Did Mars and Earth swap microbes?

Astrobiology is based on the hope that life is widespread in the universe. There are two ways this might be the case. One is that life is easy to incubate and so will pop up wherever planets resemble Earth. The other is that life’s origin requires very rare and special conditions, but that once it gets going it spreads around the universe, a theory known as panspermia, meaning “seeds everywhere”.

The basic idea of panspermia goes back to antiquity, but it was placed on a modern footing by the Swedish chemist Svante Arrhenius in the early 20th century. In the 1970s it was refined by the British astronomer Fred Hoyle and his collaborator Chandra Wickramasinghe. The theory remains highly controversial in the original form, with naked microbes wafting across interstellar space, which we now know is saturated with deadly radiation.

But there is one convincing version of the theory. From time to time, Earth and Mars take a hit from a comet or asteroid with enough force to blast rocks around the solar system. Some terrestrial rocks will fall on Mars and vice versa – my university has half a dozen Mars rocks that landed as meteorites.

If Earth and Mars can trade rocks, surely they can trade life too? Shielded within a rock, a hardy microbe could easily withstand the harsh environment of outer space and so arrive at the other end still viable.

When I suggested this in the early 1990s I received nothing but derision. It was objected that microbes wouldn’t survive being kicked off a planet, or the fiery plunge through the atmosphere. However, it has been shown that the Mars meteorites generally do not show signs of shock heating, only the outer layer of a meteorite becomes incandescent, and it all happens so fast that the interior doesn’t get hot. Today, these objections have largely melted away.

The bombardment of the planets by comets and asteroids was far more severe in the past, especially before about 3.8 billion years ago, after which it tailed off somewhat, although it has never ceased entirely. Evidence suggests that until about 3.5 billion years ago Mars was warm and wet and far more earthlike than it is today. As we know there was life on Earth at that time, it seems inevitable that the transfer of viable organisms from Earth to Mars would have occurred, thus seeding the red planet with Earth life. Of course, the same mechanism works in reverse; indeed, it is easier to knock rocks off Mars because of its lower gravity and thinner atmosphere.

All of which raises the intriguing question of whether life on Earth may have started on Mars and come here in impact ejecta, implying that we are all the descendants of Martians. Mars does have a few favourable aspects as an incubator of life; certainly early Mars was no less congenial than early Earth for biology to get started. But whichever way around it was, it seems that if we ever find traces of life on Mars, chances are it will just be good old terrestrial life.

Fascinating though that may be scientifically, it would fail to answer the much deeper question of whether life is easy or not to start. Ideally we would like to find a second genesis of life on Mars, thus answering the question in the affirmative.

The spread of life between near-neighbour planets could be common throughout the universe. But what about longer journeys?

Calculations show that some Earth ejecta will reach the outer moons of the solar system, such as Europa, although the probability of a successful transfer of life isn’t promising. It will also happen from time to time that ejected Earth rocks will be flung out of the solar system altogether by the gravitational field of Jupiter. But now the numbers are very unfavourable: the chances of a terrestrial rock ever hitting another earthlike planet beyond the solar system are tiny, and even the hardiest microbe would be unlikely to survive a journey of millions of years.

On the other hand, our sun was born amid a cluster of closely-spaced stars, so if life was established quickly somewhere in the cluster, there is a possibility that it could have spread rapidly between the nascent planetary systems.

If life on Earth did arrive from elsewhere, the problem of how and where it first arose gets shifted off into unknown territory. Whether kicking the can down the road – or across the galaxy – amounts to good science is debatable.

Paul Davies

Reference: 

https://cosmosmagazine.com/biology/did-mars-and-earth-swap-microbes

What Planetary Protection Protocols Do

Arguments calling for extra caution have permeated Mars exploration strategies and led to the creation of specific guiding policies, known as planetary protection protocols.

Strict cleaning procedures are required on our spacecraft before they’re allowed to sample regions on Mars which could be a habitat for microorganisms, either native to Mars or brought there from Earth. These areas are labeled by the planetary protection offices as “Special Regions”

The worry is that, otherwise, terrestrial invaders could jeopardize potential Mars life. They also could confound future researchers trying to distinguish between any indigenous Martian life forms and life that arrived as contamination from Earth via today’s spacecraft.

