Analog SFF, March 2008

by Dell Magazine Authors

  We know the ice is flowing, but the detailed physics is unknown because we don't know precisely what Martian ice contains. Even if we did know, field studies would be essential. On Earth extrapolation from laboratory simulations to the low strain rates found in the field is problematic, a difficulty orders of magnitude more challenging on cold, low-gravity Mars, where deformation rates are very low.

  We'll also be looking for evidence of liquid water, which could harbor life. There could be basal melting. A flaw in the cap called the Chasma Boreale perhaps results from a catastrophic outflow of basal water. Even on the surface, liquid water could be found. We'll look for evidence like meltwater features, ice fogs, volatile anomalies. And we'll look for microscale near-surface environments, droplets trapped under bits of rock: the crew will perform a “fingertip search."

  As for the weather, the poles are at the heart of the planetary carbon dioxide cycle, through its snowing-out through the winter in each hemisphere. This cycle is coupled to the dust and water cycles, and to atmospheric heat transport. The poles are therefore pivotal to the global climatic system. And so catching a Martian snowflake in a spacesuit hand will teach planetary lessons, for it is a remarkable fact that Mars's global cycles of dust, carbon dioxide, and water all intersect in the formation of each flake.

  We will be monitoring the polar climate at multiple locations for extended periods (several complete seasons). Human subjective observations of transient events, colors, cloud species, snow textures, and other features will be a value-add. During excursions we'll be gathering climatological and glaciological data continually.

  I got the chance to design a couple of equipment packages to support science on excursions, which I called the Wells-REP, the Wells Rover Exploration Package, and the WellsSEP, the Wells Surface Exploration Package. (The names are in honor of H. G. Wells, whose tripedal “war machines” bear a passing resemblance to a WellsSEP....) The designs were based on experience gained from robot landers.

  The WellsREP is a rough cylinder you could cradle in your arms. It rides on the outer hull of the rover. At an investigation station, it is picked up by a manipulator arm and applied to surface samples of interest. The idea is to allow a close-up inspection of surface samples without requiring a full EVA, or the import of samples into the interior. It is based on a cut-down Beagle 2 PAW (position-adjustable workbench); the mass is only a couple of kilograms. The key instruments are an ice core grinder, a microscope, an X-ray spectrometer to examine elemental abundance, and a Mossbauer spectrometer for analysis of iron compounds.

  The WellsSEP (see the figure) is a package of seismometry and climatology instruments named by analogy with the Apollo Lunar Surface Exploration Package (ALSEP). The idea is we'd establish a network of WellsSEPs around Pole Station and during excursions. Powered by a radiothermal generator, each WellsSEP is intended to sample its environment through at least two full Martian years, communicating with Pole Station via orbital comms systems. As part of an extended seismometry network, the WellsSEPs will contribute to a deep mapping of the internal structure of the permanent ice cap.

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  Figure: The WellsSEP fully deployed. Figure copyright Bob Parkinson

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  The WellsSEP when set up is a tripod whose head contains a science package positioned at a height of four meters above the ice surface. The legs are jointed, with eight segments each, to be unfolded and positioned by astronauts on the surface. The tripod is designed to be stable in Martian winds up to 10 m/s. You need the height, as the science instruments should be positioned above the winter snow, which can lie two meters deep. Also there can be significant climatic variations over this scale; close to the surface a drop in temperature of several degrees per meter is common, and thermistors and anemometers are installed along one tripod leg to monitor these variations. The instruments include a seismometer, a meteorology package, a gas analysis package based on a mass spectrometer, photometers to measure sky brightness, a UV sensor, a dust impact sensor, and a camera.

  The figure (prepared by Bob Parkinson) gives you an idea of the depth we went to in these studies. Throughout the exercise we made quite conservative assumptions about technology advances; we were aiming for plausibility rather than prediction. But by the nominal mission date, miniaturization trends can be anticipated to have delivered powerful packages with an economy of size, mass, and power requirements—"labs on a chip."

