Note: incredibly beautiful and hi-res images to go with this blog post can be found on the HiRISE Science in Motion page.
Late last year, NASA released results that indicated a strong possibility of liquid water on Mars. Gullies formed in the steep sides of craters looked very much like liquid water had erupted from underground, evaporated, and left a deposit of minerals behind. The features were bright, veined streaks that appeared in images.
Now, though, that's being rethought. New evidence indicates that the deposits may be from dry flows; basically minor landslides. As the scientists themselves say in their paper (subscription required):
Bright gully deposits identify six locations with very recent activity, but these lie on steep (20Â° to 35Â°) slopes where dry mass wasting could occur. Thus, we cannot confirm the reality of ancient oceans or water in active gullies
In other words, the slopes are steep enough that even in Mars' light gravity, solid matter can tumble down. In the original images, some lighter material can be seen upslope of the gullies, and perhaps the streaks came from those deposits. Spectra taken don't show the kinds of materials expected for evaporational deposits. The gullies may have formed millions of years ago from flooding of water, but that time is long past.
Bummer. That was a very cool story, but now it's looking less like water.
And then things get worse! The Vastitas Borealis Formation is a large area on Mars thought to have been an old ocean, or maybe was the result of catastrophic flooding. That looks less likely to be the case now too! New images show large (1-2 meter) boulders in the area, when it was thought to be mostly fine-grained sediment:
The origin of the Vastitas Borealis Formation (VBF), covering the lowest portions of the extensive northern plains, has been the subject of much debate, including (among other hypotheses) that it is the fine-grained residue of an ancient ocean or that it represents frozen deposits of sediment-laded water from giant outflow channels.
Here's an image of the boulders. Note the scale bar; both images are at the same scale.
The paper goes on to say:
The more than 200 HiRISE images of this unit show that rocks ranging in size from the limits of resolution (~0.5 m) to ~2 m in diameter are ubiquitous... Boulders are concentrated around circular structures of probable impact origin, but they are present over most of the VBF at uniform densities. In addition, we have seen no light-toned layered deposits within the VBF; such deposits elsewhere are thought to be of aqueous sedimentary origin. The boulder distribution and absence of light-toned layered deposits are difficult to reconcile with the hypothesis that the VBF primarily consists of a thick (~100-m) deposit of fine-grained materials deposited from suspended sediments in an ocean.
In other words, the rocks indicate that the area was not an ocean or from outflow. Bummer again.
The final blow: a region thought to have been an old dried up sea turns out to be ponded lava when it flowed over a channeled region. In other words, the smooth lava fooled the scientists when it was seen in lower-res images, but new data show the reality of the lava flows.
This is a pretty big setback for those looking for sources of liquid water on Mars (frozen water -- ice, duh -- is not hard to find, especially at the poles). It would be nice to think that liquid water could be found, but we have to go where the data tell us. Right now, they're saying that liquid water, even subsurface deposits of frozen water, may be rarer than we thought.