I’ve followed NASA’s Discovery program for low-cost planetary missions
($450M) since its inception in the early 1990s.
Missions have been proposed 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.
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The northern plains of Mars from NASA’s Phoenix lander. Credit: NASA/JPL-CALTECH/University of Arizona |
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.
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| Artist concept of the IceBreaker lander on Mars. Credit: NASA |
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.)
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| 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. Credit: NASA/JPL-CALTECH/University of Arizona |
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.)
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| NASA’s Phoenix lander found ice immediately below the surface soil. Over the course of several days, the ice sublimated and disappeared from images. Credit: NASA/University of Arizona |
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.
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:
