July 18, 2012
Standard Model Does Little To Explain Why Earth Is So Dry
Lawrence LeBlond for redOrbit.com - Your Universe Online
Despite seventy percent of the Earth´s surface being covered by water, in reality the whole of the planet is only made up of 1 percent water, making it relatively dry compared to the gas giants, such as Jupiter and Neptune. And now, that dryness, which has long perplexed scientists, has been explained by a team of scientists working at the Space Telescope Science Institute (STSI) in Baltimore, Maryland.The standard model suggests that our world should be much more watery, as the solar system formed a protoplanetary disk of swirling gas and dust around the Sun billions of years ago that would be responsible for the creation of a wet world. In essence, the earth should have formed from icy material within that zone around the Sun where temps were cold enough for ice to condense out of the disk, being born of material rich in water.
However, this had not been the case, as our planet was born out of much drier ingredients, with the possibly of comets carrying water to Earth after it was formed.
In their analysis of the common accretion-disk model explaining how planets form in a debris disk around the Sun, the team, led by Rebecca Martin and Mario Livio, uncovered a possible reason for Earth´s relative dryness.
Their study, “On the Evolution of the Snow Line in Protoplanetary Discs,” found that the Earth was formed from rocky debris in a dry, hotter region, inside the so-called “snow line.” This snow line currently lies in the middle of the asteroid belt between Mars and Jupiter. Beyond that point, the Sun´s light is too weak to melt icy debris left over from the protoplanetary disk. Previous models have suggested that this snow line was much closer to the Sun when the Earth was formed 4.5 billion years ago.
But Martin and Livio do not buy into this scenario.
“Unlike the standard accretion-disk model, the snow line in our analysis never migrates inside Earth's orbit,” said Livio. “Instead, it remains farther from the Sun than the orbit of Earth, which explains why our Earth is a dry planet. In fact, our model predicts that the other innermost planets, Mercury, Venus, and Mars, are also relatively dry.”
The standard model explains that the protoplanetary disk around the Sun is fully ionized and is funneling material onto the star, which heats the disk. During this time, the snow line is initially about 10.7 astronomical units (a billion miles) from the Sun. Over time, the disk runs out of material, cools, and draws the snow line inward, past Earth's orbit, before there is sufficient time for Earth to form.
“If the snow line was inside Earth's orbit when our planet formed, then it should have been an icy body,” Martin explained. “Planets such as Uranus and Neptune that formed beyond the snow line are composed of tens of percents of water. But Earth doesn't have much water, and that has always been a puzzle.”
The STSI study found a problem with the standard model for the evolution of the snow line. “We said, wait a second, disks around young stars are not fully ionized. They're not standard disks because there just isn't enough heat and radiation to ionize the disk.”
“Very hot objects such as white dwarfs and X-ray sources release enough energy to ionize their accretion disks,” Martin added. “But young stars don't have enough radiation or enough infalling material to provide the necessary energetic punch to ionize the disks.”
Since these disks are not ionized, mechanisms that allow material to flow through the region and fall onto the star are absent. Instead, gas and dust orbit around the star without migrating inward, and creates a “dead zone” within the disk. This dead zone exists from about 0.1 astronomical unit (9.3 million miles) to a few AUs beyond the star. This zone acts as a barrier, preventing material from moving inward toward the Sun. The material piles up in the dead zone and increases in density.
The dense matter then begins to heat up through gravitational compression and then heats up the region outside the barrier, vaporizing any icy material, turning into dry matter. It is within this hotter, dry region where the Earth forms. This altered version of the standard model explains why Earth didn´t end up with much water.
Martin acknowledged that this revised model may not explain how all disks around young stars behave.
“Conditions within the disk will vary from star to star, and chance, as much as anything else, determined the precise end results for our Earth,” added Livio.
The study is to be published in the journal Monthly Notices of the Royal Astronomical Society.