The sad consequence of these policies is that the multi-billion-dollar Mars spacecraft programs run by space agencies in the West have not proactively looked for life on the planet since the late 1970s.

That’s when NASA’s Viking landers made the only attempt ever to find life on Mars (or on any planet outside Earth, for that matter). They carried out specific biological experiments looking for evidence of microbial life. Since then, that incipient biological exploration has shifted to less ambitious geological surveys that try to demonstrate only that Mars was habitable” in the past, meaning it had conditions that could likely support life.

Even worse, if a dedicated life-seeking spacecraft ever does get to Mars, planetary protection policies will allow it to search for life everywhere on the Martian surface, except in the very places we suspect life may exist: the Special Regions. The concern is that exploration could contaminate them with terrestrial microorganisms.

Can Earth Life Make It On Mars?

Consider again the Europeans who first journeyed to the New World and back. Yes, smallpox and syphilis traveled with them, between human populations, living inside warm bodies in temperate latitudes. But that situation is irrelevant to Mars exploration. Any analogy addressing possible biological exchange between Earth and Mars must consider the absolute contrast in the planets’ environments.

A more accurate analogy would be bringing 12 Asian tropical parrots to the Venezuelan rainforest. In 10 years we may very likely have an invasion of Asian parrots in South America. But if we bring the same 12 Asian parrots to Antarctica, in 10 hours we’ll have 12 dead parrots.

We’d assume that any indigenous life on Mars should be much better adapted to Martian stresses than Earth life is, and therefore would outcompete any possible terrestrial newcomers. Microorganisms on Earth have evolved to thrive in challenging environments like salt crusts in the Atacama desert or hydrothermal vents on the deep ocean floor. In the same way, we can imagine any potential Martian biosphere would have experienced enormous evolutionary pressure during billions of years to become expert in inhabiting Mars’ today environmnents. The microorganisms hitchhiking on our spacecraft wouldn’t stand much of a chance against super-specialized Martians in their own territory.

So if Earth life cannot survive and, most importantly, reproduce on Mars, concerns going forward about our spacecraft contaminating Mars with terrestrial organisms are unwarranted. This would be the parrots-in-Antarctica scenario.

On the other hand, perhaps Earth microorganisms can, in fact, survive and create active microbial ecosystems on present-day Mars – the parrots-in-South America scenario. We can then presume that terrestrial microorganisms are already there, carried by any one of the dozens of spacecraft sent from Earth in the last decades, or by the natural exchange of rocks pulled out from one planet by a meteoritic impact and transported to the other.

In this case, protection protocols are overly cautious since contamination is already a fact.

Technological Reasons the Protocols Don’t Make Sense

Another argument to soften planetary protection protocols hinges on the fact that current sterilization methods don’t actually “sterilize” our spacecraft, a feat engineers still don’t know how to accomplish definitively.

The cleaning procedures we use on our robots rely on pretty much the same stresses prevailing on the Martian surface: oxidizing chemicals and radiation. They end up killing only those microorganisms with no chance of surviving on Mars anyway. So current cleaning protocols are essentially conducting an artificial selection experiment, with the result that we carry to Mars only the most hardy microorganisms. This should put into question the whole cleaning procedure.

Further, technology has advanced enough that distinguishing between Earthlings and Martians is no longer a problem. If Martian life is biochemically similar to Earth life, we could sequence genomes of any organisms located. If they don’t match anything we know is on Earth, we can surmise it’s native to Mars. Then we could add Mars’ creatures to the tree of DNA-based life we already know, probably somewhere on its lower branches. And if it is different, we would be able to identify such differences based on its building blocks.Bacterial species Tersicoccus phoenicis is found in only two places: clean rooms in Florida and South America where spacecraft are assembled for launch. Mars explorers have yet another technique to help differentiate between Earth and Mars life. The microbes we know persist in clean spacecraft assembly rooms provide an excellent control with which to monitor potential contamination. Any microorganism found in a Martian sample identical or highly similar to those present in the clean rooms would very likely indicate contamination – not indigenous life on Mars.

