Archive for the ‘deserts’ Tag

Life in the Universe VI: Space, the Desert & Exoplanets   8 comments

The Milky Way may be home to million or billions of other living planets, but there are enormous empty spaces between us.

The Milky Way may be home to million or billions of other living planets, but there are enormous empty spaces between us.

Space is on my mind here in the deserts of southern Utah.  It isn’t so much that when the sun goes down in the desert the stars shine brightly.  It is the very nature of the desert itself.  The way small clusters of people and houses seem to occur randomly with huge empty spaces between them reminds me of the scarcity of life in an immense void.

And during this time of year at least, the way the temperature drops so quickly at night and rises almost as quick in the morning reminds me of being on an airless planet where the nearby star’s light brings intense heat during the day and biting cold at night.

The landscapes of the American southwest can often be mistaken for alien ones.  On this morning I watched a couple rock climbers scale this pinnacle.

The landscapes of the American southwest can often be mistaken for alien ones. On this morning I watched a couple rock climbers scale this pinnacle.

This is an ongoing series on my blog, believe it or not.  Like space, there are long journeys involved in going from one post to the next in the series.  The last installment, Part V, began to explore the question of life outside the solar system by highlighting the indomitable Carl Sagan.  Part IV discussed the search for life within our own solar system.  This part will continue to explore the idea of life out in the universe as a whole – a challenging subject I admit I’ve been avoiding.

The question that I posed to begin, the one which underpins the meaning of this series, is explained in Part I.

The large expanses of desert are accentuated by the lack of trees, the bare rock, and the big sky.

The large expanses of desert are accentuated by the lack of trees, the bare rock, and the broad skies.

The Milky Way rises over rock formations in Canyonlands National Park, Utah.

The Milky Way rises over rock formations in Canyonlands National Park, Utah.

The Quest for Exoplanets

Humans have found over 1000 planets outside our own solar system to date, with well over 3000 potential candidates.  In typical parochial fashion, we call these extra-solar worlds exoplanets.  The Kepler space telescope is one of the finest tools we have in the quest to find exoplanets.  It explores a constellation-sized area of the Milky Way Galaxy near Cygnus, the Swan (aka the Northern Cross).

Kepler continuously monitors the brightness of more than 145,000 stars.  It looks for a slight dimming in brightness indicative of a planet crossing between earth and the star. Think of trying to detect the dimming of a bright streetlight a mile away when a moth flies in front of it and you have the idea.

To find exoplanets, astronomers have traditionally used the slight wobble of a star that occurs when an orbiting planet tugs on it.  This gives us good information on the sizes of the planets, along with how close they orbit to their host stars.  More recently the Spitzer space telescope has detected, for the first time, actual light coming from an exoplanet.  This is key.  In order to find out anything about the surfaces of these worlds we need to examine the light bouncing off them or skimming through their atmospheres.  Spitzer and some ground-based telescopes can do the former while Kepler is uniquely suited for the latter.

Turret Arch greets a rising Orion the Hunter.

Turret Arch greets a rising Orion the Hunter.

Candidates for Life

Most of what we’ve found thus far have been very massive exoplanets the size of Jupiter and larger.  Many of these “hot Jupiters” orbit very close to their stars, closer even than our own Mercury.  As our techniques get more refined and as more time goes by (allowing the wobble method to work on exoplanet candidates orbiting further from their stars), we are finding more and more planets that are close to the size of Earth.

Crucially, we are now finding planets that orbit their stars at a distance which allows liquid water to exist.  This orbital distance, which in our solar system essentially extends from Venus to Mars, is the “habitable zone”, also known as the Goldilocks Zone. Combining these two factors that are relevant to the search for earth-like life (the planet’s size and distance to its parent star), we have found to date 12 earth-like exoplanets.

The size and brightness of the host star makes a big difference in how close a planet can orbit and still be cool enough for liquid water and possible life.  We have found only one earth-sized, rocky planet thus far (Gliese 581-g), and happily this planet orbits about the same distance from its star as earth does from the sun.  But there are two problems.  First, Gliese 581 is a much smaller and cooler star than the sun.  So its habitable zone, where water may exist, is presumably much closer in.  Gliese 581-g still would orbit within it, but depending on the shape of its orbit it may get too hot for liquid water.

There’s a much bigger potential problem, however.  The very existence of Gliese 581-g is disputed by some astronomers.  Its discovery is somewhat clouded and controversial.  Confirmation of Gliese 581-g may take some time.

