Spectra Confirm Ice on Asteroid 24 Themis. How Did it Get There?
It seems that space age instruments are now finding evidence of water-ice in the many locations we have pointed them to look. The latest to be confirmed includes asteroid 24 Themis. But is it from subsurface reservoirs evaporating (not unlike comets) or is it locally generated by the sputtering of atoms from surface rocks and recombination with the solar wind?
Recent data appears to confirm prior measurements indicating the likely presence of water-ice on the surface of asteroid 24 Themis.
Humberto Campins of the University of Central Florida in Orlando and his colleagues recorded spectra of the asteroid 24 Themis over a seven-hour period, corresponding to 84 percent of the rotational period of the spinning rock. The spectra, taken with NASA’s Infrared Telescope Facility on Hawaii’s Mauna Kea, revealed the consistent presence of frozen water as different parts of the asteroid’s surface came into view ...
The finding corroborates earlier observations (SN Online: 7/18/08) of the same asteroid by Andrew S. Rivkin of the Johns Hopkins University Applied Physics Laboratory in Laurel, Md., and Joshua Emery of the University of Tennessee in Knoxville, who also used the Infrared Telescope Facility. Over several years, Rivkin and Emery had found evidence of frozen water in single spots on 24 Themis but had not studied the asteroid as it made one entire rotation. Together, the two teams’ findings reveal that the asteroid’s entire surface is coated with frozen water, Campins says.
The analyses of the sunlight reflected off the asteroid also show that organic compounds are widespread on the surface, he added, including polycyclic aromatic hydrocarbons, CH2 and CH3.
All our spectra confirm the shape and depth of the 3.1 micron absorption reported by Rivkin and Emery (2008) and show this absorption is present throughout the asteroid's rotation. The shape and wavelength of this absorption is interpreted by Rivkin and Emery (2008) as due to water ice, as opposed to hydrated minerals originally suggested by Lebofsky et al. (1990). Our spectra fit well the same water ice model used by Rivkin and Emery (2008), and are significantly different from spectra of hydrated silicates found in asteroids and meteorites. As Rivkin and Emery (2008) state, "If confirmed, a finding of water ice on Themis would be the first detection of ice on an asteroidal surface, and the first detection of water per se (as opposed to OH) on an asteroidal surface."
An abstract for the above finding is located here: Confirming Water Ice on the Surface of Asteroid 24 Themis.
The rub comes from the fact that water ice is not supposed to be stable on the surface of such bodies, thus the question comes up, how did it get there and how does it remain?
This likely presence of water ice opens up interesting possibilities that could transform current views of some asteroids. For example, since water ice is not stable on the surface of Themis at 3.2 AU over 4.5 Gy, what is its source?
At the asteroid’s average distance from the sun -- 3.2 times Earth’s distance to the sun-- frozen water on the surface would readily vaporize, noted Campins. That means the ice must be continually replenished...
So, if water-ice is supposed to readily sublimate (vaporize from the solid to the gas phase) at the distance of 24 Themis form the sun, how does this water-ice come to exist (and persist) at the surface? There must either be a subsurface reservoir that is slow-to disappear, or the ice must be being constantly emplaced or regenerated at the surface by some as-yet unknown active process.
One possibility that has been considered is that, according to the standard model, the asteroids are similar to comets and there must be some kind of sub-surface reservoirs of water to fuel the detections of H2O (water) and/or OH (hydroxyl radical), an assumed breakdown product of water.
One possibility is that ice lies buried several meters below the surface of the roughly 160-kilometer-wide asteroid and makes its way to the top when the asteroid is pummeled by space debris. Norbert Schörghofer of the University of Hawaii at Manoa proposed last year that ice can persist for billions of years just below the surface of a dusty space rock if the asteroid’s surface temperature is less than about 145 kelvins. The temperature of 24 Themis lies in that range.
The presence of frozen water on 24 Themis also suggests that some asteroids resemble comets, becoming active suddenly and venting material into space when pockets of ice vaporize, Campins noted.
