Over time, landers became rovers, and the experiments continued.  One of the surprise detections on Mars has been the occasional, but very clear, detection of atmospheric methane.  This gas is often associated with biological activity (biogenic methane), and may indicate the presence of microscopic organisms on, or below, the surface of Mars.  Alternatively, it may relate to geophysical processes - natural processes occurring along tectonic fractures involving heat, pressure and water.  Or thermogenic methane may have been produced by the breakdown of buried microorganisms (usually associated with sub-surface gas and oil reserves on Earth).  The detection of methane by the Curiosity rover in 2013 caused many eyebrows to raise within the planetary science community. 

The methane emission was certainly real - and has been confirmed by instruments on-board the Mars Express Orbiter (3). However, even now, 6 years on, it is not clear what the origin of the methane was.  Strangely, though, the overall levels of methane in the Martian atmosphere have been found to be very low:

"The ExoMars Trace Gas Orbiter’s first analysis of the Martian atmosphere at various points around the globe finds an upper limit of methane 10–100 times less than all previous reported detections." (3)

This still corresponds to 500 tonnes emitted over the 300 year lifespan that methane is thought to have in the atmosphere.  Scientists are now trying to figure out what is happening to the plumes of emitted methane that had been previously detected - is there a mechanism close to the surface that is removing methane?  Mars is particularly prone to short wavelength UV radiation, which may be driving speedy chemical reactions with the methane. In other words, I wonder whether methane is extracted from the atmosphere by natural processes much quicker than it is on Earth. There are mysteries about the origins of the plumes of methane detected, as well as the overall absence of the gas in the atmosphere.

Recent arguments have been offered around the trapping of methane in permafrost, which is periodically released by tectonic activity breaking the ice (4).  Could this Martian methane be associated with pockets of underground natural gas or oil?  There are good reasons to consider this as a possibility.  In 2000, NASA Ames research scientist John McGowan argued that the decomposition of ancient oceanic life on early life-friendly Mars could have built up the kinds of oil and gas reserves that are so familiar to us on Earth:

"If Mars possessed an Earth-like biosphere at one time in the past, Mars may contain sub-surface deposits of oil and natural gas which would constitute evidence of past life. Life might still exist in these deposits." (5)

Evidence for ancient lakes and oceans were suggestive of such a possibility 20 years ago.  At that point, there was also emerging evidence of movements of ground water across the Martian surface, as well as an understanding of the changing Martian climate over long time periods, suggesting that Mars was warmer and wetter as early as 300 million years ago (6).  Since then, there has been a growing recognition that water may have moved across the Martian surface in relatively recent times.  It has been shown that substantial rivers flowed intermittently across the surface of Mars until perhaps just 1 billion years ago (7), in contrast to scientific orthodoxy about the much earlier 'death' of Mars. The run-off of water feeding these large rivers must have been from precipitation - Martian rain.  Yet, the current thin atmosphere of the red planet is incapable of supporting this kind of weather.  This implies an evolving atmosphere over time, in keeping with the highly changeable nature of Mars' orbit.  Drawing upon all the most recent discoveries, I argue for this in the chapter entitled 'Meandering Mars' in my new book 'Darker Stars' (8).

   Darker Stars: New Evidence

A good proportion of the Kuiper belt consist of binary objects, and this has led to the idea that planetesimals initially form as pairs, rather than as solitary objects.  The recent flyby of Ultima Thule by the new Horizons probe has served to intensify this debate, as this object was found to be a fascinating example of a bilobate object (1).

