astrobiology

All posts tagged astrobiology

Artist’s conception of early Earth.

The Earth is about 4.5 billion years old, and it’s seen some dramatic events during that time – from the giant Moon-forming impact to the advent of fidget spinners.

The earliest phase in the Earth’s history, called the Hadean, was hellish. During long stretches, the surface was probably molten, the atmosphere was toxic, and there were no oceans. Eventually, though, the Earth transitioned to its current hospitable condition, and life got a toehold by at least 3.5 billion years ago.

But today in the Boise State Geosciences seminar, we heard about possible evidence for life going back almost a billion years earlier from Dr. Elisabeth Bell of UCLA’s Earth, Planetary, and Space Sciences Dept.

Optical microscope photograph; the length of the crystal is about 250 µm.

Dr. Bell’s work focuses on the hardiest of all mineral grains, the mighty zircons. These little rocks can be smaller than the width of a human hair, but they provide some of the strongest clues about conditions on the early Earth.

The mineral from which they’re made is very tough, and so weathering processes that usually break down other minerals barely affect zircons at all. Consequently, zircons that formed billions of years ago still retain their integrity, and, like the amber from “Jurassic Park”, often contain treasures in the form of other minerals. Dr. Bell and her team analyze these mineral time-capsules to learn what the early Earth was like.

One of the most exciting finds from her work comes from the Jack Hills geological formation in western Australia, where rocks almost as old as the Earth can be found. Inside these rocks are even older zircons, dating back to 4.1 billion years ago, which themselves have trapped small grains of graphite.

By analyzing the isotopic composition of these graphite grains, Dr. Bell has found tantalizing evidence for chemical processing of carbon that resembles biology. Although the evidence is still tentative, the results suggest life on Earth started chugging away smack in the middle of the Hadean, much earlier than has previously been believed.

Taking a step back, such a result suggests that life may get started on an Earth-like planet very quickly, which could mean that life is an almost inevitable outcome of the evolution of Earth-like planets. Given how common Earth-like planets may be, that could mean the universe may be replete with at least simple life.

Glint from a sea on Saturn’s moon Titan. From http://www.jpl.nasa.gov/spaceimages/details.php?id=PIA18433.

Our spring semester journal club opened with a nice review paper on finding habitable planets from Tyler Robinson, NASA Sagan Fellow at UC Santa Cruz.

The traditional definition of a habitable planet is “a world that can maintain stable liquid water on its surface”, but, as astrobiologists have explored for decades, this definition involves a vast flotilla of assumptions and very narrowly focuses our search for Earth-like life.

Even with all its limitations, this definition provides a very useful and practical starting point – at the first order, whether a world can host stable liquid water on its surface depends on the amount of sunlight it receives and whether it has a sufficiently thick (but not too thick) atmosphere.

Having found countless worlds outside our solar solar, astronomers are able to assess whether those worlds satisfy these conditions using observations we can already make, and a few dozen (probably) do.

In his review paper, Robinson discusses the observational and theoretical techniques astronomers can employ in the near future to take the next steps in deciding whether a world really has liquid water. Among the different approaches he describes, one is the most striking is the search for the glint from an alien ocean.

Robinson points out that Galileo was the first person to propose how to look for an ocean on another world. In his controversial Dialogue Concerning the Two Chief Worlds Systems, he says that, if the Moon had seas, “the surface of the seas would appear darker, and that of the land brighter”, just as on Earth.

Seas can also appear very bright compared to land, given the right observing geometry: seas exhibit specular reflection – this is the same effect you see looking out a plane’s window when the Sun reflects off the ocean. So looking for the glint provides a way to find large bodies of water on a distant world. Indeed, the first extraterrestrial oceans were found on Saturn’s moon Titan using this method, and one of the sea glints now frequently observed by the Cassini mission is shown in the figure at left.

Of course, we don’t have spacecraft orbiting any extrasolar worlds (yet), so we can’t resolve individual points on their surfaces. But, as discussed by Robinson, as they orbit their host stars, some of those worlds line up the right way that we could see a spike in the total amount of light coming from the planets. Observing such a spike over and over again whenever the planet was in the right geometry would be a strong hint that it had a large body of liquid reflecting sunlight. Given a little more information about planetary conditions, we could confidently infer such a planet had liquid water on its surface.

