Cameron Park (SPX) Jul 02, 2007
As the "Dawn" mission -- the first attempt to explore truly large asteroids -- moves, somewhat uncertainly, toward its hoped-for July 7 launch, we're still finding out new details about its mission. The mission's principal Investigator, C.T. Russell, outlined two of them to SpaceDaily in a recent interview.
(1) It's been assumed for some time that Dawn -- in addition to orbiting both of the two most massive asteroids, Ceres and Vesta -- would try to fly by a number of others while en route to its two main targets.
Its gamma ray and neutron spectrometer would be useless during such flybys -- it wouldn't have nearly enough time to acquire any meaningful count of gamma rays or neutrons given off by the asteroid. But its other two instruments -- its multispectral "Framing Camera" package, and its Visible and Infrared Mapping Spectrometer to look for minerals and rock types -- would be highly useful.
The possibility that it might fly by as many as a dozen more asteroids has been mentioned -- and in his 2004 article on the mission ("Dawn: A Journey in Space and Time"), Russell listed four possible candidates in a table.
Three of them, however, were tiny asteroids -- with diameters of only two to five kilometers. Only one possibility, 197 Arete, was of any major size (30 km diameter). And this was at a time when the Dawn mission was somewhat more ambitious than it now is -- a magnetometer planned for it has since been removed to avoid mission cost overruns, and its official observation time at each major asteroid has been trimmed from 11 months down to only 7 months for Vesta and five for Ceres (although, since it will still be orbiting Ceres at the end of its primary mission, it could continue observing Ceres as long as it continues to operate).
Also, its launch was delayed by a year, and its flight path radically changed -- so none of those four listed asteroid flybys is possible anymore, and different ones must be chosen.
I asked Dr. Russell just what candidates for asteroid flybys en route to its two major destinations are now being considered for Dawn. He replied that -- just as when I had asked him that question earlier, when the mission still had its original flight plan -- there is still a great deal of flexibility in such plans, and that in fact the flexibility will remain large for a long time after this mission is actually launched, because of its very long-term thrusting with its ion engines:
"We will not decide until we know what trajectory we are on. An ion-propulsion mission is intrinsically different from a chemically propelled mission. The chemical trajectory is like a billiard shot -- [a rigidly preplanned trajectory] off two of the sides and into the pocket. Ion propulsion is like playing badminton in the wind. It is the difference between a businessman flying from LA to NY, and a retired couple driving across the country.
"Every day in our launch opportunity...our optimum trajectories differ, AND the optimum trajectory [also] depends on exactly what is the [electric] output of the solar array [that powers Dawn's ion engines]. So when we get into space -- and precisely how the spacecraft operates after launch -- will have to be factored into any decision." He had previously said that they would like, if possible, to arrange for Dawn to fly by "an example of each principal asteroid [compositional] type", as we have identified them by Earth-based near-infrared spectra -- but it remains to be seen whether they can actually pull that off.
He added: "We have examined flyby opportunities many times in the past; and what we find in general is that there are few opportunities on the way to Vesta, with many more between Vesta and Ceres. Since we will also know our fuel reserves and our power situation much better [by the time of our period in orbit around Vesta], then we may just postpone any planning until post-Vesta." He also confirmed, however, that virtually all the candidate asteroids for flybys that they're now considering are still quite small.
(2) The plan is for Dawn to progressively enter lower and lower orbits around each asteroid -- and, in the original plan, the lowest-altitude orbits could be low indeed: as low as 15 km above some parts of the surface of Vesta, and 40 km above Ceres.
The scaled-down plan calls for it to never get closer than 170 km to Vesta, and 700 km to Ceres. The main reason for Dawn not ultimately getting as close to Vesta as had been planned is simply that the new flight plan gives them less time -- and xenon maneuvering propellant -- to explore Vesta at different altitudes than they had hoped.
But the time problem doesn't apply to Ceres -- the reason for the radical change in Dawn's minimal altitude around that world is due to a new factor that shows, once again, how suddenly more optimistic about life on other worlds astrobiologists have become over the last 15 years. To be precise, the possibility of Ceres as an abode of life is no longer being absolutely ruled out.