In this way, comets and asteroids can be considered close relatives (or perhaps kissing cousins). However, it doesn't seem to satisfactorily explain why some objects develop brilliant, luminous comas (comets) and some objects remain for all intents and purposes inert hunks of rock.
Comet theory has undergone many changes over the course of the last century. In the latter part of the 19th century and the earliest decades of the 20th, our level of technological sophistication was not especially advanced. Experimentation in the electrical sciences were fairly new to science. But in that newness there was also a fascination and a wish to compare the effects of electric discharges to many aspects of earth and space sciences. The spectra of comets were compared to those of low-pressure discharge tubes and found to be strikingly similar. Some likened the two directly.
However, such comparisons fell out of favor due to the difficulty of describing the behaviors of low-pressure gas discharges mathematically. Instead, a non-electrical theory took root. In that theory, comets were considered to be loose agglomerations of dust and rubble, held together by water ice which was made to sublime (vaporize from a solid directly to a gas), thus providing the source of the dust and water seen in comets' comas.
But, as more advanced observing instruments have been pointed at comets, that "dirty snowball" theory has come under increasing strain.
Comet nuclei show sharp surface features rather than the rounded features expected of hunks of ice melting in the sun.
Breaking up comets (the most likely source of confirmation of the "dirty snowball" theory) have revealed nowhere near the levels of water expected.
Comet tails remain filamentary and contiguous over millions of miles, rather than expanding explosively in all directions like inert gases in the vacuum of space.
Comets emit high-energy x-rays.
Space age data has modified our mind's eye image of comets from their conception as snowballs with a little dust mixed in, to being rubble piles covered in ices, to being relatively rocky with pockets of ice below the surface, hidden from our view. Frankly, they look almost nothing like Whipple's original conception, later termed the "dirty snowball" by others.
Are comets and asteroids really so different? There may yet be a grain of truth to the notion that comets and asteroids are relatives. After all, the Centaurs have (in some cases) been classified both as asteroids and a comets. They seem to be a step intermediary between the two extremes. They're thought to be rocky bodies, and yet several have been observed to display comas much like comets.
As of 2008, three centaurs have been found to display cometary comas: Chiron, 60558 Echeclus, and 166P/NEAT. Chiron and Echeclus are therefore classified as both asteroids and comets. Other centaurs such as 52872 Okyrhoe are suspected of showing cometary activity. Any centaur that is perturbed close enough to the Sun is expected to become a comet.
The possibility that current theory may be wrong is intriguing. Anomalies offer a chance to correct the course of theories gone astray.
Recently, observations turned up an anomaly at Mercury.
...the temperature on the surface of Mercury can range to over 400 degrees Celsius [750 degrees Fahrenheit]. Water can't really sit there. This water is clearly there [in its atmosphere]. The very first time we took a whiff of the planet, it was right there.
Anomalously, Mercury's atmosphere clearly shows shows signs of water that could not possibly exist at Mercury's metaphorically fiery surface. But, how was the anomaly to be resolved? A little bit of laboratory physics, judiciously applied.
...the process of chemical sputtering could create water where none existed before from the ingredients of solar wind and Mercury rock...
"The solar wind is highly ionized. Those are radicals -- they want to make connections with everything that they can. Imagine a solar wind hydrogen showing up and hitting the surface. It weathers whatever the mineral is, and steals an oxygen. If you do that, you get something like OH-, for example." OH-, also known as a hydroxyl group, would produce a peak at atomic mass 17 on the FIPS spectrum. "You can do it in reverse -- an oxygen from the solar wind can steal a hydrogen. The process is called chemical sputtering."
Sputtering is a relatively well-known process, whereby a surface bombarded by atoms or charged particles may give up some material from its surface. Those materials may then recombine with other materials to form molecules not originally present in either the surface material being bombarded or in that which was bombarding said surface. In this case, Oxygen may have been released from surface rocks and recombined with hydrogen from the solar wind to form the OH (hydroxyl radical) and H2O (water) observed in Mercury's atmosphere and magnetotail.
A similar process has been alluded to in reference to the recent detection of OH and/or H2O in the vicinity of the Moon's surface.
