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.[1] 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[2], 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 François 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.

With the shrinking apparent size of Mars, it dimming, and its nearness to the Sun, it seems that ground-based imaging is all but over. Granted, the current "apparition" won't end until the next conjunction in early December this year, but there haven't been any new images here since the end of July.

Thanks to all the image contributors for their great work and dedication!

On the busy side of things, Phoenix, the rovers Spirit and Opportunity, and the Mars Reconnaissance Orbiter and Mars Express continue their work.

At the Phoenix site, summer is ending which means it will start to experience sunsets. Since Mars is tilted about 23 degrees on its axis, for part of the year its north pole faces the Sun; this means that north of a particular latitude even though it spins, there are parts that are never out of the sunlight. This is the same season-making process as here on Earth. The effects on Mars are easily seen—average air pressure changes, average temperature changes, and the ice caps shrink and grow.

In fact, the Phoenix lander is far enough north that as the seasonal cap grows, it will bury the lander under a thick layer of ice. Of course, this will happen well after permanent night arrives at the site and the lander has stopped working.

But until then, Phoenix will continue its mission—NASA approved an extension from its nominal end date of 26 August 2008. Now it will keep digging and studying the martian soil until at least the end of September.

So here's something I hadn't really thought of before: since the Mars Phoenix Lander is so far north, it is above the arctic circle so for some number of days, the sun will never set (just like for folks in northern Finland, Scandinavia, and Alaska).

What made me realize this was that the camera team took time out to record this phenomenon. They put all the images together to make a really nice composite image of it.