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ASTRONOMY BEAT

ASTRONOMY BEAT

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Editor’s Introduction When students learn about the planets, most modern textbooks and images show the view of each planet from space. But what would it be like to !nd our- ASTRONOMYselves standing on one of the solid worlds with which BEAT we share our and see its geography and weather from the surface? We asked Dr. Lori Fenton, an expert on the surface features and , to help us imagine being tourists on the red planet.

ars is a truly amazing place, breathtaking in its beauty and stunning in its . New spacecra" data over the past 20 years have Mbegun to reveal what this neighboring world is really like. In many ways, Mars is much like Earth. Its axial tilt is 25.19º, compared to Earth’s 23.45º, creating a seasonal pattern similar to our own. Its day is 24 hours "TIBEFESFMJFGNBQPG0MZNQVT.POTBOEUIF5IBSTJT.POUFT GPVSPGUIF MBSHFTUWPMDBOPFTJOUIF4PMBS4ZTUFN(+."34) and 39.6 minutes long, just a bit longer than our own day. Its atmosphere is transparent to our eyes, allowing

us to see its vistas much as we would on our own plan- Its atmosphere is unbreathable CO2, with an average et. Many geologic features, such as volcanoes, dry river surface air pressure less than a percent that of Earth’s. channels, polar caps, canyons, and dunes, are familiar Mars’ mean distance from the Sun is 1.52 AU (228 mil- to terrestrial geologists. Some Mars weather patterns lion kilometers), half again as far from the Sun as Earth. are also much like those found on Earth: cold fronts, $is distance, coupled with its low atmospheric density, dust storms, water ice clouds, and snow. leads to a mean surface air temperature of -63° C, which In other ways, Mars is remarkably di#erent from our is 78° C cooler than Earth’s mean air temperature. world. With a diameter just over half that of Earth $ese di#erences have led to some unusual condi- and a density just over two thirds that of Earth, Mars’ tions in the environment, many of which we gravity is much lower (just 38% of ours). Mars has no don’t yet fully understand. Because it is smaller than global magnetic !eld, which allows radiation harm- Earth, Mars has lost much more of its internal heat ful to life (as we know it) to reach the planet’s surface. since it formed, so plate tectonics (the movement of

"TUSPOPNZ#FBU/Pt.BZ  1BHF Left:5IFFEHFPG1MBOVN#PSFVNUIFOPSUIFSOQPMBSJDFDBQ BU& /4UBOEJOHBUUIFi9w ZPVXPVMETFFUIFTUFFQTDBSQUIBUFYQPTFTMBZFSTVOEFSUIFJDFDBQ  BTXFMMBTBMBSHFEVOFöFMEUIBUGFFETJOUPUIFMBSHFTUTBOETFBPO.BST /"4"+1-.444 3JHIU5IFMPDBUJPOPGUIFJNBHFPOBTIBEFESFMJFGNBQPG.BST(+."34) large continental plates) never got a foothold there. depends on where you are standing. I’ll describe two $e primary outcome of this inactivity is that, like the exciting places that are vastly di#erent from the rocky and many other geologically stable worlds, the plains and low hills that the Mars landers have made surface of Mars is a record of billions of years of history famous. Keep in mind that these locations only begin (in contrast, most of the Earth’s surface is less than 500 to capture the range of Mars’ moods (just as describing million years old). As a result of the lack of plate dri" the deep Marianas Trench in the Paci!c Ocean and the and Mars’ lower gravity, hotspot volcanism has allowed Kamchatka Pensisula in Russia to an alien would only the Solar System’s largest volcanoes, the three $arsis begin to characterize Earth). Still, let’s imagine that you Montes and , to grow more than two are an intrepid explorer, discovering the mystery of times taller than comparable volcanoes on Earth. Mars in a high-tech space suit. What would it be like? Tectonic ri"ing created the , a system Olympia Rupes of canyons that extends for 4000 km, reaches widths of up to 200 km and depths down to 7 km. Massive out- Let’s !rst go to the northern of Mars, %ow channels, some of which are thousands of kilome- known as (Latin for “Northern ters long, indicate that billions of years ago, enormous Plateau”), to a spot at longitude 30º E, latitude 84º N. catastrophic %oods carved the surface, possibly lead- $is deposit of ice and dust stands a few kilometers ing to a short-lived ocean in the lower-lying northern above the surrounding plains, its edges o"en ending plains. $roughout this rather dramatic history, sedi- in steep cli#s. Imagine that you are standing at the mentary layers of volcanic ash and wind-blown sand, precipice of one of these cli#s, called Olympia Rupes silt, and dust have repeatedly settled, formed bedrock, (“Olympia Scarp”), facing and looking out at and been subsequently bombarded by impacts and the sweeping vista before you (see above). Strong eroded by the wind. Today, Mars is a cold, dry, and downslope winds try to pull you over the edge, but you relatively quiet place of ice and dust, having settled into are safely anchored with ice climbing gear, free to ob- a long retirement a"er its violent youth. serve the stark beauty around you. Given this knowledge of Mars and its past, what would It is late spring, just before sunrise. You are so far north you see if you were standing on Mars today? It really that at this time of year the Sun barely skims below the horizon at night. In the sky near the horizon you