NASA scientists have discovered water molecules in the polar regions of the moon. Instruments aboard three separate spacecraft revealed water molecules in amounts that are greater than predicted, but still relatively small. Hydroxyl, a molecule consisting of one oxygen atom and one hydrogen atom, also was found in the lunar soil.
"The data from Cassini's VIMS instrument and M3 closely agree," said Roger Clark, a U.S. Geological Survey scientist in Denver and member of both the VIMS and M3 teams. "We see both water and hydroxyl. While the abundances are not precisely known, as much as 1,000 water molecule parts-per-million could be in the lunar soil. To put that into perspective, if you harvested one ton of the top layer of the moon's surface, you could get as much as 32 ounces of water."
Data indicate that water exists diffusely across the moon as hydroxyl or water molecules — or both — adhering to the surface in low concentrations. Additionally, there may be a water cycle in which the molecules are broken down and reformulated over a two week cycle, which is the length of a lunar day.
The scientists were looking for a signature of water in the craters near the poles, but found evidence for water instead on the sunlit portions of the moon. This was certainly unexpected and the science team from M3 looked and re-looked at their data for several months.
the findings show that not only is the moon hydrated, the process that makes it so is a dynamic one that is driven by the daily changes in solar radiation hitting any given spot on the surface.
The sun might also have something to do with how the water got there.
The rocks and regolith that make up the lunar surface are about 45 percent oxygen (combined with other elements as mostly silicate minerals). The solar wind — the constant stream of charged particles emitted by the sun — are mostly protons, or positively charged hydrogen atoms.
If the charged hydrogens, which are traveling at one-third the speed of light, hit the lunar surface with enough force, they break apart oxygen bonds in soil materials, Taylor, the M3 team member suspects. Where free oxygen and hydrogen exist, there is a high chance that trace amounts of water will form.
So, it seems again, that the OH (hydroxyl radicals) and H2O (water) may be locally produced by sputtering and recombination with the solar wind.
Now, it seems that scientists have found and been a little stymied by water-ice on an asteroid. So, how shall this issue be resolved? And can the same process inform us about the comet issue?
Nancy Atkinson, of Universe Today, makes a wholly appropriate observation in light of recent data from Mercury and the Moon, mentioned above.
Another option is that an action similar to the recent findings of water on the Moon, where solar wind interacts with a rocky body without an atmosphere to create H2O and OH molecules. Without an atmosphere, the body is exposed to solar wind, which includes hydrogen ions. The hydrogen is able to interact with oxygen in surface of the asteroid to create water molecules.
It seems that this process, sputtering and recombination, is an increasingly useful tool in explaining the genesis of OH and H2O in the vicinity of rocky bodies in space when exposed to the solar wind. Her observation seems reasonable.
If we're willing to apply this same explanation to other bodies int he solar system it may just explain the abundances of OH and H2O in the comas of comets without recourse to the increasingly implausible "dirty snowball" hypothesis.
An alternative explanation for the generation of OH and H2O is provided by Australian plasma physicist Wal Thornhill. In his hypothesis, the water and hydroxyl radicals in cometary comas are generated locally. He posits that comets are rocky bodies just like asteroids, largely devoid of water (except in cases where the parent body was significantly icy). However, in the presence of a strong electric field, cathode sputtering will occur, leaching atoms (most likely Oxygen from silicates) from the surface. Those atoms then recombine with atoms from the solar wind (most likely Hydrogen) to locally form the observed hydroxyl (OH) and water (H2O) abundances. He supports his claim with the fact that "forbidden lines," indicative of a strong electric field, have been found in the Oxygen spectra of cometary comas.
The flaw in the conventional approach is that only gas-phase chemical reactions and reactions induced by solar radiation (photolysis) are considered. The far more energetic molecular and atomic reactions due to plasma discharge sputtering of an electrically charged comet nucleus are not even contemplated [see below]. Yet this model solves many comet mysteries that are seldom mentioned.
The hydroxyl radical, OH, is the most abundant cometary radical. It is detected in the coma at some distance from the comet nucleus, where it is assumed that water (H2O) is broken down by solar UV radiation to form OH, H and O. It is chiefly the presence of this radical that leads to estimates of the amount of water ice sublimating from the comet nucleus. The comas of O and OH are far less extensive than the H coma but have comparable density.
