ROVERS FIND EVIDENCE THAT
MARS HAD LIQUID WATER

Albert Brakel

From the time of Mariner 9 more than 30 years ago, it has been known, or at least strongly suspected, that the striking river-like channels on the Martian surface indicated the presence of water on early Mars. The channels were obviously carved by a fluid, which almost everyone was convinced was water, but there were nagging doubts that the fluid just might have been a slurry of carbon dioxide and suspended solid particles, or very fluid lava, or even mudflows. Plenty of solid water (ice) exists in the polar caps, and as the Odyssey spacecraft recently showed, also in high latitude soils and some at low latitudes. And as the carbonate minerals in the famous ALH84001 Martian meteorite demonstrated, there was at least a little water around when the rock in the meteorite formed, though probably deep in the subsurface. The big questions that remained are: was there ever a large quantity of liquid water at or near the surface of Mars, and was it there long enough to enable life to have originated? The Opportunity Mars rover on Meridiani Planum has taken a significant step to answering these questions by proving unequivocally for the first time the former presence of liquid water on the surface of Mars.

Opportunity bounced to an airbag landing on 25 January, in an area known from orbital scans to be rich in hematite, an iron oxide mineral that has now been confirmed there. By sheer good luck, it came to rest in a small crater that has a ledge of layered outcrop along its inner wall. Close-up investigation of this bedrock has yielded the evidence for the former presence of water. First up, the alpha-particle x-ray spectrometer found a very high concentration of sulphur in the rock. The chemical form of this sulphur appears to be in magnesium, iron or other sulphate salts. Chlorides and bromides have also been detected. These salts are all soluble in water, and are deposited as crystals when salt water is evaporated to dryness, hence their general name of “evaporites”. Further evidence comes from the molds of elongate or platy crystals in the rock, oriented at random angles across the bedding, indicating crystal growth within the sediment after the layers had already been deposited. Subsequently, the crystal material was removed by solution or erosion, leaving the empty molds in place. They strongly resemble evaporite crystal growth in sediments on Earth. The implication is that the sediments were saturated in salty water, and then dried out. The possibilities include deposition in a body of water, or layers of air-fall volcanic ash that later become infiltrated by ground water, the water seeping into pores between the particles like water in a sponge.

Even more telling is the detection of jarosite by the Mössbauer spectrometer, which can identify iron minerals. Jarosite, an amber-yellow to dark brown mineral, is potassium iron hydroxy-sulphate, with the chemical formula KFe3(SO4)2(OH)6. The OH (hydroxy) part can only be derived from water. On Earth, jarosite can form in a number of ways, such as in an acidic lake or acidic hot springs, or by the weathering of ores containing sulphide and iron, where water reacts with the sulphide to form sulphuric acid, which then grabs any available potassium (e.g from clays) and iron to give jarosite.

One more indicator is cross-bedding, which is internal layering within a bed that lies at an angle to the main bedding. Anyone familiar with the sandstones around Sydney will be familiar with cross-bedding. It is formed by migrating ripples or dunes, either by wind or by movement in water. Wind- and water-formed cross-bedding are distinctively different, and that found by Opportunity has the characteristics of water action.

Enigmatic features at the site are spherules of unknown composition. These are spherical objects about 3 mm across that were first found lying loose on the surface, and then found also embedded within the outcrop. They could have formed as concretions; that is, within the sediment by deposition from solution around a nucleus. They could also have originated as molten droplets from volcanism or impact, cooling and solidifying before they hit the ground. In the latter cases, they could be expected to have been concentrated in particular layers in the outcrop, but they are not. The balance of probabilities favors the concretion origin, which again requires water.

On the other side of the planet, the Spirit rover had successfully bounced down on 4 January. The target was Gusev Crater (in Terra Cimmeria), which at one time had obviously been a lake filled by inflow from the channel of Ma’adim Vallis. What is not known yet is whether the infilling of the Gusev lake was a short-term one-off event, or a longer-lasting feature. In at least one place on Mars, Holden Crater in Mare Erythraeum, the Mars Global Surveyor has found a delta deposit that was constructed in three stages, suggesting at least three episodes of flow over a significant period of time.