Amazingly, astronomers have used Earthshine reflected from the Moon to indirectly observe sea glints from the Earth. And so we’ve actually detected oceans on two worlds using distant spacecraft (if you let me call the Moon a “spacecraft”). As Robinson’s review implies, astronomers are probably on the cusp of finding oceans on extrasolar worlds. From there, it’s just a hop, skip, and a jump to finding life.

I just finished Don Canfield‘s book Oxygen, a sweeping exploration of the history of Earth’s most volatile atmospheric constituent.

Canfield starts by exploring how cyanobacteria generate oxygen during photosynthesis and how the process evolved. For instance, chloroplasts, the photosynthetic power plants of cyanobacteria, were once free-floating cyanobacteria themselves that took up residence in and eventually merged with eukaryotes that evolved into present-day cyanobacteria.

Subsequently, Canfield discusses Earth’s oxygen removal and renewal processes – decomposition of organics burns up oxygen, while their rapid burial preserves it.

The next several chapters present the geochemical evidence for changes in oxygen throughout Earth’s history, including variations in the ratios of different isotopes, sensitive to biological and abiological processes. Since I teach a class on astrobiology at Boise State, I focused a lot of attention on these parts, trying the piece together the interplay between biology and geology betokened by the isotopic variations.

One element of that story especially relevant to my class: variations in the carbon-13 isotope. As it turns out, one enzyme in cyanobacteria, RuBisCO, helps convert atmospheric C02 into organic carbon compounds, but it preferentially selects the lighter carbon-12 isotope 2.5% more often than the carbon-13 isotope. The organic compounds built using RuBisCO are therefore slightly depleted in carbon-13 relative to the atmosphere and when they are later incorporated into geological strata, the slight depletion gives a measure of how much life was around when the stratum was laid down.

Zooplankton salp pellets. From http://www.whoi.edu/oceanus/feature/will-ocean-iron-fertilization-work.

The book contained lots of other appealing details. For instance, it’s not exactly clear what caused an enormous variation in oxygen on Earth 580 million years ago, a sea change in Earth’s history matter-of-factly called the Great Oxygenation Event. But one explanation has to do with the evolution of a new kind of poop:

The idea is that zooplankton [newly evolved 580 million years ago] produce fast-sinking fecal pellets. These would decompose less in the upper layers of the ocean as they sink […] when compared to the smaller, slowly settling microbial biomass [that had previously predominated].  (pp. 135-136)

Since the old sinking biomass took a long time to sink to the ocean floor, it had a long time for bacteria to decompose it, using up a lot of oxygen in the process. But the new, faster-sinking poop made it to the ocean floor before it decomposed much and so left the oxygen dissolved in the ocean instead.

I did have to spend a lot of time reading and re-reading the discussions of geochemical cycles and signals because it’s been a long time since high school chemistry for me, but I was willing to struggle through these parts because I found the underlying story so interesting.

So a really fascinating and challenging read about the complex (and poop-filled) evolution of Earth’s bio-geo-atmosphere.

Fig. 11 from Barnes et al. (2016) showing evolution of the HZ (blue region) of Proxima Centauri, along with the orbits of Proxima Centauri b (solid line) and Mercury (dashed line).

Fig. 11 from Barnes et al. (2016) showing evolution of the HZ (blue region) of Proxima Centauri, along with the orbits of Proxima Centauri b (solid line) and Mercury (dashed line).

As a follow-up to last week’s Proxima Centauri b event, we discussed a recent analysis of the planet’s habitability by Prof. Rory Barnes and colleagues in our weekly journal club.

In this paper, the authors consider a very wide range of evolutionary scenarios for Proxima b to explore the resulting range of outcomes and decide how habitable the planet is, really.

They incorporate lots of potentially important effects, including the evolution of the host star’s luminosity and its influence on the planet’s surface temperature.

