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Of Ice and the Planetesimal

Mindful of the recent work on axial tilt I’ve reported in these pages, I was interested to learn that Vesta’s axial tilt is just a bit greater than the Earth’s, about 27 degrees. We’ve been pondering the consequences of such obliquity on planets in the habitable zone, but in Vesta’s case, the issue isn’t habitability but water ice. For spurred by the Dawn mission, scientists are looking at whether permanently shadowed craters on the asteroid’s surface would allow water to stay frozen all year long. Unlike the situation on the Moon, the answer on Vesta (on the surface at least) seems to be no.

Earth’s axial tilt is 23.5 degrees, but the Moon’s is a scant 1.5 degrees, making the shadow in some lunar craters permanent, a fact that has led to speculation that ice in these locations could be of use to future manned missions there. In contrast, Vesta’s obliquity means that it has seasons, so that every part of the surface becomes exposed to sunlight at some point in the year. Even so, says Timothy Stubbs (NASA GSFC), “Near the north and south poles, the conditions appear to be favorable for water ice to exist beneath the surface” [italics mine].

Image: New modeling shows that, under present conditions, Vesta’s polar regions are cold enough (less than about 145 K) to sustain water ice for billions of years, as this map of average surface temperature around the asteroid’s south pole indicates. (The white dashed line marks Vesta’s south polar circle.) Figure reprinted from the paper in Icarus by T.J. Stubbs, T.J. and Y. Wang. Image credit: NASA/GSFC/UMBC.

The polar regions on Vesta are under scrutiny as the Dawn mission continues its close look. Observations from the Earth have suggested a bone-dry Vesta, but in its current low orbit, Dawn is using its gamma ray and neutron detector (GRaND) spectrometer to look for hydrogen-rich deposits that might flag the presence of water ice below the surface. Because Dawn’s targets — Vesta and Ceres — are both considered remnant protoplanets, it will be important to learn whether there is sub-surface water on Vesta. The next issue to ponder will be whether any water that is discovered has arrived recently or goes back to the earliest days of the Solar System.

Modeling has shown that water ice should be able to survive in the top few meters of regolith when surface temperatures are less than 145 K or so, and while Vesta’s equator sees average yearly temperatures around 150 K — too warm to sustain water ice within meters of the surface — the regions near the asteroid’s north and south poles are cold enough to allow its survival for billions of years. The temperature dividing lines show up at about 27 degrees north latitude and 27 degrees south latitude.

As for the surface, this JPL news release quotes Stubbs on the matter:

“The bottoms of some craters could be cold enough on average — about 100 kelvins — for water to be able to survive on the surface for much of the Vestan year [about 3.6 years on Earth]. Although, at some point during the summer, enough sunlight would shine in to make the water leave the surface and either be lost or perhaps redeposit somewhere else.”

Ceres, the second of Dawn’s destinations, should prove a study in contrasts. As compared with Vesta’s dry surface, Ceres may have seasonal polar caps of water frost and even a thin atmosphere. Some models show a layer of 100 kilometers of icy material including water and ammonia, and allow the presence of liquid water beneath. Recall that this is the most massive body in the main asteroid belt, making up perhaps as much as a third of the mass of the entire main belt. Hubble observations have suggested that the topography here is low, with a lack of deep craters that indicates flow in the crust to even out the landscape. Dawn will be able to give us better estimates of Ceres’ mass and help us understand how that mass is distributed.

The paper on Vesta is Stubbs and Wang, “Illumination conditions at the Asteroid 4 Vesta: Implications for the presence of water ice,” Icarus Vol. 217, Issue 1, pp. 272-276 (January, 2012).


Comments on this entry are closed.

  • Nick February 10, 2012, 17:56

    Those are nice temperatures for ice: so low it practically ceases sublimating in vacuum. I now wouldn’t be too surprised to find a bit of underground ice in the polar regions of Vesta. Earth’s moon looked utterly bone dry, too, until we took a much closer look. It would be nice if we could slam a meteor or two into these mostly shadowed areas while Dawn is there to look at what they dig up. Maybe next mission…

    I assume Dawn’s orbit at Ceres will also take it over the polar regions?

  • Rob Henry February 10, 2012, 22:21

    Axial tilt is usually given wrt orbital plane, which for moons is not a line drawn from the Sun. If our moon’s tilt really is 1.5 degrees from the Sun, it is far from Earth’s tilt and wouldn’t that make its polar regions MORE susceptible to epochs of higher insolation. A typical tilt of at least 20 deg wrt the Earth (which has 200 times as much tidal influence on it as the sun) might cancel out most of this tilt today, but it would exasperate it yesterday and tomorrow.

    And how can there be a greater expectation of frost than atmosphere on Ceres as this article implies? cryovolcanism??

