Earthlike Mars? Options

[glennwsmith]

The work of UMSF members has, on several occasions, risen above the amateur level to represent a real contribution to space science. I refer, in particular, to the graphic work which has appeared in national publications.

May I suggest that the question of the existence of an Oceanus Borealis represents a similar opportunity for UMSF members, and that this opportunity has both a primary and a secondary aspect?

Primary, in that question of an Oceanus Borealis does not depend so much on esoteric data such as, say, methane concentrations, but is to a great extent a question of visual interpretation. What are we seeing when we look at photos of these smooth northern basins? UMSF, with its four thousand plus pairs of educated eyes, can certainly make a contribution. (Indeed, some of the Mars imaging teams have issued a general invitation to interested amateurs to help scan their photos.)

Secondary, in that we can serve as an informed sounding board. Dr. Parker, for example, seemed to enjoy the opportunity of reviewing with us various northern basin scenarios. And, ultimately, a hypothesis which can be explained clearly and convincingly to an educated public is better than one which cannot.

Indeed, what we have with a possible Oceanus Borealis is one of those eureka! moments in science, or, more properly, the emergence of a new paradigm as depicted by Thomas Kuhn in his "The Structure of Scientific Revolutions". (Another example can be found at http://www.worldenergysource.com/articles/...uty_WE_v3n2.cfm -- a lovely instance of the primacy of an idea over the wealth and fame that may flow from it.)

To this end, I hope this thread will remain current, and posts added to it as additional findings become available, and as additional insights are gained.

In fact, I have a possible such insight of my own: Dr. Parker has cited the cobbled surfaces of the northern basin(s) as militating against the frozen ocean hypothesis; but surely someone in the professional ranks has had the idea that many of these cobbles, as on the Antarctic ice sheets, are accumulated meteorites?

(And thanks for the thoughts on relevance of Phoenix data.)

[dvandorn]

My only problem with the concept going around that the *entire* Martian northern hemisphere was excavated down a few km below mean by an enormous impact, whose basin is the entire northern half of the planet, is that I'd have to think such an impact would disrupt the entire planet, causing it to re-accrete rather as Earth and Moon re-accreted after the impact of a Mars-sized body on the proto-Earth.

How could Mars retain its structural integrity during an impact whose crater is roughly half the size of the planet? I'm not a mathematician, but it seems to me that the energies released by such an impact would have to be enough to disrupt the entire planet... in other words, I can't imagine a solid body that wouldn't come apart under such an impact, no matter the angle of impact.

-the other Doug

[Tom Tamlyn]

Doug,

There's an interesting paper by Melosh in Nature Geoscience summarizing recent work on the martian giant impact theory. see http://www.nature.com/ngeo/focus/planetary...CFcZM5QodGTJbCA The paper was briefly available for free, but apparently no longer. (But I may be able to find it on my hard drive.)

If I recall correctly, Melosh said that the energy released by the putative impacter would have amounted to "only" about one percent of the total gravitational binding energy of the planet, and that accordingly disruption and re-accretion would not be expected.

TTT

[Juramike]

As part of an imaging project I've been planning, I've made a list of all the Mondo Big Impacts in the Solar System.
"Mondo Big" I defined as rim or feature diameters > 350 km. Most of the data on the list comes from the USGS Gazetteer.

 Big_Craters_of_the_Solar_System_bigger_than_350_km_20090504.xls ( 21K ) Number of downloads: 805


This list is better entitled as "List of big impact features that have been preserved."

Kinda interesting on the list that in the Jovian and Saturnian system only the outer satellites have preserved craters.
And in the inner solar system, Earth and Venus are notably absent. Either they didn't get whapped (doubtful) or surface process have done a nice job of obliterating the evidence.

Relevant to this thread, the putative Vasititas Borealis on Mars fit's nicely with other basins seen on Mercury and the Moon. It's still on the big size, but not too weird when looking at the other planets.