M-dwarf stars, like Proxima Centauri, get dimmer early in their lifetimes. As a consequence, the surface temperature of a planet orbiting such a star can drop over time.

Or, put another way, the habitable zone (HZ) around the star can move inward, meaning planets that start out interior to the HZ (i.e., planets that might be too hot to be habitable) may eventually enter the HZ.

Figure 11 from Barnes et al. (2016) shows that this is probably what happened to Proxima b: it started out way too hot for habitability and, as its host star dimmed, it entered the HZ.

As Barnes et al. show, such a history could potentially drive away all the planet’s water (assuming it started with any), leaving behind a dried husk of a planet. But the fact that the planet is CURRENTLY in the HZ could fool us into thinking it’s habitable.

This result shows that planetary habitability is a complicated idea and that the current conditions on a planet can depend in a complex (and hard-to-determine) way on its history. Time (and lots more data) will tell whether Proxima b is actually an extraterrestrial oasis for life or a barren wasteland.

The geysers near Enceladus' south pole. From https://en.wikipedia.org/wiki/Enceladus#/media/File:PIA19061-SaturnMoonEnceladus-CurtainNotDiscrete-Eruptions-20150506.jpg.

The geysers near Enceladus’ south pole. From https://en.wikipedia.org/wiki/Enceladus#/media/File:PIA19061-SaturnMoonEnceladus-CurtainNotDiscrete-Eruptions-20150506.jpg.

Saturn’s moon Enceladus has inspired fascination since Herschel found it in the late 1700s. The discovery of active cryovolcanoes geysers** on its surface by the Cassini mission in 2006 raised that fascination to a feverous intensity.

Although the source of energy powering the volcanoes geysers** was not (and is still not) understood, their implication was clear: Enceladus could have a sub-surface ocean, like his big sister Europa.

The follow-up discovery of salty particles in the geysers all but clinched the existence of a sub-surface water source. Salt in the eruptions would probably require liquid water to dissolve rocky materials to produce the salt. But it wasn’t clear whether the sub-surface source is a small pocket of water directly beneath the geysers or a more global-scale ocean.

In our journal club today, we discussed a paper that points to the existence of a global ocean. The recently published study from Peter Thomas and colleagues analyzed Enceladus’ rotation to study its internal structure. By tracking the motion of hundreds of control points on Enceladus’ surface, they found that its outer icy crust oscillates back and forth during its rotation much more than it should if the moon were solid all the way through.

Thomas and colleagues show that the large oscillations they found (called libration) require a large layer of fluid within Enceladus to lubricate the space between its outer, icy shell and its rocky interior. Otherwise, the moon would oscillate a lot less. Difficult to say exactly, but Thomas and colleagues estimate the ocean could be as thick as 30 km beneath an 20-km thick icy crust.

Just as Europa’s ocean, a sub-surface ocean in Enceladus could represent an enormous harbor for life. Even on this tiny moon, such a deep ocean is only about a tenth the size of the Earth’s oceans*.

Although Enceladus’ surface would be a tough neighborhood for life, sub-surface biota (if they ever evolved) would be protected by a thick layer of ice from the vacuum of space and interplanetary radiation. In fact, this same radiation could impinge on the surface and produce a steady supply of biologically useful oxidants, which could then trickle down into the subsurface ocean and help power the alien biosphere.

Thomas and colleagues suggest a more detailed analysis of Enceladus’ surface geology, newly inspired by their discovery, might help unravel the history of the ocean. Similar analyses of Europa’s complex tangle of surface ridges and cracks helped piece together that moon’s geological history.

Journal club attendees today included Jennifer Briggs, Hari Gopalakrishnan, Tyler Gordon, and Jacob Sabin.

*Earth’s oceans are, on average, 4 km deep, much smaller than Earth’s radius of 6,400 km. Therefore, I approximated their volume as pi*(6,400 km)^2 (4 km) ~ 500 Mkm^3. A similar calculation for Enceladus’ ocean — pi*(50 km)^2 (30 km) ~ 24 Mkm^3.

