The search for signs of life beyond Earth: gaining ground

Credit: pbs.org

This morning, the National Academies’ Space Studies Board Committee on Astrobiology and Planetary Science is receiving a briefing on a workshop last December on searching for life across space and time. I wrote a report on the workshop for posting on nasa.gov. It has not been published, so I’m posting it here, with a few updates.

In recent years, NASA planetary missions have identified a growing number of potentially habitable environments in our solar system – planetary bodies that have, or could have, liquid water: Mars, Jupiter’s moons Europa and Ganymede, Saturn’s moons Enceladus and Titan…. NASA’s New Horizons mission gathered evidence that even the dwarf planet Pluto might have a subsurface liquid water ocean. Looking for signs of habitability and life on these bodies, and on planets in other star systems, is now a major focus of NASA’s planetary exploration program.

One of the hottest prospects in the search for evidence of extraterrestrial life in our solar system is Europa, which has a global subsurface liquid water ocean that could be heated by volcanic or hydrothermal activity on the ocean floor.

Last year, responding to congressional direction, NASA established an Ocean Worlds Exploration Program including orbiter and lander missions to the Jovian moon. Planning for a NASA orbiter mission to Europa is already under way. In June, NASA appointed a science definition team (SDT) to identify scientific goals and objectives for a Europa lander mission.

Last month the SDT delivered its report to NASA, identifying three primary goals for this mission, in order of priority: 1) Search for evidence of life on Europa, 2) Assess the habitability of Europa via in situ techniques uniquely available to a landed mission, and 3) Characterize surface and subsurface properties at the scale of the lander to support future exploration. (Two town-hall meetings scheduled this spring – March 19 at the Lunar and Planetary Science Conference in The Woodlands, Texas; and April 23 at the Astrobiology Science Conference in Mesa, Arizona – will provide scientists with an opportunity to comment on this report.)

Given growing interest, inside and outside NASA, in the search for evidence of extraterrestrial life, last year NASA’s Science Mission Directorate tasked the National Academies’ Committee on Astrobiology and Planetary Science with organizing an expert dialogue on “searching for life across space and time.” This dialogue took place in California in December.

“The search for evidence of extraterrestrial life in our solar system and beyond is shifting into high gear. Before we decide where to look for it, we have to pin down what we’re looking for and how we’re going to look for it,” said Mary Voytek, NASA’s senior scientist for astrobiology. “It’s been 15 years since the Academies last reviewed and synthesized the relevant issues in our quest for evidence of extraterrestrial life. The forthcoming report on last month’s workshop will synthesize our current understanding of the limits of life and life’s interactions with planetary environments, as well as of our capabilities to extend this understanding,” said Voytek, who participated in the workshop.

In addition to places, the “time” factor is important to address. As then-NASA Chief Scientist Ellen Stofan noted at the workshop, “A habitable zone is not just a place, it’s a time.”

The aim of the gathering, said workshop chair James Kasting of Pennsylvania State University, was not to produce a consensus or formal recommendations but to gather key people in planetary exploration and exoplanet studies to share ideas.

Questions that drove the dialogue were:

• What is our current understanding of the limits of life and life’s interactions with the environments of planets and moons?
• Are we today positioned to design, build and conduct experiments or observations capable of life detection remotely or in situ in our own solar system and from afar on extrasolar worlds?
• How could targeted research help advance the state of the art for life detection, including instrumentation and precursor research, to successfully address these challenges?

Experts at the workshop considered what exactly “life” is. Is Earth life – life as we know it – the only kind of life that planetary conditions can engender? What does life require? How do life and its environments co-evolve? What does a planetary environment have that life needs? How does life use what its environment has, and how does that use change the environment?

For instance, “are plate tectonics and hydrothermal systems essential to the origin of life?” asked astrobiologist John Baross of the University of Washington. “I would say yes.”

Indications are that astrobiologists are well on the way to life detection in terms of both knowledge and know-how but need more of both.

“The reality is that astrobiology is looking for Earth-like life,” Baross observed. But we also need to be ready for a “surprise,” he said.

As to the search for fossil evidence of past life on Mars, Caltech geologist John Grotzinger, former project scientist for NASA’s Mars Science Laboratory mission, said, “Silica is the great material on Earth that survives everything.” So on Mars, we should be looking to silica deposits for possible evidence.

If astrobiologists ever do find evidence of past life on Mars, given that so much material has been exchanged between Earth and Mars over billions of years (in the form of meteorites), then they will have to determine whether it originated independently of Earth life or whether it’s related to Earth life.

The ocean worlds of our solar system – Europa, Titan, Enceladus, Triton, Ganymede, Callisto, and now Pluto – have subsurface liquid water oceans today, said Jet Propulsion Laboratory astrobiologist Kevin Hand. These ocean worlds offer “the prospect of extant life living in an environment that has not been exposed to Earth (or Martian) life.” Europa’s ocean “has been there for most of the history of the solar system.” In Hand’s judgment, “ocean worlds are possibly the best place to search for extant life” of an independent origin.

As to searching for signs of habitability and life beyond our solar system, exoplanet expert Vikki Meadows of the University of Washington said some important questions to address are, “How could we recognize the effects of life on an extrasolar planet? How do we discriminate life processes from the surrounding environment?” How do we determine false positives and false negatives?” For example, it’s not enough to detect oxygen in an atmosphere. We need to be able to distinguish between biotically and abiotically generated oxygen.

Breakout groups at the workshop addressed these questions: How could targeted research over the next 5-10 years help advance the state of the art for life detection, including instrumentation and precursor research? What do astrobiologists already know and have, and what they need, to work on in-situ detection of life as we know it, in-situ detection of life as we don’t know it (a.k.a. “weird life”), remote detection of life as we know it, and remote detection of life as we don’t know it? Good ideas abounded….

How will these good ideas be translated into good science? NASA Astrobiology has a science strategy in place to guide research over the next decade, and astrobiologists have troves of data to work with from past and current planetary missions. NASA’s Cassini mission to the Saturn system – coming to an end soon, after 12 years of operations – discovered erupting geysers and a global subsurface ocean on Enceladus and revealed Titan as a world with hydrocarbon rain, rivers, lakes and seasons. NASA now has a fleet of spacecraft operating at Mars: the Mars Reconnaissance Orbiter, The Mars Exploration Rover Opportunity, MSL, MAVEN, Mars Odyssey. NASA’s Juno spacecraft is at the Jupiter system, and planning is under way for a dedicated mission to Europa. NASA’s Ocean Worlds Program will work on the best ways to look for evidence of habitability and life on those bodies. At the same time, NASA’s NEXSS project – the Nexus for Exoplanet System Science – is bringing astrobiologists and astrophysicists together in a research coordination network dedicated to the study of planetary habitability. The search for signs of life is definitely on….