Why is Earth so dry?

Jul 17, 2012
This illustration of two different disk models shows overhead views of the structure of the protoplanetary disk that encircled the newborn Sun 4.6 billion years ago. The Sun's family of planets agglomerated from dust and ices within the disk. The major difference between the two models is the location of the so-called snow line, which divides a warm, dry area of the disk from an icy, turbulent region. In the standard disk model, shown at left, Earth formed beyond the snow line, in an icy region. Our planet should, therefore, contain lots of water because it formed from ices that would have been a major fraction of its composition. However, it's estimated that less than 1 percent of Earth's mass is locked up in water, which has puzzled scientists. In the new disk model, shown at right, Earth formed in a warmer, dry region, outside the snow line, which is much farther away from the Sun. This model explains why Earth is comparatively dry. It provides new insights into estimates of the abundance of Earth-like planets in the galaxy. Credit: NASA, ESA, and A. Feild (STScI)

(Phys.org) -- With large swaths of oceans, rivers that snake for hundreds of miles, and behemoth glaciers near the north and south poles, Earth doesn't seem to have a water shortage. And yet, less than one percent of our planet's mass is locked up in water, and even that may have been delivered by comets and asteroids after Earth's initial formation.

Astronomers have been puzzled by 's water deficiency. The standard model explaining how the solar system formed from a , a swirling disk of gas and dust surrounding our Sun, billions of years ago suggests that our planet should be a water world. Earth should have formed from icy material in a zone around the Sun where temperatures were cold enough for ices to condense out of the disk. Therefore, Earth should have formed from material rich in water. So why is our planet comparatively dry?

A new analysis of the common accretion-disk model explaining how planets form in a debris disk around our Sun uncovered a possible reason for Earth's comparative dryness. Led by Rebecca Martin and Mario Livio of the Space Telescope Science Institute in Baltimore, Md., the study found that our planet formed from in a dry, hotter region, inside of the so-called "snow line." The snow line in our solar system currently lies in the middle of the asteroid belt, a reservoir of rubble between Mars and Jupiter; beyond this point, the Sun's light is too weak to melt the icy debris left over from the protoplanetary disk. Previous accretion-disk models suggested that the snow line 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," Livio said. "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 results have been accepted for publication in the journal Monthly Notices of the Royal Astronomical Society.

In the conventional model, the protoplanetary disk around our Sun is fully ionized (a process where electrons are stripped off of atoms) and is funneling material onto our star, which heats up the disk. The snow line is initially far away from the star, perhaps at least one billion miles. 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."

Martin and Livio's 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," Livio said. "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."

So, if the disks aren't ionized, mechanisms that would allow material to flow through the region and fall onto the star are absent. Instead, gas and dust orbit around the star without moving inward, creating a so-called "dead zone" in the disk. The dead zone typically extends from about 0.1 astronomical unit to a few astronomical units beyond the star. (An astronomical unit is the distance between Earth and the Sun, which is roughly 93 million miles.) 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, much like people crowding around the entrance to a concert, waiting for the gates to open.

The dense matter begins to heat up by gravitational compression. This process, in turn, heats the area outside the plug, vaporizing the icy material and turning it into dry matter. Earth forms in this hotter region, which extends to around a few astronomical units beyond the Sun, from the dry material. Martin and Livio's altered version of the standard model explains why Earth didn't wind up with an abundance of water.

Martin cautioned that the revised model is not a blueprint for how all disks around young stars behave. "Conditions within the disk will vary from star to star," Livio said, "and chance, as much as anything else, determined the precise end results for our Earth."

Explore further: Toothpaste fluorine formed in stars

More information: Science Paper by: R. Martin and M. Livio (PDF document)

Related Stories

Planetary systems can form around binary stars

Jan 10, 2006

New theoretical work shows that gas-giant planet formation can occur around binary stars in much the same way that it occurs around single stars like the Sun. The work is presented today by Dr. Alan Boss of ...

How the Earth survived birth

Jan 07, 2010

For the last 20 years, the best models of planet formation—or how planets grow from dust in a gas disk -- have contradicted the very existence of Earth. These models assumed locally constant temperatures within a disk, ...

Ideas on gas-giant planet formation take shape

Mar 22, 2006

Rocky planets such as Earth and Mars are born when small particles smash together to form larger, planet-sized clusters in a planet-forming disk, but researchers are less sure about how gas-giant planets such as Jupiter and ...

The water reservoir in a young planetary system

Nov 15, 2011

(PhysOrg.com) -- Astronomers once thought that the process of star formation was more-or-less controlled by the simple coalescence of material by gravity, leading eventually to a new star. But they have come ...

