Tag: habitability

The Interiors of Exoplanets May Well Hold the Key to Their Habitability

Scientists have had a working — and evolving — understanding of the interior of the Earth for only a century or so.  But determining whether a distant planet is truly habitable may require an understanding of its inner dynamics — which will for sure be a challenge to achieve. (Harvard-Smithsonian Center for Astrophysics)

The quest to find habitable — and perhaps inhabited — planets and moons beyond Earth focuses largely on their location in a solar system and the nature of its host star,  the eccentricity of its orbit, its size and rockiness, and the chemical composition of its atmosphere, assuming that it has one.

Astronomy, astrophysics, cosmochemistry and many other disciplines have made significant progress in characterizing at least some of the billions of exoplanets out there, although measuring the chemical makeup of atmospheres remains a immature field.

But what if these basic characteristics aren’t sufficient to answer necessary questions about whether a planet is habitable?  What if more information — and even more difficult to collect information — is needed?

That’s the position of many planetary scientists who argue that the dynamics of a planet’s interior are essential to understand its habitability.

With our existing capabilities, observing an exoplanet’s atmospheric composition will clearly be the first way to search for signatures of life elsewhere.   But four scientists at the Carnegie Institution of Science — Anat Shahar, Peter Driscoll, Alycia Weinberger, and George Cody — argued in a recent perspective article in Science that a true picture of planetary habitability must consider how a planet’s atmosphere is linked to and shaped by what’s happening in its interior.

They argue that on Earth, for instance, plate tectonics are crucial for maintaining a surface climate where life can fill every niche. And without the cycling of material between the planet’s surface and interior, the convection that drives the Earth’s magnetic field would not be possible and without a magnetic field, we would be bombarded by cosmic radiation.

What makes a planet potentially habitable and what are signs that it is not. This graphic from the Carnegie paper illustrates the differences (Shahar et al.)

 

“The perspective was our way to remind people that the only exoplanet observable right now is the atmosphere, but that the atmospheric composition is very much linked to planetary interiors and their evolution,” said lead author Shahar, who is trained in geological sciences. “If there is a hope to one day look for a biosignature, it is crucial we understand all the ways that interiors can influence the atmospheric composition so that the observations can then be better understood.”

“We need a better understanding of how a planet’s composition and interior influence its habitability, starting with Earth,” she said. 

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The Ancient Mars Water Story, Updated

Rendering of Gale Lake some 3.5 billion years ago, when Mars was warmer and much wetter. The Curiosity mission is finding that Gale Crater water-changed rock is everywhere.

Rendering of Gale Lake some 3.5 billion years ago, when Mars was warmer and much wetter. The Curiosity mission is finding that rocke in Gale Crater changed by water everywhere. (Evan Williams, with data from the Mars Reconnaissance Orbiter HIRISE project)

Before the Curiosity rover landed on Mars, NASA’s “follow the water”maxim had already delivered results that suggested a watery past and just maybe some water not far below the surface today that would periodically break through on sun-facing slopes.

While tantalizing — after all, the potential presence of liquid water on a exoplanet’s surface is central to concluding that it is, or once was, habitable — it was far from complete and never confirmed via essential ground-truthing.

Curiosity famously provided that confirmation early on with the discovery of pebbles that had clearly been shaped in the presence of flowing surface water, followed by the months in Yellowknife Bay which proved geologically, geochemically and morphologically the long-ago presence of substantial amounts of early Martian water.

Some of the earliest drilling was into mudstone that looked very much like a dried up basin or marsh, and that was exactly what Curiosity scientists determined it was, at a minimum.  It took many months for Curiosity leaders to ever use the word “lake” to describe what had once existed on the site, but now it is a consensus description.

Since the presence of a fossil lake was confirmed and announced, the water story has taken something of a backseat as the rover made its challenging and revelatory way across the lowlands of Gale Crater, through some dune fields and onto the Murray formation — a large geological unit that is connected to the base of Mount Sharp itself.  And all along the path of the rover’s traverse mudstone and sandstone were present, a clear indication of ever larger amounts of water.

I spoke recently with geologist and biogeologist John Grotzinger, the former NASA chief scientist for Curiosity and now a member of the science team, to get a sense of how things had progressed for the Gale water story.  He said there was no longer any doubt that the crater was once quite filled with water.

“We have  not seen a single rock at Gale that doesn’t say that the planet was wet.  In the areas where the rover has driven, I’d be very comfortable now in saying that the surface and ground water was often present for millions to tens of millions of years.”

Gale crater mudstone

Gale Crater mudstone at the Kimberly site.

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The Search for Exoplanet Life Goes Broad and Deep

The scientific lessons learned over the centuries about the geological, chemical and later biological dynamics of Earth are beginning to enter the discussion of exoplanets, and especially which might be conducive to life. This is an artist's view of the young Earth under bombardment by asteroids, one of many periods with conditions likely to have parallels in other solar systems. (NASA's Goddard Space Flight Center Conceptual Image Lab)

The scientific lessons learned over the centuries about the geological, chemical and later biological dynamics of Earth are beginning to enter the discussion of exoplanets, and especially which might be conducive to life. This is an artist’s view of the young Earth under bombardment by asteroids, one of many periods with conditions likely to have parallels in other solar systems. (NASA’s Goddard Space Flight Center Conceptual Image Lab)

I had the good fortune several years ago to spend many hours in meetings of the science teams for the Curiosity rover, listening in on discussions about what new results beamed back from Mars might mean about the planet’s formation, it’s early history, how it gained and lost an atmosphere, whether it was a place where live could begin and survive.  (A resounding ‘yes” to that last one.)

At the time, the lead of the science team was a geologist, Caltech’s John Grotzinger, and many people in the room had backgrounds in related fields like geochemistry and mineralogy, as well as climate modelers and specialists in atmospheres.  There were also planetary scientists, astrobiologists and space engineers, of course, but the geosciences loomed large, as they have for all Mars landing missions.

Until very recently, exoplanet research did not have much of that kind interdisciplinary reach, and certainly has not included many scientists who focus on the likes of vulcanism, plate tectonics and the effects of stars on planets.  Exoplanets has been largely the realm of astronomers and astrophysicists, with a sprinkling again of astrobiologists.

But as the field matures, as detecting exoplanets and inferring their orbits and size becomes an essential but by no means the sole focus of researchers, the range of scientific players in the room is starting to broaden.  It’s a process still in its early stages, but exoplanet breakthroughs already achieved, and the many more predicted for the future, are making it essential to bring in some new kinds of expertise.

A meeting reflecting and encouraging this reality was held last week at Arizona State University and brought together several dozen specialists in the geo-sciences with a similar number specializing in astronomy and exoplanet detection.  Sponsored by NASA’s Nexus for Exoplanet Systems Science (NExSS), NASA Astrobiology Institute (NAI) and the National Science Foundation,  it was a conscious effort to bring more scientists expert in the dynamics and evolution of our planet into the field of exoplanet study, while also introducing astronomers to the chemical and geological imperatives of the distant planets they are studying.… Read more

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