"TUSPOPNZ#FBU/Pt.BZ  1BHF see three of the four morning stars: , Earth, and largest sand sea on Mars: (“Olympia Earth’s Moon. You won’t catch a glimpse of Mars’ two Dunes”), which skirts the edge of Planum Boreum and small , and : they orbit so close in size rivals the largest sand seas on Earth. $ere are to Mars, along the equator, that they are perpetually still some small patches of seasonal frost on the shaded below the horizon at high latitudes. $in, high altitude dune slopes, but by summer solstice the dunes will be clouds obscure much of the night sky, but they have ice-free, only to soon become covered in frost once turned violet in the morning twilight. You watch as the again — just like on Earth, winter comes quickly when sun rises o# to your le", brightening the sky and illu- you’re this close to the north pole. minating the scene. Behind you and under your feet are seemingly endless layers of sand, dust, and ice that slowly but continually Amazonis Planitia (“Amazon Plain”) is a strikingly %at strive for equilibrium in Mars’ ever-changing climate region in the northern lowlands of Mars. Loose, bright system. In front of you is a sharp cli# that drops nearly dust overlies and obscures much of the bedrock. $is a kilometer at a steady slope 49 degrees from horizon- mantle and the lack of rock outcrops makes it a terrible tal. $e cli# face reveals bands of dark and light mate- place for !eld geology. But when it comes to what hap- rials; these are the same layers under your feet, piled pens in the atmosphere, it’s quite an astounding place. up over many millennia. $e origin of the icy layers is On Earth, dust devils are short-lived vortices that form thought to be related to cyclic changes in the axial tilt when the surface is much hotter than the air above and other orbital characteristics of Mars, much like it, causing the air to convect turbulently (like a pot of the ice ages on Earth are driven by slow changes in the boiling water). Vortices are common in convecting air, Earth’s tilt and orbit. and if they are strong enough and touch the surface, they can pick up sand, dust, and small debris, form- Beginning near the foot of the cli# and stretching o# ing swirling dust devils. On Earth these typically range into the distance is a dark “river” of dunes, “marching” from one to tens of meters wide and up to hundreds of away from you at the imperceptible speed of twenty meters tall. centimeters per year. $e dunes will eventually join the