Recommended for you

Spectacular supernova's mysteries revealed

1 hour ago

(Phys.org) —New research by a team of UK and European-based astronomers is helping to solve the mystery of what caused a spectacular supernova in a galaxy 11 million light years away, seen earlier this ...

Supernova seen in two lights

2 hours ago

(Phys.org) —The destructive results of a mighty supernova explosion reveal themselves in a delicate blend of infrared and X-ray light, as seen in this image from NASA's Spitzer Space Telescope and Chandra ...

Toothpaste fluorine formed in stars

Aug 21, 2014

The fluorine that is found in products such as toothpaste was likely formed billions of years ago in now dead stars of the same type as our sun. This has been shown by astronomers at Lund University in Sweden, ...

Swirling electrons in the whirlpool galaxy

Aug 20, 2014

The whirlpool galaxy Messier 51 (M51) is seen from a distance of approximately 30 million light years. This galaxy appears almost face-on and displays a beautiful system of spiral arms.

User comments : 13

Adjust slider to filter visible comments by rank

Display comments: newest first

1 / 5 (3) Jul 17, 2012
In the new disk model, shown at right, Earth formed in a warmer, dry region, outside the snow line, which is much farther away from the Sun.

He meant inside the snowline -- he just forgot to look at the picture.


however challenging that the accretion disk is not ionized punches huge holes in star formation theory. There is currently issue with the theory as it stands, and one of its saving graces is that MAYBE the accretion disk is ionized and a magnetic field forms that funnels material into the forming star. To suggest this is not the case leaves no way for stars to form in the short amount of time of a few million years.

Unlike what you were taught in grade/high school - you learn in college undergrad gravity is not enough to create a star
1 / 5 (2) Jul 17, 2012
Now I feel both stupid and gratified. With the recent result that the initial mantle of Mars is as wet as those of Earth-Moon, I noted that the current ice line lies between Mars and Ceres (icy). I forgot that the protoplanetary disk and the weak young sun changes things.

Well, the new model is already tested by the recent planetary science results. But it isn't as good for habitability as I hoped, if this generic result still isn't general.

@ El Nose:

They discuss conditions of an already formed star: "young stars don't have enough radiation or enough infalling material" et cetera. The earlier conditions will differ.

I've looked over my text books (astrobiology, but with star formation theory) and there is no critical ionization mentioned. Protostars forms within 10^4 years.

Later on the X-wind turns on, which explains jet constraints and disk ionization and stirring. (And even isotope ratios that looks like Earth originated in a molecular cloud seeded by supernovas.)
1 / 5 (2) Jul 17, 2012
[cont] The protoplanetary disk phase is a few million years of course, enough time to form planets. Are you sure you didn't confuse those two?
1 / 5 (8) Jul 18, 2012
The most glaring problem which these theories consistently ignore is all of the data we received -- which was either normalized or thrown out -- from Venus, which specifically suggested that Venus was not in thermal equilibrium.

If you rewind back in time to when we didn't know anything about the temperatures of Venus, you had Carl Sagan proposing an out-of-control greenhouse effect in order to counter Immanuel Velikovsky's proposal that Venus was instead a newly-minted planet whose birth is recorded by a large number of mythological stories which span the entire globe.

One important Velikovskian prediction was that Venus *had* to be covered in volcanos. The mainstreamers had placed their bets on a very different desert-like Venus. Volcanos it was, but the mainstreamers decided suddenly that neither their prediction, nor the results of any of the numerous probes we sent there, actually mattered at all.

See http://www.youtub...S7JmUhl8
1.5 / 5 (8) Jul 18, 2012
The data from these probes clearly demonstrated that Venus' heat originates from its surface -- not the thick cloud cover.

But, of course, students are not told any of this within the textbooks today, leaving it up to the due diligence of critical thinkers to go out of their way to explain it to people on forums like this one.

Had researchers not been so intent on destroying Velikovsky, the Venus data could have inspired an entirely new line of investigation. The data does indeed support the possibility that Venus might be very, very new. In fact, it's very striking that there is agreement amongst many of the cultures of the world not only on Venus' recent introduction to the sky, but also on its transformation from a beautiful comet-like appearance to a vengeful ugly medusa which rained fire upon the land.

One need not be a creationist to investigate such things. All of these myths *PRECEDE* the formation of the world's religions.
4.2 / 5 (5) Jul 18, 2012
Jeez, NOT VELIKOVSKY! Hannes Alfven, can't you just go and join the global warming denialists?
3.4 / 5 (5) Jul 19, 2012
I can't believe that the thread went from analyzing the most exciting planet science result in years, to creationist trolling!
Or maybe I can, because religion poisons everything.

