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IN BRIEF

Observations from the Cassini mission to Saturn has allowed researchers to better understand how the moon Enceladus could sustain a global subsurface ocean. A new study gives more evidence that the moon could support life.

MOON LIFE

New evidence has emerged supporting the possibility that Saturn’s moon Enceladus has the potential to host life. Not only does this evidence point to the ability of the moon to harbor life, but that life could have also evolved into complex organisms.

NASA previously announced that Enceladus has many of the “ingredients needed for a habitable environment.” A new study published in Nature Astronomy explains exactly how the moon can sustain a global salty ocean under its icy crust, and how such an environment can be conducive to life developing and thriving.

Researchers have taken the Cassini mission’s observations of Enceladus and attempted to match them with computer models depicting various possibilities for how such an ocean could form. The researchers concluded that Enceladus’s rocky core is porous, water is heated while traveling through the core and is expelled through narrow openings in the crust. This process forms hot water jets, which are especially prevalent in the moon’s southern polar region.

Image credit: NASA/JPL-CALTECH

CHEMICAL REACTIONS

Studying these hydrothermal vents could be the best bet for determining whether Enceladus could foster life. Lead author Gaël Choblet, from the French National Center for Scientific Research, spoke with Newsweek, saying, “If a new theorypublished last year is correct, then powerful hydrothermal activity could have been occurring since the formation of the moon, possibly as much as the age of the solar system.” Choblet was referring to a study pertaining to how the moons of Saturn could be heating.

Furthermore, the heating provides an adequate timeline for life to develop. David A Rothery, professor of Planetary Geosciences at the Open University, U.K. explained, “Chemical reactions are going on even today. If it’s going on today it could have been going on a billion years into the past, and that’s long enough for life to get started—and to have evolved beyond the very most basic stages. It could be quite a complex microbial community down there and we’d love to study it.”

Encelauds has a global ocean sitting between its icy shell and rocky core.

NASA

Studying how life began and evolved on other planets could provide further insight into how life on our planet began. This is not by any means a confirmation of extraterrestrial life on Enceladus, or any other celestial body, yet it is compelling evidence in support of further research into the very real possibility.

Heating ocean moon Enceladus for billions of years

Over time, cool ocean water seeps into the moon's porous core. Pockets of water reaching deep into the interior are warmed by contact with rock in the tidally heated interior and subsequently rise owing to the positive buoyancy, leading to further interaction with the rocks. The heat deposited at the boundary between the seafloor and ocean powers hydrothermal vents. Heat and rocky particles are transported through the ocean, triggering localised melting in the icy shell above. This leads to the formation of fissures, from which jets of water vapour and the rocky particles from the seafloor are ejected into space. In the graphic, the interior 'slice' is an excerpt from a new model that simulated this process. The orange glow represents the parts of the core where temperatures reach at least 90°C. Tidal heating owing to the friction arising between particles in the porous core provides a key source of energy, but is not illustrated in this graphic. The tidal heating results primarily from the gravitational pull from Saturn.

Credit: Surface: NASA/JPL-Caltech/Space Science Institute; interior: LPG-CNRS/U. Nantes/U. Angers. Graphic composition: ESA

"It's really weird that a planet that's orbiting far away from the star can tell us anything about the star's interior — I think that's really bizarre but cool," said Emily Sandford, an astronomer at Columbia University in New York and lead author on the new work.

"Basically, by measuring very precisely the shape of those dimmings, the amount of light that's missing over time when a planet passes in front, you can measure the density of the star the planet's orbiting," she told Space.com

Using a method first derived by astrophysicsts Sara Seager and Gabriela Mallen-Ornelas in 2003, Sandford and co-author David Kipping, also of Columbia, analyzed data on 66 Kepler stars to determine their densities. Their precision rivalled that of asteroseismology, a technique used on bright stars that uses oscillations in their shine to reconstruct their interior properties. While asteroseismology requires very precise brightness measurements, Sandford said, the new calculation needs less pristine data.

