Life's building blocks observed in spacelike environment

Where do the molecules required for life originate? It may be that small organic molecules first appeared on earth and were later combined into larger molecules, such as proteins and carbohydrates. But a second possibility is that they originated in space, possibly within our solar system. A new study, published this week in the Journal of Chemical Physics, from AIP Publishing, shows that a number of small organic molecules can form in a cold, spacelike environment full of radiation.

Investigators at the University of Sherbrooke in Canada have created simulated space environments in which thin films of ice containing methane and oxygen are irradiated by electron beams. When electrons or other forms of radiation impinge on so-called molecular ices, chemical reactions occur and new molecules are formed. This study used several advanced techniques including electron stimulated desorption (ESD), X-ray photoelectron spectroscopy (XPS) and temperature programmed desorption (TPD).

The experiments were carried out under vacuum conditions, which both is required for the analysis techniques employed and mimics the high vacuum condition of outer space. Frozen films containing methane and oxygen used in these experiments further mimic a spacelike environment, since various types of ice (not just frozen water) form around dust grains in the dense and cold molecular clouds that exist in the interstellar medium. These types of icy environments also exist on objects in the solar system, such as comets, asteroids and moons.

All of these icy surfaces in space are subjected to multiple forms of radiation, often in the presence of magnetic fields, which accelerate charged particles from the stellar (solar) wind toward these frozen objects. Previous studies investigated chemical reactions that might occur in space environments through the use of ultraviolet or other types of radiation, but this is a first detailed look at the role of secondary electrons.

Copious amounts of secondary electrons are produced when high-energy radiation, such as X-rays or heavy particles, interact with matter. These electrons, also known as low-energy electrons, or LEES, are still energetic enough to induce further chemistry. The work reported this week investigated LEEs interacting with icy films. Earlier studies by this group considered positively charged reaction products ejected from ices irradiated by LEEs, while the work reported this week extended the study to include ejected negative ions and new molecules that form but remain embedded in the film.

The research group found that a variety of small organic molecules were produced in icy films subjected to LEEs. Propylene, ethane and acetylene were all formed in films of frozen methane. When a frozen mixture of methane and oxygen was irradiated with LEEs, they found direct evidence that ethanol was formed.

Indirect evidence for many other small organic molecules, including methanol, acetic acid and formaldehyde was found. In addition, both X-rays and LEEs produced similar results, although at different rates. Thus, it is possible that life's building blocks might have been made through chemical reactions induced by secondary electrons on icy surfaces in space exposed to any form of ionizing radiation.

Pluto, Other Faraway Worlds May Have Buried Oceans

Our solar system may harbor many more potentially habitable worlds than scientists had thought.

Subsurface oceans could still slosh beneath the icy crusts of frigid, faraway worlds such as the dwarf planets Pluto and Eris, kept liquid by the heat-generating tug of orbiting moons, according to a new study. 

"These objects need to be considered as potential reservoirs of water and life," lead author Prabal Saxena, of NASA's Goddard Space Flight Center in Greenbelt, Maryland, said in a statement. "If our study is correct, we now may have more places in our solar system that possess some of the critical elements for extraterrestrial life."

Underground oceans are known, or strongly suspected, to exist on a number of icy worlds, including the Saturn satellites Titan and Enceladus and the Jovian moons Europa, Callisto and Ganymede. These oceans are kept liquid to this day by "tidal heating": The powerful gravitational pull of these worlds' giant parent planets stretches and flexes their interiors, generating heat via friction. 

The new study suggests something similar may be going on with Pluto, Eris and other trans-Neptunian objects (TNOs).

Many of the moons around TNOs are thought to have coalesced from material blasted into space when objects slammed into their parent bodies long ago. That's the perceived origin story for the one known satellite of Eris (called Dysnomia) and for Pluto's five moons (as well as for Earth's moon). 

Such impact-generated moons generally begin their lives in relatively chaotic orbits, team members of the new study said. But over time, these moons migrate to more-stable orbits, and as this happens, the satellites and the TNOs tug on each other gravitationally, producing tidal heat.

Saxena and his colleagues modeled the extent to which this heating could warm up the interiors of TNOs — and the researchers got some intriguing results.

"We found that tidal heating can be a tipping point that may have preserved oceans of liquid water beneath the surface of large TNOs like Pluto and Eris to the present day," study co-author Wade Henning, of NASA Goddard and the University of Maryland, said in the same statement.

As the term "tipping point" implies, there's another factor in play here as well. It's been widely recognized that TNOs could harbor buried oceans thanks to the heat produced by the decay of the objects' radioactive elements. But just how long such oceans could persist has been unclear. This type of heating peters out eventually, as more and more radioactive material decays into stable elements. And the smaller the object, the faster it cools down.

Tidal heating may do more than just lengthen subsurface oceans' lives, researchers said.