  All these activities will yield invaluable results. But the real reason to send humans to the north pole of Mars is to extract an ice core.

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  Ice Cores on Mars

  On Earth, ice cores have yielded climate records with an accuracy of a year reaching back some 100,000 years into the past. Can we achieve similar successes on Mars? There seems every prospect. Some workers have already found a correlation between Mars's obliquity changes and the thickness of strata visible to Mars Global Surveyor.

  Martian ice, however, is deposited by different physical processes on a different world. And the first thing to recognize is that a Martian core will be a lot more compressed than anything on Earth.

  Just as on Earth, Mars's permanent north pole ice cap has been built up layer by layer through the annual deposition of snow. On Mars, however, the principal component of the snow is carbon dioxide ice. When the spring sun sublimes away the carbon dioxide you're left with a residue of water ice, a mere one seventh of a millimeter per year. That compares to Greenland, say, where you get an annual (water-ice) snowfall tens of centimeters thick. The layers thin further through compression and ice flow. Dating these fine layers with the precision achieved on Earth may be impossible. However the thinness of the layers offers the prospect of extracting records covering significant periods of time from comparatively shallow cores—a million years in a hundred meters.

  A further complication is ice flow, which can mix up the layers. But because of the deeper cold and lower gravity, ice flow is much slower than on Earth. And the pole itself, at the summit of the cap's parabolic profile, should be a stationary point in the flow, and may be an optimal location for coring into undisturbed layers.

  Given your ice core, how do you interpret it?

  On Earth you extract past temperature records from ice strata by using a “temperature proxy,” some measurable property that depends on the temperature when the ice was formed. In practice, ratios of isotopes, notably O-18 to O-16, and H-2 to H-1, are measured. It takes more energy to evaporate water molecules with heavy isotopes from the ocean surface, so less heavy isotopes are deposited in cold periods than during warm periods.

  Perhaps a similar temperature proxy can be established for Martian ices. The water-ice which is snowed out on Mars is not, of course, lost from an ocean by evaporation but through sublimation of exposed ice, at the summer pole and from low-latitude frost and snow residue: different physics will mandate a different proxy. Alternatively a temperature proxy based on an entirely different process may be established. As the saying goes, this is a significant area for further research.

  As regarding dating, the absolute dating of ice layers will be essential for a full stratigraphic understanding. We don't know how to do this on Mars. Methods may include the study of beryllium isotopes from known supernova events, volcanic ash dating, and luminescence dating.

  Ice core records on Earth are also correlated against independent records of historically dated events, such as volcanic eruptions, or global climatic events dated through techniques like tree-ring analysis. We humans have left global signatures too, including “nuclear horizons,” fall-out from massive atmospheric thermonuclear weapons tests of known dates. The lack of such correlative signposts will hinder the dating of Martian ice columns.

  There are, however, some significant signposts, which may be used to establish at least a skeleton timescale. Given a surface deposition rate of 0.14mm per year:

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  Scale = 1cm (decades) Major dust storms, as observed historically,
should leave traces in the ice column. The global dust storm of 1971 (Mariner 9) would leave a layer 9mm below the ice surface (by the year 2038), with the astronomically observed storm of 1956 a little deeper. There were only ten “planet-circling” events between 1873 and 1996, while only the 1971 storm was truly global.

  Scale = 10cm (centuries-millennia) Major energetic cosmological events such as supernovae or gamma-ray bursters, known from history or Earth-based astronomy, would leave spikes in the population of cosmogenic relics. The Crab supernova of 1054 would leave traces 14cm deep.

  Scale =1m(tens of millennia) An impact event on Mars just large enough to create a global microtektite deposit should occur once every ten to a hundred thousand years, so leaving layers some 1-10m apart. The distribution of tektites might give some indication of the distance to the originating crater, which if found could provide a dating calibration. Meanwhile the tektites’ composition could yield some indication of the impactor's nature. Conversely the ice column should yield a record of varying impact rates through Mars's history.

