Mars is the most studied planet other than our own, currently playing host to no less than 7 different craft currently operating both in orbit and on its surface. It’s of interest to us due to its similarity to Earth, giving us an insight into how certain processes can affect planets differently. Mars is also the easiest of our sister planets to explore, being relatively close and having an atmosphere that won’t outright destroy craft that dare land on it. Still for all that research it still manages to surprise us, most recently by revealing the fact that liquid water still flows on it. We’re still far from done with it however and the MAVEN craft has just revealed some key insights into Mars’ atmosphere and the history behind its current state.
Mars’ atmosphere is extremely thin, over 100 times less dense than the atmosphere here on Earth. To put that in perspective that’s about the same density as the air here is on Earth at an altitude of about 30KM, or about 3 times as high as your typical jet airliner flies. It’s also almost all carbon dioxide with a small smattering of nitrogen and other trace elements. However it wasn’t always this way as numerous studies have revealed that it must have held a much thicker atmosphere in the past. What has remained something of a mystery is just how Mars came to lose its atmosphere and whether those same processes were in effect today. MAVEN, a craft specifically designed to figure this out, has made some key discoveries and it seems that the long held belief that the sun is to blame is true.
For a planet to lose its atmosphere there’s really only two places it can go. In some cases the planet itself can absorb the atmosphere, driving chemical reactions that pull all the gases down into more solid forms. This specific scenario was investigated on Mars however the lack of the kinds of minerals we’d expect to see, mostly carbonates given Mars’ mostly carbon dioxide atmosphere, means that this was unlikely to be the case. The second way is for it to lose the atmosphere to the vacuum of space which can happen in a number of ways, usually through the planet being unable to hold onto its atmosphere. This latter theory has proved to be correct although it’s far more interesting than Mars simply being too small.
In the past Mars would have looked a lot like Earth, a small blue marble wrapped in protective gases. Back then the core of Mars was still active, generating a magnetic field much like that on Earth. However, after a time, the core began to cool and the engine behind the giant magnetic field began to fade. As this field weakened the solar wind began to erode the atmosphere, slowly stripping it away. Today Mars’ magnetic field is around 40 times weaker than Earth’s, no where near enough to stop this process which is still continuing to this day. For Mars it seems that its diminutive core was what sealed its fate, unable to sustain its protective magnetic shield from the relentless torment of our sun.
Whilst this has been the prevailing theory for some time its good to get confirmation from hard data to support it. Our two closest solar relatives, Venus and Mars, provide insights into how planets can develop and what changes produce what outcomes. Knowing things like this helps us to understand our own Earth and what impacts our behaviour might have on it. Mars might not ever see its atmosphere again but at least we now know what it might have looked like once, and where it has gone.
Mars is by far the most studied planet that isn’t our own, having had 46 separate missions launched to it since the 1960s and is currently host to no less than 5 active missions both in orbit and on its surface. Those missions have taught us a lot about our red celestial sister, the most intriguing of which is that it was once not unlike Earth, covered in vast swaths of ocean which could potentially have been host to all sorts of life. Even more interesting is that while it’s little more than a barren desert that’s only notionally above vacuum it still contains water ice in non-trivial quantities, leading many to speculate that somewhere its liquid form must also exist. The process by which Mars transformed from a lush landscape like ours to the wasteland it is today is still shrouded in mystery and is something that MAVEN, NASA’s latest mission to Mars, is seeking to solve.
MAVEN successfully launched yesterday atop of an ATLAS V rocket and will spend the better part of a year transiting the distance between Earth and Mars. Its primary objective is to investigate the evolution of Mars’ atmosphere to try and ascertain the factors that influenced its demise. Since the current prevailing theory is that a cooling planetary core led to a loss of a protective magnetic field which then allowed the solar wind to slow strip away the atmosphere many of the instruments aboard the craft are geared towards measuring solar particles around Mars’ orbit. The rest of the instrumentation is focused on directly measuring Mars’ atmosphere which will then allow scientists to reconstruct a full picture of it and the influences working on it.
I believe this is also (and someone feel free to correct me on this) the reason for its slightly abnormal orbit for when it arrives at Mars. Instead of taking the usual approach of having a near circular orbit (like the Mars Reconnaissance Orbiter) it instead has a highly elliptical orbit with the closet approach being a mere 150KM above the surface whilst its furthest point is 6200KM out. This would allow the craft to get good measurements of the levels of solar particles as it gets closer to the surface and how that compares to it further out. Considering the orbital period will also only be 4.5 hours it would make for some rather exciting flybys if you were aboard that craft but then again that’s not an orbit you’d use if you had people on board.
The orbit also has the rather unfortunate effect of limiting one of MAVEN’s more long term capabilities: it’s link back to Earth. MAVEN has a 10Mbit/s link thanks to an updated Electra array which is almost twice as powerful as MRO’s. However due to the rather eccentric orbit it won’t be available as often which will limit the amount of data that can be passed back. This doesn’t just impact the satellite itself though as whilst the rovers on Mars can communicate directly to Earth it’s not a very fast connection, so most offload onto a local satellite for their more data hungry applications. Since it’s currently only an augment to the other fleet of satellites around Mars this isn’t too much of an issue although it could present some contention issues later on the track when the other satellites are retired.
The science that MAVEN will conduct on its planned 1 year mission will prove invaluable in determining just what happened to Mars’ atmosphere and, by extension, what the chances are of any life existing on its surface today. It will also provide infrastructure for future missions, allowing them to be more ambitious in the goals that they attempt to reach. For now though it’s 1 day into its long trip to our celestial sister, quietly awaiting the day when it can finally start fulfilling its purpose.