A survivor in Arches National Park overlooks a desolate valley at dusk.

A survivor in Arches National Park overlooks a desolate valley at dusk.

An exoplanet called Kepler 22-b is also interesting.  The Kepler space telescope caught it passing in front of its star on just the third day of the spacecraft’s operation.  Though 22-b is some 2.5 times bigger than Earth, its parent star is very similar to the Sun (G type).  Also, 22-b orbits at an average distance very similar to Earth’s, and so its year is similar to ours.  The only problem with Kepler 22-b is that we know so little about it.  For instance, we don’t know how elliptical its orbit is.  If it is highly elongated (as most explanets’ orbits are) it might spend part of its year very very close to the star and part very far away.  Earth’s orbit is nearly circular.

The closest potentially habitable exoplanet to us is Tau Ceti-e, only 12 light years away.  That is still much too far for us to visit in anything close to a human lifetime, so we need to temper our enthusiasm.  Also, Tau Ceti-e is yet another unconfirmed exoplanet.

The Milky Way Galaxy rises vertically over Canyonlands National Park.

The Milky Way Galaxy rises vertically over Canyonlands National Park as Venus sets.

Are We on the Right Track?

You might be questioning the importance of looking for exoplanets that are earth-like, orbiting sun-like stars at earth-like distances.  You might wonder why we don’t also look for life forms that aren’t anything like ours, life that perhaps does not rely on water or based on carbon.  Also you might notice that we always speak of planets.  We know from the search for life within our own solar system that the moons around planets are in some cases better candidates for life than are the planets themselves.  Finally, life in the cosmos may in some cases be decoupled from planets or moons, living instead in space, perhaps close to large energy sources (such as quasars).

You’re right to question.  Definite biases exist in the search for extraterrestrial life.  To some extent they are unavoidable.  But consider two facts: First, it is easiest to look for earth-like planets and life.  And this is not an easy enterprise to begin with.  Second, our sort of life is all that we know for certain can exist.  Again, it is hard enough to look for our type of life trillions of miles away let alone other types.  These sound like excuses for our bias, but there it is.

And so the hunt continues for exoplanets that are candidates for earth-like life.  Based on the Kepler space telescope’s findings, astronomers estimate that perhaps as many as 20% of the sun-like stars in the our galaxy have habitable planets orbiting them.  This is a stunning estimate because it suggests that there are nearly 9 billion habitable planets in the Milky Way Galaxy.  If even a tiny percentage of these planets have developed intelligent life, then we have plenty of company in our galaxy. 

Arches National Park under the winter stars.

Arches National Park under the winter stars.

 

 

 

 

 

Death Valley IV: Geologic Features   Leave a comment

This is the first of three posts on the geology and ecology of Death Valley National Park in California.  Death Valley is Disney Land for geologists, and for anybody interested in earth science.  What isn’t as well appreciated is it’s also a very special place for desert ecologists and botanists.  But first the geology:

A colorful dawn breaks over Death Valley National Park in California.

A colorful dawn breaks over Death Valley National Park in California.

Since it is the driest place in North America, vegetation does not cover geologic features at Death Valley.  And since it lies in a place where there’s been a lot of geological action for an awfully long time, there exist a great variety of rock types and structures.  Regarding the latter, the whole region has been first smashed by mountain building and more recently torn apart by rifting.  Death Valley’s structure (meaning twisted and folded rocks, fault zones, etc.) shows this in dramatic fashion and is one of the major draws for geo-types.

I first visited Death Valley with my first year geology class.  We came down on Spring Break from drippy Oregon and boy was it nice to be in warm sunshine for a week.  We all got 3 credits for it, but it was a lark!  Since my professor was a biologist and avid birder as well as a geologist, he mixed ecology and raptor-spotting in with rocks for a really complete picture of this amazing place.

The soaring dunes at Mesquite Flat in Death Valley National Park, California.

The soaring dunes at Mesquite Flat in Death Valley National Park, California.

GEOGRAPHY AND CLIMATE

Death Valley is an enormous trench.  The vertical relief from Badwater at -283 feet elevation to the top of Telescope Peak is about 11,300 feet (almost 3500 meters)!  This giant steep-walled valley is called by geologists a graben (German for grave).  Steep fault zones, called “normal” faults, force the bordering mountain ranges up while the valley drops and fills with sediments.  This sort of faulting is repeated across the Basin and Range Province of Nevada and bordering states.