  Scale = 10m(hundreds of millennia) The planet's obliquity oscillates with a period of 120,000 years, with amplitude variations on a period of 1.6 million years. When obliquity is low, massive permanent polar caps of carbon dioxide may form. Globally the air will be thin and clear and comparatively dust-free: there may be no global dust storms. When obliquity is high, the air will be comparatively thick and dust-laden. Thus “clean” ice is deposited during times of low obliquity, and “dirty” ice in epochs of high obliquity, so there should be a layering in the embedded dust about 10m thick. This layering may already have been observed by orbiters.

  Scale = 100m (megayears) Recent discoveries made by the Mars Express orbiter indicate that there may have been flowing water on the surface of Mars in episodes as recently as a few million years ago. If so, salty deposits evaporated from the transient water bodies’ surfaces and scavenged from the air at the poles may be detectable at these characteristic depths.

  Scale = km(deep time) Extra-Martian meteorites would result from significant impact events on the terrestrial planets. Relics of known impact events on Earth discovered as meteorites in the Martian ice column might allow comparative dating. However traces of the Chicxulub dinosaur-killer asteroid strike—bits of the Caribbean sea floor blasted to Mars—may be detectable only in the very deepest layers of the ice.

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  The extraction and interpretation of an ice core is central to the human rationale of Project Boreas. But there are several doctorates-worth of further work to be done before this sort of study becomes a reality:

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  We have no accepted technique for the absolute dating of layers of Martian ice cores.

  We have no temperature proxy for Martian ice.

  We have at present no database of comparative studies (like tree rings on Earth) against which to interpret Martian ice core results.

  We could make progress on a lot of these issues with Earth-bound studies even before we get to Mars.

  Martian ice cores offer huge scientific returns, then. But the challenge of extracting them is going to be significant.

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  Drilling on Mars

  Ice core drilling on Mars has been studied since a remarkable 1977 proposal to use Viking-class technology to return a core sample of Martian polar ice (see R.L. Staehle et al, “Mars Polar Ice Sample Return Mission,” Spaceflight Nov. 1976, Nov. 1977, Dec. 1977). And it's been done in fiction: Chinese and American astronauts drilled in search of life at the Martian north pole in The Secret of Life by Paul McAuley (2001).

  You can consider going to shallow depths (meters), medium depth (a hundred meters or so), or deep (a kilometer or more—the Martian north pole cap is three kilometers deep). Shallow-depth low-power drilling has been studied in the context of robotic explorers. Beagle 2 (2003) carried a shallow drill. During the Apollo missions, core samples of 1-2m were taken with hand-driven boring devices.

  On Earth you can buy medium-depth drilling rigs off the shelf, weighing a couple of hundred kilograms and needing a couple of kilowatts of power. You can load such rigs in the back of a truck and assemble them on site. NASA's Astrobiology Technology and Instrument Development Program has been trialing a rotary drill system in the Canadian High Arctic.

  Deep drilling, however, is orders of magnitude more challenging in terms of mass, power, and manpower. The “GISP2” project (Greenland Ice Sheet Project 2), initiated by the Office of Polar Programs under the U. S. National Science Foundation in 1988, drilled through the Greenland ice cap summit to a depth of some 3km. GISP2 was a five-year project involving fifty people on-site. Heavy lifting was provided by the U. S. 109th Air National Guard. It's going to be tough assembling such resources on Mars.

  The most useful study on Martian deep drilling was presented at the Mars Society's first convention by Frankie et al ("Drilling Operations to Support Human Mars Missions,” in Proceedings of the Founding Convention of the Mars Society, ed. R. Zubrin et al, San Diego 1998 [MAR 98-061]). This used NASA Design Reference Mission technology as then defined, with the drill rig built into a dedicated lander, and liquid carbon dioxide extraction equipment (the liquid is used as a working fluid—see below). Frankie's study was in the context of drilling through rock in search of liquid-water aquifers, but it is applicable to ice drilling because, as Frankie notes, “Permafrost at Martian temperatures is as hard as basalt and as sharp as glass."