The biggest single scientific puzzle about the asteroids is how wildly differently the small protoplanets out of which they formed were heated during their initial formation, even when those protoplanets must have varied only modestly in their distance from the Sun.
(Most of today's asteroids are only the irregular fragments left when this rather small number of large initial protoplanets collided; but a few of the larger asteroids -- especially Ceres, Vesta, and Pallas -- are believed to be leftover original protoplanets that survived that early period of general smash-up.)
There is a general tendency for the asteroids in the inner parts of the Belt to be made out of "silicaceous" rocks -- similar to the silicate rocks that make up the inner planets -- that must have undergone very considerable heating while the asteroids were forming (Vesta is the most extreme case of this, which is why it has long been considered the most scientifically interesting of all the asteroids.
Despite the fact that it's a tiny world only 530 km wide, something generated enough intense excess heat in its interior for it to have actually undergone surface volcanic eruptions that spilled floods of basalt lava across a large part of its surface.)
The outer parts of the Belt, by contrast, are dominated by asteroids made out of "carbonaceous chondrite" rock that can never have been exposed to such high temperatures -- indeed, many of them seem to have been substantially chemically modified by large amounts of liquid water that must have come from ice that collected inside them when they first formed,. (Recently, at least three small asteroids have been discovered that actually seem to still be emitting jets of gaseous vapor from ices frozen inside them, like comets.)
But the difference in distance from the Sun of the asteroids that fall into these two general categories is not all that dramatic, and indeed the asteroids made out of fragments of these two types of original protoplanets have been very substantially intermixed in their orbits over the eons -- the relationship between their solar distance and their type is only a general one.
There are at least two general ideas proposed for what could have heated some of these little protoplaneary worlds so dramatically. One is that -- during the short "T-Tauri" period after the sun's fusion process first turned on, when it was flinging out a solar wind thousands of times more forceful than it has ever done since -- the magnetic field carried by that solar wind generated powerful electrical "induction currents" inside asteroids as it swept past them, greatly heating them.
The other flows from the fact that all of the Solar System shows strong evidence of having been bombarded during its initial formation by extremely powerful radiation from a nearby supernova explosion -- whose shock wave may very well have been what formed our Solar System in the first place, by compressing an initial rarified cloud of gas and dust enough for the Sun and planets to start gravitationally congealing out of localized higher-density regions within it -- and that radiation apparently caused the formation within it of large amounts of aluminum-26 and iron-60, two radioisotopes that have half-lives of only a few hundred thousand years and so give off intense heat during their decay back into stable isotopes, providing all the forming worlds of the Solar System with a brief initial burst of intense internal heat that they would never have gotten otherwise.
But there are still serious problems working out in detail how either of these possible forces could have caused such vast differences in internal heating among the asteroids.
Whatever the cause, Ceres is one of the "carbonaceous" asteroids, and not all that unusual among them in its apparent surface composition -- which is why, despite its size, it's always been a considerably lower-priority asteroid for specific study than Vesta.
But it remains by far the biggest of the remaining asteroids -- at 1000 km diameter, it contains, by itself, about a third of the mass of the entire Asteroid Belt -- and so even the modest excess of heat generated within its interior during its earliest days was rather efficiently trapped there instead of radiating quickly into space (as would have happened had it been a smaller object, thus allowing it to cool down more efficiently).
And it's also considered almost certain that the protoplanet Ceres was initially formed out of the collection of not only small bits of rock but a lot of bits of water ice -- since ice can exist stably at distances just a little farther from the Sun than Ceres, and many chunks of it must have gotten catapulted further into the inner System by the distant but disruptive gravitational tuggings of Jupiter.
Earth-based near-infrared spectra of the asteroids reveal that many of the carbonaceous ones show signs of minerals modified by exposure to liquid water early on that must have come from such fragments of ice that were incorporated into their parent protoplanets and then melted by their trapped internal heat.
Most of those protoplanets, however, were shattered by collisions during the Solar System's early days. But Ceres remains, and it's a big one -- so big, in fact, that a 2005 analysis by Thomas McCord and Christopher Sotin suggested that it might have trapped a substantial amount of heat in its interior even without the added heating from those short-lived radioisotopes or magnetically-generated electric currents.