The Window Is Closing

On top of all these reasons, it’s pointless to split hairs about current planetary protection guidelines as applied to today’s unmanned robots since human explorers are on the horizon. People would inevitably bring microbial hitchhikers with them, because we cannot sterilize humans. Contamination risks between robotic and manned missions are simply not comparable.

Whether the microbes that fly with humans will be able to last on Mars is a separate question – though their survival is probably assured if they stay within a spacesuit or a human habitat engineered to preserve life. But no matter what, they’ll definitely be introduced to the Martian environment. Continuing to delay the astrobiological exploration of Mars now because we don’t want to contaminate the planet with microorganisms hiding in our spacecrafts isn’t logical considering astronauts (and their microbial stowaways) may arrive within two or three decades.

Prior to landing humans on Mars or bringing samples back to Earth, it makes sense to determine whether there is indigenous Martian life. What might robots or astronauts encounter there – and import to Earth? More knowledge now will increase the safety of Earth’s biosphere. After all, we still don’t know if returning samples could endanger humanity and the terrestrial biosphere. Perhaps reverse contamination should be our big concern.

The main goal of Mars exploration should be to try to find life on Mars and address the question of whether it is a separate genesis or shares a common ancestor with life on Earth. In the end, if Mars is lifeless, maybe we are alone in the universe; but if there is or was life on Mars, then there’s a zoo out there.
From: The Conversation
Author: Alberto G. FairénReference:

https://futurism.com/we-shouldnt-worry-about-contaminating-mars-with-earth-microbes/

Life was scientifically detected on Mars more than 40 years ago.

In the Viking lander experiments life was detected but a conservative and erroneous interpretation of the upper level authorities of NASA  decided that it was not. This interview to Gilbert V. Levin describes this strange (and negative) approach to research that was utilized by the NASA mission management team.

One of the Principal Investigators for the NASA Viking biology team was Dr. Gilbert V. Levin who invented and built the Viking Labeled Release Experiment. His experiment tested the soil of Mars nine times at two different landing sites under different temperature regimes and environmental conditions. All his data point to microbes metabolizing a nutrient solution and giving off an indicative radioactive CO2 gas. In 1997, Levin simultaneously reported in my book MARS: THE LIVING PLANET and in an Astrobiology Proceedings paper for the SPIE, that his experiment definitely detected living organisms on the surface of Mars. He has been highly criticized by many of his peers, but certainly not all. With the recent smoking gun evidence of meandering river channels on Mars formed by liquid water, the odds that Mars once had life and still has life today have gone up significantly.

In the following conversation I talk with Dr. Levin about his early work as a Sanitary Engineer and how it got him involved with NASA and the search for life on Mars.

1) You actually started your career looking for microbes in municipal water systems correct?

My professional career started as a 'sanitary engineer.' During my senior year in high school I met a sanitary engineer who was a commissioned officer in the U.S. public health service. He told me of the many facets to this profession and the multiple scientific and engineering disciplines involved in protecting the public health. Career possibilities included water supply, wastewater, drainage, air pollution, foods, and all aspects of the environmental protection, and the development of relevant processes and products. I applied to the Johns Hopkins University to enroll in its sanitary engineering program and was accepted. The first step was to obtain a bachelor's degree in civil engineering, and then take a master's degree in sanitary engineering and public health. Upon completion of both degrees, I went to work for the Maryland state health department as a junior sanitary engineer. My assigned responsibilities dealt with municipal water supplies, waste water disposal, industrial waste disposal, shellfish sanitation, and swimming pools. Water quality analysis, especially microbial, was involved in all these activities. Early on I became especially interested in the microbiology concerned projects I was assigned.

2) You worked with a microbial detection technique called radiorespirometry in the late 1950's that was extremely sensitive for the detection microbes in water and in blood. Are you the inventor of this method and how does it work?

