Science Feature

Extra terrestrial life: where to look for it?

Four years from now, NASA will launch a capsule called TiME headed for Titan, the largest moon of Saturn. With a dense atmosphere and some evidence of surface liquid, Titan is our prime target in the search for extraterrestrial life. Biplas Das reports from a meet discussing clues to ET life.

doi:10.1038/nindia.2012.104 Published online 16 August 2012

Astrophysicists are looking at our cosmic backyard in the hope of finding clues to origin of life.

"One such place is Titan, which retains the kind of chemistry that earth had 4 billion years ago," says Bishun N. Khare from the NASA Ames Research Centre.

Speakers at the meet: (clockwise from left) Khare, Vidali, Chakrabarti, Herbst, Acharyya.

© Biplab Das

"Observational studies have revealed that Titan has rich chemistry in its atmosphere and hydrocarbon lakes", Khare told a gathering of astrophysicists and astrobiologists at a recent meet discussing the 'chemical evolution of the star forming region' and 'origin of life'.

TiME would reach Titan in 2023, parachuting onto the moon's second-largest northern sea, the Ligeia Mare. For 96 days, the capsule would study the composition and behaviour of the sea and its interaction with Titan's weather and climate. Khare said it would also seek evidence of the complex organic chemistry that may be active on Titan today and that may be similar to processes that led to the development of life on the early earth.

Titan is enveloped in a thick atmospheric haze. "Laboratory simulations have found that this haze is made of solid organic materials called tholin. Titan's tholin are mostly made up of nitrogen and methane. Tholin releases methane and ammonia at probably liquid nitrogen temperatures," Khare said.

Discussing the results of NASA's ongoing research on how tholins interact with liquid ethane and methane in the lakes of Titan, he said this will "improve our chances of detecting any possible biology on this cold and distant world".

Life beyond earth

Did the seed of life really come from outer space?

The meet — held at Kolkata's S. N. Bose National Centre for Basic Sciences (SNBNCBS) between July 10-13, 2012 — saw scientists quite affirmative on this question. They elaborated on how hydrogen, oxygen and water — molecules essential to life — were cooked up in giant laboratories known as dust clouds floating between stars, millions of light years away.

Besides forming stars, such molecules make long distance space voyages boarding stony stragglers like comets and meteorites until the gravity of earth-like planets trap them.

"These are the same molecules that Stanley Miller used to show how life on early earth may have begun," said Sandip Chakrabarti, an astrophysicist from SNBNCBS. "In his experiment, Miller took hydrogen, water, methane and ammonia gases and ran electric spark through this mixture of gases yielding amino acids, the building blocks of protein and essential to cellular life," Chakrabarti added.

However, the origin of life on earth was linked to space when scientists analysed the content of a meteorite which fell over Murchison, Australia. The meteorite was found to be rich in amino acids. Chakrabarti claimed that it is possible to form amino acids and adenine in collapsing dust clouds. Though such molecules are constantly bombarded with cosmic rays, bound amino acids are 30 times more stable under gamma ray radiation, he pointed out.

Hunting in star crevices

Dust clouds form an interstellar medium (ISM) that floats in empty space between stars. ISM is made of gas and dust particles or grains and is the birthplace of stars. "The dust particles consist of silicates and carbonaceous materials measuring between 20 angstroms and a few microns covered with common ice," said Eric Herbst from the departments of chemistry, astronomy and physics of University of Virginia, US.

Till date, about 150 neutral molecules have been detected in the gas phase of interstellar objects. These molecules are overwhelmingly organic, hydrogen being the most abundant of them, Herbst claimed.

"On dust particles, hydrogen molecules form at an extremely low temperature of 10K followed by oxygen and water molecules. This leads to formation of 100 monolayers of ice," Herbst said. At temperatures between 200 and 300K, ice melts resulting in reactions between radicals and spawning terrestrial-type molecules, he added.

Organic molecules have been detected in the active galactic nucleus (AGN) disk around a black hole in the NGC 1068, a spiral galaxy located about 50 million light years away.

The water link

Water is one of the basic raw materials that help spawn life. How water interacts with dust grains is important to understand the delivery of water to planetary bodies like earth. This is important as astrophysicists believe that water, too, has come from space. "Water is the main component of ices covering dust grains," said Gianfranco Vidali of Syracuse University, US.

To find how the first layers of ice form on bare interstellar grains, the initial stages of formation of D2O on bare surfaces was studied to mimic the formation of water on dust grains at edges of molecular clouds on amorphous silicate films, Vidali explained. "Atoms of deuterium (D) and oxygen adsorb onto silicate surface at 40k to form D2O and desorbed at 5000 K. Diffusion of oxygen on silicate surface forms molecules of oxygen and ozone," he said.

"Molecules assemble and react on dust grains through four major physical processes - accretion, diffusion, reaction, and desorption," said Kinsuk Acharyya, Bose Fellow from SNBNCBS. The outer surface of a grain is dotted with microwells that look like hemispherical wells on egg baskets.

At the slightest change in temperature, the atoms hop from one site to another on the same grain reacting and creating molecules of hydrogen, water and even methanol, he added.

However, these molecule-churning dust grains can only be studied by analysing the light they absorb and emit. It is not feasible to send space probes to such remote dusty outposts.