Looking For Life on Other Worlds

Life is abundant on Earth. From the coldest glaciers to the hottest deserts, its almost impossible to find a spot on Earth that is devoid of life. During warm summer nights, invading armies of mosquitoes and roaches make us almost wish for fewer life forms. Earthworms after rain, mold on a leftover piece of bread, seagulls diving for fish on the beach, mango trees loaded with fruit, are all examples of life thriving on our planet. But amazingly, all living beings that we know of are inhabitants of Earth.

Despite the outlandish claims of unidentified flying objects (UFOs), humans have yet to detect even a single microscopic organism of extra-terrestrial origin. Thus, one of the most sacred questions of human race remains unanswered: Are we alone in the universe?

The discovery of planets around nearby stars, the detection of surface features on Mars associated with recent flow of water, and the possibility of a liquid water ocean underneath the icy surface of Jupiter’s moon, Europa, has invigorated the efforts to search for life on other worlds. But first, we have to ask what is life? Would life everywhere be based on the same chemistry as life here on Earth? How common are the ingredients that make up life on Earth?

These are hard questions and, unfortunately, we have only one example so far. Whereas, life elsewhere may appear quite different than life on Earth, most scientists expect a similar underlying chemistry. Thus, in order to search for life in the universe, we search for similar conditions that gave rise to life on Earth.

Raw Materials for Life:

Life on Earth is primarily composed of carbon, hydrogen, oxygen and nitrogen. Fortunately, these represent four of the five most abundant elements in the universe, with Helium rounding out the top five. Thus the basic ingredients for life are easily available in the universe. Hydrogen (and Helium) is left over from the Big Bang, the beginning of the universe, whereas carbon, oxygen, and nitrogen have been cooked inside of stars. Other elements, such as phosphorus, sulfur, iron, etc., also participate in the machinery of life, but are required in relatively small quantities.

The most important element for life is carbon. It is often stated that life on Earth is "carbon-based". But why carbon? The chemical structure of carbon allows the formation of long chains and rings of atoms to which atoms of other key elements can attach. These long chains of atoms result in molecules such as amino acids, sugars, phosphates, etc. that form the backbone of life. Silicon, located just below carbon in the Periodic table, is the only other element that can form such long chains. However, the relatively weak silicon bonds are not favorable to the formation of life. Thus, because of its atomic structure, we can expect carbon to play a central role in the biology of other worlds too.

But how do we form organic compounds (carbon-based), like amino acids and phosphates, which are the building blocks of life? The atmosphere of early Earth mostly contained ammonia, methane, carbon dioxide, and water vapor. A series of lab experiments in the 1950’s showed that when an electric current, mimicking lightening, is passed through such a solution for a few days, it results in the formation of complex organic compounds, including the all important amino acids, and sugars. Organic compounds may also have been introduced to Earth from space. Comets are known to contain water and carbon-compounds. In addition to bringing water to Earth, comets may also be responsible for supplying organic molecules. Since collisions with comets were common early in the history of the Solar System, the primary ingredients of life, water and organic compounds, must have been delivered to other bodies of the Solar System too.

Origin of Life on Earth:

The story of life on Earth is a fascinating one. Earth, along with the rest of the Solar System, was formed 4.6 billion years ago. A period of heavy bombardment of comets and asteroids followed that lasted for roughly 500 million years. Such collisions would have boiled off Earth’s oceans repeatedly, aborting any efforts leading to the formation of life.

Soon after, either from chemical processes on Earth or brought in from comets, the surface of Earth was covered with organic gunk. From this soup of organic matter, a molecule arose that could make rough copies of itself and store information. This molecule was the ancestor of Deoxyribonucleic Acid, or DNA. We don’t know yet the exact steps that produced this molecule, but scientists suspect that it must have happened somewhere in shallow puddles or ponds.

DNA drives life on Earth. But DNA itself is not "alive". To replicate itself DNA requires a complex and specialized environment that is provided within living cells. Thus the smallest and simplest systems that live are cells. DNA not only governs the process of reproduction but also provide instructions on how to make proteins, the workhorse of living cells. Life on other worlds, most likely, will not be driven by DNA, but perhaps by some other molecule with similar properties. What an astonishing discovery would that be to find such a molecule!

The earliest signs of life forms have been dated to 3.8 billion years ago, only 200 million years after the end of the era of bombardment. Two hundred million years may seem like a long time from human perspective, but its only a blink of an eye in cosmic time scale. For the next 3 billion years or so life did not get more complex than some microorganisms, such as the blue-green algae. For the most part of its history Earth has been inhabited by organisms too small to be seen by a human naked eye.

However, about 600 million years ago, perhaps in response to a more favorable environment, sudden diversification of species occurred which the scientists call the Cambrian Explosion. The complexity of animals and plants increased dramatically in the next few hundred million years, eventually leading to modern humans who started wondering about their own origins.

Life on Other Worlds:

If raw materials for life are easily available, then perhaps the music of life has been played on other worlds too. The discovery of microbial colonies on Earth surviving in extreme environments, ranging from the near boiling temperatures in the vicinity of deep-sea volcanic vents to the freezing depths of the Antarctic ice sheet, have increased the odds of finding life elsewhere. However, all life forms on Earth need liquid water to survive, and liquid water is hard to find in the Solar System. Thus, the search for life is basically a search for liquid water.

Our neighbor Mars provides one of the best places to look for life. Over 3.5 billion years ago Mars had a thick atmosphere and there is ample evidence that water flowed freely on the surface. However, over the course of time, Mars lost its atmosphere and water either evaporated or seeped underground. Today, Mars is mostly a dry planet. However, there is recent evidence that suggest vast repository of underground water. If this is true, then there may be places on Mars, even today, that are be breeding grounds for life.

Interestingly, Jupiter’s moon Europa, located five times further away from the Sun than the Earth, is suspected to have a liquid water ocean underneath its icy surface. It is thought that tidal forces of Jupiter keep the interior of the moon warm enough to keep the water in liquid form. The conditions in Europa’s oceans might be similar to the under-water vents here on Earth, and like Earth, may be a thriving place for life.

In our Solar System we don’t expect to find complex intelligent life forms. In fact, Mars and Europa may only contain microscopic life. Some critics argue, what is the significance of an extra-terrestrial microbe? The discovery of even a single living cell from another world would deprovincialize biology, and forever change the way we look at life. The discovery would also suggest that if life started on at least one other place in our Solar System, then it might be common amongst the 400 billion stars in our galaxy. On some planets life may have found a way to evolve into complex species. Who knows there may be other worlds out there where, at this very instant, thinking beings are asking themselves, "Are we alone in the universe"?


The writer (shameed@earth.ast.smith.edu) is a research astronomer at University of Massachusetts at Amherst and teaches astronomy at Smith College.



Figure Caption:
Carved by past water flow on Mars? This image was taken by the Mars Global Surveyor in 1998. It shows a portion of one of the canyons on Mars that may have formed by a sustained flow of water, more than 3 billion years ago. Photo courtesy, NASA/JPL/Malin Space Science Systems.