To test the binary hypothesis, as well as to learn more about the KBO populations, the Hubble Space Telescope will undertake a lengthy project to scope over 200 objects, measuring their binary/solitary properties, and their colour.  This is the reason why these properties may be connected: 

"Recent models of small body formation suggest that binaries are leftovers of the very earliest times of our solar system, when pairs of bodies could form directly from collapsing swarms of small-scale “pebbles.” Competing theories of planetesimal formation predict different size and color distributions for binary and solitary KBOs. If objects first formed through an accretion process and were merged into binaries later, scientists expect the objects in binary systems to have dissimilar colors and to have a different size distribution than solitary objects. However, if planetesimals formed through a rapid collapse process that produced some solitary objects and some binary systems from the start, scientists would expect objects in binary systems to have a similar surface color and a size distribution similar to that of solitary objects."  (2)

NASA has awarded this programme, known as the Solar System Origins Legacy Survey (SSOLS), to the Southwest Institute (3).  It builds upon the previous work of OSSOS and CFEPS, this time targeting previously identified candidate KBOs from these large surveys.

2)  Southwest Research Institute "SwRI to conduct largest ever Hubble survey of the Kuiper belt", 2 April 2019

swri.org press-release

3)  Alex Parker "Testing Origin Theories" SSOLS

"The outer gap is deep, wide, and largely a dust-free zone, leading astronomers to believe that a giant planet almost the mass of Saturn is orbiting here — around 800 light-minutes from the central star, and more than three times the distance between Neptune and the Sun!" (1)

An international team of astrophysicists have found a correlation between comet hyperactivity (particularly among the Jupiter Family Comets) and the deuterium-hydrogen isotope ratios of their ejected waters (1).  Many of them, although not all, exhibit similar values for the D/H ratio as that found in ocean water on Earth.

Because of its position in the inner solar system, the Earth should have formed as a dry, desiccated husk: It is too close to the Sun for water to have survived on the protoplanet.  Yet, as we know, water is available in abundance on this planet.  This is a mystery which I have explored in some depth in my books 'Dark Star' (2005) and 'Darker Stars' (2018), because this could provide evidence for alternative ideas about the origin of Earth.  The science in this area is complex and multi-factorial (which is why plenty of space needs to be devoted to it within a book).  Nonetheless, the variation in D/H ratios across the solar system can potentially tell us much about the conditions under which the Earth formed, and whether the water was added later by comets and/or asteroids.

The new paper considers whether some of the variation in cometary D/H ratio values is down to how much of a comet's surface is active.  Some of the ejected water in so-called 'hyperactive comets' has been found to be tied up in ejected dust grains from the comet, which then release the additional water into the coma by sublimation.  Because these hydrated dust grains seem to preferentially combine with deuterated water, active comets ejecting the grains into space exhibit high relative abundances of deuterated water.  As a result, a correlation emerges between hyperactive comets and terrestrial water values.  Other less-active comets do not seem to undergo this fractionation of water in quite the same way, likely because of a lack of water ice on or near their surfaces.

If this finding could be extended broadly amongst comets (and there are plenty of reasons why it might not!) then the explanation for the origin of terrestrial water may swing back towards comet deposition through collision.  The location where comets form is crucial here.  If they form beyond the 'snowline' in the solar system, then their water content is explainable, and a trend towards higher deuteration could build as the distance of formation extends outwards.  Again, this is complicated by the potential for many bodies in the solar system to migrate about and change their locations over time. 

I've often argued that the Earth's initial position could have been further out than it currently is, to a location beyond the 'snowline' >3AU from the Sun. Such a possibility has one major advantage over the assumption that the Earth formed in its current location: it would readily explain the presence of its water.  I recently put this to leading astrophysicists Michele Bannister, who replied with the following point:

"The problem is not so much migrating Earth as getting it to survive Jupiter migrating, particularly if the grand tack was indeed an event. So it doesn't tend to get moved more than a bit of an AU. Inner solar system has a lot less room to play with." (2)

I'll be honest - I have a different basis for how Earth got shunted into the inner solar system... but then that's all very speculative.  I think there's some flexibility built into this, because the theoretical models generally assume that only the current set of planets are players in the early solar system, with Jupiter and Saturn (and to some extent Neptune) being major players in any major migratory events.  The Nice model (incorporating the 'grand tack' Michele mentions) may be the generally accepted model (and does, to be fair, explain a fair old bit), but there remain other tantalising possibilities, too.  Shunt the Earth out a bit at the beginning, and terrestrial water become a lot less of a puzzle. 

References:

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