  • jkittle February 11, 2012, 20:01

    if we leave out the possibility of teraforming, would ceres make a better site for a base then mars? it has water, still close enough to the sun to use solar panels, no dust storms and very low delta vee to land and take off. it is probably easier to build structures our of ice and plastics on ceres than out of dust and iron on mars, but i am not sure the trade offs here.

  • Nick February 11, 2012, 23:24

    “would ceres make a better site for a base then mars? it has water, still close enough to the sun to use solar panels, no dust storms and very low delta vee to land and take off. it is probably easier to build structures our of ice and plastics on ceres than out of dust and iron on mars.”

    Short answer: yes. It is much easier to make structures out of ice than out of dust and iron, and it probably also makes a much better propellant. (Plastics will be quite a bit more difficult).

    Slightly longer answer: machines will be small and automated enough that we can have industry in both places. Which will come first depends on what is currently unknown: what resources exist in each place that could be exported to earth. However even if we discover, say, gold or diamonds on Mars, Ceres will make a dandy source of propellant for shipping those goods back to Earth.

  • Ole Burde February 12, 2012, 11:50

    The best place for a real base (meaning an industrial base) must be in orbit around the moon , IF enough water can be found in the polar shadows . In the O’Neil scenario , raw materials are launched to such a base from the moon in railgun-buckets , using almost no input of earth materials . A small but permanent and close to selfsufficient moonbase in the polar area , is therefore the first step to any realistic atempt at a bigger and more allround capable base orbiting the moon ..

  • Eniac February 12, 2012, 18:40


    …It is much easier to make structures out of ice than out of dust and iron…

    I am not so sure. Dust makes brick and iron makes steel, both excellent materials with a long and venerable pre-hi-tech history.

  • Nick February 12, 2012, 22:58

    “Dust makes brick”

    Not very easily. Here on earth certain clays, wet with liquid water and infused with organic molecules and hydrated minerals, are fired to make bricks.

    “Iron makes steel”

    Via a complicated, hot, and energy intensive processes. These processes requires such an extensive supply chain that iron was far too expensive for structure until the industrial revolution, which involved a great elaboration in the division of labor. Granted, the meteoric iron found in some asteroids will make the job much easier (vs. starting with the iron oxide ores on Earth and Mars) but even then one has to at least melt the iron and process the melt into the desire shapes, which is far more difficult (and has much less mass throughput ratio, i.e. [mass output/unit time]/mass of equipment) than the corresponding processes for water ice. Also, water ice makes a better radiation shield, and water + organics + ammonia, not iron, are the building blocks of most of the rest of the economy.

    Of course, iron makes a better structural member, and sinter ed bricks will likely come in handy, so in the longer term we will be doing all of the above. Just that water is easier and more important and so will happen sooner.

  • jkittle February 13, 2012, 0:41

    Well it costs only about 1% of the propellant to get from the moon to lunar orbit compared with getting from earth to LEO. there is likely plenty of ice on the moon
    but it only makes sense if the overall goal of all of this is to explore, water and energy is the best commodities in space orbit,
    by the way it is relatively easy to make bio plastics if you can make methane, ( or hydrogen and use carbon dioxide) though not price competitive with $2 a gallon petroleum on earth ( @ ~$110 per barrel)

  • FrankH February 13, 2012, 16:07

    Ceres is too small and warm (230K typ. surface temp) to hold on to a significant permanent atmosphere; it’s a tiny world. It would be easier than Mars to land on ( and take off) but it’ll take longer to get there.
    We’ll have to wait until Dawn gets there to see how accessible the water ice is.
    There’s a huge difference between hydrated minerals (which may be abundant on Ceres) and easily extracted water.

  • Eniac February 14, 2012, 1:10

    Not very easily. Here on earth certain clays, wet with liquid water and infused with organic molecules and hydrated minerals, are fired to make bricks.

    Firing is just another word for sintering, and the water is there only to keep things together and allow a stable shape to be formed before firing. Another way to form shapes needs to be found, but is probably as easy as compacting the dust into a mold under high pressure.

  • wade cooper February 20, 2012, 23:50

    If we ever hope to terraform a planet like Mars we will need a tremendous water supply such as vesta or Ceres have.
    . This would mean analyzing the core and water content and then using long term thrusters, such as a nuclear steam engine planted on the asteroid or planetoid and nudging it’s trajectory until it can be made to crash into Mars. In this way the atmosphere could be made thicker with water vapor and oxygen, the planet would become larger by a small fraction and would sustain heat damage from the impact, warming it slightly.
    . After the dust settles a new planet would be awaiting the terraforming process. Without this added water/ice, I feel we would never succeed in terraforming Mars, or any similar body.