Australasian Science: Australia's authority on science since 1938

New Super-Earth May Be a Waterworld

By David Reneke

News from the space and astronomy communities around the world.

Four astronomers from the Harvard Smithsonian Centre for Astrophysics (Cfa) have taken a giant step in the relatively new science of exobiology. They’ve been successful in probing the atmosphere around the transiting super-Earth known as GJ1214b.

This exoplanet isn’t exactly our size but it’s close enough to set pulses running. It is 6.5 Earth masses and measures 2.7 Earth radii, and it orbits a small M-dwarf star whose diameter is only 21% of the Sun’s.

The Cfa astronomers – Zachory Berta, David Charbonneau, Jean-Michel Desert and Jonathan Irwin – used the Hubble Telescope’s infrared spectrometer to observe the planet as it passed across the face of the star. In doing so, the planet’s atmosphere absorbed light from the star, subtly altering the star’s intrinsic spectrum as we observe it. This spectrum was carefully measured and subtracted after the transit.

So what is a super-Earth? It’s an exoplanet (a planet around another star) whose mass is between about two and ten Earth masses. Planets larger than this are closer to our gas giants in size, and perhaps in other physical properties as well.

The “super-Earth” category at present refers only to the mass of the object – not to its radius, its orbital distance from the star, surface temperature or atmosphere. Of the 576 exoplanets whose approximate parameters are currently known, 36 are in the super-Earth category.

Earlier models tried to predict what the atmosphere of this super-Earth might contain using ground-based transit spectra of it and our current knowledge of the atmospheres of the planets in the solar system.

Astronomers are generally in agreement that unless the atmosphere has a thick top layer of clouds obscuring our view, probable compositions of these distant worlds could include abundant water vapour. Now that is an enticing thought.

Indeed, probing the atmosphere around a planet orbiting a star 42 light years away is, on its own, a remarkable achievement and a big first step in our eternal search for those elusive biomarkers that signal life.

Our Growing, Shrinking Moon
We often think of the Moon as a geologically dead body orbiting the Earth without a purpose or reason, but scientists look at it as a perfectly preserved slice of our solar system’s history.

Now images returned from NASA’s Lunar Reconnaissance Orbiter (LRO) suggest that our natural satellite isn’t dead at all. It’s actually quite active but exhibiting some unusual and strange properties: the Moon seems to have been growing and shrinking recently in its history.

How can that be? Well, the first evidence of an active Moon came in 2010 when LRO’s camera returned high-resolution pictures of landforms called lobate scarps – lobe-shaped cliffs that have now been found scattered across the lunar surface.

About 10 metres high and several kilometres long, lobate scarps form along fault lines, or inclined fractures, where blocks of a body’s crust rise vertically. It’s likely they formed when the Moon’s interior cooled and the rock contracted to force slices upward. Common as they are on our geologically active Earth, finding them on the Moon was a huge surprise.

As the Moon’s core has continued to cool in the relatively recent past, the crust has buckled under compression. It’s like when an apple dehydrates and the skin buckles and forms around the shrinking core.

Based on the sizes of the scarps, scientists estimate that the distance between the centre of the Moon and its surface shrank by about 100 metres in the process.

Now add rare linear valley features called “graben” to the picture. They formed when the lunar crust was stretched, broken and dropped down between two bounding faults. In short, it’s causing growth.

Strange stuff indeed, but there’s more! Around the same time that the Moon was shrinking around its cooling core, forces in some places were acting to pull it apart.

Taken together, these discoveries paint a very different picture of our Moon, and it all relates to the Moon’s evolution, how it formed and how it lost its heat.

The mix of scarps and graben found on the Moon suggest that only the outer portion of our satellite seems to have melted. The balance of stresses acting on that early Moon creates the right conditions for the somewhat contradictory surface features we see today.

Our Moon is shrinking in some places, growing in others, but worth further exploration everywhere.

David Reneke is an astronomy lecturer and teacher, a feature writer for major Australian newspapers and magazines, and a science correspondent for ABC and commercial radio. Subscribe to David’s free Astro-Space newsletter at www.davidreneke.com