The heat from more standard sources -- the very slowly decaying traces of uranium, thorium, and radioactive potassium-40 that exist in all the Solar System's rocks, and the simple heat from the collisions of the chunks of material that crashed into Ceres to make it grow in the first place -- would by themselves be enough to "quickly melt" the ice in a Ceres composed of three-quarters silicate rock and one-quarter ice, causing the water to circulate within Ceres' interior and extensively chemically modify the rock.
The water would find its way to Ceres' top surface to create a surface ocean of liquid water fully 100 km thick, capped by an upper crust of refrozen ice like the one over Europa's water ocean.
If Ceres had initially grown quickly enough out of chunks of debris crashing together gravitationally, its deep interior might even have briefly become hot enough to actually boil water -- and in any case generating enough heat that a fair amount of Ceres' initial water supply would simply have sublimated into water vapor and escaped completely from it.
Then as Ceres lost its remaining excess interior heat, the frozen crust would get thicker and thicker -- and eventually it would get thick enough that the excess heat leaking out of Ceres into space would be transported not just by being conducted through the ice, but by driving extremely slow "solid-state convection" of the warmer and softer ice (again like that which will exist in Europa's surface ice crust, if it's thick enough) to churn the warmer ice at the basis of the frozen layer all the way to the top over a period of several hundred thousand years. This same churning would of course also carry the substances dissolved in the underlying water ocean, and then frozen into the ice, up to Ceres' surface, including dissolved salts from the rock.
After about 2 billion years, in this scenario, Ceres' original deep surface ocean would finally freeze completely, into a layer of ice existing to this day. And the best measurements made yet of Ceres' shape, by the Hubble Telescope, indicate that Ceres' mild equatorial bulge, caused by the centrifugal force produced by its rotation, is about 7 km less tall than the one we would see if the asteroid was made of equal-density rock all the way up to its surface. This suggests that it does indeed have such a surface layer of lower-density, less massive material -- thus stretched outwards somewhat less by centrifugal force at Ceres' equator than plain rock would be.
That layer would almost certainly be a 65 to 125 km-thick layer of water ice, making up 16 to 26% of Ceres' current mass -- with a thin crust of rock debris and salts on top of the ice, left behind because Ceres' surface temperatures at its distance from the Sun are somewhat too warm for water ice to exist forever in a vacuum without evaporating into vapor and leaving behind any stuff dissolved in it. (Any recent, moderately big meteor impact craters on Ceres may have punched through that layer and temporarily exposed the ice underneath.)
But those additional heat sources I've mentioned -- short-lived early radioisotopes, and heating by the early Sun's strong magnetic field -- could prolong the life of Ceres' ocean even longer. And if there was a substantial amount of ammonia that had been mixed with the original water-ice chunks that formed Ceres, then there is a chance that the bottom part of its outer ice layer might still be liquid to this day! (Ammonia makes an excellent antifreeze; mixed in high enough concentrations with liquid water, it can lower its freezing temperature down to only -97 deg C., which is why the majority of scientists think Titan probably also has a deeply buried liquid water/ammonia ocean layer to this day.)
One absorption band found on Earth-based infrared spectra of Ceres' surface do indeed suggest to some scientists that there may be ammoniated clays on its surface -- but a more recent study suggests that other IR lines that would be expected from such clays don't exist, although it confirms that Ceres' surface does seem to feature a crust of carbonates, clays and other water-modified minerals created by its early days of near-surface liquid water.
"Dawn's" scientists were particularly sad at having to remove that magnetometer from its planned payload, because that instrument (in addition to looking for metal cores in the two asteroids) could have detected the "induced magnetic field" from such a still-existing liquid Cerean ocean layer -- the same technique that the Galileo craft used to virtually clinch the existence of a liquid-water ocean under the ice of Jupiter's three big icy moons.
Even if Ceres no longer has a liquid-water layer, though, it seems likely that it had one for a very long time -- perhaps most of its 4.5-billion year life. And that, in turn, raises the question of just how far prebiotic organic evolution could have gone in such an ocean.
