Why isn't Earth a lot wetter?

Posted by Emma Woollacott

With the weather we're currently getting in the UK, it's hard to believe that the Earth is short of water. But at less than one percent of the planet's total mass, there's not nearly as much of the stuff as scientists would expect.

The standard model for Earth's origin has it forming from icy material in a zone around the Sun where temperatures were cold enough for those ices to condense out of the disk - indicating a lot more water than we actually have.

But a new analysis has revealed a possible explanation: that our planet formed from rocky debris in a hotter region, inside the so-called 'snow line' where ices melt. In our solar system, this line currently lies in the middle of the asteroid belt, but previous accretion-disk models suggest that it was much closer to the sun 4.5 billion years ago, when Earth formed.

"Unlike the standard accretion-disk model, the snow line in our analysis never migrates inside Earth's orbit," says Mario Livio of the Space Telescope Science Institute in Baltimore.

"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. "

In the conventional model, the protoplanetary disk around our sun is fully ionized - electrons are stripped off atoms - and is funneling material onto our star, which heats up the disk. The snow line is initially a billion miles from the star. But, 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.

However, the team realized that the disks around young stars are not fully ionized, as there simply isn't enough heat and radiation.

"Very hot objects such as white dwarfs and X-ray sources release enough energy to ionize their accretion disks," says Rebecca Martin. "But young stars don't have enough radiation or enough infalling material to provide the necessary energetic punch to ionize the disks."

If the disks aren't ionized, there's no mechanism that would allow material to flow through the region and fall onto the star. Instead, gas and dust orbit around the star without moving inward, creating a so-called 'dead zone' in the disk, from about 0.1 astronomical unit to a few astronomical units beyond the star.
This zone acts like a plug, preventing matter from migrating towards the star. Material, however, piles up in the dead zone and increases its density.

The dense matter begins to heat up by gravitational compression, which in turn heats the area outside the plug, vaporizing the icy material and turning it into dry matter. And Earth could have formed from the dry material in this hotter region.

The revised model isn't a blueprint for how all disks around young stars behave. "Conditions within the disk will vary from star to star," says  Livio, "and chance, as much as anything else, determined the precise end results for our Earth."