue and cell, as well as controlling the growth of the living things on Earth. Professor Sun Kwok, Dean of the Faculty of Science in HKU, is famous in explaining the formation of planetary nebulae as well as the composition of stellar dusts. In his newest publication in 2015, he has mentioned that over 60 kinds of gas-phase molecules have been identified in the planetary nebulae and circumstellar envelope of an evolved star through spectroscopic observation, notably the organic molecules[i].

His article has placed emphasis on the evidence shown on the pre-solar meteorites that solids can be transferred from one evolved star to the Sun across the galaxy. These pre-solar grains may contain diamond, corundum, etc. Such evidence could be linked to the spectroscopic data of the chemicals in the planetary nebulae as well. These perhaps indicate that the possibility lives on earth were not solely synthesized through combining the elements in the atmosphere with the famous Miller-Urey process, but also of exotic origin. In other words, life could have been generated from the organic matter delivered from another part of the universe.

An example of meteorite

Prerequisite of life – a wonderful environment for living, though of high rareness

Stars could generate organic matter. However, when it comes to the discussion of sustainable lives, a stable and habitable environment is necessary. Such environment is rare, but wonderful for organisms to live forever, while Earth is an excellent but the only known example.

Planets situated in the habitable zone are able to receive adequate but not too much amount of radiation from their parent stars to maintain the water in liquid state on their surfaces. Liquid water is crucial for organisms to stay alive. Functions like regulating body temperature and transferring hemoglobin show the significant importance of water. Spotting out the habitable zone therefore would be a giant leap for exploring lives outside the Earth.

Dr. R. Brasser, a Taiwan-based scientist, has been studying the habitable zone for many years. In his recent papers published in 2014 and 2015, he has applied the Milankovitch cycles to the field of research for long term behavior of potential habitable planets[i].

Milankovitch cycles were proposed by Serbian scientist Milutin Milankovic in 1940s. He suggested that both orbital eccentricity and obliquity of the Earth changes periodically in 41,000 and 100,000 years respectively[ii]. Planets do not necessarily revolve around a star in circular orbit. It could be elliptical. The less circular the orbit is, the higher the orbital eccentricity. Meanwhile, for the obliquity, it has challenged our common sense that the tilting of the rotational axis of the Earth in fact fluctuates from 21 degrees to 24 degrees in its cycle.

You can imagine, when you intend to boil a large bowl of water, someone suddenly moves it away from the stove, the water could not be heated up as a result, it may even cool down! Same case occurs if you incline the bowl. Only part of it will be heated. This disturbs the water convection a lot.

Then, let us turn the eyes to planetary scale. In fact, it is all the same. Water is an excellent heat capacitor, any change in ways of receiving energy could affect its internal activity in the container. Cloud pattern and precipitation style will vary a lot, making the climate fluctuate from the pleasant one to the harsh one periodically.

The graphical illustration of HD 40307 system, where the planet HD 40307g is located

Dr. Brasser has investigated one of the candidates of the habitable planets called HD 40307g located 42 light years away from usⁱⁱ. First, because of its high orbital eccentricity, most of the time throughout its orbiting would be away from the habitable zone. What therefore implies is the incoming solar radiation varies significantly, where ice age would occur regularly and intensely on this planet. Second, along with its low obliquity, the climatic mechanism would not allow sufficient heat to transfer from the equatorial region to the polar region. Polar cap may form which further worsens the habitability of the planet, not to mention that the positive feedback mechanism would make the planet looks like a snowball in the later stage.

In short, you may find that a good environment is required for organic matter to be converted into a life, but it is very rare in the big universe. However, the rare chance may have occurred twice in our amazing solar system.

A potentially habitable place for organisms – Europa, Jupiter’s fourth largest moon

The 4.2 AU-far Europa-Jupiter system from our mother Earth is always one of the popular candidates of habitable planets. One major difference for Europa to be distinguished from other candidates is, it is just too close to us that only eight-year travelling time is required under current technology.

Observation of Jupiter and its moons by a simple telescope

Europa is the fourth largest moon of Jupiter. The snowwhite satellite is not only appealing but also famous of containing a global ocean of water beneath the ice crust. Additionally, through spectroscopic observation, oxygen is detected on its surface[i]. With these two elements, has the probability of life ever existed there?

Planetary scientist Veronica J. Bray had led his transnational team to respond to the myth of crustal thickness by measuring the extent of viscous relaxation during numerous strikes of asteroids[ii]. The work came to disclosure in 2014. Measuring crustal thickness is crucial in proving the existence of geological activities on Europa.

Older calculation shows that if Europa has a crustal thickness below 10 km, there will be plate tectonic[iii], which allows the ejection of the liquid water to the surface to make the surface warmer and more habitable.

Plate tectonic is doomed as the most essential mechanism in sustaining the biosphere on earth. Nutrients are released to supply our need while gases are expelled to keep us warm. The huge volcanic events also help regulating the climate. This mechanism can only work when the crust is situated on a fluid (but accurately, in short time scale, what we see may have no difference to solid for many planets) with certain flexibility provided. It cannot work neither the crust is too thick nor too thin. Don’t you see our Earth is just too lucky to have the intermediate one?

Europa’s image taken by NASA’s Galileo spacecraft during its operation

Available from https://www.nasa.gov/press-release/nasa-tv-to-air-announcement-of-instruments-for-europa-mission

Bray’s work can be explained by an old-school toy, paper clay. When you drop an object to the wet clay, an imprint will form. Later, it will solidify and leave a remarkable imprint behind. Nevertheless, the depth of the imprints depends on the flexibility of the bottom part of the clay – how watery it is. As mentioned, the mantle part would behave like fluid while the crust part would behave like solid. Definitely, there is a gentle and continuous change from crust to mantle, though, scientists can still pin a more accurate mark on it through years-long effort.

Example of paper clay with imprints on it

Say, for 10 km below the surface, if that region is more ductile and plastic, the greater extent of viscous relaxation for the crust thus makes the asteroids pressing deeper on the crust when it bombards the surface, and vice versa. Consequently, the crust should be defined thinner. The measurement on the extent of viscous relaxation thus could help defining the crust-mantle boundary.

From the extent of viscous relaxation, Bray deduces that the crustal thickness of Europa is 7 km^v, which is lower than the maximum allowed thickness for plate tectonic.

Such discovery helped the scientists to prove the volcanic origin of the cracks imaged by the Galileo spacecraft. This study acts as an encouragement for Europa-Jupiter System Mission (EJSM) to obtain more data in the 2020s[i]. Possibilities on the discovery of microorganisms on the satellite exist currently. Europa, the pure satellite, would probably be the second proven habitable place in the solar system very soon.

Being a little organism in the universe, human still have great dreams towards our enormous universe. They may just be just too curious on their uniqueness at the time, or they find themselves too fragile when facing the risk. They need alternatives. And they need a better environment. In turn, curiosity and crises will keep on driving us deeper in the universe!

2015年8月22日

Shedding light on finding lives outside the earth