I am the inventor. It is a very simple test, patterned after the long-used, classic method for detecting bacteria. That method placed a sample of the material suspected of bacterial contamination into a test tube containing a liquid broth designed to culture the bacteria. If bacteria were present, they would eat the nutrient and reproduce. At the same time they were exhaling gas as part of their metabolism of the food. Eventually enough gas would be expired to create small, visible bubbles. The bubbles were proof that bacteria were present. Some tests were designed to detect any bacteria. Others were designed to detect specific species. The types of nutrient used determined which bacteria would respond. Varied depending on the specific test, the length of time required to detect the bacteria ranges from one to several days, even up to a week. My invention was simply to add tiny amounts of radioactive nutrient into the nutrient(s) used in the test. Chemically there was no difference between the radioactive molecules and the nonradioactive ones. The bacteria could not tell the difference between them and metabolized them both. However, when radioactive molecules were metabolized the gas produced was radioactive. Methods to detect radioactivity are so sensitive that the gas can be detected within minutes, providing answers almost immediately compared to the length of time required by the classic method. In the standard test, bacteria have to reproduce to about a million per milliliter of culture broth to produce visible bubbles. The radioactive method is so sensitive that as few as ten bacterial cells in the sample can be detected in about half an hour, before any growth occurs. Growth is not needed. I developed the method to detect total bacteria and to detect coliform organisms (of sewage origin)for use in detecting contamination of drinking water and swimming water. This was adopted by several states as an emergency water supply public method. I then developed the method and associated instrumentation to be able to detect and identify specific pathogenic microorganisms of public health interest. The method is now used in hospitals and clinics worldwide to detect human blood infection very quickly.

3) Didn't you have a problem selling the invention initially?

My carbon-labeled microbial respirometry technique worked very well, both to detect and to identify microorganisms. However, potential user agencies feared the public relations aspect of using radioactive material. Of course, hospitals were using increasing amounts of isotopes and X-rays, but even they resisted (until sometime later) expanding that use into microbiological testing. This was frustrating.

4) How did you get involved with NASA?

In 1958, I accompanied my wife, then a reporter for Newsweek magazine, to a Christmas party at the home of the Washington bureau chief, Ernest Lindley. There I met the first Nasa administrator, Kieth Glennan and we had a nice talk about space research. I had long been interested in the possibility of life beyond the earth. When I was 9 years old, my cousin, pointing out Mars to me, told me about an astronomy course she was taking at college where the possibility of life on mars and elsewhere was discussed. An idea dawned on me at the party. Putting down my martini, I asked, only half-jokingly, whether Nasa might ever look for life on Mars. Glennan surprised me by saying he was planning to do so, and that he had just hired an M.D., Clark Randt, to head up a new Nasa biology program. Glennan suggested I go see Randt and tell him about my test. I made an appointment very soon after. Randt was most receptive and told me to submit my idea as a proposal for possible funding for me to do the research. This was very exciting, and I promptly went to work crafting a proposal explaining what needed to be done to develop my microbial radiospirometry experiment and an instrument to perform it on Mars. He said Nasa intended to fund several such experiments and to choose a number of them for a Mars lander.

5) When did NASA officially fund you for this?

In 1959, Nasa funded my proposal to develop my radiosrespirometry experiment to go to Mars. I named it 'Gulliver,' because it was to seek Lilliputian life forms on a far away land, and I hired a small team to help me in the laboratory. The development went exceedingly well. Within the first year we had developed a suitable nutrient for detection of a broad array of microorganisms, selected and incorporated the radioactive carbon label, and demonstrated the sensitivity and quickness of the technique. Later, Nasa changed the name to 'Labeled Release' to indicate the seriousness of its purpose. Before the end of the year we had a working instrument that a subcontractor manufactured to meet our concepts. We tested the instrument on a nearby playground and it promptly detected microorganisms.

6) Can you describe how the Gulliver worked?

The instrument shot out 2 greasy strings that fell onto the ground with their free ends landing about 100 feet from the instrument. The strings were then reeled in, collecting tiny particles of soil that adhered. A glass vial of the nutrient was broken over each reel. The soil organisms promptly attacked the nutrients and produced radioactive gas. Geiger counters measured the radioactivity of the gas as it rose above the reel, providing evidence that a reaction had taken place. When one reel showed a positive response, the other was promptly doused with a poison to kill any microorganisms on it in order to serve as a control. The monitoring for radioactive gas arising from each reel continued. In our very first field test, the poisoned reel produced very little gas, while the test reel produced thousands of counts per minute in about half an hour. The difference between them proved that the first reel was responding to living organisms.