The negatively charged oxygen atom, or negative oxygen ion, has been detected close to cometary nuclei. And the spectrum of neutral oxygen (O) shows a "forbidden line" indicative of the presence of an "intense" electric field. The discovery at comet Halley of negative ions puzzled investigators because they are easily destroyed by solar radiation. They wrote, "an efficient production mechanism, so far unidentified, is required to account for the observed densities." And the intense electric field near the comet nucleus is inexplicable if it is merely an inert body ploughing through the solar wind.
Let's see how the electrical model of comets explains these mysteries. The electric field near the comet nucleus is expected if a comet is a highly negatively charged body, relative to the solar wind. Cathode sputtering of the comet nucleus will strip atoms and molecules directly from solid rock and charge them negatively. So the presence of negative oxygen and other ions close to the comet nucleus is to be expected. Negative oxygen ions will be accelerated away from the comet in the cathode jets and combine with protons from the solar wind to form the observed OH radical at some distance from the nucleus.
The important point is that the OH does not need to come from water ice on, or in, the comet. Of course, some water is likely to be present on a comet or asteroid. It depends upon their parent body.
Thornhill's electric model of comets also apparently resolves the discovery of anomalous x-rays emitted by comets in one fell swoop.
Copious X-rays were discovered by accident coming from a comet. No one expected them from an inert body rushing through the solar wind. An ad hoc explanation was devised that required protons from the Sun to combine with electrons from the comet. No one sensed the irony. that moving protons combining with electrons is the defining characteristic of an electric current flowing between the Sun and the comet.
X-rays are generated naturally by high-voltage discharges.
A comet's tail arises from the interaction between the electric charge of the comet and the solar discharge plasma. The comet spends most of its time far from the Sun, where the plasma charge density and voltage with respect to the Sun is low. The comet moves slowly and it easily accumulates enough charge to balance the ambient voltage.
As the comet approaches the Sun, the nucleus moves at a furious speed through regions of increasing charge density and voltage. The comet's surface charge and internal polarization, developed in deep space, respond to the new environment by forming cathode jets and a visible plasma sheath, or coma. The strong electric field in the comet's plasma sheath generates x-rays.
A charged comet nucleus moving through the solar plasma will form a number of plasma sheaths or double layers, where the comet's electric field is concentrated. The Giotto mission to comets Halley and Grigg-Skjellerup were surprised by their sharp plasma boundaries and one "mystery" boundary. There should be no mystery when they are considered as electrical plasma sheaths and not simply mechanical "bow shocks." The so-called "near nucleus ion pile-up" is simply a manifestation of a cometary plasma sheath, or double layer. It is a region with a strong electric field and consequently capable of generating strong x-rays. The cometary x-rays were found coming from a region that didn't conform to simple hydrodynamic collision calculations. It was remarked that it was like finding the shockwave from a supersonic aircraft several kilometres to one side of the aircraft. A plasma sheath is controlled by electromagnetic forces and is not expected to conform to bow-shock physics.
It is beginning to seem that a reassessment of asteroids and comets in terms of low pressure plasma discharge phenomena may be a reasonable course of action and bear scientific fruit.
Granted, such a reassessment may require a bit of an uncomfortable rethink of certain aspects of solar system dynamics, since the "electric comet" hypothesis does not stand in a vacuum (no pun intended). It requires that the sun possess a weak radially oriented electric field which permeates interplanetary space. Which itself has implications for the function of the sun and its interaction with other bodies (planets, moons, asteroids, comets and spacecraft) inside the heliosphere.
It is already accepted that there exists a heliospheric current sheet. It doesn't seem like much of a stretch to consider that such an electric current may be driven by an electrical potential within the heliosphere, that is to say a radially oriented electric field.
It will be interesting to see whether physicists try to generalize the recent data and whether it leads to a reassessment of the composition and function of comets and asteroid. Are they considerably closer related than we've led ourselves to believe? Time will tell.