@ HannesAlfven:

No data has been "thrown out", but Venus is little understood (retrograde slow rotation, superrotation of atmosphere, water loss, tectonics, et cetera).

What is understood is that Velikovsky, a psychiatrist so adherent to nutty theory [sic!] already there (Freud was a fraud), and his theories on physics were nuts. You can't refer to non-existent research (not peer review published).

The other side of the coin is that Velikovsky was a mythologist based on christian creationist texts among other things. So this topic doesn't belong on science blogs, it belongs on creationist blogs.
SnowballSolarSystem _SSS_
1.7 / 5 (6) Jul 19, 2012
Here's another example of using nuance to address a scientific problem. True, most science is accomplished incrementally between scientific revolutions, but maybe we're due. Science can get stuck in the rut of local maxima or minima if it fails to occasionally expand its purview.

An alternative (revolutionary) model for the formation of terrestrial planets, Snowball Solar System (SSS), suggests that the planets may have formed in an icy state only to be subsequently scorched to rocky-iron cinders during the common envelope phase of a binary sun which merged in a luminous red nova (LRN) at 4.567. The LRN created the short-lived radionuclides of the early solar system, and the super-intense solar magnetic field remelted interplanetary dust bunnies into the chondrules of the ordinary chondrites.

Scientific revolutions are surprising and address scientific problems and phenomena as a matter of course, overturning former ad hoc explanations. SSS also neatly explains the slight retrogr
SnowballSolarSystem _SSS_
1 / 5 (5) Jul 19, 2012
Furthermore, SSS suggests that the central binary pair formed as a result the collapsing stellar mass ridding itself of excess angular momentum, a process which also simultaneously formed the gas giants Jupiter and Saturn for the same reason and by the same process. Then core collapse caused Saturn and Jupiter to rise in orbit (likely from a formation distance of < .04 AU) at the expense of the energy and angular momentum of the central binary pair, causing their orbits to decay.

Again, this revolutionary model explains away the ad hoc necessity of hot-gas-giant exoplanets forming further out by super-accelerated accretion and subsequently migrating inward.
SnowballSolarSystem _SSS_
1 / 5 (5) Jul 19, 2012
Are there any adjectives for 'moon' other than 'lunar' and moony, and can 'lunar' also refer to moons of other planets?
2.3 / 5 (6) Jul 19, 2012
This solar system with binary suns? You matching your screen name to support it? Are you out of your mind? First off ... the center of mass of the two suns is the rotation point for the suns and all the planets. Existing planets too close to the either sun will be ejected or consumed. Future planets won't ever seed because that same ejection/consumption clears out any material that could later form them. So you say the suns merged, but has mass been lost or gained? I say no, you say yes.

Angular momentum shedding off mass and creating Jupiter and Saturn? Gas giants start with a big rocky metallic core. The suns shedding low density outer layers isn't going to provide enough of these heavy elements for planet formation. Your worst contention is that the spiraling outward orbits of Jupiter and Saturn could ever settle down into the nearly circular orbits they have now. I can't go on ...

I've seen swiss cheese with less holes then your theory has.
SnowballSolarSystem _SSS_
1 / 5 (6) Jul 20, 2012
xen uno, thanks for responding. The binary pair would have formed well inside the orbit of Mercury, and Mercury may have once been the progenitor moon for the 4 larger jovian Galilean moons before Jupiter lost it to the Sun. Progenitor moons and planets may form around 2:1 to 3:1 inner resonant nurseries like the asteroids at Jupiter when a decaying binary pair creates orbit inflation through perturbation.

Both Mercury and Io appear to be sulfur enriched, showing a possible connection and a mechanism (liquid sulfur) for forming a progenitor moon in the absence of its own resonant progenitor. Likewise Titan may have been the far-larger progenitor moon of Saturn with hydrocarbons forming the glue.

Jupiter may have successively formed Venus, Earth, Mars and the asteroid belt which is still in its original resonant nursery.

(Coincidentally, my folks are also from Iowa, and that's why the family crest has a bible and an ear of corn.)
1 / 5 (1) Jul 23, 2012
@ SnowballSolarSystem:

There is no such "alternative model" that has gone through peer review. For one, the orbital momentum of our sun shows it has always been single. You are pattern searching, and have no idea how to formulate a testable theory.

Stop lying on the science. Especially when the article describes the most exciting planet science result in years, we don't need to be reminded of the downsides of science success such as crackpots interest.