I'll resume some discussion of the DPS presentations shorty; I'm currently at a workshop on Mars Atmospheric Modeling which is really interesting and I'll share some of what I'm learning here. It's amazing how much "we" really do know and understand about the physics of climate!
In the mean time, I thought I'd let everyone know that the Mars Phoenix mission had officially ended.
Since it is at such a high latitude, as autumn progresses the days get shorter and the Sun gets lower in the southern sky, just as it does for folks in northern Canada, Scandinavia, and northern Asia here on Earth. This means less sunlight to charge the batteries and thus less overall power to keep the craft operating. The engineers had begun a program of systematically shutting down heaters and instruments in order to keep it running as long as possible. But on 2 November, they lost contact with it and as of yesterday, with no further contact, they declared the mission over.
I'll have more on the incredible science that has been done—which I learned about at the DPS meeting—soon. I've got to get some breakfast and then head off to conference sessions.
We've known for a long time now that there is water on Mars. Well, more specifically, we've known there is water vapor in the atmosphere and water ices at the poles and in the clouds. In the late 70's the Viking 2 lander confirmed surface frosts existed. The big question about water is not whether on not its there, but whether or not liquid water existed and if so, how long.
As science and technology progressed scientists been able to confirm various minerals on Mars that, at least on Earth, usually form in the presence of liquid water. We call these hydrated minerals and many of them start out as other minerals but then get chemically altered by water. Typically, the water breaks up into H+ (hydrogen) and OH- (hydroxyl) with the latter bonding to the mineral. Other times, the entire water molecule gets stuck into the mineral. One important class of these is the phyllosilicates.
Silicates are minerals that are derived from the SiO4 tetrahedral molecule—it's sort of like methane but with silicon and oxygen instead of carbon and hydrogen. This makes sense if you look at a periodic table; silicon is in the same column as carbon which means it behaves similarly in general chemistry—this fact is what leads to sci-fi writers talking about "silicon-based life". You can stick these triangular pyramids together into a huge single crystal where each Si shares all of its O's with another Si, so you have Si + 4 "half" O's or SiO2 which is quartz. You can also connect them into pairs or chains or double chains or sheets or even rings. The sheet form is the phyllosilicate group that contains things like mica and clays.
So one of the DPS talks by Eldar Noe Dobrea, now at the Jet Propulsion Laboratory, discussed his study of an outcrop of phyllosilicates in the highlands around Mawrth Vallis. Using new high resolution images and spectra from the Mars Reconnaissance Orbiter he hypothesizes that different types of phyllosilicates weren't put there over different times, but rather that the layering suggests a primary layer of iron and magnesium phyllosilicates was put down and the the upper level interacted with liquid water leaching out some elements and leaving behind aluminum phyllosilicates. Eventually, this was covered up by some other rock and then parts of this layer eroded away by the wind "sandblasting" some parts of this top layer.
A second talk on hydrated minerals by James Wray was about trying to infer formation times and conditions of sulfate minerals. Sulfates tend to form out of the salts left behind when water containing them evaporates—more evidence of liquid water on Mars. However, in this case his work seems to indicate that all these sulfates formed during the earliest geologic period on Mars, called the Noachian Epoch which ended about 3.5 billion years ago.
Last week I was at the 40th annual meeting of the Division for Planetary Sciences of the American Astronomical Society. This is one of the major professional conferences for planetary sciences and I try to get there every year to see what's new in- and outside of my Mars area of expertise. It's also a chance to meet up with some of my fellow scientists and chat in person to swap ideas. So I thought I'd try to discuss here some of the latest Mars science.
A caveat: the things discussed here are the talks and papers that jumped out at me as "easily translatable". Much of the state of the art in Mars science is incremental in nature and those results just don't make for a good story for the masses. Thus, over the next few days I'll be picking and choosing and writing about just a few of the things I heard about.
The first talk was on a new look at a martian chronology. Trying to get a good time-history of the various geological formations on Mars is very difficult. In general a technique called stratigraphy is used—in general "things above" are younger then "things below" such as a crater in a lava flow bed would mean that the crater is younger. Seems simple enough but it gets complicated very quickly. Craters can create flows; they can have secondary cratering and/or rays; flows and future impacts can "erase" older features.
In addition to this technique we have crater counting where you add up the numbers of crater in one area and compare to the count in another area. If we assume that surfaces start crater-free, then the surface with more craters is older than the one with fewer craters. However this is complicated by crater sizes. The current model of solar system formation says that as time goes on there are fewer and fewer "large" impactors so if you have two surfaces with the same number of craters but one of them has both large and small ones, its probably older than the one with only small ones.
