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Earth, Earth science, planetary history, planetary climates

Water World Earth: Understanding our Wet Planet

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Earth as a Planetary Science Topic

earth water
The ocean supports life on within itself as well as on land. Water is our most precious life-supporting resource. Copyright 2016, Carolyn Collins Petersen

When I travel, I often pay close attention to the geological landmarks in the regions I visit. I do this because I’ve always been fascinated by rock formations, mountain ranges, and how features form. If you’ve never studied geology, it’s a great science to know. With a little practice, you can look at a landform and know the history of that region pretty quickly. It’s not so easy with water, particularly when I’m traveling via the ocean. However, this important substance plays a big role on our planet (and on other worlds, too). It’s a complex topic, but worth taking a brief look at here and then getting into some more details in future posts.

Earth and Its Water

Planet Earth is a water world. During a recent talk to a general public audience, I explained how Earth’s oceans are thought to have formed, and how oceans appear to exist (or have existed) on other worlds in the solar system. I ended by talking a little bit about how Earth’s atmosphere, oceans, and land all interact with each other. These are topics familiar to planetary scientists, and of course, to climate and atmospheric sciences.

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Gullfoss waterfall in Iceland. Copyright 2016, Carolyn Collins Petersen.

Earth’s systems interact in ways that we don’t always think about as we go about our day-to-day lives. For example, if it rains or snows, we all know that the water ends up in ponds, lakes, streams, and rivers. It makes its way to the oceans and exists in snow drifts and glaciers (in the case of snow). It also seeps underground and it can stay there for a long time. If you drink well water, you know that what you use comes from deep underground, basically from rocks. For many city dwellers, the idea that water can come from rocks is a complete surprise. I had a lengthy conversation with a woman on board the ship I just lectured aboard about this. After a talk about Mars, where I talked about the possibility of water underground on the Red Planet, she came up afterwards and asked about how it could exist in rocks. We had a great conversation about that. In the process, we also talked about how everyone from farmers to technology firms, transport companies, and many other entities have a vested interest in how we use our water on the planet.

The Role of Water for Life

Water is pretty important to life, so much so that it is one of the three main “requirements” that astrobiologists look for when they talk about life existing on other worlds, the others being warmth (energy), and organic material (food for the life to consume). All forms of life need to be wet in some way. We tend to focus on human needs for water, but every organism has some need of it to live and evolve and reproduce. That’s why so many people recognize the need for clean water to sustain life. It’s part of a healthy environment on any planet, but particularly on Earth.

Where Did the Oceans Come From?

On one of my most recent trips, I was crossing the Atlantic. On nearly every cruise trip where I am a science speaker,  someone asks me where the water came from. This time, I did a talk specifically about that. The truth is, however, scientists are still figuring out all the ways that Earth got its water. We used to say it came from comets. It’s possible that some of Earth’s water did come from specific families of comets, but it’s still an open question about how much of it did. Some comets did NOT contribute their water to early Earth, as we found out from the Rosetta mission when it studied Comet 67P/Churyumov-Gerasimenko. Comets of its type existed when Earth and the other planets were formed and being bombarded by solar system leftovers, but they didn’t contribute a lot to Earth’s water supply. Other families of comets may well have been suppliers, however.

Water and the Newborn Solar System

Earth water
Bombardment early in solar system history brought metal- and water-rich meteoroids and asteroids to Earth (and Venus and Mars).

Infant Earth likely formed from water-rich planetesimals, and as this young world solidified and cooled, the water vapor from those protoplanetary objects condensed to form the first pools, rivers, lakes, and oceans. We also can’t ignore the contribution that water-rich asteroids and meteoroids made to Earth’s water budget. Early in Earth’s history, these objects slammed into the infant planet Earth (and possibly Venus and Mars), and contributed their water to the growing worlds.

If that delivery mechanism worked across the planets (and it’s pretty clear it did, due to the number of worlds (including moons and dwarf planets) that have (or had) water) then water is a ubiquitous part of the solar system. It holds the clues to a lot of things we still need to understand about the early solar system, and is an important part of the chemistry of the worlds where it exists.

Protecting Our Water

All that being said, water on Earth — particularly the clean variety — is a precious resource. It fuels us, but it also keeps the planet’s ecosystems in balance. Swamps and wetlands provide ways to filter water, bring oxygen to the atmosphere, and sustain a huge variety of organisms that we depend on (such as fish) for food and other products. Fresh water in rivers and aquifers provide drinking water for millions of people, and our bodies evolved to use that fresh water. Polluting it pollutes our bodies, and the life around us.

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This ghostlike octopod is almost certainly an undescribed species and may not belong to any described genus. Image courtesy of NOAA Office of Ocean Exploration and Research.

