Where did Earth get its abundant water? Was it really delivered by asteroids and comets? New evidence from Mars supports a different idea — that our planet has had its water all along, no impacts required.
As the only planet we know of with abundant oceans of liquid water on its surface, scientists have invested considerable effort into figuring out exactly where Earth's water originated. We know that there's water locked up in asteroids and comets. At the same time, we also know that Earth suffered heavy bombardment from these objects shortly after it formed over 4.5 billion years ago. Suggesting that there could be a possible link between these two facts isn't too much of a stretch.
Still, it's not the only hypothesis that's been considered.
"There are two hypotheses about the emergence of water. One is that it arrives on planets by accident, when asteroids containing water collide with the planet in question. The other hypothesis is that water emerges in connection with the formation of the planet," said Martin Bizzarro, from the University of Copenhagen's Centre for Star and Planet Formation, said in a press release.
Bizzarro is co-author of a new study led by Zhengbin Deng, an assistant professor at the Centre for Star and Planet Formation, which carefully examined the mineral structure of an ancient Mars meteorite known as NWA 7533 (aka "Black Beauty"). The research team's conclusions suggest that the second hypothesis — where water is simply a natural byproduct of planet formation — may be the correct one.
Meteorite 'Northwest Africa 7533', discovered in Morocco in 2012, was found to be an ancient piece of the planet Mars, which was blasted into space by an immense impact before eventually crossing paths with Earth. Credit: University of Copenhagen/Deng et al.
Black Beauty was revealed to be an ancient piece of Mars after scientists examined gases trapped in bubbles inside the meteorite. Previous studies have also shown that this meteorite was ejected from the surface of Mars during an impact that took place around 4.4 billion years ago.
During their study of NWA 7533, Deng and his colleagues discovered signs that there was already water on Mars's surface at the time of that impact.
"We have developed a new technique that tells us that Mars, in its infancy, suffered one or more severe asteroid impacts. The impact, Black Beauty reveals, created kinetic energy that released a lot of oxygen. And the only mechanism that could likely have caused the release of such large amounts of oxygen is the presence of water," Deng said in the press release.
This false-colour elevation map of Mars reveals the likely location where the Black Beauty meteorites originated. Credit: NASA/University of Copenhagen
"If that is true, it is extremely exciting, because it means that the presence of water is a bioproduct of the planet formation process," Bizzarro said.
A WARMER MARS?
Additionally, as study co-author Takashi Mikouchi pointed out in a University of Tokyo press release, "such an impact would have released a lot of hydrogen, which would have contributed to planetary warming at a time when Mars already had a thick insulating atmosphere of carbon dioxide."
This combination may solve the mystery of why we see abundant evidence of liquid water on Mars' surface, despite the planet being colder in the distant past. Any water on Mars now is either completely frozen or in the form of briny deposits buried under its thick glaciers. Billions of years ago, the Sun was even cooler than it is today and would have provided less heat to Mars. So, even with a thicker atmosphere, the planet wouldn't have been warm enough for liquid water without some added help.
Related: ROM scientist shows off meteorites from Mars and the Moon
IMPLICATIONS FOR LIFE, HERE AND BEYOND
This new 4.4 billion year timeline for water on Mars' surface is roughly 700 million years earlier than previous estimates. According to Bizzarro, it would have also been long before water-rich asteroids could have bombarded Mars. So, where did the water come from? This study provides a compelling answer.
"It suggests that water emerged with the formation of Mars. And it tells us that water may be naturally occurring on planets and does not require an external source like water-rich asteroids," Bizzarro explained.
The TRAPPIST-1 planetary system, compared here to our own solar system, could be an ideal location to search for life, with three rocky planets in its 'habitable zone'. Credit: NASA/JPL-Caltech
This shifting of the narrative for water — from an accident requiring very specific conditions to simply something that happens naturally during planet formation — could make the emergence of biological life far less complicated. This has implications for Earth and Mars, and possibly even Venus. It also could change our perspective slightly as we continue to discover more rocky planets around other stars.
If this study's findings are correct, it becomes far more likely that most rocky exoplanets will have abundant water on their surface. Since water is essential for the development of life — at least life as we know it — that also increases our chances of discovering that we're not alone in the universe!