**Dr. Thomas kindly pointed out that there is some debate over the nature of the eruptions on Enceladus, and volcanoes is probably not the term to use.

Artist's conception of Kepler-452 b. From https://en.wikipedia.org/wiki/Kepler-452b#/media/File:Kepler-452b_artist_concept.jpg.

Artist’s conception of Kepler-452 b. From https://en.wikipedia.org/wiki/Kepler-452b#/media/File:Kepler-452b_artist_concept.jpg.

Exciting discovery reported last week of a planet a little bigger than Earth orbiting a star very like our Sun.

The planet, Kepler-452 b, was discovered by the Kepler mission and has a radius 60% larger than the Earth’s. It receives about 10% more light from its star than we do here on the Earth, and it’s probably about 2 billion years older. Together, these qualities mean it may be the most Earth-like exoplanet found to date (although there are lots of other similar planets).

Unfortunately, the host star is so distant, 1,400 lightyears from Earth*, that the usual method for directly estimating the planet’s mass, radial velocity observations, is not feasible. Instead, the planet’s discoverers constrain the planet’s mass by considering a range of compositions, calculating the radius expected for each of those compositions, and comparing it to the observed radius. Based on this analysis, they estimate at least a 49% probability that the planet is rocky, like the Earth.

Based on the amount of light it receives from its host star, there’s a good chance Kepler-452 b is habitable. This means, given a long-list of assumptions about the planet and its atmosphere, liquid water would be stable on its surface. Thus, Kepler-452 b joins a short but rapidly growing list of planets that might host life.

With our success finding potentially habitable planets, it’s probably only a matter of time (maybe just a few more years) before we find a planet that’s not just habitable but inhabited. Children in school right now might be the first generation to grow up in a universe where they know we’re not alone.

Today’s journal club attendees included Jennifer Briggs, Hari Gopalakrishnan, and Jacob Sabin.

*This website is the only reference I can find that gives the distance to Kepler-452 b from Earth. The paper itself doesn’t say 1,400 light years. The exoplanet.eu catalog gives a stellar magnitude V = 13.7 (also not given in the discovery paper). Converting that V magnitude to a flux and then using the stellar parameters given in the paper, I estimate a distance of 2,400 light years.

The recent study has removed Lord Helmet's original skepticism about buckyballs as the originator of DIBs.

The recent study has removed Lord Helmet’s original skepticism about buckyballs as the originator of DIBs.

At journal club today, we discussed the recent paper from Maier and colleagues which has solved the long-standing mystery of diffuse interstellar bands or DIBs.

These light absorption features were originally discovered by Heger way back in 1919, only a few months after the end of World War I. Their discrete absorption peaks are pervasive throughout visible wavelengths, indicating they are not simply due light-scattering by interstellar dust. The fact that they appear unchanged no matter the nature of the star whose light they absorb also suggests they don’t arise from the star itself. Instead, they must lay somewhere in the vast space between the Earth and the star.

Astronomers proposed DIBs might arise from dust grains, carbon chains, and even floating bacteria. Bucky balls, large soccer-ball-shaped carbon molecules, had also been proposed as candidates since they were discovered in white dwarf stars.

But deciding which candidate was actually the culprit required meticulous and highly sensitive lab work to recreate the extreme conditions of outer space, where temperatures are near absolute zero and gas pressures can be 10 million times smaller than at Earth’s surface. After twenty years of work, Maier and his team in Switzerland and Germany finally managed to create a little pocket of interstellar space in their lab.

By carefully ionizing buckyballs and introducing them into a cold He gas, they showed the spectral features created by the buckyballs in association with He matched almost exactly the spectral features of some DIBs.

The upshot of this is that the spectral forest of DIBs found at other wavelengths likely points to the prevalence of other large and complex molecules self-assembling in space, so this discovery is just the tip of the chemical iceberg. It has even been suggested that the complex molecular precursors for life originated in interstellar space in the same way as the buckyballs.

Whether that’s true or not, this discovery shows that the vast and lonely spaces between the stars aren’t quite as empty as they seem.

Journal club attendees included Jennifer Briggs, Emily Jensen, and Tyler Wade.