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| Close up on the heights (-923.8 elev.) of Hansteen α (AKA, "Mons Hansteen" and "the Arrowhead"), a triangular berg, 25 km long on its three margins, composed of intrinsically bright material and rising here 1030 meters above its neighborhood in south Oceanus Procellarum. LROC Narrow Angle Camera (NAC) periapsis observation M166175569LR, spacecraft orbit 9623, July 24, 2011; 61.74° incidence, slew 16.45° west, resolution 0.5 meters from 41.05 km (Enlarged image HERE.) [NASA/GSFC/Arizona State University]. |
Mons Hansteen (12.2°S, 50.21°W) is a familiar nearside landmark, when viewing a Moon that's almost Full through a modest telescope. As the reader can see in the picture of the Full Moon at the end of this post, it stands out from its surroundings in the far south Oceanus Procellarum.
It's nicknamed "the arrowhead" because it looks like one, like, long-ago, it was knapped to a point by the patient hand of a hunter and now rests
half buried in the darker mud of a trail, perhaps uncovered by a recent downpour.
Close investigations of the Moon over the past half century reveal what investigators call "Hansteen Alpha," or Hansteen α, stands out geologically and in other ways. The small mountain is made of different stuff than most of what is found on the lunar surface and the volcanism that flooded and re-flooded the Moon's basins. It's optical brightness is complimented by differences at other wavelengths as well, presenting a spectral profile found in only a few other locations on the Moon.
Hansteen α is one of the Moon's red spots, bright with an albedo similar to the lunar highlands but spectrally red, brighter in shorter wavelengths and characterized by absorption in the ultra-violet (UV).
Moreover, it must be a younger feature than it might seem on first glance, younger than certain of the larger craters nearby and other features in its neighborhood that were clearly overrun repeatedly by the darker basaltic lavas that periodically flooded and re-flooded the lower elevations over a period nearly three billion years long.
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| The heights in the first image are at lower right in this 3.94 km-wide field of view, the full width of the area captured from LRO in LROC NAC M166175569LR, and showing a cross-section of Hansteen α from that central area north to more lower elevations, nearer the mountain's northwestern margin. (View larger sizes HERE.) [NASA/GSFC/Arizona State University]. |
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| Hansteen α is younger than craters Billy (45.57 km across and 3.88 billion years old, to the south) and Hansteen (45 km across and 3.87 billion years old, to the west), because both are less than one and a half times their respective diameters in distance, and the impacts that excavated these features should have at least partially covered the bright mountain. Instead, no evidence of such a direct effect has been found, only peppering of more recent impacts. LROC Quickmap at 250 meters resolution [NASA/GSFC/Arizona State University]. |
Hansteen α resembles the Moon's highlands but, beginning early in the post-Apollo era, investigators noted differences in texture, color and measured albedo. In the close-ups at the beginning of this post, showing some of the highest elevations of Hansteen α, depressions can be seen clustered on terraces. These seem to have once been volcanic vents. Volcanic vents are not particularly rare on the Moon, but the kind of material that emerged from these is clearly not the same stuff that flooded its surroundings.
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| A closer look at Hansteen α elevations, with locations marked of areas shown in LROC NAC observations posted here, the highest elevations and, further along along, some solidified flows on the steep southeastern margin, included below. LROC Quickmap at 64 meters resolution, together with the LROC WAC-derived digital terrain model [NASA/GSFC/Arizona State University]. |
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| Using an experimental 3D visualization tool, accessed through the LROC Quickmap. a 152 square km area, centered on Hansteen α, animated between 0 and 10 X vertical exaggeration, further illustrating highest elevations, southwest of the formation's center; something more difficult to measure using 2D overhead photography [NASA/GSFC/Arizona State University]. |
The margins of Hansteen α seem abrupt, with steeper slopes than is found around the edges of the more common basaltic domes. This might be in keeping with suggestions that the kind of lava emerging into and out of this feature was thicker, related to its composition and the heat necessary for melting and transport. It also provides geologists with clues about its age relative to the volcanism that flooded Procellarum in this region.
The Hansteen α might
have formed from a "secondary," and "more evolved" volcanism, something certainly less common
than the basaltic lavas that flowed out into the
Moon's broad basins, not once but repeatedly, over a 2.7 billion year period,
from the Nectarian age, when the Moon and Earth were only 600 million
years old, until just prior to the Copernican period 1.2 billion years
ago.
These more recent high-resolution images from LRO show groupings of blocky boulders, mostly related to mass wasting along slopes though some of these clusters are found on level areas and don't appear to be the result of impacts.
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| An intrusive flow, clearly visible on the abrupt southeastern margin of Hansteen alpha, right where it solidified, This is a
3.93 km square field of view from LROC NAC observation M1129816872R, orbit 18636, July 30, 2013; 42.35° incidence, 80 cm per pixel resolution from 80.7 km over 12.2°S, 310.14°E [NASA/GSFC/Arizona State University]. |
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| Contextual
view of the full 7.8 km-wide field of view swept up in the same LROC NAC observation, showing a wider view of southeast margin of Hansteen α. Such close-ups reveal that the margin here is more abrupt than it seems at a distance. (Enlarged views and various other sizes available HERE.) [NASA/GSFC/Arizona State University]. |
So, just what is Hansteen alpha? In two papers presented in 2011 and 2012 to the Lunar and Planetary Science Conference, Hawke, et.al., representing some of the more noted investigators working with data returned from LRO (the more recent of these being "The Geology and Composition of Hansteen Alpha," 43rd Lunar and Planetary Science Conference (2012), #1754), wrote, "Non-mare volcanism is the only viable process for the formation of Hansteen α."
That paper, along with others those investigators cite, along with still others cited as references below, present truly fascinating discussion of how instruments on-board Clementine (1994), Lunar Prospector (1998-99) and both the LROC cameras and the Diviner instrument, flying on LRO since 2009, are actively being used to weed out the history of this unique feature and how it relates to the complicated volcanic stratigraphy of the Procellarum basin.
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| Finding Hansteen α through a modest telescope is relatively easy, from about four days after First Quarter through three days after Last Quarter, on the south edge of Oceanus Procellarum, as shown in this well-crafted mosaic by Stephan Lammel. Look for it left of center in the inset and in the Full Moon, above. |
References:
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