I’ve followed NASA’s Discovery program for low-cost planetary missions ($450M) since its inception in the early 1990s. Missions have been proposed to study worlds from Mercury to Saturn. In the current competition to select the 13th mission, there’s a new class of mission proposed – missions that would directly seek life, or failing that directly assess the habitability of two worlds that appear to have the ability to support life.
Scientists believe three conditions are needed for life as we know it: liquid water, access to a range of elements and minerals essential for metabolic activity, and energy that can be exploited to power the chemical reactions needed to power life. Currently, only three locations beyond our Earth are believed to combine all of three. Jupiter’s moon Europa appears to have the right combination with a vast ocean under its ice shell, key elements contained in the rocky inner world at the bottom of the ocean, and energy provided by the immense tidal heating created by Jupiter. Saturn’s moon Enceladus similarly appears to have a liquid ocean beneath its icy shell that is in contact with a rocky core, but the source of the energy that keeps its ocean liquid is debated. The third location is the vast northern plains of Mars where a layer of near-surface ice is believed to warm periodically to allow liquid ice that could support life that exploits the chemistry of the surrounding soil.
NASA / JPL-Caltech / University of Arizona
The northern plains of Mars from NASA’s Phoenix lander
The vast oceans of Europa show great promise, but they are locked beneath an icy shell that is at least several kilometers thick. It will take a series of expensive missions to unlock that world’s mysteries.
Enceladus, however, conveniently has active plumes that spew the contents of its ocean into space where a spacecraft can taste its contents simply by flying through the jets. The Enceladus Life Finder mission proposes to search for life in this moon by doing just that. (One observation suggests that at least occasionally Europa may also have plumes, but diligent follow up studies have failed to confirm that finding.) You can read about the Enceladus proposal in a previous post.
Artist’s concept of Mars Icebreaker Life spacecraft
The IceBreaker mission would seek life on the northern plains of Mars. In many ways, this mission is a direct follow on to NASA’s Phoenix mission that previously explored this expanse. The Phoenix mission confirmed that an ice layer lies just a few centimeters below a coating of soil. A class of chemicals, perchlorates, could provide a source of chemical energy to sustain life. As the orientation of Mars’ axis wobbles, the climate changes approximately every 125,000 terrestrial years to allow substantial periods of warmer summers at the Phoenix site – possibly forming liquid water. In between these warm periods, life could hibernate or subsist in films of briny water on soil particles. (A number of terrestrial microbe exist in briny films in extremely cold environments.)
NASA / JPL-Caltech / University of Arizona
Distribution of water in Mars' soil
The distribution of water in the upper layers of Mars’ soil based on gamma ray measurements made from Mars orbit. The northern and southern regions of Mars appear to have substantial ice layers below a covering layer of soil.
Mars, even in on its northern plains, is a harsh word where any life likely survives in extremely marginal conditions. On the Earth, life is so ubiquitous that it literally everywhere and changes the chemistry of our soils, atmosphere, and oceans. Ubiquitous life is unlikely at Mars or we’d have seen its chemical signature. (The presence of trace amounts of methane gas in the atmosphere may be a chemical signature of life but it could also result from abiotic geologic processes.)
A few locations on the Earth, however, are so harsh that life barely hangs on makes few changes to the surrounding chemistry. Two such locations are the extraordinarily dry desserts of the Atacama Desert along the central Pacific coast of South America and the dry valleys of Antarctica. Another habitat is the deep subsurface of the Earth’s crust where metabolic rates are so slow that the microbial organisms may reproduce only every 100 to 1000 years.
The team proposing the IceBreaker mission bets that life arose in the distant, early eons of Mars when the climate was warmer. Or, alternately, life from Earth may have been delivered to Mars in meteorites blasted from the surface of our early world by asteroid strikes. As Mars lost its atmosphere and became dry and cold, the adaptability of life may have allowed it to persist in locations such as the northern plains where the climate periodically becomes mild. The Curiosity rover’s discovery of organic molecules in its currently dry equatorial location provides support for the hypothesis that Mars once supported either life or at least pre-biotic chemistry.