The pictures of the surface at Gusev do not look like a typical dried-up lake bed. A long-lasting lake would have a bottom draped by mud, with coarser material deposited in the shallower water near the shore line. The mud would consist of clay and silt settling out from suspension, after being brought in by water inflows or blown in as dust. The longer the lake lasted, the thicker the mud deposits would be. Instead we see a surface strewn with boulders. Possibly the boulders were carried in by a major one-off flood event, with the water disappearing too soon to leave much of a mud drape behind. More likely, the boulders are ejecta thrown out from the many small craters that pepper the floor of Gusev, excavating the volcanic bedrock below any thin lake sediments. One such crater, informally named Bonneville, sits about 250 m from the Spirit landing site, and would be a natural drilling site to expose what lies below the surface. The rover is slowly heading there, and its main job on arrival will be to see if any lake sediments exist, how thick they are, and if they hold any evidence for the former presence of water.

In the meantime, Spirit’s rock abrasion tool (RAT) has drilled 2 mm into a 60-cm tall dark volcanic rock dubbed "Humphrey," and found bright material in interior crevices and cracks that looks like minerals crystallized out of water. When molten rock solidifies, the high-melting-point rock-forming minerals crystallize out first, until eventually the only liquid left is a little water that contains materials in solution like silica. These materials then precipitate out to fill any interior crevices. The amount of water involved is tiny, and nowhere near the amounts that were present in Meridiani Planum. Alternatively, water could also have interacted with the rock much later, leaving minerals behind in any cracks.

While the NASA rovers continued on their way, British scientists finally gave up on their Beagle 2 lander in mid-February. The lander was supposed to come down in Isidis Planitia last Christmas, but has not been heard from since. However, Beagle 2’s mother craft, Mars Express, successfully entered Mars orbit and has been sending back some very detailed images.

PLANETARY NEBULAE FOR AUTUMN SKIES

This month we can see the bright Milky Way band stretching overhead like a starry, celestial bridge, and deep within it, are numerous planetary nebulae (PN). Of these PN there is a rich diversity in brightness, sizes, and shapes; where the shape may be stellar-like, a disc, or doughnut (annular). In this article I will mainly concentrate on those that are less talked about, a collection from the PK (Perek, L., L. Kohoutek), and Minkowski PN catalogues.

Most of these PN are very small, about 3.0” in size, and are seen through the telescope as little more than star like; so it is a good idea to use a filter. I generally use an Ultra-High Contrast (UHC) filter, or an Oxygen – III (OIII) filter for planetary nebulae, but these filters are also very good for bringing out detail in emission nebulae. Where I say bringing out the detail, they don’t actually make the object brighter, but instead block out unwanted light which increases the contrast. Though, this process does make the object “look” brighter.

The following observations were made with a 12.5” f/5 Newtonian, form my semi-rural home site at Lobethal, SA. The skies here typically have a limiting naked-eye magnitude of 6.1, with the seeing rated at ANT II-III.

PK 275-4.2 : 09 13.9 –55 28 (Vela)
With a photographic magnitude of 13.5, and a catalogue size of 2.4 arc-seconds, I just couldn’t pick the tiny planetary in the busy star field at 105X. At 150X, I had more success! While it appeared stellar-like in size, and equal to a 13 mag. star for brightness, it could be distinguished from nearby field stars with the OIII filter using the “blinking” method. By this, I mean, without the filter, the planetary was the same brightness as nearby 13 mag. stars. But with the filter in front of the eyepiece, it seemed half a magnitude brighter to these same stars. In the general wider view, there is a group of four bright 7-8 mag. field stars, lying 9.0’ SE, shaping an upside down “Y” shape.

