Солнечная система и ее тайны

Планеты Созвездия НЛО

Liquid Water and Stability

Imagine a planet without water. This would not be the same as a “desert planet,” where there is very little water, but rather a planet on which the molecule H2O simply does not exist.

It would be difficult to imagine that life as we know it could thrive in such an environment. As in other examples of the search for life in the universe, astronomers look for evidence of water on planets in the solar system and beyond because we know that this is the critical element in the origin and sustaining of life on Earth.

Some 70 percent of the Earth is covered by water, our own bodies are about 60 percent water, and the entire life cycle depends on water to continue. All life that we know of requires water. Plants require water for drawing minerals from the soil and exchanging gases with the atmosphere, and for photosynthesis. Animals use water to transport nutrients and to discard waste, and some, of course, dwell in aquatic environments.

Water in the Solar System

We long believed that our solar system was essentially a dry place, except for the Earth. Seen through telescopes or by our early robot probes, the moon and Mars looked barren, pockmarked by meteor strikes. Apollo astronauts said they had a greater appreciation for the Earth in part because it was such a contrast with the dead, lifeless moon.

However, more recent unmanned missions have shown us that there is quite a lot of water in the solar system, and it shows up in a variety of ways.

Mars, for example, was almost certainly a planet where water ran freely in the past. The “canali” that the Italian astronomer Schiaperelli observed in the 19th century are most likely the dry riverbeds and canyons that Martian water created millennia ago. Recently, robot probes have revealed water under Mars’ ice caps and in its soil, though it is only liquid under transient conditions. Some scientists have speculated that there may be vast underground oceans there as well.

The moon, contrary to what the astronauts thought, also shows evidence of water - albeit in very small quantities. By looking at how light is absorbed at certain wavelengths, planetary scientists were able to show the presence of water (H2O) or hydroxyl (OH) at the Moon’s poles. India’s Mini-SAR radar instrument detected water-ice in craters, which indicated the presence of substantial amounts of solid water on the Moon. Sending a probe to impact one of these craters, NASA’s LCROSS satellite later found evidence of water ice, again through its absorption signature.

Two other places in the solar system stand out for their water content: the abundant comets and asteroids, and the moons of the giant planets.

Comets, often described as “dirty snowballs,” are about a third water ice, and recent studies have indicated that asteroids too may contain ice reserves. Beyond the orbit of Jupiter, water is more abundant: this far from the Sun, it’s cold enough that water condenses to form solid ice, which can then be used along with rocks to build up solid bodies.

The moons of the gas giants contain a significant amount of ice: many have icy surfaces, and some are thought to have liquid oceans beneath. Enceladus, a moon of Saturn, is literally gushing icy geysers outward from its surface.

Now that we know the solar system harbors quite a lot of water in different forms, but our exploration so far shows that life exists only on the Earth, what can we infer as we move on to search for water on Super Earths and other exoplanets?

Our principal conclusion is this: liquid water is a necessary, but not sufficient, condition for life to arise. This means that, as far as we know, living things do need water to emerge and thrive, but the presence of water alone does not guarantee the presence of life.

Water on Exoplanets

The search is on for evidence of water on exoplanets, and the results so far are somewhat mixed.

For example, astronomers discovered a planet in 2009, GJ 1214b and later made measurements of its atmosphere. Their data indicated that the planet’s atmosphere could either have high-altitude clouds, or be comprised of heavy molecules, most likely water. If it were to have a water-dominated atmosphere, we might suppose that it is therefore a haven for life, but this Super Earth orbits quite close to its star, a red dwarf 60 light-years from Earth. Since it is so hot and is outside the “habitable zone,” it is not considered a good candidate for extraterrestrial life.

Researchers used the transit method to investigate the atmosphere of GJ 1214b. A new method, which uses the “wobble method” that will also be presented in future units, recently found success. Using this approach, researchers found water vapor in the atmosphere of Tau Bootis Ab, a “hot Jupiter.” This planet has a mass larger than that of Jupiter, but orbits very close to its host star, whirling once around in just 3 day - also not a great candidate for life.

These results seemed quite promising, but when scientists turned their attention to three similar exoplanets using the same technique used on GJ 1214b and looked for water vapor in their atmospheres, they were surprised to find the planets unexpectedly dry. The planets were targeted because, like Tau Bootis Ab, they are hot Jupiters, and with the high temperatures they experience, astronomers expected water to have been turned to vapor in their atmospheres.

The study used the Hubble space telescope, and NASA reported the unexpected findings in this way:

“Our water measurement in one of the planets, HD 209458b, is the highest-precision measurement of any chemical compound in a planet outside our solar system, and we can now say with much greater certainty than ever before that we've found water in an exoplanet,” said Nikku Madhusudhan of the Institute of Astronomy at the University of Cambridge, England. “However, the low water abundance we have found so far is quite astonishing.”

Current techniques are not capable of viewing the “holy grail” of Earthlike exoplanets to detect water in their atmospheres, or ultimately, on their surfaces. The atmospheres of Super Earths, however, are within the reach of current technology (especially when they orbit small stars like GJ 1214). In the next decade with new telescopes like JWST, the search for water on planets outside of our solar system will start to move towards increasingly more Earth-like (and more promising) venues.

Considering the fact that missions like Kepler are finding hundreds of exoplanets and planet candidates, and early investigations are confirming that some of them have water, we can be encouraged that the hunt for life beyond our solar system will eventually be successful.

Солнечная система и ее тайны