To explore for life, the IceBreaker mission would carry three instruments that perform overlapping measurements to explore the landing site’s chemistry. The key instrument would be a laser desorption mass spectrometer built by a team at NASA’s Goddard Spaceflight Center. Mass spectrometers “weigh” molecules within samples, and the distributions of weights allow scientists to estimate what the original substances were.
Four previous landers – the two Vikings, Phoenix, and Curiosity – have carried mass spectrometers to Mars to search for organic molecules. Their efforts have been skunked, however, by the ubiquitous presence of perchlorates. These molecules form on the surface of Mars when UV radiation cause chlorine and oxygen to react. The resulting family of molecules make good rocket fuels, but are also food sources for many terrestrial microbes.
Unfortunately, when soil samples with perchlorates are heated, the perchlorates chemically react with any organic molecules and destroy them. (To put it more colloquially, the perchlorates and organics burn up together.) All the mass spectrometers delivered to Mars to date have attempted to use heat to bake the organics out of the soils so they could be measured, which triggered the reaction with perchlorates that destroyed any organics in the samples. (The detection of organics at the Curiosity site by its mass spectrometer is the result of clever instrument manipulation to work around the perchlorate problem.)
The European 2018 ExoMars rover will use the first mass spectrometer sent to Mars designed to work around this perchlorate trap. Its mass spectrometer, built by the Goddard team, will use a laser to vaporize tiny portions of samples to release organic molecules without triggering a reaction with the perchlorates. The IceBreaker lander would carry a copy of this mass spectrometer.
A second instrument, the Signs of Life Detector (SOLID) would search for a several dozen complex organic molecules that are key to the functioning of the simplest unicellular organisms on Earth. These molecules are essential to storing genetic information, performing basic metabolic functions, and building cellular structures. If any of these molecules are present, then they will react with antibodies within the instrument’s sample slides. A laser beam will scan the slides, and any positive reactions will result in a bright glow detected by an imager. The position of the glow(s) on the slides would indicate which molecules are present.
The operation of these two instruments would be synergistic. An analogy helps explain how they would work together. Imagine that you wanted to determine whether a possible race of Martians were bakers making breads, cakes, tortillas, etc. The mass spectrometer would be the generalist instrument that could distinguish whether the building blocks in food samples were for a meat, vegetable, or a baked good. You might suspect that the Martians might make baked goods similar to those on Earth – if a recipe worked in one place it might work well in another. The SOLID instrument would detect if specific baked good – cupcakes or bread -- were present. If the instrument detected one or more of these baked goods, you can be pretty certain that bakers exist on Mars. Of course, Martian cooks might invent their own unique families of baked goods without terrestrial equivalents. Then the mass spectrometer could suggest the presence of baked goods by detecting the basic ingredients necessary.
The IceBreaker’s third instrument would be an updated version of Phoenix’s Wet Chemistry Laboratory. As the name implies, this instrument would add water to soil samples and then determine what chemicals in the soil would be available for use by any life. The availability soluble forms of the building blocks of life as we know it – carbon, hydrogen, nitrogen, oxygen, and sulfur – as well as oxidants that can serve as energy sources would imply a habitable site. (The Phoenix version of this instrument discovered the presence of perchlorates.)
NASA / JPL-Caltech / University of Arizona / Texas A&M University
Phoenix finds ice
NASA’s Phoenix lander found ice immediately below the surface soil. Over the course of several days, the ice sublimated and disappeared from images.
The lander’s drill would allow scientists to explore the relationship between the periodic swings in climate on the northern plains and both its habitability and the preservation of the organic biosignatures of life. Over time, the axis of Mars wobbles – the technical term is precession – much as the axis of a top does. For our home world, the gravity of our large moon dampens the precession. Mars lacks a stabilizing moon and the tilt of its axis can swing chaotically from 0 to 60 degrees (it is currently 25 degrees, almost the same as the Earth). The result is that the climate on the northern plains can range from frigidly cold to being able to melt the layer of ice beneath the coating soil down to depths of a meter or more.