PK 275-4.1 : 09 15.1 –54 52 (Vela)
105X: Obvious enough, as a relatively faint, fat, fuzzy star-like object, and with the OIII filter in place, it was even more pronounced. 150X: Slightly improved – very small, 13 mag. in brightness, 10” in size, and very obvious as non-stellar. Nearby is an attractive little group of three stars forming a small triangle. The planetary lies 6.5’ SW from it. At 360X (with Ian Bedford’s 30” Newtonian): very bright, very small, round disc, 10” in diameter. Enhanced with the UHC filter. Catalogues gives this planetary a photographic mag. of 12.7, and a size of 10.2”, making it an easier target than the previous.

PK 275-2.2 : 09 24.8 –54 36 (Vela)
105X: Difficult to pick out at this power, being almost stellar-like in size, and lying among field stars of the same brightness. 150X: Much improved view – very small, relatively bright, almost star-like, but can be easily noticed as slightly larger in size to nearby 12 mag. field stars. The OIII filter enhances it even more.

PK 279-3.1 : 09 43.4 –57 17 (Carina)
150X: Obvious as a very faint, very small, fat, fuzzy star, equal in brightness to an 11 mag. field star. OIII filter shows it best as a bright, fat, fuzzy star.

PK 283+2.1 : 10 31.3 –55 21 (Carina)
105X: At first, the eye is drawn to what appears as three 12 mag. stars in a triangle, but on a closer inspection, the southern star seems a little soft around the edges, or fuzzy. With the OIII filter in place, the planetary brightens half a mag. over it’s two stellar companions. 150X: Improved view – very small, round, 7” in size, and strong response to the OIII filter.

PK 288+0.1 : 10 54.6 –59 10 (Carina)
150X: Best with OIII filter – relatively bright, round, smooth disc, 25” in size. Without the filter, I cannot detect it. The planetary lies in a rich star field, and the background sky seems uneven which is probably from the nebulous extensions of the nearby Eta Carina nebula.

The following observations were made with the 30-inch f/5 Newtonian telescope, belonging to Ian Bedford of Riverton, SA. Typically, the skies here have a limiting naked eye magnitude of 6.5, with the seeing rated at ANT II-III.

McNeil’s Nebula: near M78. (Orion)
360X: Relatively bright, large, round, with one – two faint stars involved. 480X: Bright, large, round, 1.0’ in size, with one star, and a small, round, hazy knot.

Minkowski 1-14 : 07 28.0 –20 13 (Puppis)
480X: Relatively bright, very small, almost star-like, 3” in size, and brightens towards the centre. Best with the UHC filter to pick it with full confidence. The catalogues give this planetary a photographic magnitude of 13.4, and a size of 13.0”.

Minkowski 1-16 : 07 37.3 –09 39 (Monoceros)
240X: Very faint, and star like in appearance, but the UHC improves the view. The planetary is a bright, very small, round, hazy spot. The catalogues give this object a photographic magnitude of 13.2 and a size of 3.0”.

Minkowski 1-17 : 07 40.4 –11 33 (Puppis)
480X: The planetary appears star-like and is difficult to find, but the UHC picks it out easily. Without the filter, it is little more than a faint star, but with the filter it is relatively bright, very small, round disc, 3” in size.

Minkowski 1-18 : 07 42.1 –14 21 (Puppis)
240X: Extremely faint, relatively large, round, 30” in size. With the UHC filter, the planetary appears bright. 480X: Faint, smooth glow throughout, with a faint stars on the NW and SW edges.

Minkowski 3-4 : 07 55.2 –23 38 (Puppis)
With a photographic magnitude of 11.8 and a size of 20.0” this planetary was easy at 120X, appearing as a bright, very small, round, hazy spot. There is a very strong response to the UHC filter. 240X: bright, 20” in size, with the 15.7 central star suspected.

Longmore 5 : 11 13.9 –47 57 (Centaurus)
120X: Obvious enough, as a faint, large, round, hazy glow. Much improved with the UHC filter, showing a bright, doughnut shape, 3.5’ in diameter. 240X: Relatively bright annular, with a faint star at the centre, which may or may not be the true central star. Seems to be a little brighter along the western edge. 360X: There can be seen some six stars around the rim. Some catalogues place this planetary some 8.5’ too far south.