During the ensuing years, Nasa funded about 10 mars life detection experiments, including two additional ones of mine: the 'Dark Release' experiment - which detected photosynthetic microorganisms by demonstrating their uptake of radioactive carbon dioxide in the light, and their release of the gas in the dark; and 'Diogenes,' based on the enzymes in the firefly lantern that light up in the presence of adenosine triphosphate, a chemical that is the immediate energy provider in all known metabolism. All the experimenters went full tilt in developing their experiments and enabling robotic instruments in the hope of making it aboard a Mars lander whenever it might be designated.

Reproduced and adapted from Space Daily

Author: Barry E. DiGregorio
Ref.  http://www.spacedaily.com/news/mars-life-03l.html

Comet's 2014 Mars Flyby Caused Most Intense Meteor Shower Ever Recorded

By Mike Wall, Space.com Senior Writer | September 21, 2017 

·          

An artist’s illustration of the Martian meteor shower caused by Comet Siding Spring’s flyby of the planet on Oct. 19, 2014. The comet has passed Mars in this view and is shown heading back toward the outer solar system. Mars’ atmosphere is exaggerated in this illustration to highlight the presence of a coherent group of meteors due to the comet’s debris stream.The annual Perseid meteor shower may be great, but it's got nothing on the brief sky show a comet gave Mars a few years back.Comet Siding Spring produced the most intense meteor shower in recorded history when the object flew by the Red Planet in October 2014, according to newly analyzed data collected at the time by NASA's Mars Atmosphere and Volatile Evolution (MAVEN) orbiter.

MAVEN'S observations suggest that the Siding Spring shower boasted about 108,000 meteors per hour at its peak and lasted up to 3 hours, scientists led by Matteo Crismani, of the University of Colorado Boulder, reported today (Sept. 21) in a presentation at the European Planetary Science Congress 2017 (EPSC 2017) in Riga, Latvia. 

For comparison, viewers with dark skies can usually count on seeing about 80 meteors per hour during the mid-August peak of the Perseid meteor shower, which is perhaps the most famous and reliably impressive of Earth's annual showers.The Perseids and other such showers occur every year when our planet plows through streams of debris shed by comets over the eons. Each shower is caused a particular debris stream. (In the Perseids' case, this debris comes from Comet Swift-Tuttle.)

The Siding Spring Martian shower, however, was a one-off event. The comet zoomed within 87,000 miles (140,000 kilometers) of the Red Planet's surface on Oct. 19, 2014, sending huge numbers of particles careening into the thin Martian atmosphere.

"This is one of the most exciting planetary events that we'll see in our lifetime," Beatriz Sanchez-Cano, of the University of Leicester in England, said in a statement. "Mars was literally engulfed by the coma, the comet's outer atmosphere, for several hours."

Scientists therefore observed the flyby using a number of instruments, including the spacecraft orbiting the Red Planet and roving across its surface. (MAVEN arrived just in time for the show, entering orbit around Mars in September 2014.)

Reproduced from space.com 

Mars Satellites

Phobos and Deimos were predicted by Jonathan Swift

Obviously the Viking missions were a watershed in the study of Mars. Since then, three more spacecraft have been to the planet. In July 1988, two Russian spacecraft, Phobos 1 and Phobos 2, were launched. Contact was lost with Phobos 1 on its way out from Earth, but Phobos 2 successfully entered Martian orbit in January 1989. During the next fifty-nine days it obtained enough photographs to map nearly the entire planet---unfortunately, the full results have not yet been published in the West. There was also the American Mars Observer, which, in a stunning setback, went dead in August 1993, just as it was entering the final phase of its approach to the planet---only three days from its destination!

The primary objective of the Russian Phobos mission had been not the planet itself, but Phobos, the larger of the two Martian satellites. Plans called for placing a small lander on the surface of Phobos, but unfortunately, contact was lost in March 1989, just as Phobos 2 was starting to image the small moon and approach it for the landing phase.

Tiny as they are, the moons are intriguing worlds in their own right. The events leading up to their discovery by Asaph Hall in 1877 have already been discussed, but, strangely, their existence had been guessed on several earlier occasions, including by Jonathan Swift in 1726.

