Tag Archives: craters

The soft craters of Epimetheus

The soft craters of Epimetheus

Cool image time! The image on the right, reduced in resolution to show here, is the highest resolution image that Cassini has taken of the Saturn moon Epimetheus, taken from only 9,300 miles away on February 21, 2017.

Epimetheus (70 miles or 113 kilometers across) is too small for its gravity to hold onto an atmosphere. It is also too small to be geologically active. There is therefore no way to erase the scars from meteor impacts, except for the generation of new impact craters on top of old ones.

Below is the inset at full resolution, showing several craters, with ponds of dust on their floor. Overall, the surface of this tiny moon looks soft. The craters are all shallow, as if any impact merely plunged into a blob of ice cream. Any ejecta from those impacts eventually rained back down, and then settled slowly in the moon’s low points, forming those ponds of dust.

close-up of soft craters

In many ways this image is very revealing, as it shows what the early accretion process of any planetary body will look like. Nor is this unique. Earlier images taken of the asteroid Eros by the NEAR probe saw many of these same features, as have images of Saturn’s other small moons. In the early stages, new material gets absorbed easily because it finds it easy to bore into the body of the newly formed and not very dense planetary body. There isn’t much ejecta, and what there is doesn’t fly that far away so that it can settle back down on the surface and add to the new body’s total mass.


New research suggests that, in general, Mars has always been too cold to harbor liquid water on its surface.

The uncertainty of science: New research suggests that, in general, Mars has always been too cold to harbor liquid water on its surface for long periods.

Mars’ atmosphere was probably never thick enough to keep temperatures on the planet’s surface above freezing for the long term, suggests research published today in Nature Geoscience1. Although the planet’s topography indicates that liquid water has flooded Mars in the distant past, evidence increasingly suggests that those episodes reflect occasional warm spells, not a consistently hospitable phase of the planet’s history.

The research does not say that liquid water never flowed on the Martian surface, only that such events were short-lived. They looked at craters and noted that the surface has impacts from meteorites that would not have survived to the surface had the atmosphere been thick enough for liquid water.

The research however did not address Mars’ relatively smooth northern hemisphere, where there are not a lot of craters and where some scientists think there might once have been a shallow ocean. If Mars never had liquid water for long periods, why does this area lack craters?


The Moon: a desert after all?

LEND data of lunar south pole

The uncertainty of science: A new science paper, published Saturday in the Journal of Geophysical Research – Planets,, has found that there is much less water ice trapped in the permanently shadowed craters of the lunar poles than previously thought. From the abstract:

This means that all [permanently shadowed regions], except those in Shoemaker, Cabeus and Rozhdestvensky U craters, do not contain any significant amount of hydrogen in comparison with sunlit areas around them at the same latitude.

And from the paper’s conclusion:

[E]ven now the data is enough for definite conclusion that [permanently shadowed regions] at both poles are not reservoirs of large deposits of water ice.

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The strange hollows on the mountain tops of Mercury

hollows on Mercury

Another spectacular planetary science image, this time from Messenger orbiting Mercury. This close-up image of the hollows of Mercury only illustrates their mystery. The insert shows the context of the close-up image. These irregular sinks are here found on the mountain top ridge of an inner crater rim. Also, some but not all of the hollows have bright interiors.

Scientists have proposed that some form of impact melt process caused these hollows. At impact, the ground literally rippled like water when you toss a stone into a pool. Here, however, the molten ripples quickly froze, creating the inner and outer crater rim rings. To my untrained eye, the hollows look like collapse features where the surface hardened first, then collapsed when the molten inner material drained away as it became solid.

Why some hollows are bright, however, is not yet understood.


Sponge in space


On August 25 Cassini did a close fly-by of the small Saturn moon Hyperion, getting as close as 15,500 miles. The mission has just released images from that fly-by.

Looks like a sponge, doesn’t it? This moon is small, only 168 miles across, which makes it about half the size of the asteroid Vesta that Dawn is presently orbiting. Why it is so peppered with craters is of course the big science question. I would guess this has something to do with the environment around Saturn, with its rings and the innumerable particles that come from it. Yet, other moons of Saturn are not as crater-filled, so there is obviously more to this than meets the eye.

This fly-by was the second closest of Hyperion that Cassini has done, the first passing over the the moon’s surface by only 310 miles. Because the irregularly-shaped moon’s rotation is more like a chaotic tumble, scientists could not predict what part of the surface they would see. To their luck the new images captured new territory.

Another fly-by is scheduled in only three weeks, on September 16, 2011. This time, however, the spacecraft won’t get as close, passing at a distance of about 36,000 miles.


Craters, craters, everywhere

Below the fold are two images released today, one from Dawn at Vesta and the other from Messenger at Mercury. What makes them interesting to me is that, though the surfaces of both Vesta and Mercury are crater-packed, there are definitely distinct differences between them that one can spot if you look closely, all highlighting the fundamentally different environments of both worlds.

First, the Vesta photograph. The image looks out past the asteroid’s horizon, showing clearly that this dwarf planet is not spherical, with the south pole depression that puzzles scientists just on the planet’s limb. The parallel long deep grooves that are associated with this depression can be seen on the right. Notice also that the inside walls of all the craters slope downward in a very shallow manner. This gives the impression that the impacts that formed these craters smashed into an almost beachlike sandy surface. Note too the that the center of some craters have what appear to be flat small “ponds,” a phenomenon seen by the spacecraft NEAR when it orbited the asteroid Eros. These ponds are not liquid, but are actually made up of fine-grained particles that settle in the hollows of the asteroid.
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Strange craters on Vesta

strange craters on Vesta

The images from Dawn keep rolling in. The picture on the right, released two days ago, shows the asteroid’s terminator. What makes it intriguing is the weird looking crater near the bottom of the image. It appears to have formed at impact on the wall of a cliff, something that at first glance seems impossible.

This is what I think happened: The impactor sliced down the wall of the cliff, but because of Vesta’s low gravitational field the impact scar never collapsed downward, filling in.

I once wrote an article about asteroids for Astronomy where I described these objects as having the consistency of mashed potatoes and ice cream sundaes. This image illustrates this nicely. The asteroid’s weak gravitational field limits the density of its material, so that puffy strange formations such as this crater can form.