So it seems that the Mars Odyssey THEMIS team may have found openings to caves on Mars! They show up as really dark spot in visible images—dark because they have steep, nearly vertical, walls so no sunlight gets in at all from the fixed solar-viewing geometry of the orbiter. But that's not quite enough to convince anyone.

The real clincher comes when you look at them in the thermal infrared (TIR). At these wavelengths, warm things are brighter than cold things. A cave tends to remain at a constant temperature (and roughly equal to the annual average local temperature of the area it is in). This means that a cave is colder than the local area in the day, but warmer at night.

So in comparing images from the Odyssey THEMIS instrument that are in the visible band, the TIR in the day and the TIR at night, these cave entrances can be discerned: black in visible, dark TIR in the day, bright TIR at night.

So, what's be big deal about caves? Well, they are relatively easy access to the underground where we figure any sort of life (if it exists on Mars) must have migrated in order to be shielded from the deadly UV and cosmic rays that bombard the surface. Of course, all the caves seen in this discovery are on the flanks for Arsia Mons, the southernmost of the three Tharsis Volcanoes. It seems unlikely that life would have migrated to these altitudes in the first place before having to go deep underground. But, if there are caves here, there may be caves elsewhere.

According to the latest press release on Opportunity, the dust has cleared enough that they have taken the first steps to continue exploration. In fact, on 21 August Opportunity shuffled forward just over 13 meters towards Victoria Crater—remember, the goal is for the rover to work its way down into the crater. While there has been significant clearing, it could take months to be back to normal. Right now the rover is working on about half-power (which is double what it was getting just a few weeks ago!) so the rover team will be taking that into account as they move forward. You can see the gradual clearing throughout August and September in the MarsWatch image submissions. Thanks to all of you who make this possible! You can also compare it to the MRO maps from June and July.

Spirit had recovered from the dust storm a little earlier than Opportunity, but it, too, is working at about half-power. Although the skies have begun clearing there's the issue of dust having settled on the solar panels. What we really need now is a good wind to blow it clear! There is also some question on what effect this settled dust will have on the instruments, especially the Mini-TES. As the spectrometer measures the thermal infrared light from targets, it could be corrupted by the light emitted from the dust on its optics making the job of identifying the target mineralogy much more difficult. The rover team has actually backtracked somewhat to image a target previously studied in order to get a feel for how this contamination will look.

OK, it's been awhile since I've posted anything here—I'll try to be better about that in the future. I'm finishing up all the grading involved with a 2-week intensive Space Science course I was doing for in-service middle-school teachers and that's been taking a lot of my time.

However, as everyone probably already knows by now, the Mars Phoenix mission has successfully lifted off (4 AUG 2007) and is on its way. On 10 AUG 2007 it made its first course correction and all systems are still go.

I have to admit that I'm really excited about this mission; the lander will be digging in the martian soil and running several life-searching experiments on what it scoops up. Depending on how deep they have to dig to find ices, they'll either dig up one long, deep trench or several shallower trenches. The lifetime of the mission is really going to be controlled by the onset of martian winter. Since Mars is tilted about 24 degrees, its north pole pointing at a spot about halfway between Deneb and ν Cephei, as it revolves around the Sun at some point it will be tilted away from the Sun and in seasonal darkness.

Although during northern summer, the region where Phoenix will be will receive copious amounts of sunlight, once autumn begins, the Sun will begin getting very low to the southern horizon. Eventually the it will be too low in order to provide enough power to run the lander. After that it gets more interesting—the latitude where Phoenix will be is normally covered by the seasonal polar ice cap in winter!

Earlier this month, JPL released a podcast discussing some of the science goals of Phoenix. It's a nice overview and was done by my friend and Mars colleague Leslie Tamppari.

In Rover news, I hear that the teams are far more upbeat about the survival chances of both Spirit and Opportunity. In fact, Spirit has even been able to do some science as the skies begin to clear. While they're not out of danger completely, folks are optimistic. MRO continues to monitor the rover sites to get the birds-eye view of the area while the rovers themselves wake up long enough to snap a few pictures of the Sun to monitor cloud optical depth. We're all waiting for the dust to clear enough so the rovers can continue their missions.

Just a quick note tonight: it seems as if Florida is going to be having some severe weather so the launch of the Phoenix mission (a redo with some extras of the Mars Polar Lander) is being postponed. It was originally planned to lift-off on 3 August, but there are still two more launch windows the next day. Here's hoping for clearing weather!

It figures, I go on vacation for a week, and that's when all the official news on the dust at the rover sites breaks! You can read the NASA PR report directly—it was updated on 23 July. In summary, it says that the dust in the area around Opportunity had thickened to the point where almost 99% of the sunlight is being blocked. It's almost as bad over at the Spirit site.

The reason this is critical is that the rovers use sunlight to charge the battery packs that run them. When the output was cut in half, the teams suspended all activity (which means Opportunity's climb down into Victoria crater is on hold). As the storm worsened, the output was cut even further so communications were even cut back. Right now, the power output is just enough to keep the batteries charged to allow the rover to stay warm enough over night to stay alive. The latest communication seems to indicate that things are getting better, but the behavior of these dust storms is very hard to predict.

For a while there, the rover had been gathering images of the local scene as the dust increased. NASA put together a composite image showing the darkening skies. Up at the top of the image you'll see the parameter τ which is the optical depth of the dust cloud and is a measure of the diminishing of the light. For the mathematically inclined, the light is dimmer by a factor of e^-τ, where e= 2.71828… is the exponential base. So, at τ=4.7, the light is at 0.62, or 62% of its nominal brightness.

Mars Odyssey is still in orbit, going strong, and the THEMIS instrument (Thermal Emission Imaging Spectrometer) is measuring the atmospheric brightness as many wavelengths. With those data the team is able to model the atmospheric temperature as well as the dust cloud thickness. These results are being put into a daily map. Although the maps claim to be of dust opacity, the numbers on the scale make me believe they are really dust cloud optical depth—opacity is a sort of light-absorbing efficiency factor for a substance and optical depth is the total opacity along a path through the substance and thus a measure of the total light blocked. The maps have also been combined as frames in a nice animation. There are some major jumps early on, but once the storm takes off, you can see it evolve almost daily.

Since we have Mars Reconnaissance Orbiter right there imaging the planet, we can see the visual effect of the storms in this mosaic. The decrease in regional contrast is stunning! This image is useful for giving you a visual reference for what those optical depth numbers "look" like.

Of course, you can also keep an eye on the dust storms yourself, either through your own observing program, if you have a telescope (and if you make images, you can submit them here) or by visiting the MarsWatch images section and seeing what our army of observers is collecting.

And let's all wish the rovers "clear skies"!