-Mike

[EDIT: 20090504 2030 Updated coordinates for South Pole/Aitken basin]

[nprev]

That's an interesting observation about Jupiter & Saturn, Mike. Shooting from the hip, I wonder if the fact that both planets probably have had a significant amount of small stuff orbiting in their equatorial planes (the ring systems being mere tattered remnants of the originals) has contributed greatly to erosion of large impact features on their moons, which would presumably tend to form more infrequently and therefore also generally be older.

FWIW, I think you're spot on assuming that Earth, Venus & Mars wipe out really big craters pretty fast, although Argyre & Hellas are proportionately huge. Plate tectonics cleans up Earth rather well, and even the largest expanse of old terrain (the Canadian Shield) preserves only a few hefty ancient craters. Venus' surface looks a little like warm taffy at macroscales, and I bet that it's pretty malleable as silicate planetary crusts go over short geological timescales.

[serpens]

I thought that MOLA had detected a heavily cratered Northern landscape buried by a comparitively thin resurfacing layer - eolian or aqueous deposition or possibly both? If so this would indicate that even if the northern basin was the result of a very early impactor, this occurred before the LHB. So an impact origin and the later formation of a sea would not seem to be mutually exclusive events.

But the northern basin doesn't look particularly circular, nor does it seem to have the depth or the surrounding (kilometer deep) ejecta material that characterises Helles.

[glennwsmith]

Relative to the current discussion, check out this picture of the Rembrandt impact basin on Mercury:

http://antwrp.gsfc.nasa.gov/apod/ap090504.html

This "mare" is clearly, to me, the result of lava flows -- and my money is still on actual H2O, though now frozen, forming the smooth surface of the northern Mars basin.

And as has just been pointed out, correctly I believe, there is no contradiction between an impact basin and a subsequent sea.

[marsbug]

Some more modelling on the idea of early mars being cold and wet.

[serpens]

"]Some more modelling [/url]on the idea of early mars being cold and wet.

I have some difficulty with the concept that the current temperature conditions (cold) applied to Mars in the beginning. To put this in perspective Earth was apparently warm, then we had the snowball earth followed by continuous warmth (always areas above freezing regardless of cyclical swings) . Lots of speculation on sun activity levels, interstellar dust, tectonics etc but no definite explanation for this. So why do we think Mars has always been stable at sub zero temperatures and pass off the clear evidence of major liquid water effects as brines or short term impact phenomona?

Given the evidence of large impactors (including the one that formed Earth's moon) these, and near misses would have caused variations in orbits of the inner planets, particularly for Mars given its size in comparison to Earth and Venus. If we can accept that a Mars sized planetoid collided with Earth then we should accept that there is a possibility that it also had a close encounter with Mars which at that time had an orbit closer to the sun. Potentially such an encounter moving Mars away from the warmth and setting the other body on a collision course for Earth.

Flight of fantasy perhaps. But no more so than trying to explain the water features on Mars purely in terms of impact or brines

[marsbug]

As I understand it we can explain the evidence of water using brines and impact phenomena at the average temperatures we see today, so why do we need to invoke some unknown force to warm the planet up?

I think this discussions been done before on this forum by people more knowledgable than me: Here and here. Have a read if you've got the time (lots of time) , it's interesting and quite heated in places!

Edit: Based on the last couple of paragraphs of this report (I don't have access to the full nature article) the significance seems to be that, in the right combination, salts found at the landing sites of the MER's and viking could depress the freezing point of water enough for a stable liquid to form at those locations. There might be some room to discuss how that gels with the theories already discussed, but we'd need to be carefull not to run the discussion into the ground or cross any lines. Doug and the other mods run a zero tolerance / benevolant dictator regime here. You'll notice one of the threads I linked above has been locked for causing the mods headaches!

[Doc]

As I understand it we can explain the evidence of water using brines and impact phenomena at the average temperatures we see today, so why do we need to invoke some unknown force to warm the planet up?