The steep mountains left by the normal faults to stand high above valley floors block moisture coming in from the Pacific and cause an extreme form of the “rainshadow effect”.  The Sierra Mountain Range, which tops out at over 14,000 feet at Mount Whitney, gets most of the rain and snow.  The Panamint Range, which borders Death Valley to the west, also gets its share.  This leaves almost no moisture for Death Valley.  That is why years can pass without any rainfall.  It is extremely arid, and this of course causes the plant and animal life to be sparse.  But the fascinating adaptations that have evolved in the life forms at Death Valley more than makes up for the paucity of biomass.

Basin and Range structure has led to two types of features.  These features, both of which are displayed at Death Valley, determine much of what goes on geographically, ecologically and even with human history here.

The extensive salt flats near Badwater in Death Valley National Park, California.

The extensive salt flats near Badwater in Death Valley National Park, California.

PLAYAS

 First thing you’ll notice are the playas (or pans), which are dried up lake beds.  These flat surfaces, which can be floored in white salts or a tan clay surface, are caused by internal drainage.  Because of the normal faulting described above, water that washes from the ranges into the basins of the Basin and Range often never makes it out along a river course. Instead, the water collects in large, shallow lakes.

When the water evaporates, salts (chlorides and sulfates of sodium, calcium, phosphorous, etc.) are left behind in the lakes.  These so-called evaporites are too heavy to be lifted into the air with the water vapor.  (This is why rainwater is fresh and why the oceans are salty.)  The salts come from weathering of the minerals in rocks of the surrounding mountains.

The full moon sets just as morning light hits the cracked salt flats near Badwater, North America's lowest point, in Death Valley, California.

The full moon sets just as morning light hits the cracked salt flats near Badwater, North America’s lowest point, in Death Valley, California.

The evaporite minerals are inevitably concentrated into the shrinking pools of water, where they crystallize into fascinating patterns.  This happens during most seasons (winters are wet and summers very dry), and so salt layers build up.  Gypsum and borax are also formed in this way.  Death Valley’s human history includes the charismatic 20 Mule Team borax story.  Near Badwater in Death Valley proper, a huge salt pan is spectacularly developed.  Take the West Side road for the best access.

 Go over to Panamint Valley in the western part of the park to see and walk on a great playa.  It was formed when fine sediment was deposited instead of pure salt.  Certainly Death Valley’s best-known example of this is Racetrack Playa, where stones appear to have skated across the playa, leaving behind their tracks.  It’s still uncertain how they move, but winds and a thin layer of ice probably have something to do with it.  Note that to visit the Racetrack in the far northern part of the park requires driving a long, long washboard gravel road.  And to make things worse, the road bed is made of especially sharp gravel, so you’ll need very good tires (and two spares).

A close view of the ridges that form the salt polygons at the Badwater salt flats, Death Valley N.P., CA.

A close view of the ridges that form the salt polygons at the Badwater salt flats, Death Valley N.P., CA.

ALLUVIAL FANS

But mostly what you’ll see in Death Valley are the other feature that result from Basin and Range faulting.  As you drive through the park, one thing you’ll notice is that this is a rocky desert, not so much a sandy one.  As you look across the valley, you’ll notice large semi-circular (fan-shaped) gravel features that narrow to a point at the canyon mouths.  These are alluvial fans, and they form everywhere that rapid uplift of mountains overwhelms the ability of rivers to transport the debris out of there.

Try walking up an alluvial fan and you will get a feel for their deceptive steepness and difficult, loose surface of cobbles.  But it’s a great education on how they form.  You’ll also see desert varnish, a dark, sort of rust that forms on the rocks when they sit undisturbed for a long time.  I rarely link to Wikipedia, but heck, go ahead and check out desert varnish.  It’s an  interesting, part living feature of the Mojave.

A black and white rendition of the simple beauty of Death Valley's sand dunes.

A black and white rendition of the simple beauty of Death Valley’s sand dunes.

When alluvial fans merge into a wedge of debris that flanks the entire range of mountains, it is called a bajada.  Eventually the mountains disappear and all that’s left is a gravel plain.  Namibia has extensive ancient gravel plains, but the American West is really much younger.  Large outcrops that stick up island-like out of alluvial fans or bajadas are called inselbergs.  Great words in geology!

I’ll get to the “rest of the story” in my next post.  I miss Paul Harvey!

The pre-dawn hours in Death Valley's sand dunes promises a beautiful sunrise.

The pre-dawn hours in Death Valley’s sand dunes promises a beautiful sunrise.

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