That year Swift published Gulliver's Travels, which describes the imaginary exploits of Lemuel Gulliver. Though his visit among the tiny Lilliputians is perhaps the best known, Gulliver made other explorations. On his "Voyage to Laputa," Gulliver learns that the scientists there

have . . . discovered two lesser stars, or satellites, which revolve about Mars; whereof the innermost is distant from the center of the primary planet exactly three of its diameters, and the outermost five; the former revolves in the space of ten hours, and the latter in twenty one and a half; so that the squares of their periodical times are very near in the same proportion with the cubes of their distance from the center of Mars; which evidently shows them to be governed by the same law of gravitation that influences the other heavenly bodies.1

Swift's prediction is surprising in that he not only had the number of moons right, but he also placed them close to the planet---the distances of the actual Martian moons are 1.4 and 3.5 diameters of Mars, compared with 3 and 5 as given by Swift. One would almost be tempted to think that Swift obtained an actual glimpse of the moons through a telescope, were it not for the fact that there was no telescope at the time anywhere close to being powerful enough to show them. Voltaire, in his 1750 story Microm�gas, which tells of the visit by an inhabitant of the star Sirius to the solar system, also credited Mars with two moons, but here, at least, there is no mystery; he must have been influenced by Swift's tale.

The idea that Mars might have two satellites harks back still earlier, however, to Kepler's misconstrual of the anagram in which Galileo announced the discovery of what we now know to be the ring of Saturn.² Probably Swift had learned of Kepler's earlier surmise. Moreover, since at the time he wrote it was believed that Mercury and Venus were companionless, Earth had one satellite, Jupiter had four, and Saturn had five, Mars's place in this progression seemed to call for two moons. Since they remained hidden, the moons had to be very small, and if they were very close to the planet they would be lost in its glare. However Swift arrived at his prediction, there can be no doubt that it was simply a lucky guess.

After the proper discovery of the satellites by Asaph Hall in August 1877, it was immediately apparent that they are highly unusual objects. Phobos lies at a distance of 9,400 kilometers from the center of Mars, or only 6,000 kilometers from the Martian surface. Mars seen from its surface would be an astounding sight; its disk would subtend an angle of 43�, and it would fill nearly half the sky from horizon to zenith! The present period of revolution of Phobos around Mars is only seven hours and thirty-nine minutes. Thus it completes three full revolutions in the time that Mars takes to rotate once on its axis---a state of affairs so surprising that Hall at first thought there must be two or three inner moons! Owing to its rapid motion, Phobos rises in the west and sets in the east, and it remains above the horizon for only four and a half hours at a time.

Because its orbital inclination is only about Phobos, for all practical purposes, lies in the equatorial plane of the planet. It is eclipsed by the planet's shadow 1,330 times every Martian year, managing to escape only for brief periods around the times of the summer and winter solstices. Observers on the Martian surface above 70 north and south latitudes would never catch sight of it at all, since it would never clear the horizon.

Deimos lies 23,500 kilometers from the center of Mars, and its orbit, too, is nearly equatorial. The period of revolution is about thirty hours, and it remains above the Martian horizon for sixty hours at a time. It never rises above the horizon in the polar regions above 82 north or south latitude.

In 1945, after analyzing measures of the positions of the satellites made since their discovery in 1877, B. P. Sharpless announced that Phobos appeared to be rapidly spiraling inward toward Mars.³ Such an acceleration could only be produced by some sort of drag, and in 1959 a Russian astronomer, Iosif Shklovskii, concluded that the drag was due to friction with the outer atmosphere of Mars. This was reasonable enough; however, in order to explain the rapid rate of its acceleration, Shklovskii went further and proposed that Phobos must be hollow inside---and that it might even be an artificial space station!⁴ Subsequently, someone suggested that the reason the satellites were not discovered until 1877, despite careful searches by William Herschel and Heinrich d'Arrest, was that they did not yet exist!

Needless to say, Shklovskii's view was always regarded with considerable skepticism, and later studies have shown that although Phobos is indeed spiraling inward toward Mars, the rate of its acceleration is only about half that derived by Sharpless---about 15 in orbital longitude since 1877. This is a small enough quantity to be accounted for by frictional forces due to tides raised by Phobos in the solid body of Mars. The acceleration will continue for another 40 million years or so, until the moon immolates itself by crashing into the planet.⁵

Owing to similar tidal forces, Deimos, whose period of revolution is slower than the period of Mars's spin, is spiraling very slowly outward from Mars; however, the effect is very slight and actually produces very little change in its orbit.