The oceans teem with life, with many forms not even yet cataloged (and some only recently being discovered). As we swept along the ocean, I kept thinking about all the life below us in that aquatic realm, and what it has meant to the planet. We know so little about that “world beneath the waves” even though we’ve learned a lot in recent years. We owe it to ourselves to protect and understand that world because, for all we know (and we know a lot), the world above the waves depends in many ways on the “aquasphere” of planet Earth.

Understanding our Planet

It’s important to study our planet (just as we do the other worlds of the solar system) as if we were exploring a new world (as we do in planetary science). Understanding its systems and features helps us understand other worlds. And, in a most interesting way, understanding THEM gives us new insights into the planet we call home.  Our lives depend on what we learn. Just ask a farmer, a sailor, a fishing boat worker, or any of the many hundreds of people in professions who depend on sources of water, and the people whose lives are affected by access to clean water. This goes way beyond politics and opinion; the fact is, we are still learning about this place we live on and that’s an important part of humanity’s education.

planetary history, planetary climates, planetary science, planets, venus, Venus Express

Planetary Habitability

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What Does it Mean?

In my last blog entry, I talked about oceans on ancient Mars and the evidence for them that planetary scientists are finding in craters on the Red Planet. All the questions about water on Mars really point to a big question about habitability — that is, a world’s capability of sustaining life.  There’s a sort of rote list of things that planetary scientists recite when it comes to assessing what a world has to support living beings. It has to have water, warmth, and organic material (food, essentially) for the life to exist. Those things are important for habitability.  So, if Mars had water in the distant past, and if it had warmth (from volcanism or heating from its core or if it had an atmosphere that could trap heat), then two of the three conditions for life would have been met. Food — organic material — would be a simple chemical problem to solve. Here on Earth, food for life ranges from the stuff you and I eat every day to the needs of such one-celled beings as bacteria that munch on sulfur.  Obviously, early Mars didn’t have gourmet delights that we could eat, but it could well have had plenty of delicacies for one-celled organisms.  So, the planet could have been habitable.  If we decided to live there in the future, it could still be termed habitable, but only just barely and we’d have to bring along habitats to take advantage of the barely habitable landscape. But, it could be done. And, it likely will be done.

So, we know Earth is habitable (still). And, Mars was and could be. But, what about Venus?  You know, that beautifully bright starlike object that’s making a nightly curtain call in our western skies after sunset?  Yeah, that one.  It’s gorgeous to look at, but if you landed on Venus, you’d fry in an instant, if you weren’t crushed to death first by the hellishly heavy and hot atmosphere.  So, how could this volcanic, sulfurous world be habitable?  Clearly it isn’t right now, but it may have been in the past.

What early Venus could have looked like. Could it have had a water ocean? Or was it more likely a molten world with a wet atmosphere? What happened to it? Credit: J. Whatmore.

Scientists at the European Space Agency are operators for the Venus Express orbiter, which has sent back data suggesting very strongly that early Venus could have water — perhaps even an ocean of it — and may have begun its planetary life as a much more Earthlike world. The spacecraft measured the escape of molecules of hydrogen and oxygen from Venus out to space.  The rate of escape of hydrogen is roughly twice that of oxygen, and this indicates that water is the source of these escaping materials. There’s also a tracer element called deuterium that also tells scientists that water has been escaping the planet. Deuterium is a heavy form of hydrogen, and it would have more difficulty escaping the planet’s gravitational pull. The presence of large amounts of it in the upper atmosphere of Venus tells us that water has also escaped and left the deuterium behind.

It’s probably unlikely that Venus had Earthlike oceans as shown in the artist’s concept above.  If it did have standing water, those pools and/or small oceans could have been formed when comets slammed into the molten surface.  If that happened, and if conditions were right, Venus could have been habitable for a short time in its early history. If that’s true, then it begs the question of whether life could have arisen on the planet, only to be snuffed out by Venus’s subsequent changing climate.  It’s an interesting idea and one that needs to be explored more.

However, the more likely scenario is that the newborn Venus had no oceans, but sported a very wet atmosphere overlying the molten surface.  Over time, sunlight broke the water molecules into hydrogen and oxygen (a process known as “photodissociation”). The newly freed gases fled to space, leaving behind the deuterium. The escape process cooled things down enough, and the surface cooled.

There’s still a lot of “ifs” in these scenarios, but the evidence for water (past and present) is strong, based on the Venus Express data.  It’s a good hint that the Venus we see today — hot, arid, miserable, and volcanic — wasn’t always this way.  And, it adds more to our store of knowledge about just when a planet can be habitable in its history — provided the conditions are right.

Stay tuned!