That is the big problem with in our exploration of Mars. I myself took part in the 'great debate' with Professor Dburt and even though the idea that Mars was probably warm and wet in the past looked ok, I couldn't help but feel that maybe we are on the loosing side

For example, I used to wonder; if Mars was indeed warm in the past, how do we consider the fact that the solar output at that time was probably significantly less than now and probably we should be talking about a tundra Mars instead of an Earthly paradise. The announcement of a cold and wet Mars model ushers in yet another and seemingly more accurate way of how we view the martian geological record.

As for the locked topics; it was probably for the best (we would just be going in circles anyway )

[glennwsmith]

Whoa! Marsbug and Doc, thanks for alerting me to the fact that the question of an Oceanus Borealis is entangled (as of course it must be) with the heated debates regarding basal surge versus water-based processes. I have been involved in that frustrating loop myself when I, along with Dvandorn and many others, remarked on the incredible layering of Meridiani. So part of my goal with this thread is to approach things from a different, simplistic angle: was there (or is there still, in frozen form) a vast ocean in the northern basin? When Phoenix landed on a sheet of ice, and when meteorites at widely spaced intervals are turning up ice, the presence of such seems likely to me. Interestingly, even Dburt advances the possibility of a northern ocean, in post #36 from the thread which Marsbug turned me on to, "Welcome Professor Brine Splat":

"Large amounts of water apparently survived in the subsurface, however, as both ice and (probably) deep brine (as evidenced by occasional catastrophic releases to outflow channels that possibly formed ephemeral seas in the northern lowlands)."

And I will now succumb to the temptation to use an emoticon:

[dburt]

Thanks for the emoticon, Glenn, but do you have a question? If Mars has almost always been rather cold and icy compared to Earth, owing to a much greater distance from the Sun and a paucity of atmosphere, this does not prohibit temporary surface warming (i.e., for perhaps several thousands or hundreds of thousands of years) owing to major meteorite impacts or groups of impacts, nor does it prohibit liquid water from existing on present-day Mars as concentrated brines or as very ephemeral snowmelt in low elevations containing dark (easily heated) basaltic rocks or dust. It also does not exclude local warming and brine beakouts near volcanic centers, although these centers seem to have rapidly declined in number after the end of major meteorite bombardment (the so-called Late Heavy Bombardment or LHB). Given how ice-rich Mars seems to be, soon-to-be-frozen-over lakes filling impact basins or even a temporary sea filling the Northern Lowlands could easily form following a really major impact event or series of events.

That said, 5 years of two rovers wandering across the present-day surface of Mars has as yet revealed no direct geological evidence of standing or flowing liquid water (such as a single shale bed or single pebbly stream channel) in the bedded rocks that make up both rover sites, although various interpretations have been made, entirely on the basis of preexisting expectations and putative terrestrial analogs. All the exposed fine layering at both rover sites is consistently cross-bedded, generally at low angles, and both sites contain enigmatic concentrations of generally unclumped tiny spherules (in distinct layers) and of acid sulfate salts. One site, in which most of the layering is rather coarse (breccias with abunandant lava fragments), contains a distinct horizon with silica-rich fragments, such as might originally have been produced in a boiling (easy to do on low pressure Mars) hot spring related to an impact crater or volcano. AFAIK, both sites contain abundant evidence of meteorite impacts, including evidence of very recent impacts and of actual fragments of meteorites on the surface, but neither site contains locatable volcanic vents.

I don't care to discuss further my own rather obvious and by now way over-explained (to most readers) interpretations of these highly interesting and valuable scientific observations. Occam's Razor, the Rosenthal (experimenter expectation) effect, and all that. Nuff said, although new contrary observations and interpretations remain highly welcome (send me a private message if you wish).

-- HDP Don

[marsbug]

....very ephemeral snowmelt in low elevations containing dark (easily heated) basaltic rocks or dust

I'd suggest that buried ice exposed by small impacts (as seen in this LPSC paper) at low elevations might be a better source of ephemeral liquid water, as it's an idea I've taken a shine to.

[dburt]

Thanks. No problem with that very good idea either, although keep in mind that on very low-humidity Mars, exposed white ice or snow (alone) is much more likely to sublime (simply evaporate) than to melt. That's why I hypothesized dark rock or dust.

-- HDP Don