  In trying to come up with a feasible design for Boreas, we were faced with a series of tough choices. How are you going to do the drilling in the first place? Rotary drilling is familiar, relatively low power, mechanically simple, and easily fixed in case of failure. But it requires a heavy support infrastructure, and in the dusty, cold, high-friction Martian environment any moving-part system would be vulnerable to many failure modes—lubrication failures, abrasion of bearings, metal fatigue, loss of seal integrity. But “advanced” drilling methods—such as the use of lasers, electron beams, and microwave jets—are power hungry, perhaps three to five times as much as traditional techniques.

  Though shallow-depth and medium-depth drills can be run dry, a deep borehole will require stabilization, for it would otherwise collapse because of the weight of the ice. The way this is done on Earth is to pump in a “working fluid,” such as water or mud slurry. But if contamination is an issue, an inert hydrocarbon may be used; GISP2 used butyl acetate. Water or mud is not going to work in Martian conditions. Possibly some low-temperature vacuum grease or lubricant oil would be suitable, but mass constraints prohibit importing fluid from Earth: a Lake Vostok bore utilized fifty tonnes of kerosene, for example. If lost, such a fluid load could not be replaced.

  Can working fluids be produced from local materials? Frankie argued for the use of liquid carbon dioxide as a working fluid, extracted through ISRU. Carbon dioxide is the only readily available fluid, has low viscosity, and has the right thermal properties in Martian conditions, though pressure would have to be maintained in the borehole. Though there would be a cost in extracting, liquefying, and pumping the carbon dioxide, this minimizes the weight of fluid to be transported to Mars, and it is replaceable if lost. But carbon dioxide plus liquid water yields corrosive carbonic acid. You would have to keep temperatures low enough throughout the borehole that ice chips do not melt, which will affect drilling rates.

  In the end, such considerations simply broke our budgets. Frankie's design would have weighed thirty tonnes and eaten five hundred kilowatts; the total landed mass for our study was to be just over sixty tonnes. And whereas GISP2 had fifty people on site, we will have a maximum of ten astronauts, with perhaps two guys available to drill at any one time, in spacesuits, working under the constraints of planetary protection protocols. We just couldn't do it; we had to compromise.

  So we settled on a medium-depth drill, capable of reaching to depths of a couple hundred meters or so. At such depths we can run dry, without a working fluid.

>   As it happens this doesn't wreck our science objectives. Because of the lower deposition rate on Mars, even a hundred-meter core would reach back deep in time, covering several precessional cycles. We wouldn't be able to sample the deepest and oldest layers, perhaps including any trapped bodies of water, any basal melting, and the lithography of the sub-cap rock. But the ice core results could be complemented by other studies, such as sampling exposed strata in the walls of the spiral canyons, visited during EVA excursions. Of course for this to work you would need to establish a reliable stratigraphy for the polar deposits.

  The drill would be small enough to pack on the back of a rover trailer, run off a rover power supply, and to be operated by a two-person team. We will use rotary technology to drill a dry hole into the Martian ice. A core of diameter 82mm is taken to a depth of 250m. The winch tower is 2.65m high. The system's total mass is about 250kg. This includes a winch with a 250m cable, a drill sonde, a containing tent, and science processing equipment. The total power requirement is about 4kW. The core is removed in sections 2m long by tipping the winch tower to a horizontal position and extracting the inner barrel containing the core and ice chips. Each section is transferred to a science processing fold-out workstation, large enough for one core section at a time to be handled, sampled, and analyzed.

  All operations, including drilling and sample processing, take place within a “tent,” a hemispheric dome supported by inflatable struts. The dome is there for environmental protection from such hazards as dust, carbon dioxide snow, and UV degradation, and for planetary protection.