Cassini HD. Thinx Media. Published Saturday 9/15. iPad. Free on 9/15, then $1.99.

Reviewed by Carl Zimmer

Mars Curiosity, with its sky crane and all, is certainly impressive. But I'm a Cassini fan myself. It left Earth in 1997 and arrived at Saturn in 2004. It has winged around the ringed planet ever since, hurtling by Saturn's many, many moons along the way. Every year it has delivered images back to Earth that are not simply gorgeous but deeply informative about the outer zone of our Solar System. Its success and its stamina have gotten its mission extended twice. It will keep snapping pictures of Saturn and its moons until 2017, when it crashes into the planet.

I've tried to keep up with the flow of images at the Cassini web site, but after eight years of daily footage, there is just too much to handle. To appreciate everything it has found, we need curation.

One form of curation can be found at sites like Bad Astronomy, where Phil Plait regularly posts Cassini images that he finds particularly worthy. Thinx Media now provides curation of a different sort, by loading some 840 images into an app they call Cassini HD.

The images are organized into categories–starting with the planet itself, followed by images of its moons, and then its rings, and finally by false-color pictures. You can plow through the photos one at a time, swipe after swipe, or jump to two navigation systems: either a drop-down menu, or a gallery linked to a diagram of Saturn and its moons.

I preferred jumping around the Saturnian system. At the moment, my favorite moon of Saturn is Daphnis, a five-mile-wide rock that draws a thin path of ice and dust out of the planet's rings. If you like what you see, you can use Cassini HD's nicely integrated functions to email or tweet a photo, save it to the iPad's photo app, or send it to Tumblr or Facebook.

There are two big shortcomings of Cassini HD. For one thing, it's not particularly HD. You can't zoom in on details of the photos on the app. For a closer look at Titan or Enceladus, you will need to look at NASA's biggest versions of their images on their web site.

The other shortcoming is the text–the reason that I'm reviewing this app at Download the Universe. Each section of Cassini HD kicks off with a paragraph, or a few. Each photo comes with a one sentence caption, which you can expand into a longer version. But, as far as I can tell, all the text comes verbatim from NASA's web sites. I won't call this plagiarism, but I will call it disappointing. Every caption comes with an identical paragraph about the Cassini mission. One caption I came across informs you what Cassini will be doing in 2005–because it was written by someone at NASA in 2004. The captions point you to other pictures for further information, but they include the original links, so that you end up on NASA's web site instead of jumping to other images in the app. There's useful information to be gathered the text, but you will get little pleasure along the way. In this respect, Cassini HD is a far cry from The Solar System, which featured original text by the science writer Marcus Chown.

On balance, however, Cassini HD is a good value. The Solar System will set you back $13.99. Cassini HD will cost you just $1.99–and it's free tomorrow, when the app goes live. I like to know that even if I'm out of Wi-Fi range, I can always take a trip through Saturn's rings. The text may not sing, but the pictures are still transporting.

Carl Zimmer writes frequently about science for the New York Times and is the author of 13 books, including Evolution: Making Sense of Life.

Moon Rocks: An Introduction to the Geology of the Moon by Andrew G. Tindle and Simon P. Kelley. Published by The Open University. iPad (iBooks 2 and iOS5 required), free.

Guest review by Veronique Greenwood

In 1988, after 12 men had walked on the surface of the Moon and nearly 850 pounds of lunar rocks had been ferried back to Earth, 13% of Americans were purportedly still under the impression that the Moon was made of green cheese. While I hope that number has shrunk in the last couple decades, I can testify that I know precious little more about the Moon than that it is indeed made of rock and that during the Cold War, bits of it were glued to plaques and passed out as gifts. Also, at some point, I believe some golf was played there.

If you're looking to sound like less of a dunce at astronomer cocktail parties, you might want to check out The Open University's free 70-page ebook on the basics of Moon geology. It's well, if plainly, written and provides links to the original research that underlies our understanding, though on several important counts it falls short of fulfilling its promise as an interactive textbook.