Any life present in the northern plains presumably would thrive during the periodic warm spells (estimated to occur about every 125,000 years in the current epoch). As the climate cools, the ice refreezes, locking any biosignatures in place. Then two processes work to destroy any organic material. Ultraviolet light would destroy any organics immediately on the surface, and as the covering soil layer is churned, much of it is likely cleansed of any signs of life. Deeper down in the ice layer, cosmic radiation penetrates to slowly destroy organics.
The IceBreaker lander would carry a drill that could bring material to the surface from as deep as one meter. It will provide samples of both the soil and the ice at different depths. Shallower ice would have melted during moderate climate warming but also would lose its organics more quickly to the accumulated damage from cosmic radiation. Deeper ice would represent periods when the warmest climates melted the buried ice and would also protect organics longer. By taking samples at different depths, the mission’s scientists would be exploring the questions of life, habitability, and the preservation of biosignatures across time.
NASA / JPL-Caltech
Subsurface Structure of Enceladus’ Plumes
The Enceladus Life Finder would sample the gases and ices emitted by the plumes of this moon, which are believed to come from an ocean beneath the icy shell.
As I mentioned at the beginning of this post, IceBreaker is one of two proposed missions that would directly search for life. The other, the Enceladus Life Finder (ELF), would fly through the plumes that are venting the contents of that moon’s ocean into space. While the specifics of these two missions are very different given the differences in the worlds they would explore, the scientific approach would be similar. The ELF mission also would carry a mass spectrometer that would directly measure the presence of any complex organic molecules that would suggest the presence of life. This mass spectrometer plus a second one designed to measure the composition of dust particles would explore habitability of Enceladus’ interior ocean by looking for key resources. The combined measurements also would help scientists better understand the conditions within the moon. (The Cassini orbiter at Saturn carries versions of these two instruments, but the ELF’s instruments would be far more sensitive.) An optional third instrument would, like the IceBreaker SOLID instrument, analyze the plumes for more complex organic molecules, although the method (microchip capillary electrophoresis) would be quite different.
I don’t envy the teams of scientists evaluating the Discovery proposals for NASA. How do you, for example, decide that the science that can be performed by a radar mapper of Venus is more important than long term telescopic observations of the bodies of the outer solar system or the exploration of a class of asteroids that has never previously been visited (to mention just a sample of the diversity of missions in the current competition). If either IceBreaker or ELF found solid evidence for life, then it would among the top discoveries in human history. But what if they don’t find life or the results are ambiguous? How would their potential results minus a discovery of life rank against those of other missions? Those review panels have a tough job.
Both IceBreaker and ELF have their own unique challenges, too. The IceBreaker mission would use the same lander design as the Phoenix and upcoming Mars InSight missions, so it would seem to me that it has a high probability of fitting within the budget cap ($450M) for Discovery missions. However, when the InSight mission was selected as the previous Discovery mission, there was serious grumbling in the planetary science community about NASA having too much of a focus on Mars since the 2020 Mars rover mission had just been selected. Selecting a third Mars mission in a row may present a political problem. The ELF proposal, on the other hand, has the opposite problem. No Discovery mission has flown past the asteroid belt. Its proposal team has the challenge of convincing NASA that a mission to the Saturn system can be done within the cost cap.
Evaluating these missions will fall to review teams and NASA managers with the expertise to make the tough calls. (My favorite proposals reflect my personal and idiosyncratic preferences about which worlds most interest me.) This time, they have two choices for missions to directly search for life. At least one, IceBreaker, has a high probability of being able to fit into the Discovery program.
Betting on the fortunes of any of the 28 Discovery proposals is a risky proposition. The message I take from IceBreaker and ELF is that we know where and how to directly search for life. Whether either proposal prevails in this competition, I expect that in the next decade or so a mission will be selected to taste the ices of the northern plains of Mars or Enceladus for the signature of life.
While many teams proposing missions for the Discovery program are reluctant to say much about their proposals, the IceBreaker team has been more open than any other team that I recall. Do an internet search for 'Mars IceBreaker' and a number of documents, many of them extended abstracts for conferences, will come up. Here are two links you might start with: