A road trip from Cassini to Plato via Valles Alpes and return via Mare Imbrium.

Our journey commences in Cassini Crater on the bottom-right, with its modest ramparts and a couple of significant impact crater that lie entirely within the rim of Cassini. The drop from the crater rim to the crater floor of Cassini is 1200m.

Heading NNW (conveniently the image is looking towards lunar north) we pass two prominent capes: Promontorium Agassiz and Promontorium Deville that project into Mare Imbrium. These lie at the southern end of the Montes Alpes mountain range which defines the NE border of Mare Imbrium. The Montes Alpes is part of the middle ring of the multi-ringed Imbrium impact basin, the other mountain ranges around the impact basin are Montes Caucasus, Montes Apeninnus and Montes Carpatus. Heading further north along the range, Mont Blanc with a base of around 25km, is the third highest peak in the Montes Alpes and rises to a height of 3.7-3.8 km above the floor of Mare Imbrium.

Travelling further north we encounter the western end of the spectacular Vallis Alpes, a lunar valley that bisects the Montes Alpes, and runs for a full 166 km, linking Mare Imbrium (Sea of Showers) to Mare Frigoris (Sea of Cold). It is a graben that was most likely filled with lava flowing in from Mare Imbrium and Mare Frigoris. In the centre of the Vallis Alpes is a rill that runs along the floor of the valley and is longer than the valley itself. It is often hard to spot by telescope, but can be teased out with good image processing.

From the eastern end of Vallis Alpes we move NNW along the western shore of Mare Frigoris until we encounter Rima Archytas that meanders westward and leads to the 180 km long Rimae Plato, the remains of a collapsed lava tunnel. The low lighting angle nicely accentuates this sinuous rille that leads up the eastern flank of Plato crater.

Plato Crater is the giant (101 km wide) lava-filled impact crater in the upper part of the image, and at 3.84 billion years old it is only slightly younger (about a 100 million years younger) than Mare Imbrium. Whilst circular in shape, viewed from earth it is slightly oval shaped due to fore-shortening. Five prominent craterlets are seen on the floor of Plato, and the higher resolution image that I have placed on Astrobin lets you see a further 9 smaller craters (just; my limit of resolution in this image is about a kilometre). The western rim of Plato crater has a large block that has partly slumped eastward onto the crater floor. Heading west from this point is a rille that is sourced form a small volcanic vent and which directed lava flow westward then southward into Mare Imbrium. For truly spectacular images of this region you can visit - http://lroc.sese.asu.edu/posts/1077 (go to bottom of that page) to see an LROC image that lets you zoom in almost to ground level on this slump block.

Our return route takes us out onto the floor of Mare Imbrium as we head SE passing a series of mountain peaks that project through the Mare Basin. Mare Imbrium formed around 3.9 billion years ago by the impact of a likely proto-planet which liquefied the lunar crust on impact to form the 1145 km wide Mare Imbrium basin. U-Pb age dating on samples of Mare Imbrium basalt have yielded radiometric ages of 3938 million years (+/- 4 Ma). Mare Imbrium is second only in size to Oceanus Procellarum amongst the maria, and is the largest impact mare. The size of the impactor (the colliding proto-planet) is modelled at 250 km (+/- 25km). Peaks that project through the smooth floor of the mare are Montes Teneriffe, Mons Pico and Mons Piton. Between Mons Pico and Piazzi Smyth we traverse a couple of subtle compression ridges (wrinkle ridges) that run across the floor of the basin.

As we pass Mons Piton, we see Promontorium Agassiz on the horizon just 50 km away, and drive across washes of light-coloured fine ejecta that form part of the radial ray system that emanates from the Aristillis crater located south of this image, before completing the loop of our tour back at Cassini Crater.

Imaged 27 September 2020 from Lysterfield, Melbourne.

Skywatcher 8 inch/f5 Newtonian, ASI24mc, Televue 5x Powermate, total focal length 7000mm.

4 spliced image panels. Each panel comprises a stack of 750 sub-frames.

Data source – 4 x 2 minute video runs with ASI224mc planetary cam.