Bizarre microbes flourish in the most punishing environments on Earth from the bone-dry Atacama Desert in Chile to the boiling hot springs of Yellowstone National Park to the sunless sea bottom vents in the Pacific.
Could such exotic life emerge in the frigid arctic plains of Mars?
NASA's Phoenix spacecraft could soon find out. Since plopping down near the Martian north pole a month ago, the three-legged lander has been busy poking its long arm into the sticky soil and collecting scoopfuls to bake in a test oven and peer at under a microscope.
There hasn't been a eureka moment yet. But Phoenix turned up a promising lead last week when it uncovered what scientists believe are ice flecks in one trench and an icy layer in another.
Scientists hope experiments by the lander will reveal whether the ice has ever melted and whether there are any organic, or carbon-containing, compounds.
"We're looking for the basic ingredients that would allow life to prosper in this environment," chief scientist Peter Smith of the University of Arizona in Tucson has said in describing the mission's goal.
The discovery of extreme life forms, known as extremophiles, in unexpected nooks and crannies of the Earth in recent years has helped inform scientists in their search for extraterrestrial life.
"It's very suggestive that there are lots of worlds that may support life that at first glance may look like fourth-rate real estate," said Seth Shostak, an astronomer at the SETI Institute, a nonprofit dedicated to the search for extraterrestrial intelligence.
While the possibility for E.T. seems to grow with new extremophile discoveries on Earth, the truth is there's no evidence that life ever evolved on Mars or if it even exists today.
But if there were past or present life on the red planet — a big if — scientists speculate it would likely be similar to some extreme life on Earth — microscopic and hardy, capable of withstanding colder-than-Antarctica temperatures and low pressures.
"It's going to be microbes. It's not going to be a little green man," said Kenneth Stedman, a biologist with the Center for Life in Extreme Environments at Portland State University.
Under a microscope, extremophiles vary in size and shape. Some resemble miniature corkscrews while others are rods or irregular shapes. Scientists use a dye to distinguish the living ones from the dead.
The Phoenix mission has its limitations beside a shoestring budget of $420 million. It doesn't carry instruments capable of identifying fossils or living things. Rather, the lander has a set of ovens and a gas analyzer that will heat soil and ice and sniff the resulting vapors for life-friendly elements. Its wet chemistry lab will test the pH, or acidity, of the soil much like a gardener would. And its microscope will examine soil granules for minerals that may indicate past presence of water.
Most living things on Earth thrive not only in the presence of water, but also need sunlight, oxygen and organic carbon. But the range of conditions in which life can survive has been expanded with recent discoveries of microbes trapped in glaciers and rocks or living in volcanic vents and battery acid-like lakes.
These extreme conditions on Earth mirror the harsh environments found on Mars and other parts of the solar system. Present day Mars is like a desert with no hint of water on its weathered surface, although studies of rocks suggest the planet was wetter once upon a time.
Most researchers agree life likely cannot develop on the Martian surface, which is bombarded by lethal doses of radiation. But satellite images have revealed a softer side, spying hints of a vast underground store of ice near the Red Planet's polar regions. Phoenix last week hit what's thought to be an ice layer 2 inches below the surface.
Even if Phoenix uncovers microbe-habitable conditions, a more sophisticated spacecraft would be needed to determine if life was ever there or is present now.
The last time NASA looked for organics was during the 1976 twin Viking missions, which sampled soil near the Martian equator but turned up empty.
Scientists chose to dig in Mars' far north this time because they think it's an analog to Earth's polar regions, which preserve life's building blocks and sometimes even life itself in ice.
Researchers have shown microbes on Earth can be inactive in a deep freeze for thousands of years and resuscitated under the right conditions.
In 2005, NASA researchers announced they revived bacteria that were apparently dormant for 32,000 years in a frozen pond in central Alaska. Earlier this month, Penn State University scientists said they were able to grow in the lab an ultra-small species of bacteria trapped in a Greenland glacier under high pressure and low oxygen for at least 120,000 years.
"There's a lot of amazing things that survive in the cold environments," said Jennifer Loveland-Curtze, a senior research associate at Penn State.
What that means for Mars and other hostile environments is debatable. But scientists are plumbing the depths of Earth for clues to possible life that may exist elsewhere in the universe.
"We need to continue to try to understand what's going on with the extremophiles here on Earth," said Stedman of Portland State University. "The more we learn how extremophiles here are functioning, the more that will inform any kind of future mission."