Is there water on Mars?  We know the answer is “yes”, since Mars has ice caps at both poles that grow and shrink with the seasons.  The real, harder questions are:  how much water is on Mars, and where is it located?

On Earth, of course, a lot of the water is in liquid form on the surface, with a significant amount in ice, also near the poles.  Water vapor also is a small but important component of the atmosphere.  Mars, on the other hand, has no persistant liquid surface water, and its atmosphere is drier than the driest desert on Earth.  However, features on the Martian surface strongly suggest that Mars used to be wetter in the past:  various telescopes and probes have found terrain that looks like seashores, river valleys, and other hallmarks of water erosion.

Enter the Phoenix robotic lander.  The probe’s mission is to dig underneath the surface to analyze the soil, looking specifically for subsurface water and anything else interesting.  Almost immediately, Phoenix’s scoop uncovered ice, which can be observed to melt over a period of four Martian days:

This finding is important for several reasons.  First of all, if there was surface water on Mars in the past, where did it all go, and how long did it take to change Mars from a wet planet to a dry one?  Secondly, all life on Earth contains water as an essential part of its chemical make-up, so subsurface water may be the best place to look for life on Mars.

Detecting life isn’t as simple as it might seem at first, not least because the environment on Mars is substantially different than on Earth, so Martian life might not resemble Earth life.  As Phoenix does its complete chemical analysis of soil samples, we will learn a lot, whether life is found or not, so keep an eye on the news.

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Over the last 100 years or so, our view of the universe has changed dramatically.  It was possible in the early 20th century to think that our galaxy (literally meaning Milky Way, same root as “lactose” and “lactate”) might be the entire universe.  The famous “Great Debate” between astronomers Harlow Shapley and Heber Curtis was over whether the observed “spiral nebulae” were part of our galaxy, or separate galaxies in their own right. The issue wasn’t settled during the debate, but several years later by Edwin Hubble, who used techniques developed by Henrietta Swan Leavitt to measure the distance to the Andromeda galaxy.  Needless to say, he found the distance to be much larger than the size of the Milky Way, thus showing the universe to be a very large place.

Edwin Hubble (1889-1953) and Henrietta Swan Leavitt (1868-1921)

All this is background for today’s story.  Once the universe could be divided into parts—the part inside our galaxy, and the part outside—astronomers could start studying the structure of galaxies in earnest.  Harlow Shapley himself located the center of our Milky Way and got a reasonably accurate measure of its diameter.  The challenge then became to map out the complete structure, which is a difficult task:  unlike (say) the Andromeda galaxy, we’re inside the Milky Way, so we can’t see the whole thing.  If you go out to a place with a dark sky, you can see the milky streak of light that is the disc of our galaxy, but without special equipment, you can’t find the galactic center or discern that our galaxy is a spiral galaxy.

Andromeda galaxy (M31), the spiral galaxy closest to our own galaxy

Specifically, the special equipment we need includes radio telescopes and infrared telescopes.  Radio telescopes look at the longest wavelengths of light, while infrared instruments pick out the kind of radiation we feel as heat. Radio waves help us find hydrogen, the most common chemical element in the universe, while infrared radiation highlights dust, which to astronomers means heavier molecules and aggregates of molecules, often containing carbon or oxygen atoms.  As it turns out, dust traces the spiral arms of a galaxy very well, so if you have a good infrared telescope, you can map the spiral structure even while viewing from within the Milky Way.

Which leads us to the news of the day:  the Spitzer space telescope, which is an infrared telescope, has done an extensive survey of the galaxy, and the results were announced at this week’s American Astronomical Society meeting in St. Louis:

This artist’s depiction of the structure of the Milky Way is based on the Spitzer observations, and it shows that our galaxy has two major spiral arms and a number of smaller structures. Previous observations had concluded our galaxy has four arms rather than two, which raised the question of why our galaxy seemed so much more complex than others we observed. (I guess I need to revise my lecture notes now.) With the Spitzer map of the Milky Way, the picture we can draw looks like many other two-armed spirals such as M51, the Whirlpool Galaxy:

To an alien astronomer in a distant galaxy, perhaps our Milky Way looks as beautiful.

On a personal note, I will be at the Great Lakes Cosmology Workshop at Carnegie-Mellon University next week. I will do my best to post anything exciting I find out during these proceedings.

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The Phoenix Mars probe successfully landed on Mars yesterday evening, and sent back its first pictures:

The Phoenix Mission website will have frequent updates over the course of the mission.

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The Phoenix Mars Mission has almost reached Mars, and the probe will land tomorrow: Sunday, May 25. This is one of the new generation of Mars missions, including the wildly successful robotic rovers and the orbiting Mars surveyor probe. From the Phoenix website, Phoenix

may provide important answers to the following questions: (1) can the Martian arctic support life, (2) what is the history of water at the landing site, and (3) how is the Martian climate affected by polar dynamics?

To answer these questions, Phoenix uses some of the most sophisticated and advanced technology ever sent to Mars. A robust robotic arm built by JPL digs through the soil to the water ice layer underneath, and delivers soil and ice samples to the mission’s experiments.

If all goes according to plan, Phoenix will land at 6:36 PM CDT, so keep an eye out. I will try to post an update when it is known whether the probe has landed successfully or not.

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From the Astronomy Picture of the Day, a movie simulating what the Columbia Hills of Mars would look like if you could fly over them:

Click on the picture to see the whole movie and read what this is all about.

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John Archibald Wheeler, one of the great minds of theoretical physics and astronomy, died on April 13 of this year.  He is perhaps best known for popularizing the term “black hole” to describe the collapse of a massive star core upon its death.  He worked with many of the greatest scientists of the 20th century, including Niels Bohr and Richard Feynman.

For those of us who have studied Einstein’s general theory of relativity, Wheeler’s massive book Gravitation (written with Charles Misner and Kip Thorne) holds an important place in the canon.  Wheeler also helped develop the S-matrix technique for calculating the scattering properties of subatomic particles, and (with his student Feynman) an intriguing theory of light emission and absorption known awkwardly as “delayed action-at-a-distance”. He spent most of his career at Princeton University.

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The brightest object ever seen in the universe was detected last week: a gamma ray burst 7.5 billion light years away. For a brief time, the afterglow of the huge explosion could be seen with the unaided eye in the constellation of Boötes. To put 7.5 billion years in perspective: the Earth is 4.6 billion years old, and the entire universe is 13.7 billion years old. This means that, when the light was emitted from this gamma ray burst, the Earth hadn’t even formed!

This image shows the afterglow in X-rays (left) and in ultraviolet light (right). The March 28 Astronomy Picture of the Day has more information.

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The 250 meter (919 foot) asteroid 2007 TU24 passed Earth on Tuesday, January 29, but it didn’t come close enough to pose danger. It was small enough that very few people would even be able to see it, but large enough that if it had hit Earth, it would be seriously worrisome. For more information, see the Astronomy Picture of the Day for January 30.

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The MESSENGER spacecraft has returned the first new photos of planet Mercury (the closest world to our Sun) since Mariner 10’s mission over 35 years ago. Up until MESSENGER’s mission, only about 40% of Mercury’s surface had been mapped, so a lot is unknown.

Here is one of the new photos; clicking on the the picture or the link following will bring you to a page with more information:

MESSENGER Website

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Mars escaped being struck by an asteroid, which up until recently was believed to be on a collision course. More information:
http://neo.jpl.nasa.gov/news/news156.html

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