Titan and Mercury: Challenges to Billions of Years
News from Titan and Mercury continue to challenge current theories that they formed billions of years ago.
Science Magazine highlighted new results from the MESSENGER spacecraft, now orbiting the inner planet, about deep ice found in shadowed craters in its polar regions. For ice to be stable, it would have to be at a very low temperature to avoid loss by sublimation, requiring it remain permanently blocked from the heat of the nearby sun, and blanketed with an insulating layer of dust. Theoretically, such temperatures can be achieved in shadowed craters, but can such deep deposits of water ice and organics be expected to survive for billions of years?
1. Planetary scientists are never at a loss to rescue the old ages with various “scenarios” of how the ice stays there. Neumann et al. published evidence for bright and dark deposits at the poles. The dark areas may have a protective blanket over the ice, but light areas appear to be “deposits of nearly pure water ice up to several meters thick.” To account for these, they had to invoke comets as delivery vehicles to replenish the ice over geologic time:
Detailed thermal models suggest that surface temperatures in the majority of the high-latitude craters with RB [radar-bright] deposits that MLA [Mercury laser altimeter] has observed to date are too warm to support persistent water ice at the surface, but the temperatures in their shadowed areas are compatible with the presence of surficial dark organic material. Modeled subsurface temperatures in these dark regions are permissive of stable water ice beneath a ~10-cm-thick layer of thermally insulating material. In contrast, thermal modeling of the bright areas is supportive of surface water ice. This interpretation of the surface reflectance at 1064 nm is fully consistent with the radar results as well as with neutron spectroscopic measurements of Mercury’s polar regions. The bright and dark areas can be ascribed collectively to the deposition of water and organic volatiles derived from the impacts of comets or volatile-rich asteroids on Mercury’s surface and migrated to polar cold traps via thermally stimulated random walk.
2. In another paper in the same issue of Science, Lawrence et al. agree with the comet hypothesis. There’s a lot of water up there:
Combined neutron and radar data are best matched if the buried layer consists of nearly pure water ice. The upper layer contains less than 25 weight % water-equivalent hydrogen. The total mass of water at Mercury’s poles is inferred to be 2 × 1016 to 1018 grams and is consistent with delivery by comets or volatile-rich asteroids.
Even so, depletion of the ice by sublimation is rapid enough that they said, “The average thickness of the upper layer inferred from neutron spectrometry therefore suggests that Mercury’s polar water ice was emplaced sometime in the last 18 to 70 My” [million years], a tiny fraction of the assumed 4.5 billion years Mercury has supposedly been orbiting the sun.
3. Paige et al. took on the task of modeling how long water ice could survive for billions of years. Yes, sublimation can be very slow (1 mm per billion years) if the conditions are right. A lot depends on the assumptions and the temperature:
…the temperature at which a water ice deposit can be considered thermally stable depends on the time scale under consideration. At a temperature of 102 K, for instance, a meter-thick layer of pure water ice would sublimate to space in 1 billion years, whereas at a temperature of 210 K, a meter-thick layer of pure water ice would sublimate in 35 days.
But under certain measured conditions, a one-meter depth of ice would sublimate in just 1000 years, they said:
Today, thick deposits of ground ice are found near 75°N in areas with biannual maximum surface temperatures in excess of 150 K. At these temperatures, pure exposed water ice deposited by a cometary impact would sublimate at a rate of 1 m per 1000 years. The ice deposit would disappear on time scales of tens of thousands of years if not thermally protected by a ~10-cm-thick layer of overlying ice-free material, but this geometry is problematic because the time scales for burial to these depths by impact-gardened soil from adjacent regions is estimated to be on the order of tens of millions of years.
In short, they decided that the dark deposits require replenishment. “Because metastable ice deposits must accumulate on time scales that are shorter than those at which they sublimate, the formation of the MLA-dark deposits by sublimation lag is compatible with episodic deposition of water and other volatiles by asteroids and comets.”
4. In a review of these three papers in the same issue, Paul G. Lucey described a “wet and volatile Mercury” that requires ongoing dynamical processing to last very long. His description of what is required sounds like a stretch, a composite explanation of maybe this, maybe that:
Mercury’s polar cold traps appear to have been filled by one or more comet impacts that introduced massive quantities of water and other volatile vapors in the tenuous atmosphere that promptly migrated to the polar cold traps. Ices began to immediately sublimate, and to acquire organic lag deposits, probably from radiation-induced chemical synthesis. The colder parts of the poles now exhibiting radar anomalies retained water ice below the lag deposit, while in warmer portions the ice entirely sublimed away, leaving the low-reflectance organic residue. Not depicted are the rare very-high-reflectance spots that are confined to the coldest portions of the pole. These may indicate a slow continuous production of water from small wet meteorites, solar wind proton interactions with oxygen in Mercury’s surface, or inhibition by the very low temperatures of the organic synthesis occurring elsewhere.
He says “the new data reveal a dynamic history of these deposits,” meaning that theory requires a dynamic history, not that the data actually reveal it. The water ice won’t last long. It requires continual resupply:
The results also show that the charging of the cold traps can temporarily overcome thermal instability and can be used to derive a high lower limit on the amount of water vapor that can be at least transiently retained in a transient atmosphere of Mercury in a comet impact to account for the distribution of the dark deposits.
Far out in the cold at Saturn, Titan presents other long age problems. After 8 and 1/2 years in orbit, the Cassini spacecraft has radar-mapped about 50% of the surface. A study of Titan’s 30 or so craters discovered so far, compared with those on the comparably sized moon Ganymede at Jupiter, shows Titan’s craters generally shallower, suggesting infilling of some sort. Some suggest the sand from the ubiquitous dunes that belt the equatorial regions. A NASA press release echoed on Science Daily describes the head-scratching required between finding sources of methane (rapidly depleted by the solar wind) and crater-filling mechanisms, as revealed by the number of “However” clauses:
“Since the sand appears to be produced from the atmospheric methane, Titan must have had methane in its atmosphere for at least several hundred million years in order to fill craters to the levels we are seeing,” says Neish. However, researchers estimate Titan’s current supply of methane should be broken down by sunlight within tens of millions of years, so Titan either had a lot more methane in the past, or it is being replenished somehow.
Team members say it’s possible that other processes could be filling the craters on Titan: erosion from the flow of liquid methane and ethane for example. However, this type of weathering tends to fill a crater quickly at first, then more slowly as the crater rim gets worn down and less steep. If liquid erosion were primarily responsible for the infill, then the team would expect to see a lot of partially filled craters on Titan. “However, this is not the case,” says Neish. “Instead we see craters at all stages; some just beginning to be filled in, some halfway, and some that are almost completely full. This suggests a process like windblown sand, which fills craters and other features at a steady rate.”
For those who want to relive the exciting landing of the Huygens Probe on Titan 8 years ago this month, Astrobiology Magazine posted the European Space Agency’s latest animation re-creating its bouncy, skidding landing from various angles. Viewers will notice the cloud of dry dust cast up into the atmosphere, re-emphasizing the contrast between the predicted global ocean and the reality of a mostly dry world.
One measure of the strength of a scientific hypothesis is the number of auxiliary hypotheses required. A recent creation of these worlds requires very few additional assumptions. But when secular astronomers have to bring in boatloads of comets and “wet asteroids” to keep Mercury’s poles icy, or an unknown mechanism for keeping Titan’s methane “replenished somehow” (see the hands wave), the burden is on them.