Essentially, the process works because how long a planet spends in front of its star is tied to the star's density. Planets that are far away from their stars spend less time in front of them — imagine a planet right at the star's surface, which would spend half of its orbit in front of it, versus one that is very far away, and only crosses in front for a quick part of its orbit, Sandford said. But at the same time, a planet has to move at a certain speed to maintain a steady orbit based on how massive the star is — if the planet is going too slowly, it will fall into the star, and if it's going too fast, it will fly away.

"We can then use that knowledge of [proportionally] how far the planet is from its star, combined with the duration of the transit, to solve for the stellar density," Sandford said. "If a planet is far away and yet takes a long time to transit, we know that the star must be big, puffy, and low-density; if a planet is close-in and yet transits quickly, we know that the star must be small, compact, and high-density."

In other words, the denser a star is — the more massive it is compared to its radius — the less time the planet spends in front of it.

Treasure trove

While researchers already knew about this relationship between transiting planets and their parent stars' densities, it's only since Kepler's launch in 2009 that more than a handful of transiting planets have been available for analysis, according to Sandford.

"We have this enormous treasure trove of data, and we can start to apply some of these ideas that people thought of way in advance before the data was ready," Sandford said. "That's what we've done here." [How Do You Spot an Alien Planet from Earth? (Infographic)]

This method requires that the researchers know the planet's orbit well, or that they can predict that it's circular. (Circular orbits tend to be common in systems with multiple planets.) It also requires researchers to know the planet's full orbit length — so it must have passed in front of the star more than once.

Once those criteria are established, researchers can use these calculations in two key ways: to determine if a transit is really caused by a planet or if it's a false positive, and to analyze the other planets in a multiplanet system.

That false-positive case was one of the uses Seager had in mind when the equation was first developed, she told Space.com: "If you get the stellar density from the light curve alone, and then if you get the stellar density because you already know something about its mass and size … and if those densities don't match up, the thing you're looking at is not a planet," said Seager, who was not involved in the new study. "It's a blended object; it's a false positive; it's something else."

"All the applications are exciting, whether you want to just measure the densities of the set of stars, or whether you're using it as a kind of comparison tool," she added.

During its primary mission, Kepler looked at the same set of stars for four years, but future exoplanet hunters such as NASA's Transiting Exoplanet Survey Satellite (TESS), set to launch in 2018, will spend a much shorter time on each part of the sky — just a month or so. In systems with multiple planets, if one orbits quickly enough for researchers to see more than one full orbit, that planet could act as an "anchor" to pin down the star's density, letting researchers learn about the orbits of the other planets, too.

"There's going to be a lot of habitable-zone planets that fall out in that window where their orbital period is longer than a month," Sandford said. "If we only see one transit, if we can't measure the period directly, it might be a very, very faraway planet, in which case it's too cold to have liquid water, or it might fall right in the middle of the habitable zone."

"If we can use our method to constrain the star, and from there to constrain the outer planet's period, that could be really interesting for prioritizing certain planets for follow-up if we know that they're likely to be habitable-zone, potentially the right temperature range for liquid water, type planets," she added.

Know the planet, know the star

According to Vincent Van Eylen, a researcher at Leiden University in the Netherlands who is uninvolved with the study, there's a saying that goes "Know thy star, know thy planet." Often, researchers use what they can glean about stars to identify what their planets might be made of and what the orbits look like.

Van Eylen's own research uses stars' densities and the durations of transits to learn about the planets' orbits — a reversal of Sandford's calculations.

"We don't know enough about stars, and everything we learn about planets somehow is indirect information that's related to stars," Van Eylen said. "In that way, it's kind of a sweet result that they can turn things around and actually learn about the planets, and thereby learn about the stars."

The new work has been accepted for publication in The Astronomical Journal and is available on ArXiv.org.

Email Sarah Lewin at slewin@space.com or follow her @SarahExplains. Follow us @Spacedotcom, Facebook and Google+. Original article on Space.com.