Such hydrothermal reactions of liquid water with rock are exactly the sort of thing that synthesizes large amounts of methane and other simple organic compounds in the volcanic vents on Earth's deep ocean floors, and they may have been what generates both the large amount of methane in Titan's atmosphere and the tiny traces of it that we think we've found in Mars' air.
Mix enough nitrogen-containing compounds with that brew (as would happen if Ceres' ice contained even a small amount of ammonia), and no one is sure quite how far these processes could have gone in creating very complex organic molecules.
It's exactly this same set of hydrothermal, organic-producing chemical reactions that have stirred the recent great interest of astrobiologists in the newly discovered water-vapor jets still erupting from the south pole of Saturn's little moon Enceladus, which may be hooked up to a large still-existing pocket of subsurface water below its ice.
Even if Ceres' water is now frozen solid, could it have stayed liquid long enough -- billions of years -- for primitive microbes to start to evolve in its buried ocean? In that case, their frozen remains would still be preserved in its current-day ice layer, and could have been transported up to its surface by those very slow convective currents of slightly plastic warm ice (as may also be the case for Europa).
We have already found some fairly sophisticated organic compounds -- including amino acids -- that must have been produced by the same type of processes, in the small number of carbonaceous meteorites that have been recovered on Earth, but none of them comes anywhere near the complexity of the compounds that must have allowed development of the first living cell. But Ceres' ocean probably lasted much longer than any such subsurface liquid-water pockets on the other protoplanets, which were all either a good deal smaller or shattered early on by the Belt's collisions.
It's just these thoughts that have led Dawn's planners, in the last couple of years, to drop their plans to have it orbit at any low altitude above Ceres' surface. The craft is unsterilized, and -- if it crashes into the surface and embeds pieces of itself in Ceres' near-surface ice, and if Ceres does still have an ammonia-sustained subsurface liquid ocean -- any still-living germs riding on it just might get transported by the solid-state convection of its upper ice crust down into that ocean and quickly and disastrously biocontaminate it, ruining any search for native Cerean germs.
This is exactly why NASA plans to sterilize the first Europa orbiter mission -- and Dawn, if it crashed onto Ceres from orbit, would hit at a much slower speed than the Europa orbiter and would not have been exposed first to a bath of intense sterilizing radiation from Jupiter.
We are virtually sure now that Europa does still have a subsurface ocean; we don't know whether Ceres still does, and indeed the odds seem to be against it. But why take the chance? So Dawn will remain in that final "quarantine orbit" 700 km above Ceres' surface, keeping it orbiting for at least the 50 years prescribed by NASA's biological protection rules for non-sterilized spacecraft orbiting a world that might possibly possess biological interest.
And the more one thinks about Ceres, the more potentially biologically interesting it looks. Even if its ocean has been long-frozen, it probably remained liquid for billions of years first.
Any small patches of ice exposed on its surface by meteor impacts can be examined by Dawn's VIMS spectrometer to look for substances dissolved in that ice; and it's a safe bet that one of the things the team members will be looking for -- just in case -- is either simple organics or more complex ones.
In short, Ceres has given us still another example of perhaps the single most remarkable change in the way scientists have looked at the Solar System over the last few decades; suddenly it seems that many of the small icy worlds of the outer Solar System are as interesting as Mars in our quest for fossil -- or present-day -- life.
[A late update: Dr. Russell informs "SpaceDaily" that Dawn's controllers do hope to bring it down to an orbit as low as 200 km above Ceres' surface for its studies, before raising it back into that final 700-km altitude quarantine orbit. But this is still a much higher minimal altitude for the spacecraft than the 40-km minimum altitude originally planned for Dawn in its earlier flight plan.]
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Plan G For Mars Offers Much For Geological And Geophysical Exploration
Cameron Park CA (SPX) Jun 22, 2007
The third general type of Mars Science Orbiter mission proposed by the MSO science advisory group's just-published report is "Geological and Geophysical Exploration" ("Plan G"), which would focus -- as its name suggests -- on the actual features and processes on Mars' mineral surface, rather than on its atmosphere or on the structures of its polar caps and their connection to Mars' climate record.
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