All posts tagged astrobiology

The geysers near Enceladus' south pole. From

The geysers near Enceladus’ south pole. From

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

Artist’s conception of Kepler-452 b. From

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 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.