Both Martian satellites are tiny, and this, together with their proximity to the bright planet, explains why they were not discovered earlier. In Earth-based telescopes they are mere glints of light, and only with the advent of the spacecraft era have we begun to find out what they are really like (appendix 4).

The first close-up pictures of Phobos and Deimos were obtained by the Mariner 9 spacecraft in 1972; since then, they have also been imaged by the Viking orbiter spacecraft and the Russian Phobos, which sent back some useful results from Martian orbit in March 1989 before suddenly losing contact. Phobos, which measures 27 by 19 kilometers, is shaped rather like a potato; Deimos too is oddly shaped, though less so than Phobos, and measures 15 by 11 kilometers.

Both moons have suffered heavy bombardment and have numerous impact craters to show for it. Phobos has a particularly large one, named Stickney (after the maiden name of Asaph Hall's wife, who encouraged him to continue his flagging search for the moons). It is 10 kilometers across, and the impact that formed it must have come close to smashing Phobos into pieces. Radiating in all directions from Stickney are a series of ridges and grooves. The grooves are widest (700 m) and deepest (90 m) close to the crater itself, and they converge again near the crater's antipode, which is nearly groove-free. Obviously these features are intimately associated with Stickney itself, and seem to be deep-seated fractures formed during the impact. After Stickney, the largest craters on Phobos are Hall, Roche, Todd, Sharpless, and d'Arrest.

Deimos's surface appears different because most of the craters are partially filled with debris; in many cases they can be identified only because of their bright rims. The two largest, Swift and Voltaire, measure about 3 kilometers across.

The surfaces of both satellites are quite dark, so they are not very effective for lighting up the lonely Martian nights. From Mars, Phobos would appear only about as bright as Venus does from Earth, and Deimos would resemble the bright stars Vega or Arcturus. The Martian moons are thought to be captured asteroids (or asteroid fragments), and in many ways they resemble the asteroids that have thus far been imaged at close range; 951 Gaspra and 243 Ida even have grooves like those around Stickney. There can be little doubt that they are related kinds of objects.

But if Phobos and Deimos are captured asteroids, the details of their capture remain rather murky. Most asteroids stay within the main asteroid belt, but at 2.5 astronomical units (a.u.) there is a clear zone; asteroids there are in a resonance position with Jupiter---that is, they complete exactly three revolutions for every revolution that Jupiter completes. They are, then, regularly disturbed, and as a result their orbits are chaotic. Their orbital eccentricities can become so great that they can even cross the orbits of the other planets---many cross the orbit of Mars, and a few, known as the Apollo group, veer inside that of the Earth.

Rarely, one of these asteroids might be captured by Mars, but if so, it would first have to lose energy, perhaps through aerodynamic drag. Soon after its formation, Mars may have been surrounded by a nebula; an asteroid passing through this nebula would have been slowed enough by friction for its orbit to decay, first into a closed elliptical path around Mars, and later into a more circular orbit. It would continue to spiral quickly in toward Mars until it reached the point where its period became synchronous with the rotation of the planet, after which there would have been little relative velocity between the captured object and the nebula. At this point it would have been stabilized. This would have occurred early in the history of the solar system, when space was still cluttered with rubble. An impact with a stray object later may have broken the synchronous moon apart---the fragment which then became Phobos landed inside the synchronous position, and owing to tidal forces has continued to spiral inward ever since, while that which became Deimos landed outside, close to its present position.

This is plausible enough, but is it true? At the moment we simply do not know; it remains equally possible that the satellites are planetesimals left behind within Mars's gravitational sphere of influence after the planet itself was formed---examples of the kind of objects whose impacts on Mars created the Hellas and Argyre basins during the violent bombardment of the Noachian Age.

We still have a great deal to learn about the Martian moons, but it is sobering indeed to realize that we now have detailed maps of the surfaces of these objects, which for almost a century after their discovery appeared in even the largest telescopes as mere specks of light.

From "The Planet Mars, a History of Observation and Discovery by William Sheehan

© 1996 The Arizona Board of Regents