Why Is Earth So Dry?

A Tale of Two Disk Models
Credit: NASA, ESA, and A. Feild (STScI)

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 Earth's water deficiency. The standard model explaining how the solar system formed from a protoplanetary disk, 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 rocky debris 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 accretion-disk 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."

CONTACT

Donna Weaver / Ray Villard
Space Telescope Science Institute, Baltimore, Md.
410-338-4493 / 410-338-4514
dweaver@stsci.edu / villard@stsci.edu

Mario Livio
Space Telescope Science Institute, Baltimore, Md.
410-338-4439
mlivio@stsci.edu

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New system would assess odds of life on other worlds

Earth-like? Habitable?

PULLMAN, Wash. – Within the next few years, the number of planets discovered in orbits around distant stars will likely reach several thousand or more. But even as our list of these newly discovered "exoplanets” grows ever-longer, the search for life beyond our solar system will likely focus much more narrowly on the relatively few of these new worlds which exhibit the most Earth-like of conditions.

For much of the scientific community, thesearch for alien life has long been dominated by the notion that our own planet serves as the best model of conditions best suited to the emergence of life on other worlds. And while there’s anundeniable logic to seeking life in the same sort of conditions in which you already know it to be successful, there are scientists like Dirk Schulze-Makuch, an astrobiologist with the Washington State University School of Earth and Environmental Sciences and Abel Mendez, a modeling expert from the University of Puerto Rico at Aricebo, who also see such a model as the product of a potentially limiting form of earthling-biased thinking.

To Schulze-Makuch and his nine fellow authors – an international working group representing, NASA, SETI,the German Aerospace Center, and four universities– the search for life on other worlds is really driven by two questions.

"The first question is whether Earth-like conditions can be found on other worlds, since we know empirically that those conditions could harbor life,” Schulze-Makuch said. "The second question is whether conditions exist on exoplanets that suggest the possibility of other forms of life, whether known to us or not.”

In a paper to be published in the December issue of the journal Astrobiology, Schulze-Makuch and his co-authors propose a new system for classifying exoplanets using two different indices – an Earth Similarity Index (ESI) for categorizing a planet’s more earth-like features and a Planetary Habitability Index (PHI) for describing a variety of chemical and physical parameters that are theoretically conducive to life in more extreme, less-earthlike conditions.

Similarity indices provide a powerful tool for categorizing and extracting patterns from large and complex data sets. They are relatively quick and easy to calculate and provide a simple quantitative measure of departure from a reference state, usually on a scale from zero to one. They are used in mathematics, computer imaging, chemistry and many other fields.

The two indices proposed by the group mark the first attempt by scientists to categorize the many exoplanets and exomoons that are expected to be discovered in the near future in accordance with their potential to harbor some form of life.

"As a practical matter, interest in exoplanets is going to focus initially on the search for terrestrial, Earth-like planets,” said Schulze-Makuch. "With that in mind, we propose an Earth Similarity Index which provides a quick screening tool with which to detect exoplanets most similar to Earth.”

But the authors believe that focusing exclusively on earth-based assumptions about habitability may well be too restrictive an approach for capturing the potential variety of life forms that, at least in principle, may also exist on other worlds.

"Habitability in a wider sense is not necessarily restricted to water as a solvent or to a planet circling a star,” the paper’s authors write. "For example, the hydrocarbon lakes on Titan could host a different form of life. Analog studies in hydrocarbon environments on Earth, in fact, clearly indicate that these environments are habitable in principle. Orphan planets wandering free of any central star could likewise conceivably feature conditions suitable for some form of life.”

The paper’s authors concede that attempting to rate the probability that life of some unknown form could exist on any given world is an intrinsically more speculative endeavor. But the alternative, they argue, is to risk overlookingpotentially habitable worlds by using overly restrictive assumptions.

"Our proposed PHI is informed by chemical and physical parameters that are conducive to life in general,” they write. "It relies on factors that, in principle, could be detected at the distance of exoplanets from Earth, given currently planned future (space) instrumentation.”

The paper, entitled A Two-Tiered Approach to Assessing the Habitability of Exoplanets, was written by Alfonso Davila, of SETI; Alberto Fairen, of NASA; Abel Mendez of the University of Puerto Rico at Aricebo; Philip von Paris, of the German Aerospace Center; David Catling, of the University of Washington; Louis N. Irwin, of the University of Texas-El Paso, and Marina Resendes de Sousa Antonia, Carol Turse, Grayson Boyer and Dirk Schulze-Makuch, all of Washington State University.

Source:
Dirk Schulze-Makuch,
WSU School of Earth & Environmental Science,
509-335-1180,
dirksm@wsu.edu

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