For Mars this gives us three major epochs: the Noachain (large and small craters and lots of them), the Hesperian (only small craters, but lots of them), and the Amazonian (only small craters but fewer of them). The surfaces from these general ages are fairly contiguous. These general ages can be subdivided somewhat based on other geologic processes and features and can even be compared to Earth, Venus, Mercury, and our Moon, although since we have not been able to do any radioisotope dating for Mars, Venus, and Mercury, the absolute dating is still uncertain.
There is another wrinkle. Some mid-sized craters that kicked up material so as to create secondary cratering so now some of the small stuff may really be caused by this debris and not "real" craters.
One of these craters is called Zunil and the discovery of its secondary system calls into question the crater-count chronology of Mars and throws it off by factors of 700–2000. That's a big uncertainty even in a fairly uncertain science.
Well, the very first Mars talk at DPS was by William K. Hartmann was on a new assessment of these errors. He and his collaborators used newer data from images of Zunil-type craters from the Mars Global Surveyor Mars Orbiter Camera. Essentially, they went back to these images to search for the small (10–25 m sized) craters that previous studies did not find. Well, the found them. And they found that the ages they get using their numbers seem to fit the older chronology estimates to within a factor of 2—4. Thus, they conclude that there is no major problem with our age estimates.
The big news from Mars is the first real sighting of snow! Although the highs are still a balmy -35°F (-35°C) so the snow sublimes (changes from ice to vapor) before it reaches the ground.
Back when I lived in Wyoming we'd see this effect with rain. You could look off in the distance (the high plains can be quite flat in areas) and see rain falling from storm clouds that never reached the ground due to the extremely low relative humidity near the surface and the greater relative pressures between ground and cloud---compressional heating. This phenomenon is known as virga.
The way Phoenix "sees" this martian virga is though the use of its LIDAR instrument provided by the Candian Space Agency. LIDAR stands for LIght Detection and Ranging (c.f. RADAR = RAdio Detection and Ranging) which effectively shines a (in this case, green) laser into the air then detects the reflected beam.
Since the LIDAR uses short wavelength visible light (instead of longer wavelength microwaves) it can "see" much smaller objects—in this case aerosols of ice and dust. By measuring the amount of returned light they can get an idea of how dark the material is so as to tell the difference between dust and ice. By effectively timing how long it takes for the return beam to arrive, the get altitudes. Presumably, they could also measure shifts in the laser wavelength to measure vertical speeds of aerosols, but I'm not sure they are doing this. This is how the new "laser radar" works that various highway patrols are now using—since the beam is narrow, by the time you detect it's in use, your speed has been measured as they don't "leak" like RADAR does.
Nominally, the LIDAR team gets to take 15 minutes of data 4 times per sol (a martian day, which is about 30 minutes longer than a terrestrial day). The data from sol 99 at around 05:00 (5 am) Mars Local Time, the LIDAR picked up bright aerosols that were below the clouds and appear to be falling and being blown sideways. Since it is still far too warm for there to be any CO2 ice in the atmosphere, they conclude it must be water ice. And falling water ice is... Snow!
Interestingly, there has been a lot of work by atmospheric and planetary scientists that infer the seasonal polar ice cap is, at least partially, created by falling CO2 snows. Recent (1998) work by FranÃ§ois Forget and his colleagues attempted to model the energy balance for the entire atmosphere of Mars. It's a very difficult problem to solve (for those who care, it starts with what's called an integro-differential equation for radiative tranfer) and solution found that conditions should exist such that CO2 should condense in the air and fall. They were even able to accurately match some confusing infrared measurements over martian poles taken by the Viking orbiters.
I don't think Phoenix will still be "alive" by the time it could see CO2 snows.
Since the two rovers are south of the martian equator, they have been going through their winter (while Phoenix enjoys its northern hemisphere summer). It's now past the solstice and the days are lengthening and warming up for Spirit and Opportunity.
Since early August Spirit has been biding its time, keeping its batteries charged, keeping warm, and working on a the Bonestell panorama—a 360° picture in all 13 filters of its PanCam instrument. It's waiting out the winter sitting on the southern end of Home Plate plateau.
As reported last year at a major scientific conference by Steve Squyres, principal investigator of the rover missions, this plateau is composed of broken up rocks that were most likely formed in a volcanic explosion then worked over by wind erosion. Spirit also found that much of the local "soil" is very high in silica (SiO2) which could be an indication of rock alteration by very high temperature flowing water. On Earth, such an environment is more than capable of sustaining microbial life!
Opportunity has completed its nearly one Earth-year long investigation of the interior of Victoria crater. After driving around the rim the rover entered and began its study of the excavated layers of rock. It then began its climb out, and by the beginning of this month was back on the rim and is now getting ready to, once again, put the pedal to the metal and drive off 12 km to an even bigger crater south of Victoria. Although it may never get there, its worth the try as this crater has an even thicker exposed rock layer to investigate. This means we may get a look at even earlier rocks than we've seen so far which can give clues about an even younger Mars environment.