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"TUSPOPNZ#FBU/Pt.BZ  1BHF Dust devils turn out to be common on Mars, but $ese are just two of the many kinds of surface environ- the ones that form during the summer in Amazonis ments on Mars. With an area that is just a little bit less Planitia take the cake, with some dust devils topping than all the continents of the Earth, there is so much out at 8 kilometers, maybe even higher! $ese mon- on Mars to explore. I would encourage you to browse sters are can be as much as a kilometer wide, and can through the many thousands of images of the red plan- survive for more than an hour (see the remarkable pic- et and imagine your own tourist experience on Mars. ture that accompanies this section). What would it be like to be in Amazonis Planitia on About the Author a summer a"ernoon when a giant dust devil forms? Imagine standing on a plain with small, gentle hills, Planetary scientist Lori Fenton seeing nothing but the pale dusty ground under you joined the Carl Sagan Center and the sky above. $is is “big sky” country on Mars, at the SETI Institute as a prin- with its hue ranging from a pale orange-yellow near cipal investigator in 2006, and the horizon to dark gray at the zenith. You may see a was awarded NASA’s Carl Sagan few bright stars where the sky darkens, and you might Fellowship for Early Career catch sight of the moon Phobos dri"ing slowly east- Researchers that same year. Lori’s ward (the opposite direction from everything else in primary research interests in- the sky!). O# in the distance is a small group of dust clude aeolian geomorphology — devils slowly moving towards you, pushed by the wind. how wind shapes a planetary surface — for both Mars Most are fairly small, perhaps a few meters across, but and the Earth; recent and ongoing climate changes on one of them stretches far into the sky. Mars, and the mobility of wind-blown sand and dust. Her research makes use of many di#erent types of in- Curious about these beasts, you stand your ground as formation, including visible and thermal imagery from they approach, and the largest one is headed straight spacecra" and atmospheric models such as the NASA for you. It is the size of an Earth tornado, but it is not Ames Mars Global Climate Model (Ames MGCM) nearly so powerful or terrifying. You watch as the and the Mars Regional Atmospheric Modeling System smaller vortices dance around one another, and as (MRAMS). Some of her recent publications describe the largest one spawns new, smaller vortices. $e tops how dunes and dune !elds record climate change on of the columns lean toward you, indicating stronger Mars, the !rst evidence for dune migration on another winds blow alo". As the big dust devil passes over you, planet, and how atmospheric models can be used to you feel bu#eted by the winds and you are surrounded account for wind gustiness and its e#ects on sand by a wall of dust that blocks out sight of the sky. You movement. She maintains a blog at http://cosmicdiary. hear the hiss of small grains hitting against your hel- org/lfenton/, and posts images of Mars daily. met, and you notice small electrical shocks snapping against your arms. Checking your sensors, you see that they register a slight drop in air pressure and a tem- Resources for Further Exploration perature increase of a few degrees. When you emerge from the downwind edge, the winds !nally fade, and See Astronomy Beat #59: “Never-Ending Postcards you !nd your space suit covered in !ne dust. from Mars” by Jim Bell $ese pillars of dust are thought to contribute to the Examples of dust devils imaged by the Mars Orbiter background dust haze that keeps Mars’ atmosphere an Camera aboard the spacecra": orange-yellow color. Why are they so big in Amazonis http://www.msss.com/mars_images/moc/8_10_99_re- Planitia? We still don’t really know. $ere is certainly a leases/moc2_171/ large di#erence in the surface and air temperatures dur- ing the day, which is the key factor in creating convec- Don Davis’ website explaining the true color of Mars: tive turbulence, and it is an area with a lot of loose dust http://www.donaldedavis.com/PARTS/MARSCLRS. sitting on the surface. But their extreme size, like so html much about Mars, is a mystery.

"TUSPOPNZ#FBU/Pt.BZ  1BHF $e HiRISE instrument website, where you can down- load a free tool to view any of the spectacular color HiRISE images, or just save their pre-generated !les as background images for your desktop: http://www.uahirise.org/ For more on the work of the scientists at the Carl Sagan Center for the Study of Life in the Universe at the SETI Institute, see: http://www.seti.org/carlsagancenter )

Send Your Favorite Teacher to Mars — Astronomy Beat is a service exclusively for members or at least to Tucson! of the Astronomical Society of the Paci!c. For more information about becoming a member, visit A hands-on astronomy workshop August 4-5 for www.astrosociety.org/membership.html. teachers in grades 3–12 from the Galileo Teacher Training Program and part of the 124th ASP Annual One copy of this article may be downloaded on any single computer and/or printed for your personal, non-commercial Meeting use. No part of any article may be reproduced in any form, Celebrating NASA’s Year of the Solar System & the sold, or used in commercial products without written Landing of the Curiosity Rover on Mars! permission from the ASP. Join us for two days of active engagement in and $e Astronomical Society of the Paci!c increases the exploration of the universe and how to bring it all to understanding and appreciation of astronomy by engaging your students! More information. scientists, educators, enthusiasts and the public to advance science and science literacy.

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