I learned some interesting tidbits, especially in the first chapters, that made me look at the Moon differently. For instance, we're still not really sure how it formed; each of the leading theories explains some, but not all, of what we've observed about it. One of the most widely accepted models proposes that the Earth was struck by an object the size of Mars about 4.6 billion years ago, and the debris flung off by the cataclysm coalesced into the Moon. Alongside this rather dry description was a truly alarming figure showing sequential shots of our planet in the 23 hours after the impact, lurching on its axis and spraying out matter like a soaked tennis ball shedding water.

But the breathtaking violence wasn’t over yet. As it turns out, after the planets of our solar system themselves coalesced from surrounding debris millions of years earlier, there had been quite a lot of stuff left over. This floating junk was cleared from the inner part of the solar system by the planets sweeping around their orbits. Once the giant planets—Jupiter and Saturn—started to creep outwards, things got nasty fast. “Resonance effects caused orbital eccentricities that destabilised the entire planetary system,” the text relates. “Rapid and dramatic movement of the giant planets then occurred, causing 99% of the mass of the primordial disc to be ejected from the solar system and for much of the remainder to be thrown inwards to cause an influx of asteroids and thus a surge of impacts on the inner planets.”

To translate: as the planets shifted into a new alignment, they pulled on each other gravitationally such that their neat, concentric orbits went all to hell, and they careened around in a way that sets my teeth on edge just thinking about it, in the process flinging a punishing rain of giant boulders onto the inner planets, which is how our Moon got so bunged up.

The next chapter fast-forwards several billion years. Things are much quieter. It's 1971, and the Apollo 15 astronauts are collecting pieces of the moon when one of them, David Scott, gasps and cries, “Guess what we just found! Guess what we just found.” As related in a transcript of the mission audio, he's found a crystalline rock, which is beautiful proof that when the Moon was young, it formed a crust like Earth's.

I was excited to see that the book contained an embedded video of the Apollo 15 astronauts making their discovery. I love these fuzzy old recordings, these time capsules of men with gentle mid-twentieth century American accents exclaiming “Oh boy!” when they come across a chunk of lunar feldspar. But whenever I clicked on the video (I tried several times), it froze up, and all I got was the audio.

Audio from the Moon is better than no audio from the Moon, but I was much more disappointed when I got to the book's interactive activities. The book makes several mentions of petrology—the geology equivalent of pathology. Basically, you take fine slices of rock and look at them through a microscope in various kinds of light and identify the minerals within. With each mention of petrology I looked forward to trying it out for myself with the book's “virtual microscope.”

When I reached it, I skimmed the first activity's description, which laid out how the different kinds of light revealed the identifying properties of minerals, and jumped right into the interactive element. Once I was in, though, I saw that there was no interpretive text. There were zoomable views of the sample in various lights, but all of the explanations of what I was looking at were stranded back out in the main text. To learn anything about what to look for, I had to close the interactive element, read from the text, then open the element and try to click back to exactly the place where I had been before.

This was roughly as frustrating as being told you can look at either a guidebook or a city map, but never both at the same time.

As a result, I got very little from the seven virtual microscope activities, aside from some aesthetic enjoyment. I would have learned more if I'd had a paper textbook and a companion app that could be used together at the same time. I’m surprised that the Open University, a 41-year-old UK institution whose focus is distance learning, would have bungled this point.

The disappointment of the virtual microscope aside, the book succeeds fairly well as a teaching text for curious amateurs. I know quite a bit more about the Moon now than I did before. I can say with certainty that it's not made of green cheese but of things that–to a Moon n00b like me–can seem just as fanciful: substances like the mineral olivine, which is a remarkable canary yellow under cross-polarized light; and moondust, which, for reasons that are still mysterious, smells just like gunpowder.

Veronique Greenwood is a staff writer at DISCOVER Magazine. She writes about everything from caffeine chemistry to cold cures to Jelly Belly flavors, and her work has appeared in Scientific American, Technology Review, TheAtlantic.com, and others. Follow her on Twitter .